‘J HE’fil'fi 'L‘A“UW¢. ."-'- .wao‘v This is to certify that the dissertation entitled PHOTOIONIZATION MASS SPECTROMETRIC STUDY OF HALOGENATEI) METHANES presented by Frank Cheng-Yu Wang has been accepted towards fulfillment of the requirements for Ph.D. degree in Chemistry a jor professor Date September 12, 1983 MSU is an Affirmative Action/Equal Opportunity Institution 0‘12771 _ 'u. -.- -‘ ,_._, ...‘..Z.n~' -. _ .. .m iflaAfx. .1.- 351"” - . _ _ '- v- ‘ _ . .1..me - —._—, P456 “- ——_—— RETURNING MATERIALS: Place in book drop to remove this checkout from your record. FINES will be charged if book is returned after the date stamped below. ti 9 “extra ? 4-; PHOTOIONIZAIION IASS SPECTROIEIRIC STUDY OF HALOGENAIED IETHANES By Frank Cheng-Yu Ian; A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 1983 ABSTRACT PHOTOIONIZATION IASS SPECTROIETRIC STUD! OF HALOGENATED IETHANES By Frank Cheng-Yu Wang Photoionisation mass spectrometry(PIIS) is a powerful technique for the investigation of ionization and fragmentation processes of molecules. Accurate ionisation potentials(IP) for parent ions and appearance potentials(AP) for fragment ions can be measured directly and the fragmentation mechanism elucidated. In this dissertation, a PIIS investigation of fourteen different halogenated nethanes is presented. The energetic and dynamic information obtainable from these results is discussed. lass spectra-obtained under full illumination of the helium discharge-of CF,ClBr, CF .. cr,c1, CF,Br, CF,I, cr,c1 crc1,. cc1,. a! CCl,Br. CF33r3, CflF,, CHF3C1. CHFCla and CHCl3 are reported and demonstrate the purity of the samples. Photoionization efficiency(PIE) curves-corrected for detector background counts, stray light, and sample pressure variations-have been obtained for all the parent ions Frank Cheng-Yu Wang (except CF‘. CCl4 and CEF,) and nest of the abundant fragment ions. Appearance potentials extrapolated from fragment PIE curves may be too high due to activation energy of the fragmentation reaction. kinetic shift. competition with other fragment channels. etc. Thus. the heats of formation calculated from experimental APs will always be upper ~ limits to the desired adiabatic value. An improved estinate of the adiabatic value can be extracted by comparing. for a given fragment ion. ango obtained from several precursors; the lowest. and thus "best” such values are presented. Fragment ion appearance potentials are generally consistent with the dissociation mechanism implicit in the quasi-equilibrium theory of nass spectrometry. However. consideration of alternative decomposition pathways for parent halomethane ions produced in excited electronic states suggests that dihalogen cation fragments can be formed in a few vibrational periods. i.e.. under kinetic control. A qualitative discussion of the relative stabilities of the trihalomethyl and dihalomethylene cations is also given. To Tsui-Cheng To Mom To Dad ACINOILEDGIENTS Iy heartfelt thanks and gratitude are given to two special people; without them this work could not have been accomplished. Hy wife. Tsui-Cheng has accompanied ne throughout my graduate studies and has “supported and encouraged me in my research endeavors. ly preceptor. Professor George E. Leroi. provided support in nany ways. His warmth and his guidance. on matters academic and personal. are particularly appreciated. He helped instill self-confidence and independence. and I feel most fortunate to have had the chance to work with him. I thank Professor R.H.Schwendeman for his great help in the preparations for my first seminar and second-year oral. and for his careful reading of the dissertation. I am also grateful to Professor J.Allieon for ‘his many helpful suggestions and discussions during the course of this research. and for the considerable effort he expended as the second reader of my thesis. Many members of the ISU Chemistry Department service staff were of particular help in solving problems relating to and in the maintainance of the instrumentation used in this work. I wish to thank especially Tom Atkinson. Iarty Rabb. lanfred Langer and Russ Geyer. Financial assistance from the National Science Foundation and from the Chemistry Department in the form of teaching assistantships and the 1983 Onion Carbide Corporation Summer Research Fellowship are also gratefully acknowledged. iii TABLE OF CONTENTS Li‘t Of T‘blo‘OOOOOOOOOOOOOOOOOOOOIOOOOOOOOOO0.0.0.0...OOOOOOOOOOOOIix L1.t 0f Fi.ur°‘........0.0.0.0...0.....0...0.0.0.0000...OOOOOOOOOOOxii m m. MODUflIWOOOOOOI.0...0...0..O0.00.00.00.0000000000001 CHAPTER T'O. PBOTOIONIZATION MASS SPECTIOIETII: AN OVERVIEW.........8 A. Photoionixation...............................................lO 1. Direct Ionization.........................................12 2. Antoionixation(Preionixation).............................19 3. Fragmentation Processes...................................30 Experimental Tbchniques for Measurement of Ionization and Appearance Potentials.........................................45 1. Photoionisation Mass Spectroscopy.........................46 2. Photoioanhotoelectron Coincidence Spectroscopy...........51 3. Other Tbchniques for Measuring Ionization and Appearance Potentials................................................55 (1) Optical Spectroscopy.................................55 (2) Threshold Experiments................................57 (a) Photoionixation................................57 (b) 'Electron Inpact................................58 (i) Monoenergetic..........................59 (ii) Quasi-monoenergetic....................59 (iii) Non-monoenergetic......................60 (3) Electron Spectroscopy................................61 (a) Photoelectron Spectroscopy.....................61 (b) Auger Electron Spectroscopy....................62 iv (c) Resonant Photoionization.......................63 (d) Penning Ionization.............................63 (4) Other Methods........................................64 (a) Surface Ionization.............................64 (b) Charge Transfer Spectra........................65 m m. mam BEAVIOR...O..0.OO.0.0.0....0.0.0.0000000067 A. C. D. Quantum Theoretical Considerations............................68 1. Ionization Threshold Law..................................68 2. Autoionization Bffects....................................10 3. Franck-Condon Factors.....................................7l 4. Degenerate Ionic States...................................78 (a). SpinfOrbit Coupling.................................79 (b). Configuration Instability...........................81 Thermal Bffects...............................................83 1. Hot Bands.................................................83 2. Thermal Telling...........................................85 Kinetic Effects...............................................90 1. Activation Energy and Heat of Reaction....................91 2. Kinetic Shift.............................................92 3. Reaction Path and Reaction Competition....................93 In‘tm.nt Effect'OOOOOOOO0.0.0.0....000......0.0.00.00000000093 CHAPTER FOUR. EXPERIMENTAL APPARATUS AND PROCEDURES................96 A. IntrOdnctionOOIOOOOODOOOOOOOOOOOO0..0.00.00.00.00.00.0.000000096 n. In‘tmont. O C O O O O O O O O O C O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O .96 1 O Li‘ht souc. O O C O I O O O I O I C O O O O O O I O O I O O O O O O O O O O O O O O O O O O O O O O O .96 2. bmchrm‘torOOOOOOOOOOOOOIOOOOOOO0.00.0.0.0...00.0.0.0..104 Ionization Region and Sample Inlet System................105 Photon Transducer........................................106 Ion Optics and Mass Spectrometer.........................107 Ion Transducer...........................................108 Vacumm System............................................108 Interlock System.........................................109 Instrument Control and Data Acquisition..................110 Ce hpcrh.nt.1 PIOCOdur.eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeellz 6. Samples..................................................112 Systen Pump-Down.........................................113 Lamp Spectra.............................................ll4 Mass Spectra.............................................115 Experiment Set-Up........................................116 D‘t. corr.°ti°n..O.O.OOOCIOOOOOOOOOOOOOOOOOOOOO0.00.00.00111 MEMO mm!“mDISWSSIWOOOOOOO0......0.00.00.00.00000119 A. ”sult‘OOOCOOOOOOOOOOIOOOOOOOOOOOOOOOOOOCOOOOOOOOOOO000......119 cs,c1sr..................................................122 cr‘133 cr,c1....................................................144 cr,sr....................................................153 cp,1.....................................................162 cr,c1,...................................................173 CFC1,....................................................183 cc1......................................................193 cc1.Br...OOOOOOOOOOOO0......00....OO00.0.0000000000000000198 10. manta...00.0.0.0...0.0.00.0.0....OOOOOOOOOOOOO00.0.00000208 vi 11. M'OOOOOOOOOOOOOOO00.00.000.000...0.0...0.0.0.0000000000215 12. mael...0..OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO0.0.00.00000222 13. mc13000OO..0O.0O0.0...0......O0....0.0.0.00000000000000229 14. mc1.000000000000000000000000000....00.00.000.00000000000238 B. Di‘cn.'ion..0...OCOOOOOOOOOOOOOIOOOOOO0.0.0.0...0.0.0.0000000245 1. The Heats of Formation of Fragment Ions..................245 4. 5. (1) a.+OOOOOOOOOOOOOOOOOOOOOOOOOOCOOOOOOOOOOO00.000.00.245 (2) CF3CI+OOOO.0...00......OOO...O.0.00.00.00.00000000002‘9 (3) wela+00000000OOOOOOOOOOOOOOOOOOOOOOOOOOOOOIOOO0.00.252 (4) CCI.+OOOOOOOOOOO.0...OOOOOOOOOOOOOOOOOOOO0.00.00.00.25: (s) CF33:+OOOOO.O.OOOOOOOOOOOOOOOOOOOIOOO.00000000000000257 (6) CF,*................................................2co (7) c3c1*...............................................263 (s) cc1,*...............................................263 Qualitative Consideration of Parent Ion Fragmentation Energetics..............................................268 Discussion of the Heats of Formation of Trihalonethyl and Dihalomethylene Cations.............................274 Mechanism for Formation of Dihalogen Cations............278 Rationalization of Observed Fragmentation Channels......287 CHAPTER SIX. SUNMAR! AND SUGGESTIONS FOR FURTHER VORK.............294 A. sm‘q000000OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO0.0.00.00.00.29‘ B. sn“.’tion' for Furth.ri'ork.OO0.0000.00.00.0.000000000000000295 1. 'ithont Q‘n‘. in tho In.tm°nt..C.0.0.0.000000000000000295 (1) noorotic.1 Inv°.ti‘.t1°n...COOOOOOOOOO0.0.00.0...00295 (2) no‘t' 0f Fom‘tionOOOOOOOOOOOOOOOOOOOOO00.0.00000000295 vii (3) Parent Ion Stability................................296 2. 'ith a Small Change to the Instrument....................296 (1) Temperature Control of the Sample Inlet and Ion Source Systems.....................................296 (2) Negative Ion Detection..............................297 (3) Addition of a Second Reaction Chamber...............297 (4) Laser-Induced Fluorescence of Ions..................298 EnumWOOOOOOOOOOOOOIOOOOOOO.0.O0......0.000000000000000.0.00.299 viii table 1-1. 2-1 0 2-2 0 3-1 0 S‘A-l e SArl-l. SArl-Z. 5Ar2-1. 5&2‘2 e 5Ar3-1. 5Ar3-2. 5Ar4-l. 5Ar4-2. 5A‘5'1 e 5AP5“2. 5k6-1 e SAPS-2. 5A-7-1 e LIST OF TABLES Ratio of photoionization cross section to photoionization efficiency as a function of percent absorption...............7 Some processes following absorption of a vacuum.ultraviolet photon by ‘ .01°°u1.00OO.O0.......0.OCOO...0.0.0.0000000000013 Experimental techniques used to measure positive ion .n.r‘i.'..OOOCOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOCOOOOOOOOOOO.56 Observed and calculated spin-oribit splitting for the he- logen acids and halogens....................................80 Standard enthalpies of formation (0':. kcal/mole) utilized in thermochemical calculations of AHEO of fragmente ion.....OOOOOOOOOOOOOOOO0.00.00.00.00.0.0.0.000000000000000124 Photoionization nass spectrum of CF3ClBr...................126 Summary of IP. AP. and heats of formation data for all ions It“ a’CIBIO0.00......000......OO0.00000000000000IOO0.0.0.128 Photoionization mass spectrum of CF........................140 Summary of IP. AP. and heats of formation data for all ions ~frm a‘....0..00...0.0.0.0000...0.0.0000000IOOOOOCOOOOIOOOIN Photoionization mass spectrum of CF,C1.....................146 Summary of IP, AP. and heats of formation data for all ions fr“ w.CIOOOOOOOOOOOOI0.0.0.0....0....0.00.00.00.000000000146 Photoionization mass spectrum of CF,Br.....................155 Summary of IP, AP, and heats of formation data for all ions it“ “.32...OOOOOOOOOOOOOOOO0.0.0.0....0000000000000000000155 Photoionization mass spectrum of CF,I......................164 Summary of IP. AP, and heats of formation data for all ions fr“ 0.1.0.0...OOOOOOOOOOOOOOOOOOOOOOOOO0.0.0.000000000000164 Photoionization mass spectrum of CF,C1,....................175 Summary of IP, AP, and heats of formation data for all ions fr“ azCI‘OOOOOOOOOOOOO0.0.0....0.0.I0.0.00.00.00.00000000177 Photoionization mass spectrum of CFC1,.....................185 5A-7-2 e SA-s—l e SA-s-z e 5Ar9-l. SA-g-z e SAPIO-l. SAPIO-Z. 5Ar12-l. 5A913-1. SArl4-2. 53-1-1. 5H-1-2. 58-1-3. SBP1-4. 53-1-5. 53-1-6 . 53-1-7. 53-1-8 e Summary of IP. AP. and heats of formation data for all ions fr“ we1.00000000I.0.00.0000...O...0.0.0.0.000000000000000187 Photoionization mass spectrum of CCl‘......................195 Summary of IP. AP. and heats of formation data for all ions fr“ CCI‘OIOOOOOOOOOOOOOOOOOO0..OOCI.0.0.0.0....0.0.00.0...195 Photoionization mass spectrum of CC1.Br....................2OO Summary of IP. AP. and heats of formation data for all ions fr“ cc1.BrOO....0.I...0.0.0....O.IOO...00.000.000.000000000202 Photoionization mass spectrum of CF,Br,....................210 Summary of IP. AP. and heats of formation data for all ions fr“ Q’BraOOOO000......0.0...0.00000000000...00.00.00.0000212 Photoionization mass spectrum of CHF,......................218 Summary of IP. AP. and heats of formation data for all ions fr“ m’OOOOOOOCO0....0.0...00......OOOOOOOIOOOIOOOO0.0.0.218 Photoionization mass spectrum of CHF,C1....................223 Summary of IP. AP. and heats of formation data for all ions fr“ m’clOOOOOOOOOOOOOOOOOOOOOI...O0......0.00.00.00.0000213 Photoionization mass spectrum of CHFCl,....................231 Summary of IP. AP. and heats of formation data for all ions fr“ mel10000OOOOOOOOOOOOOOOOOOOO.00.0.00...0.00.00.00.00233 Photoionization mass spectrum of CHCl,.....................24O Summary of IP. AP. and heats of formation data for all ions It“ mcl'OOOOOOOO00......OOOOOOOOOOOOOOOOOOOO0.0000IOOOOOOZm E'ti.‘t. 0‘ A40 for a.+. O O I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O .247. E.ti-‘t° Of “:0 for W3CI+O O O I O O O O O O O O O O O O O O O O O O O O O O I O O O O .250 E.ti-‘t. 0f “:0 for mclz+0 O O O O O O O O O O O O O O I O O O O O O O O O O O O O O O .253 B‘ti-‘t. 0: “20 for cel’+OOOOOOOOOOOOOOOOOOOOOOOOO00.0.000256 B‘ti-‘t. Of “20 for waBr+O O O O O O O O I O O O O O O O O O O O O O O O O O O O O I O .259 E'ti-‘t. 0‘ “20 for a:+. I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O .262 B‘ti..t. Of “go for 0C1+0 O O O O O O O O O O O O O O O O O O O O O O O O O O O O O I O .264 E.ti..t. of “:0 for cel3+00O...0.00.00.0.0.000000000000000267 53.2.1 0 53-3-1 . 53-3-2 . 58-3-3 . Relative parent ion stability..............................270 Systematic trends in the heats of formation of chloro- fluorocarbon cations and their isoelectronic boron 'ulo‘............0..00...0.0.0.0.....0....0000000000000000275 Systematic trends in the heats of formation of bromo- fluorocarbon cations and their isoelectronic boron ‘n‘IOI‘OOOOOOOOOOOOOOOOOCOOOOOOOOOOOOOOOOOOOOOOO0.0.0.0....277 Heats of formetion (kcal/mole) of mixed CFxBry cations.....280 x1 figure 1-1. 2-1. 2-2. LIST OF FIGURES Ratio of photoionization cross section to photoionization efficiency as a function of percent absorption................7 Schematic diagram of potential curves of electronic stdtes and vibronic transitions in a diatomic molecule..............27 The ionization threshold region of NO+; pronounced auto- ionization peaks superposed on the step structure............29 Potential energy curves illustrating some important dissociation mechanisms......................................34 Block diagram of a photoionization mass spectrometer.........47 Spectral distribution of four light sources commonly used in photoionization mass spectrometer.........................48 Simplified scheme of a photoelectron-photoion coincidence 'P.°trm.t.roOOOOOOOOOOOOOOOO0.0.0.0...0.0.00.00000000000000053 The effect of Franck-Condon factors and autoionization on a PIE curve and corresponding photoelectron spectra..........12 Effect of Franck-Condon factors on direct ionization thr°.h°ld. Of di‘tmic .°1°cn1°8000000000.0.00.000000000000007‘ Schematic diatomic molecule potential curves, Franck- Condon factors. and corresponding photoelectron spectrum.....76 HeI photoelectron spectra of halogens and halogen halides....82 Example of the Iahn-Teller effect............................84 The population of the vibrational levels of Br3 at 300 I.....86 Hot band structure of Br,+ at several temperatures...........87 The population of the rotational levels of HCl at 300 I......89 The effect of a triangular-slit function on a step function..95 The PIMS instrument..........................................97 The lamp gas delivery system.................................99 Helium continuum spectrum...................................100 mdrosen.‘w lino sp°ctm000000000.00.0.0.00.0000000000000103 xii 5"A'1 e 5-A92. SA-l-l e 5k1-2 e 5Arl-3. 5Ar1-4. SArl-S. SArl-G. SArl-7. SArl-S. 5&1-9 e 5A-1-10 . 5Ar2-l. 5Ar2-2. 5Ar2-3. 5Ar2-4. SAP3‘1. 5Ar3-2. SAPS-3. SAPS-4. 5Ar3-5. 5Ar3-6. 5A-3-7 e PIE 0‘ W3CI+ ft“ W‘CIBrOOOOOOOOOOOOOOOOOOOOOOO00.00.000.0121 Thermochemical procedure for determining reaction equations approate to experimental appearance threshold...............l23 Photoionization mass spectrum of CF,ClBr....................125 Photoionization efficiency curves for parent and daughter ion‘ fr“ W'CIBrOOOOOOO0.0.0.0....O'IOOOOOIOOOO0.0.00.000000127 913 of cr,c1sr+ from cp,c1nr................................129 PIE of CF3C1Br+ in the threshold region.....................130 PIE of cr,c1+ from cr,c1nr..................................132 PIE of cr,n:+ from cr,c13x..................................132 PIE of cec132* from cr,c13:.................................134 PIE of cp,* from cr,c1nr....................................134 913 of arm+ from cr,c1nr...................................136 PIE of CFBr+ from cr,c1sr...................................136 PIE of Br+ from cp,c1nr.....................................137 Photoionization mass spectrum of CF‘........................139 Photoionization efficiency curves for daughter ions fr“ @4000.OOOOOOOOOOOOOOOOOOOCCO00....00.0.000000000000000141 PIE 0‘ m.+ fr“ W‘COOOOOOOOOCOOOOO...OOOOOOOOOOOOOOOOOOOOCI43 PIE 0‘ wg+ fr“ Q‘OOOOOOOCCCCOOOOOOOOIOOOOOOO0.0.0.0000000143 Photoionization mass spectrum of CF,Cl......................145 Photoionization efficiency curves for parent and daughter ion‘ fr“ W’CIOOOOOOIOOOOOOOOOOO0.0.0.000...0.0.0.000000000147 PIE of cr,c1+ from cr,c1....................................14s PIE of CF,Cl+ in the threshold region.......................149 PIE of cr,c1+ from cp,c1....................................151 PIE of CF,+ from cr,c1......................................151 PIE Of “3+ fr“ CF.C1.000000000000IO0..0.0.0.00000000000000152 xiii 5Ar4-1. 5k4-2 e 5Ar4-3. 5Ar4-4. 5Ar4-5. SArd-G. 5Ar4-7. SArd-S. SAPS-1. SAPS-2. SAPS-3. SAPS-4. SAPS-5. 5AP5’6. SAPS-7. SAPS-8. 5AP5'9. 5Ar6-1. 5Ar6-3. 5Ar6-4. 5Ar6-5. 5A-6-6. 5Ar6-7. SA-6-8 e Photoionization mass spectrum of CF,Br......................154 Photoionization efficiency curves for parent and daughter ion. fro. CF’BrIOOOOOOOOOOOOOOOO.I.I0.00.0.00000000000000000156 PIE of CP,Br+ from CP,Er....................................157 PIE of CF,Br+ in the threshold region.......................158 PIE of Cigar+ from cr,Er....................................159 PIE of CP,+ from CP,Er......................................159 PIE of CP,* from CP,Er......................................161 PIE of Er+ from CP,Er.......................................161 Photoionization mass spectrum of CF,I.......................163 Photoionization efficiency curves for parent and daughter ion. fro. CF.IOOOOOOOOOOOOOOOOOO0.00...00.0.0000000000000000165 PIE of CP,I+ from CP,I......................................156 PIE of CF,I+ in the threshold region........................167 PIE of CP,* from CP,I.......................................169 PIE of CP,I* from cr,I......................................169 PIE of I+ from CP,I.........................................17o PIE of IP+ from CE,I........................................17o PIE of cr,* from CP,I......................,................I72 Photoionization mass spectrum of CF,C1,.....................174 Photoionization efficiency curves for parent and daughter ion‘ fro. CFaCIzOOOOOOOOOOOOOOOOO0.0..00....OOOOOOOOOOOODOOOI76 PIE of CF,C1,+ from CP,c1,..................................17s PIE of CF3C11+ in the threshold region......................179 PIE of cr,c1+ from cr,c1,...................................Iso PIE of CPCI,+ from CP,c1,.....;.............................Iso PIE of CF,+ from CP,c1,.....................................182 PIE Of CFCl+ fro. CF:C1:.OOOOOCOOOOO...0.00.00.00.0000000000182 xiv 5A-7-1 e 5Ar7-2. 5Ar7-3. 5A-7-4. 5Ar7-5. 5Ar7-6. 5Ar7-7. $Ar7-8. SAPS-1. SAPS-2. SAPS-3. 5A-8-4 e 5AP9-1. 5Ar9-2. 5Ar9-3. 5Ar9-4. 5Ar9-5. 5Ar9-6. 5A-9-7 e SAPIO-l. 5ArlO-2. SAPIO-S. 5ArlO-4. 5Ar10-5. 5Ar10-6. Photoionization mass spectrum of CFC1,......................1S4 Photoionization efficiency curves for parent and daughter ion‘ fr“. CFCI’eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee186 PIE of CPCI,+ from CFC1,.......................:............188 PIE of CPCI,+ in the threshold region.......................189 PIE of CPCl,+ from CPc1,....................................190 PIE of col,+ from CPc1,.....................................190 PIE of CPCI+ from CPc1,.....................................192 PIE of cel,+ from CPCI,.....................................192 Photoionization.mass spectrum of C01........................194 Photoionization efficiency curves for parent and daughter ion. fro. CCI‘OIOOOOOOOOOOO...000......0.0.0.000000000000000196 PIE at CCI.+ fro. CCI‘...0.00.0.0...0.0.00.0...0.0.00.0..000197 PIE 0‘ cel’+ fro. CCI‘OOOOI...OOOOOOOOOOOOOIOOO00.0.00000000191 Photoionization mass speotrum of CC1,Br.....................199 Photoionization efficiency curves for parent and daughter ion. fro. cel’BrOOOO0....OOOOOOOOOOOOOOOOOOOOOO0.00.00.00.00201 PIE of cc1,nr+ from cc1,Er..................................203 PIE of col,Er+ in the threshold region......................204 PIE of cc1,Er+ from cc1,nr..................................206 PIE of col,+ from cc1,Er....................................206 PIE of eel,+ from CCl,Br....................................207 Photoionization mass spectrum of CF,Br,.....................209 Photoionization efficiency curves for parent and daughter 103‘ fro. cszr.OOOOOOOOOOOOIOOOOOOOOOOOO0.0.00.0...0.0.00.0211 yIE Of cpznra+ fro. CF‘Br‘OOOOOOCCIOCOCCOOOOOOOOOOOO00......213 PIE of CF,Er,+ in the threshold region......................214 PIE 0: CFaBr+ fro. CF,B:....0..OOOOOOOOOOOOOOIOOOOOO0.0.0.0021, PIE Of CFBr3+ fro. CFzBraOOOOOOO0.0000000000000000.00.00.00.216 XV 5Ar11-1. 5A-11-2 . 5AP11‘4e 5Ar12-1. 5A-12-2. 5Ar12-3. 5Ar12-4. SAPIZ-S. SArlz-G. 5Ar13-1. 5A913-2. 5A913-3. 5Ar13-4. 5Ar13-5. 5A913-6. 5Ar14-1. 5Arl4-2. 5Ar14-3e SAP14-4e 53.1-1 e 53-1-2 e Photoionization mass spectrum of CflF,.......................217 Photoionization efficiency curves for daughter ions It“ m.0000000000000.0.0.0000...OOOOOOOOOOOOOOOOOOOOO0.0.0219 P130: a’+ fr“ M'OOOOOO...00....OOOCOOOOO00.00.00.000000221 PIE 0‘ @1+ fr” M’OO0.0...0.0.000.0......00.00.000.00000221 Photoionization mass spectrum of CEF,C1.....................222 Photoionization efficiency curves for parent and daughter 10" It- m,CIOOOOOOI.00.0.00...COO...0.00.00.00.00000000022‘ PIE of cur,c1+ from CEP,CI..................................zzo PIE of CflF3C1+ in the threshold region......................227 PIE of carol+ from cup,c1...................................zzs PIE of CEP,* from CEP,c1....................................229 Photoionization.mass spectrum of CEFCI......................230 Photoionization efficiency curves for parent and daughter ion. fr“ MCI’OOOOOOOOOOOOIOI...O....0...0.0.0000000000000232 PIE of carol,+ from cupc1,..................................234 PIE of CEFCI,+ in the threshold region......................235 PIE of carol+ from CEPc1,...................................237 PIE of anal,+ from CEEc1,...................................238 Photoionization mass spectrum of CHCl,......................239 Photoionization efficiency curves for parent and daughter ion: fr” Gel...OOOOOOCOOOOOOOOOOOOO..0.0.0.00.000000000000241 PIE Of mel'+ fr“ mel’OOOOOOOOOOOOOOOOOOOOOOOO00.0.0000000242 PIE 0‘ mcl’+ in th. thro'hOId r°'i°nOOOOOOOOOOO0.0.0.000000243 PIEO‘ mel:+ fr“mc1.00000000IOOOOOIOOOOOOOOOOOIOOOOO0.0.2“ Photoionization efficiency curves for CF,+ formed by fragmentation of various percursors.........................246 Photoionization efficiency curves for CF3C1+ formed by fragmentation of various percursors.........................249 xvi 53'1-3 e 53—1-4 e 53-1-5 e 55-1-6. 53-1-1 a 53-1-: e 53-3-1. 53-4-1. 53-4-2. 53-4-3. 53-4-4. 53-5-1 e SDI-5-2 e Photoionization efficiency curves for CF01,+ formed by fragmentation of various percursors.........................252 Photoionization efficiency curves for CCl,+ formed by fragmentation of various percursors.........................255 Photoionization efficiency curves for CFaBr+ formed by fragmentation of various percursors.........................258 Photoionization efficiency curves for CF,+ formed by fragmentation of various percursors.........................261 Photoionization efficiency curves for CFCl+ formed by fragmentation of various percursors.........................263 Photoionization efficiency curves for CCl,+ formed by fragmentation of various percursors.........................266 Procedure used to estimate the heats of formation of CF Br+ . mt, ' mt... 11.t in th. T‘bl. Ska—2.0...00.0.000000000000278 Thermochemistry of BrCl+ formation from CF,ClBr.............281 lechanisms of BrCl+ formation from CF,ClBr..................283 Dependence of BrCl+ ion count rate on CF,ClFr pressure......285 Dependence of IF+ ion count rate on CF,I pressure...........286 Appearance potential for competitive fragmentations of SOIOOtOd h‘1m°th.n°.loO00....O0.0.0.0000...0.0.00.000000000288 Schematic reepresentation of vibrational motion leading to BrCl formation on CF ClBr+ excited electronic .t‘t. pathti.l suf‘°°0000000000.0000000IOOOOOOOOOOOOO0.0.0291 xvii CHAPTER ONE INTRODUCTION Halogenated methanes are in widespread and daily use. For example. they are regularly employed as refrigerants. foaming agents. aerosol propellants,etc.1 Because neutral halocarbons are themselves relatively stable and innocuous. their use and disposal was not until recently regulated or even monitored. Many of these compounds (and the halomethanes in particular) are gases or liquids with very high vapor pressure at room temperature. so they are not generally contained, and they spread even into the troposphere and stratosphere. When exposed to light(especially vacuum ultraviolet radiation). these compounds will ionize or dissociate into positive ions. negative ions. free radicals and other neutral fragments, all of which are highly reactive. When these highly reactive species interact with other molecules in the stratosphere. the troposphere or even the atmosphere. what kinds of reactions will occur? What are the mechanisms for these reactions? What are the products? How do these products influence life? All of these questions are important and of interest to scientists. For example. some years ago it was reported2 that chlorine and bromine atoms formed by the dissociation of chlorine- and bromine-containing halomethanes by vacuum ultraviolet radiation in the stratosphere and trOposphere may be related to the destruction of the earthb protective ozone layer. There are many other effects yet to be determined. In order to initiate such investigations, scientists first need to have *4 detailed. accurate. complete thermochemical data such as the ionization potentials for parent ions.appearance potentials for fragment ions. and heats of formation for all ions. This information will allow thermodynamic predictions to be made regarding possible reactions and most favorable pathways. A knowledge of the relative stability of ions may provide important clues regarding possible reaction mechanisms. With thermodynamic and kinetic information at hand. better judgments can be drawn about the volatile halocarbons. especially with regard to which reactants need to be carefully controlled to prevent contamination of the environment or destruction of natural resources. Photoionization mass spectrometry(PIMS) has become an important experimental method in recent years.3 The most successful and important application of this technique has been to determine thermochemical information for ions. Because of its high resolution capability and the energy independence of the photoionization cross section.4 photoionization mass spectrometry is recognized as the best technique for such investigations.5 Photoionization mass spectrometry is a spectroscopic technique in which the yield of a selected ion from a sample gas is measured as a function of the incident photon energy. Thus. relative photoionization and fragmentation cross sections are measured as a function of energy. They provide a great deal of information. such as ionization and appearance potentials. vibrational and electronic level spacings. and the heats of reaction for different dissociation pathways. From these primary data. coupled with tabulated thermochemical data for neutral species. heats of formation of parent and fragment ions. fragmentation mechanisms and a variety of other information can be deduced. PINS. in combination with a few other techniques. can often fully characterize molecular ionization and ionic fragmentation processes. In this dissertation. fourteen often-used. commercially-available halogenated methanes have been selected for investigatation by the photoionization mass spectrometry technique. Ionization potentials of the parent ions and appearance potentials for associated fragment ions have been measured. Beats of formation of the corresponding ions have been calculated and the discussion includes possible decomposition pathways for several reactions and the relative stability of parent and some fragment ions. The thermochemical information derived from the measurements and the discussion of the results should prove helpful in later investigations in this field. Chapter two of this dissertation presents a general overview of photoionization mass spectrometry. It includes a general discussion of the interaction between light in the 10-20eV energy range and gas phase atoms and molecules. It also includes a brief description of several different techniques to measure ionization and appearance potentials for ions, with emphasis on the advantages of the photoionization mass spectrometry technique. Particular attention is paid ,to threshold behavior in Chapter three. Effects on the shape of the threshold ranging from the nature of quantum theory through thermodynamics. kinetics and instrumental contributions are considered. In Chapter four. the MSU PIMS instrument is briefly described. All of the experimental work performed in the course of this research was done on this instrument. and this chapter includes a relatively detailed procedure to introduce future workers to some of its complexities. The data are reported and discussed in detail in Chapter five. which forms the major portion of this disseration. Chapter six provides a conclusion and suggestions for further investigation. To complete this. introduction. the term "photoionization efficiency". "which is widely used in this dissertation. is introduced and clearly defined. Since most of the experimental data are obtained as photoionization efficiencies at selected photon energies. this term is particularly important for the research which will be described. In the description of absorption or ionization phenomena one typically uses the term "cross section" to describe the "transition probability" or "the probability of interaction with radiation". In photoionization mass spectrometry. one wishes to know the probability of ionization perhaps leading to fragmentation after absorption of a photon or the probability of absorption; these are expressed as the photoionization cross section and absorption cross section. respectively. Because of instrumental restrictions in the energy region of interest. the absorption cross section or photoionization cross section cannot be measured directly. Thus scientists report the photoionization efficiency(PIE). which is defined as: PIE 8 ion intensity / transmitted photon intensity. at a given photon energy for an ion of selected mass. The ion intensity and the transmitted photon intensity are easily and directly measurable. The purpose of the following derivation is to show that under typical PIMS experimental conditions the PIE is. with negligible error. directly a proportional to the desired partial photoionization cross section. From Beer's law: log(P./P) - A =ebc. (l-l) where P. is the incident light intensity. P is the transmitted light intensity. I is the absorption coefficient. b is the path length of the radiation through the sample. and c is the sample concentration. Alternatively one may write: ln(P/P.) = -obc ' (1-2) where c I 2.303s is defined as the absorption cross section. Equation (1-2) can be rewritten in exponential form: P = PoExm-obo) . (1-3) The photoionization cross section is defined as: “i 5 cNi/(Po-P) = oNi/P.[1 - EXP(-abc)] (1-4) where Ni is the ion intensity produced by absorption of incident light. The photoionization efficiency(PIE) is defined as: PIE 3 Ni/P a Nil POEXP(-cbc). (1-5) Hence oi/PIE - cEXP(-obc)/[l - EXP(-abc)] (1-6) or ci/(PIE/bc) = [-ln(P/P.)](P/P,)/[1 - (P/P.)] (1-7) where [l - (P/P.)] is the fraction of the light absorbed. Perfect preportionality of the PIE to the photoionization cross section would give a value of one for the ratio in equation _(1-7). independent of sample absorption. Table 1-1 shows how this ratio actually varies as a function of the percent absorption and Figure 1-1 shows the same results graphically. In almost all photoionization mass spectrometry experiments the light absorption is less than one percent. so that the PIE is an accurate representation indeed of the photoionization cross section. Table 1-1. Ratio of photoionization cross section to photoionization efficiency as a function of percent absorption. (1 - 339x100; 0.1 0.5 l 5 10 20 30 40 50 P : --.-----.-...--.“"- 6iffPfE3753'3035995""““0.9950 '"" 0.9482 0.8322 0.6932 ' 0.9975 0.9746 0.8926 0.7662 ‘LOSO f? . u! . m. b \ . g 1.000 "' t ;+: .2 h k A A 5 - w - .. 4 Lu T ‘0 t (I) m ,. <3 a: L EL ‘0 (1950 —- Lu _ :s l: . A 8 .. v 0.900 1 l l l l L | L l J l l i L l 1 l l l L 1 l l I [A 1 1 l 0.00 0.25 0.50 0.75 ‘l .00 PERCENTAGE ABSORPTION (1 -P/Po) Figure 1-1. Ratio of photoionization cross section to photoionization efficiency as a function of percent absorption. in range 0.0-1.0 percent. CHAPTER TWO PHOTOIONIZATION MASS SPECTROMETRY: AN OVERVIEW The purpose of the photoionization experiment is to measure. as a function of photon energy. the rate of production of specific ions that result from absorption of photons by a neutral molecule. The name photoionization mass spectrometry(PIMS) includes mass spectrometry. but the main aspect of the experiment is the variation of the photon energy. The experiments described in this dissertation are really vacuum ultraviolet absorption experiments. in which the mass spectrometer acts as a powerful ion filter and detector. The photoionization mass spectrometric method was initiated by Ditchburn and Arnot in 19296 when potassium vapor was irradiated with photons from an iron are. the light passing through a quartz window into the ion source of a simple magnetic mass analyzer. However. the first 7 who used the valuable results were obtained by Terenin and Popov. resonance lines emitted by cadmium. zinc or aluminium arcs. in connection with a simple magnetic mass analyzer. to study the ion-pair formation process. 9 Tix + hy ---—> 1'1" + x' (X=Cl.Br.I). The distinction between direct ionization and ion-pair formation illustrated the advantage of the coupled mass analyzer in comparison with normal optical absorption results. Simple measurement of the total photoion current would lnot show whether ionization alone or both ionization and fragmentation took place during the experiments. However. early investigators were restricted by the experimental difficulties associated with photoionization mass spectrometry: in particular. the problems of inadequate light sources and stringent vacuum requirements precluded the realization of the full potential of this new method. In the late 1950's and mid 1960's. Hurzeler et al.8'9 introduced the vacuum monochromator to photoionization mass spectrometry systems. and the situation improved somewhat. Still. the wavelengths available were limited to 2 1050‘ by the lithium fluoride windows which were employed. The installation of faster pumps to allow windowless operation subsequenty overcame this limitation.10 Thereafter. no major design improvements have been made. although considerable sophistication of the basic design has evolved. particularly as far as the ion detection system is concerned. Even though photoionization mass spectrometry has been widely used for some time. complete. general and helpful references are few. No comprehensive review of photoionization mass spectrometry has been published since the one by Reid3 in 1971. twelve years ago. 5 11 Nevertheless that review. plus the work of Rosenstock and Chupka provide a useful introduction to the experimental methodology of PIMS. 10 Other works discuss photoionization and fragmentation processes.5'11’16 applications of the techniqu.,5:11.17 5.15.18 and its relationship to other methods. Since the introduction to PIMS presented here is just an overview. the reader is urged to consult these references for more detailed information. In recent years. PINS has proven to be powerful in investigations in three different areas: the study of the kinetics of unimolecular reactions of polyatomic molecules. the study of the thermodynamics and kinetics of ion-molecule reactions. and the determination of ionization potentials of atoms and molecules and of appearance potentials of fragment ions. All of the work described in this thesis has involved the measurement of ionization potentials and appearance potentials of fourteen halogenated methanes: most of the concepts developed in this chapter relate specifically to such studies. Section A describes the relevant interaction between photons and chemical species (atoms and molecules). with particular emphasis on the processes of ionization and fragmentation. In section B different experimental techniques for . measuring ionization potentials and appearance potentials are discussed: first photoionization mass spectrometry(PIMS). then photoionrphotoelectron coincidence spectroscopy(PIPECO). and finally a general description of some other techniques. A. . Photoionization An atom or molecule can absorb radiation and be transferred from the ground state to an excited state. This process is called ll photoexcitation. For an atom. due to the limited degrees of freedom. the transition can only take place between ground and excited electronic states. However. for molecules the absorption spectrum is very much more complex because of the additional degrees of freedom present. The discrete energy states are specified not only by the electronic energy. but also by the vibrational and rotational energies of the entire molecule. Subject to the appropriate selection rules. discrete transitions in free molecules can be observed between rotational levels of the same electronic and vibrational state. giving pure rotational spectral lines in the microwave or far infrared: between rotational levels of the same electronic state but different vibrational states. giving vibration-rotation spectra in the 'near infrared: or between rotational levels of different electronic and vibrational states. giving band systems in the visible and ultraviolet regions of the spectrum. If the exciting radiation is of sufficiently high energy. the atom or molecule can also be excited into a region of continuous states lying above the bound states. In this case the absorption spectrum is continuous because the final state can take on continuous energy values over a significant range. This region corresponds to ionization and/or fragmentation processes: the neutral or ionic products can accommodate any energy in excess of the threshold for the reaction as translational and internal energy. In the following pages of this section. those processes which result in an ion. and are consequently important to photoionization mass spectrometry. are discussed. The first part will deal with direct ionization: a description of autoionization and finally a discussion of fragmentation follows. These processes are summarized 12 in Table 2-1. 1. Direct Ionization Direct photoionization is the process whereby an electron is ejected from a molecule M(X.v") in its ground electronic state I and vibrational level v" to form the corresponding molecular ion M+(x.a.b.c...)(v'-O.l.2...) in its ground or one of its excited electronic states x.a.b.c... and vibrational level v'. according to the expression: mu") + hv ---—> M+(x.a...)(v'=0.l...) + e‘. (2-1) Direct ionization is analogous to an ordinary one-electron transition observed in the visible or ultraviolet region of the spectrum: the one important difference is that the transition terminates in a continuum state instead of a bound state. The probability of such a transition between the molecular ground state characterized by the eigenfunction F" and the final state(ion + electron) characterized by F' is determined by the absolute square of the transition moment integral: T = ip°)F° dg and Fe" describes both the ion and the free electron and may be represented as a product of a free electron wavefunction and a Slater determinant for the ion.19 Now. one of the assumptions of the Born-Oppenheimer approximation is that the electronic distribution varies only slightly with 9. Consequently. it is often possible to write the combined vibration-electronic integral as: T - pelaref] Ev"Ev"dQ (2-11) where Qref is a nuclear configuration chosen to be of high probability for both states Fv' and Fv”. 0n the basis of the above considerations one may write the photoionization transition probability. to a good approximation. as 17 P . lpetorefll'll’. (2-12) Whenever the integrals of equation (2-12) are different from zero. direct photoionization transitions are allowed. For direct photoionization transitions the operator in equation (2-10) can be approximated as a one-electron dipole operator whose components transform in the molecular point group as the Cartesian vectors x. y. and E. If the product of the irreducible representations R of the components in equation (2-10). R(F°'.)xk(pc)xR(Fe") is totally symmetric with respect to all symmetry elements of the molecular point group for at least one component of Be or. in other words. if the product R(F°'.)xR(Fe") belongs to the same symmetry species as one of the components of E(3°).- then a transition is allowed. In direct photoionization transitions. the final state wavefunction F." consists of antisymmetrized electronic component wavefunctions of the positive ion Fe+ and the unbound electron £9. The function f0 is a one-electron continuum wave. The necessary derivation process leads to the result that all one-electron direct photoionization transitions are allowed.16 After taking into account the spin selection rule. the final conclusion is: the most common direct photoionization transitions are from singlet molecular ground states to doublet states of the positive ion. The predictions of electronic selection rules are very much simplified compared with those for absorption and emission between two bound states because the case now involves the initial molecular ground state and a final ionic state plus free electron. The transition of the system from molecule plus photon to ion plus electron is governed by the 18 usual dipole selection rules. but the free electron can leave the ion carrying whatever angular momentum and spin are needed to satisfy these rules. As a result all one-electron direct photoionization dipole transitions are allowed.15 The vibrational part of equation (2-12). the Franck-Condon factor. places some restrictions on the allowed vibrational transitions within a direct photoionization transition. In order for this integral to be non-zero. the integrand(Fv'.Fv") must be symmetric with respect to all symmetry Operations of the point group. i.e. R(Fv'.)xR(Fv")-R(totally symmetric). Therefore. only vibrational levels of the same vibrational symmetry species in the initial and final state can combine. Most molecules exist in the totally symmetric zero-point vibrational level of the ground electronic state at room temperature. thus allowing direct photoionization transitions only to totally symmetric vibrational levels of the ionic state. In polyatomic molecules. where there may be several symmetric vibrational modes. interlocking progressions of these symmetric vibrations are often observed.20 In transitions to degenerate vibrational levels it is necessary to consider the angular quantum number li-vi.vi-2...1 or 0. which 21’22 For this characterizes the different possible sublevels that occur. quantum number one has the selection rule A180. Because 1 is even or odd when v is even or odd. only v'=0.2.4...are symmetric. This selection rule is rigorous for point groups like D2d' D4h' D6h.and Duh. but is relaxed in point groups such as D3h' C3v' and Td. where all overtones of degenerate vibrations contain at least one totally symmetric component. The rule remains rigorous for the 091 transition 19 in all point groups because the v=l level of a degenerate vibration is never totally symmetric. In the event that there is a change of symmetry between the molecule and ion. one must apply the vibrational selection rules using only the common elements of symmetry. Conversely. from the observed structure. conclusions can be drawn about the difference in symmetry between the molecule and ion in the two equilibrium positions. For example. the degenerate bending vibration of n symmetry in a linear molecule is split into two normal modes in the corresponding bent molecule. Since at least one of these split vibrations can be totally symmetric in the point group of the bent species. progressions in every quanta of this mode can be Observed. 2. Autoionization(Preionization) In contrast to direct ionization into the continuum. which is the basis of a photoionization efficiency curve. autoionization is an indirect process. Atoms or molecules are first produced in a neutral excited state with energy above an ionization lhnit. and then spontaneously emit electrons: .- i .. M + hr ----—-) M —-——) M + e . (2—13) The first step is a resonance process; it can be brought about only by light of the correct energy. and the nature of the accessible excited state is governed by the normal electronic transition selection rules. Autoionization proper is the second step. In other words. if an atom or 20 molecule is raised to a highly excited state and radiationless transitions take place from the discrete state to the ionization continuum. such a process is called autoionization. In photoionization efficiency curves. autoionization will often appear as sharp. peak-like structure superimposed on the normally featureless direct photoionization continuum. especially for atoms. Autoionization can present a great variety of asymmetrical line shapes. and it may appear as a "window resonance" in which there is a decrease or dip in the photoionization cross section. especially for diatomic and polyatomic 12.23.24 molecules. The lifetime of autoionizing levels can vary a great 0.14 seconds all the way to about 10-6 seconds. For deal. ranging from 1 states with short lifetimes. the autoionization structure may be so broadened that it is indistinguishable from the continuum of direct photoionization. If the lifetimes of the autoionizing states are greater than one vibrational period. then the relative intensities of the superimposed structure in the photoionization efficiency curves can be expected to follow the Franck-Condon factors for the radiationless transitions from the autoionizing states of the molecule to the final state of the ion. In contrast with the direct photoionization case. the value of v' in this case is generally not zero. As a result. vibrational structure in autoionized photoionization efficiency curves cannot be used to infer what type of electron (bonding. antibonding. nonbonding) was ejected. Moreover. autoionization features in the photoionization efficiency curves characterize the bound. highly excited states of the neutral. not the final ionic state reached following autoionization. However. from this information resonance photoelectron 21 spectroscopy experiments can be carried out. which may provide information about the final vibrational states of the ion. This subject will be considered further in chapter three. In equation (2-13) M’ is the atom or molecule in a highly excited state. Highly excited states with energies in the vacuum ultraviolet region are always Rydberg states that converge to the second. third. or higher ionization potentials. The energy of the these discrete Rydberg states can be above the threshold of the ionization continuum. Molecular Rydberg orbitals are analogous to atomic orbitals with large principal quantum numbers. They have a large spatial extent. and in a somewhat simplified but useful approximation Rydberg states can be thought of as one-electron orbitals in which the electron is on the average at so large a distance from the remaining ion core that the core can be approximated as a point charge.25 This approximation is substantiated by the fact that nearly all transitions to Rydberg states (observed in photoionization mass spectrometry experiments as autoionization structure) can be fit into a series described by the Rydberg equation: En = IP - R/(n - 6)’. (2-14) where En is the energy difference of the transition. IP is that ionization potential of the molecule to which the series converges. n refers to the principal quantum number. R is the Rydberg constant (13.605eV). and 6 is called the quantum defect. which is essentially an adjustable constant which accounts for pentration of the Rydberg orbital 22 into the ion core. The selection rules for autoionization are derived from the radiationless transition probability from the superexcited (quasi-discrete) states. F‘. to the final (continuum) states. Ff. The lifetime of the initial state can be expressed as : r‘l . 4n2P2p(Ef)/h. (2-15) where p(Ef) is the density of the final state and P is the matrix element of the perturbation. P-(FfISIF‘). Based on the treatment given by Berkowitz.26 the nature of the perturbation matrix elements in autoionization will be divided into five cases: the first three cases all concern electronic autoionization. the fourth case is related to vibrational autoionization. and the fifth case involves rotational autoionization. Case(l):Excitation of an inner-shell electron to a Rydberg level This mechanism can be understood as the migration of energy from the Rydberg electron to a valence shell electron as the Rydberg electron reoccupies its inner shell hole. or as a valence shell electron drOpping into the inner shell hole and releasing its energy to the Rydberg electron. (These are obviously intended as pictorial descriptions. the indistinguishability of electrons must be recognized.) In either event. the interaction is of the form °2ltjk' where rjk is the distance between the Rydberg electron and some core electron. and the perturbation matrix element which corresponds to the transition probability takes the form 23 f f IFeFv(e3Irij)F:F:dqu (2-16) where F: includes the wavefunction of the departing electron. Within the framework Of the Born-Oppenheimer approximation. one can rewrite this matrix element in the form “f s ‘a f a _ [90(e Irij)F°quPvdeQ. (2 17) where if and E: are specified states. The second integral is a vibrational overlap and is like a Franck-Condon factor. but refers to the radiationless transition between the quasi-discrete state and the ionization continuum. If the vibrational distribution predicted by this integral could be observed. it would constitute good evidence for this mechanism. Since the quasi-discrete state is frequently not well characterized. the problem is often inverted to determine the properties of the quasi-discrete state from the observed vibrational distribution.27 Case(2):Valence shell excitation involving a spin flip This behavior has been observed primarily in the noble gas atoms Ne. Ar. Kr. and Xe. but should also be applicable to such molecules as HCl. HBr. and HI. Since the compounds investigated by this work were much more complicated than those. this phenomenon is less important and unresolvable in the photoionization efficiency curves. This case will therefore not be discussed here.r The reader is referred to Berkowitz's book26 for a good description of this case. Case(3): Simultaneous two-electron excitation 24 In this case. the excitation energy of each electron. by itself. is sufficient to cause ionization. but the sum of their excitation energies exceeds the ionization threshold. To account for the initial excitation. one must go beyond the independent particle approximation and include either configuration interaction or correlation. Hence. when the latter effects are important(as in some alkaline earth metal atoms). one may anticipate this type of autoionization to be prominent. Case(4):Excitation of a valence electron to a Rydberg level and concomitant excitation of vibrational energy This problem has been treated rather exhaustively by Berry and collaborators.28’29 particularly for the case of 3,. The vibrational energy of the core must somehow be transmitted to the rather distant Rydberg electron. This transfer involves the interaction of vibrational and electron coordinates. and implies a breakdown of the Born-Oppenheimer separation. Two significant conclusions of the analyses for diatomic molecules by Berry. et al.28'29 are apprOpriate for summary here. (a) As the Rydberg level n increases. the average distance of the Rydberg electron from the nuclear core increases. and. hence the interaction should be expected to decrease. Thus. the autoionization transition rate should decrease. and the lines should become narrower with large n. (b) The major perturbation element is of the form JF£(a/8R)Fidg where R is the internuclear separation Of the diatomic nuclei. The operator (d/OR). when acting on a Hermite polynomial. has the effect of 25 raising or lowering it by one. The vibrational wavefunctions of both the initial (Rydberg) and final (ionic) states can be well approximated by Hermite polynomials. and each set is orthonormal. Since the vibrational potentials of the two states are quite similar. the primary contribution to the integral is one where vf and vi differ by one unit. i.c.. Avail. This is similar. to the selection rule for infrared absorption. Higher order terms will affect this conclusion. but the first-order result (the so-called "propensity rule”) is seen to differ drastically from the conclusions of case (1). which yields a broad vibrational envelope as a consequence of the autoionization; vibrational autoionization can be expected to yield predominantly one vibrational member.27 Case(S):Rotational autoionization This mechanism has thus far been identified only in the case of 33.28'29 so it will not be discussed here. From the disscussion above. it is clear that in molecules autoionization mechanisms can be of at least three varieties: (a) electronically-induced. due to intermediate states with one or more electrons excited: (b) vibrationally induced. involving an intermediate state described by a single highly excited electron outside a vibrationally excited molecule-ion core. with autoionization involving some vibrational relaxation of the core. and (c) rotationally-induced. analogous to the vibrationally-induced case but involving only a rotational transition of the core in the autoionization step. Electronically-induced autoionization is possible only when the total electronic energy of a Rydberg state is greater than the lowest 26 ionization potential of a molecule. In this case the electron in the Rydberg orbital was excited from a ground state orbital lower in energy than the highest occupied ground state orbital. Autoionization occurs when the ion core of the Rydberg state relaxes to a lower energy configuration. The perturbation causing the relaxation and the subsequent ejection of the Rydberg electron is electron-electron repulsionzs. The ion is formed in a different electronic state from that of the core of the autoionizing level. There is no restriction to the ejection of an electron with low kinetic energy. and any final state of the ion with a lower energy than that of the autoionizing state can be produced. In vibrationally-induced autoionization. a Rydberg state in which the molecule-ion core is vibrationally excited may eject the Rydberg electron by vibrational relaxation of the core. This process is caused by a coupling of the oscillating multipoles of the ion core with those of the Rydberg electron. The electron energy of a Rydberg state undergoing vibrational autoionization does not necessarily have to be greater than the lowest ionization potential of the molecule: it is only necessary that the sum of the vibrational and electronic energy be greater than the lowest ionization potential. Figure 2-1 shows vibrationally-induced autoionization. In the figure. the vertical line from M1 to upper states indicates the excitation process. Curved arrows labeled A. B. and C. indicate. respectively: a Av=-3 inter-Rydberg transition from M,’ to M1... which has low probability: a Av--1 transition from M3.“ to M,+ + e-. a rapid process in general: and a two-step process involving a (presumably slow) Av=-2 step from 1‘1;n to it. see - Mz . followed by a transition with Av=-1 from M, to M3+ + e . It P 27 *‘fi M2 Mz++8 101+ M”+e “M+M’**‘ ¥_M+M*. {'1' .5! M2 . ‘11“ 55’3- EWM =VAV ~12" '3 C N / V~=7/ Q) L mm W Figure 2-1. R Schematic diagram of potential curves of electronic states and vibronic transitions in a diatomic mole- cule. [ From R.S.Berry. J. Chem. Phys. 1;. 1228 (1966)]. 28 is interesting to note that vibrational autoionization. which is believed to be the predominant mechanism in small molecules.28'30 constitutes a breakdown Of the Born-Oppenheimer approximation. Rotationally-induced autoionization. caused by the relaxation of a rotationally excited ion core. arises from the coupling of the rotational angular momentum of the ion core with the orbital angular momentum of the Rydberg electron. The role of rotational contributions to autoionization is not well understood due to the paucity of experimental data where rotational structure can be resolved. All parent ions produced by photoionization will have contributions from both direct ionization and autoionization. The parent ion photoionization efficiency curves will not only reflect these contributions. but they will also be influenced by decomposition reactions such as: predissociation into neutrals. fragmentation into a fragment ion plus neutral(s). and spontaneous emission of photons. Autoionization may be indistinguishable from the direct photoionization continuum in the photoionization efficiency curve. In fact. it has been recently proposed that direct photoionization be theoretically treated as very fast autoionization.31 The photoionization efficiency curves of diatomic molecules. where few channels for predissociation may be open. 24'30'32 For larger polyatomic are often dominated by autoionization. molecules. where several channels for predissociation may be open and many changes in geometry are possible. photoionization efficiency curves are often featureless. showing only hints of autoionization.13'33’34 Figure 2-2 shows a photoionization efficiency curve of N0 measured by Paul Killgoar. a former member of this research group. in experiments 29 ( QZBCV Fifi \ \ If I W fl Y 7 \~ 1228 1248 I268 |288 1308 1328 1343 WAVELENGTH (A) Figure 2-2. The ionization threshold region of N0+; pronounced autoionization peaks superposed on the step structure. [From P.C.Killgoar.Jr.. et al.. J. Chem. Phys. fig. 803 (1973).] 30 carried out at the Argonne National Laboratory.35 It depicts pronounced autoionization peaks superposed on the staircase structure which arises as new vibrational channels Open. 3. Fragmentation Processes Fragmentation processes are always more complex than direct ionization or autoionization. so they are often more difficult to analyze. For example. consider a tetratomic molecule. ABCD. In direct ionization the overall process is: ABCD + hy ----9 AECO+ + e'. The energy of the photon is precisely determined by the monochromator setting. and the internal energy of parent ion is easily calculated by subtracting the electron's kinetic energy and the adiabatic ionization potential from the photon energy. Thus. the energetics and composition of the system are well characterized. When fragmentation occurs. for instance if fragment ion A+ is detected. then the reaction can be written as: ABCD + hy -----9 Al + (BCD) +e'. The photon energy and the kinetic energy of the electron can be measured precisely. But this is only part of the system. The neutral fragment or fragments cannot be detected by the ionic mass analyzer. and their composition is uncertain. (They could be any combination of B. C. and 31 D.) Moreover. each species leaving the interaction region carries translational energy and perhaps internal energy as well. and these factors are not measurable by photoionization mass spectrometry. The total system is thus not easily characterized compositionally or energetically. Sometimes. thermochemical information can help' in reducing the complexity of the processes: on this basis many possible fragmentation pathways can be eliminated. and kinetic energy releases can be estimated. However. accurate estimations require precise thermochemical data. which may not be available for all species of the system. The following questions are among the objectives of ionic fragmentation investigations. If the parent ion is initially in a particular quantum state and thus has a specific energy. will it dissociate? If it does. which of the energetically possible products will be formed and how fast will they be formed? How will any excess energy be distributed amoung the products as translational energy. or as electronic. vibrational or rotational energy of the fragments? In considering dissociation mechanisms. one is concerned with how the parent ion excitation energy is distributed in the course of time until. and indeed after. fragmentation occurs. Fragmentation mechanisms can be classified as direct dissociation or predissociation. According to Herzberg.36 predissociation may be further divided into three cases: (1) electronic predissociation. in which the parent ion makes a radiationless transition into the dissociation continuum of another electronic state: (2) vibrational predissociation. in which the parent ion remains in the same electronic state and only vibrational motion is 32 involved; (3) rotational predissociation. in which only the higher rotational levels of a particular vibronic state may predissociate. Rotational predissociation processes accOunt for only a very small fraction of observed fragmentation and are not significant to the main thrust of this section. A more general kind of predissociation involves internal conversion followed by vibrational predissociation. Whether or not an ion can dissociate at all is a question of energy. because for dissociation to be possible the energy of the ion must be higher than the ground-state energy of some set of possible products. If sufficient energy is present. whether dissociation will take place depends on competition from the only other processes that can remove the excess energy of an isolated ion without fragmentation. namely. internal conversion or fluorescence to a stable ionic state. The characteristic lifetime for allowed transitions that emit light in the visible or near ultraviolet spectral region is of the order of 10-8 second. so if a particular ionic state has an allowed transition to any stable lower state it must dissociate within 10"8 second. if fragmentation is a significant process. If all transitions out of the excited ionic state to stable states are forbidden and internal conversion is relatively slow. or there are no lower-lying stable ionic states. then slower dissociation can occur. Thus. whether a dissociation which does occur will be observed in a particular experiment is also a question of rate. In mass spectrometry. time between formation of ions. and their mass separation is typically about 10-5 second and all dissociations that occur within this time will be detected. This includes almost all elementary ion decomposition 33 processes. and in addition special mass spectrometers (which employ ion trapping techniques) can be built in order to study the few ion 3 second. reactions with dissociation lifetimes as long as 10-4 to 10- Direct dissociation is the first fragmentation mechanism to be described. If the state of a diatomic ion reached directly in photoionization is repulsive(unbound) or on the repulsive part of a bound surface. the parent ion will dissociate immediately. In the latter case the potential energy surface reached in ionization is effectively unbound even if. according to a normal model of the electronic structure. it is expected to be a bound state. An Otherwise bound state may be effectively repulsive because of interaction with a repulsive state involving curve crossing or an avoided crossing. The products will dissOciatc with most of the excess energy of the parent ion(i.e. energy greater than the the energy Of the dissociation limit) released as translational energy. as there is insufficient time available for it to be redistributed among internal modes before the fragmentation occurs. The potential energy curves representing this process in Figure 2-3 are appropriate only for diatomic molecules: for polyatomic molecules. they must be imagined as two-dimensional cuts through a multi-dimensional surface. with the reaction coordinate as the horizontal scale. The diatomic potential curves retain their simple meanings for dissociations and spectra of polyatomic species only if the reaction coordinate and the vibrational motions excited by ionization are the same. It is unlikely that the change in equilibrium molecular geometry produced by ionization of a polyatomic molecule will correspond exactly and exclusively to motion in a reaction coordinate: direct 34- 4 r t l ;— A o B . AEO~B >~ —— A.’ B I B’OA 0" >. E 9 Lu 2 hv “4 hv A O 3 A98 (a) (b) (aha faAE ‘) ————-——> > > c) E g E’ h o g hv c hv m m J (c) (d) 01 Q2 Q1 02 Figure 2-3. Potential energy curves illustrating some important dissociation mechanisms:(a) direct dissociation; (b) electronic predissociation: (c) vibrational predi sociation; (d) internal conversion with vibrational predissociation. In (c) and (d) the coordinates 01 and Q: represent different molecular motions in a polyatomic molecules. [ From J.H.D.Eland, Photo- electron Spectroscopy (John Wiley and Sons, New York 1974).] 35 dissociation in the strict sense can be defined as a process in which it very nearly does so. True direct dissociation as defined above can occur only if the removal of an electron has a strong and specific influence on the bonding in the molecule. A close relationship between the identity of the products and the bonding character of the ionized electron is therefore to be sought in such instances. Unfortunately. there are as yet no clearly-characterized direct dissociations of ions for which this idea can be tested. The detection of a particular product can be considered to be significant only if an alternative decomposition is energetically possible. and this is not so in ‘the few direct dissociations of polyatomic ions that have been recognized up to now. If a molecular ionic state is definitely bound. when dissociation takes place the process is called predissociation. As noted above. only casc(l) and casc(2) will be discussed here. In casc(l) predissociation occurs by rearrangement of electronic energy: there is a non-radiative transition from the bound state to a continuum state. that is . from one potential energy surface to another. If the two electronic states have the same symmetry. the predissociation is homageneous: if they are of different symmetry. it is heterogeneous. Electronic predissociation is probably the most common mechanism by which small ions formed in photoionization decompose. The way in which the excess energy is divided among the degrees of freedom of the products. including translation. depends on the details of the potential energy surface. especially at the configuration where the transition takes place. It is possible for a large fraction of the energy to appear as kinetic energy. 36 but alternatively internal vibrational or rotational energy of the products may be favored. Some correlation between the bonding power of the electron ionized and the identity of the products formed may be sought. as the nature of the electron ionized determines the forms of the initial ionic potential energy surface and of the initial vibrational motion. However. the existence Of such correlations will also depend on several other factors. particularly the forms of surfaces that lead to different products and potentially cause the predissociation. and the strengths of their interactions with the bound potential surface. In casc(2). predissociation takes place on a single electronic energy surface by rearrangement Of the vibrational energy. The excited molecule has sufficient energy to dissociate. but the energy is initially distributed among vibrational modes that do not correspond to the reaction coordinate for dissociation. Most thermally-induced unimolecular reactions are such vibrational pro-dissociations and involve the ground electronic state of the parent ion only. There is very little evidence to show how common this mechanism may be among molecular ion decompositions. but it is probably less important for small ions than electronic predissociation. A more distinct relationship between the identity of the products and that of the electron removed is expected in vibrational predissociation than is possible in Case(l). as the vibrational energy flows from the mode excited upon ionization into other modes. which presumably correspond more closely to different reaction coordinates. According to theories of unimolecular reactions. the excess energy should be distributed among all degrees of freedom of the products. in favorable instances in a 37 statistical manner. Under these circumstances the rate constant is given to a first approximation by:17 k = k,[(E—E,)/E]N-1 - h.[l-(E,/E))N‘1. (2-18) where E is the energy available. E0 the threshold energy. N is the number of vibrational modes in the parent ion and k. is of the order of a vibrational frequency. say 1013 second-1. and is characteristic of the particular dissociation and the form of the activated complex. Because the factor in parentheses is necessarily less than unity. the larger the molecule the slower is its dissociation for given excess energy above the threshold. Near the threshold there is a minimum reaction rate for parent ions that possess only one quantum of vibrational energy above the threshold. For triatomic molecules. this minimum rate is about 109 second-1. for larger molecules. it rapidly become less. being about 105 second.1 for a penta-atomic molecule. Equation (2-18). however. is only a first approximation as it is derived not only on the assumption of statistical equilibrium. but also of classical harmonic oscillators. all of which have the same frequency. For details of the more precise forms of equation (2-18) the reader is referred to reference 60. The next mechanism to be described is internal conversion plus vibrational pro-dissociation. Because of its supposed prevalence. this "mixed" mechanism must be considered separately. although it is a sub-case of predissociation. According to the quasi-equilibrium theory of mass spectra (GET). all excited parent ions relax rapidly by 38 conversion of their electronic excitation energy into vibrational energy of the parent ion in its electronic ground state. after which fragmentation follows by vibrational predissociation. The first step is an internal conversion from one bound electronic state to another. a process that is strictly controlled by selection rules in small 12 seconds in molecules but thought to reach completion in 10‘.11 to 10- large molecules.37 In this mechanism. all traces of the identity of the electron originally ionized are lost. and the abundances of different products should depend only on the dissociation \limits for their formation and on statistical factors. The excess energy must be distributed statistically among all the internal degrees of freedom of the fragments. Potential energy curves intended to illustrate these three mechanisms are shown in Figure 2-3. but it must be emphasized that the representation is very crude. At least two outs must be made through the multidimensional potential energy surface in order to indicate vibrational predissociation at all. whereas the molecules themselves have a choice from an infinite number of such cuts. Very many more sub-cases of predissociation can be imagined than are represented in Figure 2-3. and a wider selection has been illustrated by Mulliken.38 Which of the models of ionic dissociation best describes a given decomposition depends first on the nature of the energy surface--particularly whether direct dissociation is possible or not-and then on the rates of the competing elementary processes. Hints to the rates of the elementary processes are provided by the correlation rules. by selection rules. and for vibrational predissociations. by 39 statistical theories of unimolecular reactions. These last are unfortunately the main clues that are available to the rate of dissociation of most large polyatomic parent ions. because the application of correlation rules or symmetry gives little useful information. Since this dissertation deals only with polyatomic molecules. radiationless transitions. internal conversion and vibrational predissociation are the elementary processes most relevant to the discussion of observed fragmentation reactions. Both predissociation and internal conversion are examples of radiationless transitions in which the system passes from one electronic state to another at the same energy. The rate of such a process depends on two factors: the strength of the interaction between the electronic states and the agreement between the nuclear positions in the two states at that energy. The rate is proportional to the squared magnitude of a matrix element. which may be separated into an electronic part and a vibrational overlap integral. The two parts cannot always be treated separately because the interaction between the states may be brought about by coupling between nuclear and electronic motion. Deviations from the Born-Oppenheimer approximation must then be taken into account. and potential energy curves are an incomplete model for the dynamic process. Nevertheless. it is helpful to discuss predissociation in terms of deviations from the classical model of distinct potential energy surfaces. If the lifetime of the initial state is t. there may be many accessible final states that are isoenergetic within the energy uncertainty h/2nt and the lifetime then depends also on the density of final states. p(Ef). These ideas are combined by the "golden rule" Of 40 perturbation theory into an equation for the lifetime: {1 = hp'pusf). (2-19) Here p is the matrix element of the perturbation. p-(z, is in the form of an overlap integral between vibrational eigenfunctions of a neutral parent electronic state and a parent ion electronic state. This integral. called the Franck-Condon factor. is largely responsible for the relative intensities of the vibrational bands in photoionization transitions. This overlap integral does not 72 Figure 3-1. The effect of Franck-Condom factors and autoionization on a PIE curve and corresponding photoelectron spectra. A. PIE curve. 8. PES spectrum with source energy b. C. PES spectrum with source energy c. D. PES spectrum with source energy d. 73 vanish by orthogonality because Fv' and Fv" are vibrational functions belonging to different electronic states. The effect of the Franck-Condon factor on the threshold Of a photoionization cross section is to attenuate the electronic step-like threshold by introducing additional step-like structure to the photoionization efficiency curve. The threshold for an electronic state Of the parent ion becomes a series of steps. where each step represents a threshold for a vibrational state. The relative heights of successive vibrational steps are proportional to the relative intensities for the corresponding transitions. The overall shape of the threshold can vary. depending on the difference in equilibrium geometry and bond length between the neutral molecule and the ion. This is illustrated in Figure (3-2) for a diatomic molecule. If the potential energy curve of the ion is very similar in shape and has an equilibrium internuclear separation identical to that of the parent curve (as might be the case following photoionization of non-bonding electron). then essentially only the vibrational ground state of the ion will be accessible from the molecular ground state. The threshold corresponding to this electronic transition will be only a single step. If the ion internuclear distance is a little greater or smaller than that Of the neutral molecule (e.g. the electron is removed from a slightly bonding or antibonding orbital). then additional ion vibrational levels become accessible and a vibrational progression will be observed. with a (06-0) transition and progressively less intense transitions to the higher vibrational levels. The threshold will be characterizeded by a staircase with a large first 74 / I I I \ ’ / ‘ ’ a U I \ ’ I 8 I I ‘i‘ I \/ : U. 8 0.11156 an r. = 0.11171 non r. 3 0.14002 mm ‘ = 2 U .- O s r. 8 0.12075 mm L 1L7 1, 1 1 1 1 1 1 1 0.10 0.12 0.14 0.10 0.12 0.14 0.10 0.12 0.14 0.16 0.10 r. I'll "ANCK - CONDON IACIOIS ..0000'.... .. 0123030709101”!!! F- f f EH06? O C C o— “- .- a ION VIIIO Figure 3-2. Effect of Franck-Condom factors on direct ionization thresholds of diatomic molecules. [From E.M.Rosenstock. Int. J. Mass Spectrom. Ion Phys. 39. 139 (1976).] 75 step and progressively smaller steps to higher energy. If there is a large difference in internuclear distance. for example when an electron is removed from a bonding or antibonding orbital. the vibrational ground state (06-0) transition is no longer the most intense and the maximum transition probability shifts to a higher vibrational level. Sometimes the (06-0) transition may be so weak as to be unobservable. and the threshold region will exhibit a long progression of vibrational steps. Figure (3-3) shows schematic potential energy curves for a hypothetical molecule AB in its ground state and the corresponding parent ion AB+ in several ionic states. The vibrational eigenfunctions drawn into the upper and lower potential wells are intended to approximate those of an anharmonic oscillator. The wavefunctions for the v=0 levels are bell-shaped curves whose maxima lie at the equilibrium internuclear separation for the particular electronic configuration. The eigenfunctions of the higher vibrational levels have broad maxima or minima near the classical turning points of the motion. The maxima and minima between these termini are smaller and narrower than those at the terminal positions. The contribution to the overlap integral from these intermediate crests and troughs will roughly cancel one another. For a transition in which v"=0. the overlap integral has large values for the upper vibrational levels whose eigenfunctions have their terminal maximum or minimum roughly vertically above the maximum of the eigenfunction of the lowest state. When the minima of the two potential curves lie at the same internuclear distance. the overlap integral for the (06-0) transition is large. But the (16-0) or higher vibrational level bands are obviously small since the positive and 76 (eV) .5 1. Ionization Energy AB v.80- 4' ---------------- — o 1 RO Internucleor Distance (R) -—- Figure 3-3. Schematic diatomic molecule potential curves. Franck- Condon factors. and corresponding photoelectron spectrum. [ From I.W.Rabalais. Principles pf Ultraviolet Photoelec- tron SpectroscOpy (John Wiley and Sons. New York. 1977).] 77 negative contributions to the integral effectively cancel each other. If the potential curve for the upper state is translated to higher or lower R values. the intensity of the (16-0) band increases and that of the (06-0) band decreases. The maximum in the vibrational transition shifts to correspondingly higher v' values as the potential curves are shifted further. The photoelectron spectrum. which also reflects the Franck-Condon factors in the vibronic transition moment. is included at the right of the Figure (3-3). In Figure (3-3). the internuclear separation Of the ion in its ground state. X. is the same as that of the molecular ground state. (This implies that the ion was formed by the removal of one nonbonding electron.) The resulting photoionization efficiency curve would exhibit a very sharp jump corresponding to a very intense (06-0) transition. followed by several small step increases. related to a relatively short vibrational prOgression. Curve 5 represents a potential surface in which the internuclear distance R is increased from its ground state value. In the corresponding vibrational transition. the Franck-Condon maximum appears near the middle of a long vibrational progression. In the corresponding photoionization efficiency curve. the highest step will lie near the middle of the threshold area. and two groups of small steps will be found at each side. For schematic curve 2 the electronic transition consists of a vibrational progression which finally converges into a continuum. This represents ionization into the region both above and below the dissociation limit of the ion in state p. The extrapolated appearance potential from the fragment ion PIE should correlate with the energy onset of the b-state continuum. Curve 3 shows ionization tO a repulsive 78 potential surface. resulting in broad featureless structure in the photoionization efficiency curve. Curve d corresponds to the crossing or close approach of a repulsive potential surface and a bonding potential. In such a case. the wavefunctions describing the two states are mixed and the ion is subject to the lifetime limitation of the repulsive state. Such a situation is called predissociation. It results in a local sharp decrease in the parent photoionization efficiency curve; but at the same energy a related local sharp increase in the corresponding fragment ion photoionization efficiency curve is Observed. The Frank-Condon factor has the same effects for polyatomic molecules. except that many bond lengths may change and the threshold may involve a superposition of several progressions of steps if more than one vibrational mode Of the ion is excited. The structure may be complicated. In the absence of competing processes. step-like structure is always observed in the threshold region of the parent ion photoionization efficiency curves.62-64 4. Degenerate Ionic States Whenever an electron is ejected from a fully occupied degenerate orbital in a molecule. it will form an orbitally degenerate doublet state of the corresponding ion. The degeneracy of such a state can be lifted either by the coupling between the spin and orbital angular momenta Of unpaired electron--spin-orbit coupling. or by a change -in molecular shape --configurational instability. For linear molecules the latter is called the Renner effect21. for nonlinear molecules it is termed the Jahn-Teller effect.21 When both Of these effects (spin-orbit coupling and configurational instability) are at work. the threshold of 79 a photoionization efficiency curve will have steps and these steps will show complex vibrational structure. When the interactions are stronger the threshold may exhibit as many steps as there were electron pairs in the original degenerate orbital. but not more. In some unusual photoionization processes. 'orbitally degenerate ionic states can be produced that are not also spin degenerate; such states would be susceptible to the configurational instability effect only. and not to splitting by spin-orbit coupling. (a) Spin-Orbit Coupling When an electron is ejected from a degenerate orbital of a closed-shell atom or molecule to produce an ion in an electronic state which has orbital angular momentum. the spin angular momentum and orbital angular momentum can combine in different ways to produce new orbitals that are characterized by the total electronic angular momentum. The new states have different energies because the magnetic moments due to electron spin and orbital motion may Oppose or reinforce one another. All states with multiplicity greater than one and a non-zero orbital angular momentum are split by this phenomenon called spin-orbit coupling. When this phenomenon is weak. it produces steps in the threshold; when it is strong. under ideal conditions several thresholds will be seen in the overall photoionization efficiency curve. Spin-orbit coupling is generally stronger when heavy atoms are present in the molecule. Table (3-1) lists the observed and calculated spin-orbit splittings for the hydrogen halides and halogens. The data clearly reveal the heavy atom effect. Since the PIE threshold of most of the 80 Table 3-1. Observed‘ and calculated spin-oribit splitting for the ba- logen acids and halogens. [ From J.I.Rabalais. Prinpiples pg Ulpraviolep Photoeleppron Spepproppopz (John Wiley and Sons. New York. 1977).] [Mk SI“ . 51 AE...‘ Agni-I £1 4560-: AE-me £1 AB..- AE-ma 51 55am AEem-a . HF HG Hm' . HI (2:1. ’11... 16.02 12.74 11.67 10.38 0.04 0.034 0.00 0.076 0.33 0.333 0.67 0.671 (2'1.‘u.,. 16.06 12.82 12.00 11.05 F! G. 8': [3 (1113311.... 15.70 11.51 10.51 9.22 0.03 0.035 0.00 0.067 0.35 0.309 0.65 0.607 (11,1311... 15.73 11.59 10.86 9.87 11mm... 13.96 12.41 10.74 18.39 - 0.038 0.08' 0.088 0.34‘ 0.382 0.30 0.788 (11. 1. ’n.,.,. 1404' 12.75' 1 1.54 ‘Expedneetaldataterfll’mehomRef._l7:§1atalorfla.fllr.amd1flmttunllel.l9:dalalorthebalognaanetromlkef. 23. 'Uncertm‘nvaluorvalueaobtu‘nedhomexunpolation. 81 halogens and hydrogen halides also show superimposed autoionization structure. it is hard to definitively attribute the steps at threshold to spin-orbit coupling. Experimental verification is clear in the photoelectron spectra reported in Figure (3-4). (h) Configurational Instability The electronic transitions which are reflected in a photoionization efficiency curve satisfy the Franck-Condom principle; i.e. they are vertical transitions to points on ionic potential surfaces where the nuclear configuration is identical to that of molecular ground state. If the electronic state of the ion is orbitally degenerate. retention of the ground state nuclear geometry and the orbital degeneracy may not be simultaneously possible. and a nuclear displacement will occur that destroys the orbital degeneracy. Such displacements are called Jahn-Teller or Eenner distortions. and can lead to considerable spectral complexity. Only the Jahn-Teller effect will be sketched here. since only non-linear polyatomic molecules are discussed in this disseration. There are two kinds of Jahn-Teller effects. When there exist stable distorted configurations. the electronic energy is lowered and there is more than one position of equilibrium with equal energy. In 1 other words. the electronic degeneracy is replaced by vibronic degeneracy. Thus a Jahn-Teller-distorted molecule 'has a permanent distortion that lowers its symmetry and produces an observable anisotropy. This is called the static Jahn-Teller effect. Alternatively. a change in the electronic energy can be caused by the excitation Of one or more degenerate vibrational modes from an Relative Intensuty Ionucnon Energy(eV) Figure 3-4. HeI photoelectron spectra of halogens and halogen halides. [ From J.W.Raba1ais. Principles 2; Ultraviolet Photoelec- tron Spectroscopx (John Wiley and Sons. New York. 1977).] 83 originally degenerate electronic state of a molecule which is unstable towards distortions which remove the degeneracy. The total energy can no longer be separated into electronic and vibrational parts; that is. there is strong vibronic coupling. This is called the dynamic Jahn—Teller effect. Figure (3-5) shows a qualitative picture of the Jahn—Teller effect as it influences the threshold of CK‘+. and the experimental photoelectron and photoionization spectra. Steps on the PIE of CK‘+ due to Jahn-Teller distortion are clearly evident. There are many 's26 book publications which relate to the Jahn-Teller effect; Derkowitz contains detailed information of relevance to experimental photoexcitation studies. 8. Thermal Effects 1. hot Bands Particularly when molecules have low vibrational frequencies. there may be significant pOpulation of excited vibrational states of the ground electronic state at typical experimental temperatures. The threshold of a photoionization efficiency curve may be complicated by this effect. called hot bands. Hot bands are produced by transitions from excited vibrational levels of the ground molecular electronic state. v")0. to various vibrational levels of ionic states. When hot bands are observed in the threshold of a PIE. they always contribute intensity to the low energy portion. since transitions v”>09v'=0 will have lower energy than the desired 090 origin. If a low temperature photoionization mass spectrometric study of the same compound is ENERGY IeVI 16.00 1500 ILDO 1300 coo-5 Figure 3-5. I I I I T I I I I I I 84 CW 0; U i s .3. CH; c3v T fi f f l 1' c”; I 3' a L .E a . D2d a Ia? 2:3 AI 9. ‘I 3! I I‘ 5: II. (C) B 0‘» 27: a I U, 0 g ’2‘. ‘.\/’73." U 0o 9 z % ‘ 9 ° . z 5." 2‘ wooK/O % 5 3, k . CH‘ T .9 M: a a 3. a (b) 4‘ 9 TON ENEQGI ‘e-J) Example of the Jahn-Teller effect. (a) Qualitative configu- ration coordinate diagram of methane along the distortion coordinate p. The upper levels are drawn to be consistent with the photoelectron spectrum at the right. (b) PBS spec- trum of Cl‘ in the threshold region with 21.2eV incident radiation. [From labslias. et al.. Phys. Scr.‘§. 13(1971).] (c) lass-selected photoion yield curve of CB‘+ in the threshold region with the gas sample at 78 and 300 K. [From W.A.Chupka and J.Berkowita. J. Chem. Phys. g1. 4256(1971).] Note: The energy scales of (b) and (e) have been matched so that corresponding structures and thresholds can be di- rectly compared. 85 possible. the hot band contribution may be discerned by comparing thresholds of the photoionization efficiency curves obtained at different temperatures. From statistcal thermodynamics. it is possible to calculate the relative thermal pOpulation of low energy vibrational states in the ground electronic state at room temperature.65 Figure (3-6) shows the fractional population of several vibrational states of Br3 at room temperature. The values were calculated from the equation f(n) = EXPl-hv(n + l/2)/hT]/qvib(T). (3-2) qvib = kT/hv for (kT)>hv). (3-3) where f(n) is the fraction of molecules in the vibrational state designated by vibrational quantum. number n. qvib is the vibrational partition function. and v is the vibrational frequency. The result is an exponentially decreasing curve. For polyatomic molecules the threshold vibrational distribution is complicated. but the overall shape is similar. Dibeler. et al.66 show the hot band contribution to the Br, -9 Br3+ + e- threshold at different temperatures. If the temperature is lowered sufficiently such that only v"=0 is significantly pOpulated. the hot band effect disappears. 2. Thermal Tailing Thermal tail is also an effect arising from the thermal 86 10m _ J 0.75 - _ C h 8 0.50 — _ 5 o 2 LI. 0 2 E3. 0 E 0.25 - - 0.00 — - I I L l l l 0.0 1.0 2.0 3.0 4.0 VIBRATIONAL LEVEL (n) Figure 3-6. The population of the vibrational levels of Br2 at 300 K. [From D.A.Mc0uarrie. Statistical Mechanics (Harper and row Publishers. New York. 1976).] 87 fiqz'tl1v'--|3néI'+'e 378 K 297 K 222 K l98 K I z“:- . (D t I z .‘I a — c’E . . I E III I . :5 ’z "' II I I m _ - I: M: II' E I' . I'I l0.57ev I ' ° I (I. 0) \II I 3 ' '3 I g I h - I .I \‘ I' I I .I. J II 5 I . "793 I " III I IO.52eV I 07913 I P :III I In I ~ “I I0.52eV . ., 1 ”793 I: . ' ”795 [FIE 1“! WI} (0.0) \\ I. It; .. ‘\I ' I I, I " I \3 ~..’. 7 .fl‘ «009'. ,... .I' “'7". - I .gL-I-fi'”. . I I a" ' l 1 JW.. '7 I ' IIas II75 IIes II75 IIes HTS IIes II75 WAVELENGTH. A Figure 3-7. Hot band structure of Br2+ at several temperatures. [From V.H.Dibeler. et al.. Int. J. lass Spectrom. Ion Phys. 1. 209 (1971).] 38 distribution of populated rotational and vibrational states of the neutral, which are available for fragmentation at lower photon energies than molecules in the ground rovibronic state. Experimentally. the most obvious effect of the thermal energy is to introduce a low intensity, slowly-rising onset--a thermal tail--to the fragment ion photoionization efficiency threshold. If one scans from high to low photon energy, the "thermal tail" approaches the base line asymptotically. and if the data are acquired with a sufficiently sensitive instrument. the tail will continue for several hundred millielectron volts below the thermochemical threshold. Chupka‘s7 has studied the effect of thermal energy on photoionization efficiency curves of fragment ions; Guyon and Berkowitz68 have shown that the internal thermal energy of the neutral shifts 'the fragment ion threshold to lower energy by an amount equal to the average thermal energy of the neutral. Figure (3-8) shows the relative population of the rotational levels of HCl at room temperature (298°K) calculated by statistical 65 thermodynamics. The equations used in the calculation are: {Iii/N = (21 + naxpt-erunnm/q (T). (3-4) rot qrot(T) = (T/er)(1 + 1/3(9r/T) + ...). (3-5) where Nj is the number of molecules in jth rotational state. at is the characteristic temperature of rotation. J is the rotational quantum number, and qrot(T) is the rotational partition function at temperature 89 19.0 - 18J)'- 17JJI- 16x) -~ 1511'- 14J)- 1313- 12.0 - 1L0 - 1013- [V PERCENTAGE 913 r 110 711 61) j w w l 1 I 51) ‘k0 ' 1 311'- ZJJI- 113- [4141411 1 l L L l J l l l l l l l l 1 l l l ._ 01) Figure 3-8. 0&3 2.0 4.0 6.0 8.0 10.0 12.0 14.0 ROTATIONAL LEVEL (n) The population of the rotational levels of HCl at 300 K. [From D.A.McQuarrie. Statistical Mechanics (Harper and row Publishers. New York. 1976).] 90 T. Even in the absence of vibrational hot bands. the thermal rotational energy distribution must be deconvoluted from the experimental PIE. C. Kinetic Effects Kinetic effects on the threshold of photoionization efficiency curves are related primarily to fragmentation 'processes. The generally-accepted, semi-quantitative description of the fragmentation of polyatomic molecules is based on quasi-equilibrium theory(QET).69-71 According to this theory: (1)Parent ion fragmentation can be considerd in two parts: one is the act producing the ion and the other is internal excitation energy distribution. (2) The fragmentation processes can be described as a series of competing unimolecular reactions. ‘(3) The unimolecular reaction rate constant can be calculated quantitatively by means of activated complex theory. The term "quasi-equilibrium” refers to the assumption that a parent ion produced directly in an excited electronic state via a transition from the ground electronic state of the neutral molecule will have enough time to undergo internal conversion to form a vibrationally excited ion in its ground electronic state prior to dissociation. Three distinct factors introduced by GET must be taken into account in interpreting thermochemical data from experimental fragmentation thresholds: (1) The relation between activation energy for decomposition and the heat of reaction for the process; (2) the relation between activation energy and the minimum energy to produce observable fragmentation in the mass spectrometer ion source. i.e. the so-called kinetic shift; (3) the effect of competing and consecutive reaction 91 paths on the shape and extrapolation of fragmentation threshold curves. 1. Activation Energy and Heat of Reaction Some ionic decomposition processes occur via pathways involving the surmounting of a barrier on the potential surface. Surmounting such a barrier would require an activation energy greater than the heat of reaction of the process. Thus, the determination of thermochemical information from fragmentation threshold energetics is always subject to an uncertainty equal to the activation energy. Thus heats of formation calculated from experimential PIEs will be upper limits. It is sometimes possible. from observations of fragment particle kinetic energies. to deduce the existence of a potential barrier in the reaction coordinate for the decomposition. For example. analysis of the peak shape of an ion formed in 'metastable transitions, or measurement of delayed kinetic energy release for dissociation processes in the mass spectrometer can be related to the kinetic energy released in the dissociation processes. The translational kinetic energy of the fragmentation partners arises from the excess energy present in the dissociating ion at the excitation energy of the experimental fragmentation threshold.72-75 The peak shape observed in a time—of-flight spectrometer is related to the travel time distribution of fragment ions of a particular m/z value. mhich is determined by their initial translational energy distribution.76'77 Investigations to date indicate that fragmentation processes may be accompanied by kinetic energy release ranging from essentially zero to nearly one electron volt. Thus the extrapolated threshold for a fragmentation processes may be shifted to higher energy by an amount in 92 this range. However. detailed processes for correcting an experimental threshold value for the presence of translational energy distribution remain to be worked out. 2. Kinetic Shift In order to observe fragment ions in the mass spectrometer, the fragmentation process must occur before the departure of the parent ion from the ion source. (If fragmentation occurs within the quadrupole mass spectrometer. the ionic fragments will still be counted as parents.) The residence time of an ion in the source is roughly several microseconds.78 As noted earlier. an ion must contain enough excitation energy to equal or exceed the activation energy for the fragmentation process. However. if the activation energy for the fragmentation is high. or the number of degrees of freedom of the parent ion is large. the rate of ion decomposition at the thermochemical threshold may be too slow to lead to observable fragmentation. i.e.. fragmentation while the parent ion is still in the ion source. Additional excitation energy must be supplied to increase the decomposition rate. Thus. under some conditions the measured fragmentation threshold energy will overestimate the activation energy of the process. The term "kinetic shift" has been defined as the excess energy required to produce a measurable current of fragment ions before the parent ion leaves the ion source (i.c... about 5 10- sec). The presence of a kinetic shift in the fragmentation threshold can be tested by obtaining fragment PIEs at several different temperatures and can sometimes be inferred from changes in the PIE with acceleration voltage. From the disscussion above, it is clear that the 79.30 fragmentation threshold will vary with temperature. Eosenstock et 93 al."1 have emphasized that for large-molecule fragmentation processes the kinetics of decomposition lead to a gradual increase in fragment ion current, so that there is no well-defined threshold. The kinetic shift effect has been studied by many scientists; the subject has been 0 0 reviewed by Vestal.82 Cooks."3 Harrison.84 Rosenstock.5 Levsenos and Lifshitz.36 3. Reaction Path and Reaction Competition According to quasi-equilibrum theory. the fragmentation processes are a set of competing unimolecular reactions. The parent ion will dissociate through a number of different channels. producing different fragments. The energy dependence of the first-order rate constant may be somewhat different for the various processes, so that some fragment ions will be produced in experimentally detectable amounts only at energies somewhat in excess of the activation energy for the process. The result of reaction competition is therefore a shift. in addition to the kinetic shift, of the minimum observable decomposition rate for the less favored process to high energies. At a given excess internal energy of the parent ion. a dissociation path involving relatively high activation energy will be less favored then all lower energy decomposition channels. As a result. there is no sharp threshold. but instead a very gradual increase in the fragmentation cross section for the higher energy process. For all pratical purposes this prevents the determination of precise threshold values for such processes. D. Instrument Effects The photoionization threshold is directly influenced by the finite 94 resolution of the monochromator and the finite widths of the entrance and exit slits. These instrumental effects have been discussed in the . 87 . . . . . . literature, and the slit-width contribution is presented in Figure (3-9). A step function is assumed for the photoionization cross section for a single. ideal transition from the neutral to the ion. This is combined with a trianglar slit function. which has been shown to be a 0 good approximation.” 3 Convolution of the preper integrated form of the triangle slit function with the theoretical photoionization step function leads to two parabolic curves (one normal and one inverse) connected at the inflection point. which is the true threshold position. A line drawn tangent to the inflection point in the PIE intercepts the energy axis at 5/2 below the threshold (5 is the slit width in energy units). Application of this result to the threshold region of photoionization efficiency curves shows that the slit widths account for about half the breadth in the rise of the step. 95 PIE 5,-8 "é’él ‘ E,+8 .Figure 3-9. The effect of a triangular-slit function on a step function. (a) Triangular-slit function; (b) Tri- angular-slit function convoluted with a step fun- ction. [From D.M.Rider. Ph.D. Thesis. M80. 1980.] 96 CHAPTER FOUR EXPERIMENTAL APPARATUS AND PROCEDURES A. Introduction The purpose of this chapter is: (l) to present a brief description of the MSU instrument in its current operating mode; (2) to document the experimental conditions under which the data presented in next chapter were recorded: (3) to discuss experimental problems which might be encountered in the use of this complex instrument; and (4) to serve as a reference guide for future Operators. The instrument has been described in detail in the experimental chapters of the Ph.D. dissertations of two_ previous workers-Ed Darland13 and David Rider89-and a brief overview is included here to facilitate the discussion in this chapter. A diagram of the apparatus is shown in Figure 4-1. Many instrumental problems were encountered during the course of this investigation. To solve these problems one needs experience and patience. All these problems were overcome: however. some of them are likely to occur again due to the age of the instrument. It is hoped that the following description will be of help to future users of the instrument. B. The Instrument 1. Light Source The light source is a Hinterregger-type windowless discharge lamp with a water-cooled anode and cathode. The discharge tube is a Pl EN LP 1 V Figure 4-1. HA: EN: EX: GR: IS: 11': LE: LP: P1: P2: P3: P4: P5: PT: QP: Q8: 97 P4 ’1_—'/ F r ex 7/ ”Q .5 4:.- LQ G \ L \\u U PT [.5 a A IT 11 LJC'):\ 6 .11.. 1“ The PIMS Instrument. baffle entrance slit exit slit grating ion source ion transducer ion lens lamp first differential pumping port second differential pumping port monochromator pumping port sample chamber pumping port quadrupole chamber pumping port photon transducer quadrupole rods quadrupole support 98 water-jacketed quartz capillary (25 cm long x 4 mm ID) to which the discharge gas is admitted at the anode and back-pumped at the cathode. Two kinds of gas served as light sources: helium and hydrogen. The all-glass lamp gas inlet system has been designed for easy maintenance and testing. and is mounted on a metal frame located so as to minimize the chance of breakage. The purpose of the lamp gas delivery system shown schematically in Figure 4-2. is: (1) to adjust and control the lamp gas pressure; (2) to purify the commercial-grade cylinder helium. which passes through a molecular sieve trap which is immersed in liquid nitrogen, thus condensing argon. oxygen and most other impurities; (3) to adjust and maintain a small differential back-pumping pressure at the cathodic end of the lamp. which stabilizes the discharge and minimizes sputtering of the cathode onto the monochromator entrance slits. Emission from rare gas continua is generally produced by a high-power pulsed discharge through the pure gas: in the work to be described, the Hopfield continuum of helium was employed. The lamp gas Operating pressure is about 70 torr, and the discharge electronics include a home-built. single vacuum tube and high-power switching circuit to pulse the output of a high voltage d.c. power supply. The switching circuit is in turn driven by a Cober A model 605P high-power pulse generator.(For details. see references 16 and 88.) The emission from this source under the conditions listed in the caption is shown in Figure 4-3. The intensity of the helium continuum is controlled by several variables: helium pressure and purity. pulse width and frequency. d.c. discharge voltage. Cober pulse generator peak output voltage. and the 99 Lamp i 5:187 ' 1L : iF‘Tzf . 5 I ' - : Lt::::®':| . 5 ‘ I L ...... l 4:]: ' g (its Lamp gas Thermocouple " adjustment _g I >- . 4 1 Cum]! valve J A back pumping Q adjustment valve From Interlock tank-> control valve II Manometer (working fluid: diffusion pump oil) .Lmecnanical To pump Figure 4-2. The lamp gas delivery system. In order to run the lamp. valves 1.5.7.8.10 and 11 must be open: additionally: for hydrogen. valve 2 is opened. for helium. valves 3 and 4- are opened. PHOTON counts/ second 100 WAVELENGTH (Angstroms) 1000 900 800 700 I I I I I F T l—r I l T T I I f T I r l 1 l l. 6500.0 - in 5500.0 #- 4500.0 +- r 3500.0 - 2500.0 3} :ljlllllllllllllllllllllfllulllIllIllllllllLllllllllllllllllllllllLlJLllljllllll'LLl 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 ENERGY (electron volts) Figure 4-3. Helium continuum spectruum. DATE: 15-SEP-82 LAMP GAS: Helium PRESSURE: 70.0 torr MEGAVOLT SUPPLY: 10.0kv, iOOmA PULSE VOLTAGE: 1.40kV PULSE WIDTH: PULSE PERIOD: MONOCHROMATOR: PHOTOMULTIPLIER: CURRENT AMPLIFIER: V-F CONVERTER: 0.50 microseconds 50.0 microseconds 600L/mm grating 100/100 micron slits 1200V 1.0E-05 20kHz 101 condition of the lamp electrodes. Higher helium pressure generally produces higher lamp output intensity. For the MSU instrument. maximum pressure is limited by the differential pumping between the lamp and monochromator and by the pumping speed of the monochromator pump. Typically. 70 torr of helium in the lamp and 100 pm entrance slits are used. The pulse frequency. pulse width and pulse voltage are adjusted by the Cober pulse generator. The capabilities of the Cober and the limitations and requirements of the switching circuit constrain these parameters. The Optimal set of parameters are: a pulse width of 0.5 as, a pulse period of 50 ps(translating to a one percent duty cycle). and a Cober high voltage setting of 1400 volts.(The Cober meter read 1400 volts. but the voltage measured was 1050 volts.) A narrower pulse width would allow a higher frequency repetition rate. which would increase the lamp intensity. However. the pulse width is limited to 0.42 as by the turn-on time of the switching circuit. With the above settings on the Cober and 70 torr of helium. the maximum intensity is attained with the Megavolt high voltage d.c. power supply set at 10 kilovolts. Under these conditions a current of 100 to 110 mA is drawn from the megavolt power supply when the lamp gas is pure and the lamp electrodes are clean, Increasing the voltage above 10 kilovolts increases the current drawn from the Megavolt power supply. but hardly improves the lamp intensity. The hydrogen many-line pseudo continuum is produced by a d.c. discharge through a pressure of 2 to 4 torr. The switching circuit is disconnected in this application, and the hydrogen discharge is powered directly from the Megavolt high voltage d.c. power supply. A 1500 Q. 102 1500 W resistor limits the current drawn from the supply. The output from this light source is shown in Figure 4-4; the experimental conditions are listed in the caption. Many of the potential problems with the lamp have 'been discussed in detail by David Rider,89 and will not be repeated here. Under typical Operating conditions, the molecular sieve trap will function well for about five days. or three tanks of helium: thereafter. the molecular sieve will be full and inadequate purification will result. This is revealed by checking the color of the lamp: if the original light-pink color changes to purple. the helium gas contains significant impurities. These impurities add sharp atomic lines to the helium continuum output. The aluminum cathode of the quartz high voltage discharge tube is quickly contaminated when helium is used. After continuous Operation for about three months(maximum time), the intensity Of the light drops if the lamp is not cleaned. The entire discharge system should be disassembled and cleaned as a part of routine maintenance. The detailed procedures (including aligment of discharge tube) can be found in the PIMS operations manual. At the same time, the Delrin insulators in the lamp housing should be carefully checked. since the high voltage discharge will corrode them quite quickly. Be sure that each insulator in this part is still usable. The high voltage power supply, the switching circuit, the Cober pulse generator anu cue .etrode discharge tube have been working very wel. .ez duxe than a year, and caused no problems during the course of this research. If a problem should arise. turn off all the power in the order specified in the Operations manual (inverse of turn-on procedure). PHOTON counts/ second 103 WAVELENGTH (Angstroms) 1300 1250 1200 1150 1100 1050 1000 950 900 15000 '- 10000 '- 5000 I '1 P- aco 10.00 10.40 10.50 11.20 11.60 12.00 12.40 12.30 1.3.20 1.3.60 ENERGY (electron volts) Figure 4-4. Hydrogen many line spectruum. DATE: 25-MAY-82 LAMP GAS: Hydrogen PRESSURE: 3.0 torr MEGAVOLT SUPPLY: 2.75kV, 700mA MONOCHROMATOR: BOOL/mm grating 100/100 micron slits PHOTOMULTIPLIER: 1200V CURRENT AMPLIFIER: 1.0E-05 V-F CONVERTER: 20kHz 104 Never try to make repairs in the lamp power supply circuits by oneself. especially if one lacks experience with high voltage devices. Indeed. one should never work on the high voltage circuits alone. and it would be best if someone who is familiar with electrical hazards is present. ‘The lamp system is not only dangerous. but also very expensive and it is not easy to obtain replacement parts. If the vital parts are damaged by imprOper treatment. it may take more than three months for replacements to arrive. 2. Monochromator The monochromator is a McPherson model 225. l-meter. near-normal-incidence instrument. The dispersing element used for this research is a concave osmium-Overcoated aluminum grating. ruled with 600 lines/mm and blazed at 900A.(Another grating. magnesium fluoride overcoated. ruled with 1200 lines/mm and blazed at 12001 is also available in the laboratory.) The grating has a reciprocal dispersion of 16.8A per mm. Two interchangeable entrance slit plates are available. Each plate has a blank port plus three fixed slit widths: 10pm. 50pm and 100nm. or 100nm. 300nm. and 500nm. For a given plate. the entrance slit can be changed without breaking vacuum via a knob located outside of the vacuum chamber. Interchangeable exit slit tracks are also available. each Of which has four fixed slit widths: lOum. 50pm. lOOpm and 300nm. or 100nm. 300nm. 500nm and 1000um. Any one of the four slits on a given track can be selected from outside the vacuum via a knob-chain-gear system.89 The procedure for interchanging entrance and exit slit mounts 105 is given in the PIMS operations manual. The grating is driven by a stepping motor which can be Operated manually or with a computer. The grating should be cleaned when the helium lamp spectrum shows significant scattered light below 5001. To clean the grating. first rinse with Freon 11 (CFCl,). followed by high purity methanol. The methanol rinse is best done by simultaneously applying the methanol from a squeeze bottle and drying the grating with low pressure. clean. compressed air. working from tap to bottom. The removal. cleaning. replacement. realigment and focusing procedures are listed in the McPherson monochromator manual. Because realignment of the grating requires adjustment of many parameters. a great deal of time can be saved if the grating is cleaned in situ. 'In this case. care should be taken to direct the cleaning solvents only on the grating (some absorbing materia1-such as Kimwipes-can be placed around the bottom of the grating mount). and sufficient time thereafter(about one day) should be allotted to allow vaporization of the solvents from- the loosely-covered monochromator under moderate nitrOgen gas flow. 3. Interaction Region and Sample Inlet System The ion source is a cubical stainless steel box. one inch per side. located one centimeter beyond the exit slit of the monochromator. It has two rectangular apertures on apposite sides to enable transmission of the photon beam. The widths Of the rectangular apertures are adjustable to allow uninhibited passage of the photon beam with any chosen exit slit. 0n the horizontal perpendicular to the Optic axis is a one-quarter inch diameter ion exit hole. The ion aperture is 106 covered with a high-transmission fine wire grid to minimize penetration into the source of electric fields from the ion focusing lenses located just outside. The repeller. an electrically-insulated stainless steel plate. is positioned inside the ion source across from the ion exit aperture. An adjustable voltage is applied to the repeller to accelerate the ions from the source into an ion lens system. The sample is admitted through a tube located behind the repeller and the sample pressure is measured through a tube in the top Of the ion source. The sample pressure is controlled with a Granville-Phillips model 203 adjustable leak valve and measured with a Datamatrics model 1014-A Barocell capacitance manometer. The leak valve and manometer are outside of the high vacuum. 4. Photon Transducer The photon transducer is a sodium salicylate phosphor whose emission intensity is measured with an RCA 8850 photomultiplier tube. Because sodium salicylate possesses a nearly constant quantum yield throughout the helium continuum. and that portion of the hydrogen many line output of interest in this work. it is superior to ordinary(bare) photomultiplier dynode chains. The sOdium salicylate emission intensity can be accurately measured with a standard photomultiplier and the emission band does not shift with the exciting wavelength. The output current of the photomultiplier is amplified with a Keithley 1800 current amplifier and converted to a voltage. Then it is digitized with a 13 voltage-to-frequency converter and a computer-interfaced counter. When used with the Hopfield helium continuum. the sodium 107 salicylate transducer detects a great deal of scattered light. This effect is corrected by making automatically repeated measurements of the scattered light during the course Of an experiment. then subtracting this contribution from the Observed intensities. The scattered light problem is much less bothersome when the hydrOgen lamp is used. since this source emits less visible and near ultraviolet radiation and since the many-lined spectrum does not begin until wavelengths greater than 8003, which is a region where the scattered light distribution has fallen off considerably. 5. Ion Optics and Mass Spectrometer The ions are guided from the ion source and focused onto the quadrupole mass filter with four electrostatic aperture lenses. The electrostatic lens system was supplied with the commercial mass filter. and has been modified by removal of the electron impact source. The mass filter is an Extranuclear 324-9 quadrupole mass filter. with rods 1.9 cm in diameter times 22.0 cm long. It is powered by a model 311 power supply. equipped with a model B (mass range 0-200amu) or a model 13 (mass range 0-400amu) high Q head. The quadrupole (as well as the lenses and ion transducer which are connected to it) is mounted on a support which is easily moved several inches in a direction parallel to the quadrupole axis. This permits placement of the quadrupole at an appropriate distance from _the photon beam for use with single or dual chamber ion sources of various depths. The mass Of the ions transmitted. by the mass filter is controlled by a dial on the front panel Of the quadrupole power supply.or by a voltage applied to a connector on the 108 back side of power supply. To obtain a mass spectrum. the transmitted mass is scanned by a mass scanner which applies a voltage ramp to the connector on the back panel of quadrupole power supply; the mass scanner was designed and constructed by David Rider.89 When the high Q head is changed or after about six months of continuous use of a given head. the mass filter needs to be "tuned up" to rematch the RF frequency Of the quadrupole power supply and the high Q head. The detailed procedures are listed in the Extranuclear quadrupole filter manual. 6. Ion Transducer The ion transducer. located on-axis at the exit hole of the quadrupole. is a Channeltron continuous-dynode electron multiplier. model CEM 4816. supplied by Galileo Electra-optics. The channeltron output is measured either in a direct-current or a pulse-counting mode. In the former mode the output is amplified with a Keithley model 417 electrometer. However. since the direct-current method has several disadvantages compared to pulse counting when the output current is small. it was used only for setting up an experiment. All data were collected with the pulse-counting technique. where the output pulses are amplified and discriminated with a circuit developed here. 7. Vacuum System In order to maintain the required pressure differential between the lamp (=102 torr) and the monochromator(=10-5 torr). two stages of differential pumping are used. The monochromator is equipped with a 109 McPherson model 820 differential pumping assembly. the first stage of which is pumped with a 300 liter/sec Roots blower pump. The second stage is pumped by a 300 liter/sec. four-inch diffusion ejector pump. and the monochromator with a 2400 liter/sec. six-inch diffusion pump. The sample and quadrupole chambers are also pumped with six-inch diffusion pumps. rated at 2400 liter/sec and 1800 liter/sec respectively. A baffle around the quadrupole also provides some measure of differential pumping of the sample gas in these chambers. The pumping speeds quoted above are for untrapped pumps. All four diffusion pumps are trapped with Freonrcooled baffles to reduce the amount of oil back-streaming into the chambers: thus the actual pumping speed in the various chambers will be significantly lower. When the system was pumped down. the best pressure Obtained was 2.0x10-7 torr. Nevertheless. these pumps are sufficient to maintain a pressure on the order of 10-5 torr in the monochromator with 70 torr of helium in the lamp (100 an entrance slits). The pressure in the quadrupole region can be maintained below 2x10-5 torr for a sample pressure below 2x10.3 torr. even with the relatively Open ion source currently in use. 8. Interlock System The vacuum and utility service systems in the PINS instrument are quite complex. Even through each component in the system may be rather dependable. there are so many components involved that the possibility that something will fail in any given time period is quite high. Moreover. even if the individual vacuum components were completely reliable. the necessary electrical and water utilities supplied by the 110 university are not. Thus occasional failures of the vacuum system are inevitable. If proper procedures are not taken. the expensive components inside the vacuum chambers (the grating. channeltron. and quadrupole rods) may be irreparably damaged. All of these components are extremely sensitive to the presence of even very thin contaminating films of pump oil. Thus a security interlock system which monitors the foreline pressure of all diffusion pumps. water pressure of the diffusion pump cooling system. baffles. lamp gas pressure. electric part of the lamp and all mechanical pumps has been implemented13 to reduce the chance Of catastrOphic damage to the PINS apparatus. The interlock system for this instrument was designed as a "stand-alone" system which is not computer Operated; it can thus be used to protect the instrument between experiments as well as during them. allowing the instrument to remain safely under full vacuum conditions at all times. Even so. the interlock system is not sufficiently sophisticated to totally automate the pump down procedure. However. it does mandate the correct sequence of Operations during start-up. which greatly reduces operator's errors--although it cannot eliminate them. When used to protect the unattended instrument. the action taken by the safety interlock system when ‘an error condition is detected is very simple: any device which. if left on. could conceivably be harmed or cause harm to another device is deactivated. The vacuum system then remains shut down until an operator decides that it is safe to start again. 9. Instrument Control and Data Acquisition 111 The computer used to control the PINS instrument during the course of this research was an antiquated PDP 8/I minicomputer. which uses a 12-bit word length and has 12K of memory. After this investigation was completed. SE of the memory was inexplicably damaged and required repair. The PDP 8/I was then scrapped. and the PIMS instrument is now controlled by a surplus PDP 8/F minicomputer (still 12—bit word length). which has 16K of memory. A PDP 8/M (12-bit word length. 12! memory) is also available for PIMS experiments. Data and program storage peripherals include a dual flappy disc drive(Sykes 7200 series) and an interchangeable hard disc(WANGCO F-1221). Interaction between the user and the computer takes place via a Heath Z-19 terminal or a Tektronix 4010 graphics display terminal., The data are collected by using a variable integration time technique which permits recording data of the desired quality in an Optimum amount of time. The monochromator wavelength is stepped in preset intervals. and. after each step. ion and photon counts are accumulated until the datum has the desired signal-to-noise ratio. Photon and ion count rates. wavelengths and integration times are stored on a floppy or hard disc. The course Of the experiment is continuously monitored via a computer-interfaced. stepped. stripchart recorder on which the real time PIE is plotted. Periodic measurements Of 'fiight" and "dark" photon and ion count. rates are made at a reference wavelength so that the data can be corrected for sample pressure and instrument drift. Approximate corrections for stray light are made by measuring the light intensity at two wavelengths where the helium continuum does not emit. Final corrections of the data for sample pressure variations. 112 instrument drift and stray light. as well as the final plot. are discussed in the procedure section of this chapter. C. Experimental Procedure This section presents a flow chart to show how to run a photoionization mass spectrometry experiment. The difference between this section and the Operations manual is that this discussion indicates when and why to do each procedure. whereas the operations manual describes how to do it. The experimental prOcedure introduced will follow the order of an experiment. Sometimes several steps may be started at the same time; higher priority steps will be listed first. Tb continue an aborted run. or to start a new experiment upon termination of another. the entire sequence may not have to be repeated. Under these circumstances. just skip unnecessary steps and continue to the next one. Finally. the procedure described here contains just the main concepts; for the detailed step-by-step procedures one must consult the operations manual. 1. Samples The gas samples: CF CF,Cl. CF,Br. CF,I. CF3C13. CHF,. CEF3C1. .. CHFClz. CFzBrz. and CF3ClBr were all purchased from PCR Research Inc. They were used without futher purification: a pressure regulator and gauge was attached to the commercial cylinder. and then directly connected to the ion source leak valve via a Cajon O-ring fitting through 1/4-inch 0.D. polyflow tubing. The liquid samples: CECl, and CC1.. were obtained from Fisher 113 Scientific. CFCl, from PCR Research. and CCl,Br from Eastman Kodak. All liquid sample purities were reported by the manufacturer to be better than 99.9%. Each liquid sample was placed in a pyrex bulb. degassed through several freeze-pump-thaw cycles. stored under liquid nitrogen. and later used without further purification. The samples in the pyrex bulbs(at room temperature) were connected to the leak valve via a Cajon O-ring fitting through 1/4-inch outside diameter glass tubing. The temperature of the room in which the instrument is located varies by as much as 2.5'0 during the course of an experiment. The data were collected with sample pressures between 6.0 to 10.0x10"4 torr; for a given sample. the pressure varied by less than 10% throughout an experiment. 2. System Pump-Down If the PINS instrument is completely shut down. the following procedure is required. Two parts of the overall system must be initially pumped down. the main part of the system (monochromator. sample and quadrupole chambers) and the lamp gas system. In the main part of the system. after all the diffusion pumps have been turned on and all the gate valves have been Opened. the system pressure should be lower than 1.0x10-5 torr. Normally it is easy to reach 2.0x10-6 torr with a proper degas: sometimes the pressure can be as low as 2.0x10-7 torr. After the main chambers have been pumped down. the sample gas line should be evacuated. The purposes of doing this are: (1) to pump out the air in the sample gas line. (2) to remove any residue of previous gas samples. and thus avoid contamination of the sample; and 114 (3) to check for leaks in the sample gas line connection. Any leak in the sample gas line will allow air to enter the system. which will change the sample gas purity and lead to extra peaks in the mass spectrum. The lamp gas system is pumped by a mechanical pump only. This part of the system must be absolutely leakproof: this is tested by closing the valve between the line and the mechanical pump. and monitoring the pressure change via a thermocouple gauge. If the rate of pressure increase is less than 1.0x10-2 torrlmin. the lamp gas system is considered sufficiently leakproof. Any leaks in the lamp gas system will contaminate the lamp gas. and produce dirty lamp spectra. including many sharp atomic lines. A dirty lamp gas system (especially when the helium discharge is used) will make data correction harder. The molecular sieve trap in the lamp gas system has to be cleaned periodically when helium gas is used. To clean the sieve. heat the heating tape around it up to ZOO-210°C and continue pumping until the pressure of whole system returns to normal. 3. Lamp Spectra The main purpose of running a lamp spectrum is to check the lamp system quality. Are the emission intensities as expected. and do the wavelength settings in the computer coincide exactly with several calibration lines? From a lamp spectrum. one can determine the condition of the lamp. the calibration of the monochromator and the purity of the lamp gas. Different observed phenomena infer different kinds of problems. A lamp spectrum provides not only the information mentioned above. but also information about the Optimum voltage setting 115 Of photomultiplier. the scale of the amplifier. the photon signal. etc.. which are parameters to be set before an experiment is initiated. The lamp spectrum can be generated with the grating rotation either under manual or computer control. 4. Mass Spectra Before photoionization efficiency curves from a given precursor can be Obtained. the mass spectrum of the sample gas must be acquired on the PINS instrument. Photoionization of the sample is accomplished under full illumination of the helium light source. Tb obtain the mass Spectrum. set the monochromator to the I"central imaget i.e. zero Angstroms. which is offset by 2100A on the wavelength counter. whereby all of the light from source is reflected from the grating into the reaction region. If the entrance and exit slits are Opened to their widest position. maximum photon intensity from the light source impinges on the sample gas. The mass spectrum serves several purposes:(1) It provides a test of the sample gas purity. Because most standard mass spectra are Obtained under bombardment of 70eV electrons. one must be careful when comparing PI and EI mass spectra. Although there was generally good agreement between the mass spectra obtained in this work and those tabulated in the literature. the expected variation in the relative intensities of the peaks (both parent and fragment ion) was observed. and highly fragmented daughter ions produced at high ionization energies were sometimes missing from the PI mass spectra. If the photoionization mass spectrum shows several extra peaks compared to the standard mass spectrum. it is probable that the sample gas may be 116 contaminated by some impurity. or that the sample gas inlet line has a leak. (2) One can easily determine how many ions can be produced from this sample gas by the light source. In principle. a PIE curve is obtainable for every ion revealed by a peak in the mass spectrum; the intensity of each peak gives a good idea about how difficult it will be to do that experiment. (3) It is convenient to change the conditions to start the next procedure. Simply change the entrance and exit slits to the desired positions. For a given photon energy range. the lamp gas(helium or hydrogen) can be maintained while PIEs are measured for several different ions from a single sample gas. 5. Experimental Set-Up After the mass spectrum has been obtained. the next step is the choice Of an ion for which the photoionization efficiency curve is to be measured. After a particular ion is chosen. the mass filter is set to the corresponding mass .region. and the ion Optics are adjusted to maximize the ion intensity. Scan the mass in the apprOpriate region to confirm that the transmitted ion is indeed the one selected; this is facilitated if the ion of interest contains atoms which have characteristic relative isotopic abundance patterns. Sometimes improper adjustment Of the ion optics will still allow ions to pass through the mass filter. but the isotopic ratios will not be correct. Once the desired ion is properly identified. reduce the resolution to broaden the peak and thus increase the ion signal. (However the resolution must not be lowered so far that ions of different m/z are transmitted.) Find a relatively smooth portion of the broadened mass spectrum and lock the 117, mass filter at this m/e value. Then small drifts in the quadrupole tuning during the course of long experiments will not affect the photoionization efficiency. All these procedures above can be done either by the computer or manually. Hereafter. all the procedures will be done under computer control. Many parameters must be considered when setting up a photoionization mass spectrometry experiment; these parameters have a great influence on the resulting photoionization efficiency curve. It is necessary to do a quick experiment (or perhaps several) to help decide how to choose these parameters. From the quick run. one can draw conclusions about the following aspects of the final experimental photoionization efficiency curve: (1) the desired signal-to-noise ratio (consistent with time constraints); (2) the duration of the experiment; (3) the energy region which is to be scanned: (4) the wavelength to be chosen for the reference measurement; (5) the desired wavelength resolution; (6) the wavelength interval between successive data points. and (7) the sample pressure. For the actual experiment one also needs to decide: (1) how frequently and for how long to measure the background and reference signals. and (2) where and for how long to measure the stray light. After all these parameters are determined. one can start an experiment. 6. Data Correction Once an experiment is set up. data collection and storage are controlled with a PDP 8/M or PDP 8/F minicomputer. The periodic reference measurement is made to correct for sample pressure variation 118 and instrument drift. A shutter is closed in front of the monochromator exit slit under computer control so that periodic background measurements can be made. Dark counts generated in the detectors are also stored and will be subtracted from the ion and photon signals. which are obtained when the shutter is open. in the later procedures. The stray light measurement is made to correct for the stray light contribution to the photon data. After these three corrections. which are made at the department computer facility (PDP 11/34). a comparison of the ion signal with the photon signal is made to ascertain whether corrections are necessary for artifacts caused by the finite slit width. scattered light. or detector misalignment. (For more detailed information on these points. see reference.13) After all of these corrections are made. the final photoionization efficiency curve is plotted out and the experiment is complete. CHAPTER FIVE RESULTS AND DISCUSSION A. Results In this section. all of the data obtained in this investigation of fourteen halomethanes will be presented. For each compound. a figure showing the mass spectrum and a table listing the relative intensity assignments will be given first. This is followed by a tigure depicting all of the PIE curves measured from the neutral precursor. plotted together. and then a summary table of all IP. AP and heat of formation values corresponding to each ion. A brief summary is provided for each ion. including a plot of the individual PIE and a description of any unusual features. plus a statement of the reaction equation upon which the heat of formation calculation was based. The ionization potential of a parent ion is Obtained by fitting (the fit program is called KINFIT4. which is developed by the MSU Chemistry Department) the threshold region of the PIE curve with an error function.89 which represents the Gaussian distribution of instrumental and equilibrium rotational contributions to the ion signal. A precise value for the adiabatic ionization energy is Obtained from the inflection point of the error function. which corresponds to the peak of the Gaussian distribution. Since many factors influence the threshold region of fragment PIE curves. there is no single suitable fitting function. Each fragment ion PIE curve is first smoothed for easier and more accurate extrapolation. 119 120 and then a straight line is drawn from the higher energy part of the threshold down to the base line. The smooth program. written by P.noffman and P.Aiello of the MSU Chemistry Department. employs a modified Savitsky-Golay alOgrithm where the data points in a selected sliding window are fit to a linear equation. The energy at which the extrapolated line intersects the base line. plus the equipartition value of the rotational energy. is chosen as the appearance potential. An example is shown in Figure 5-Ar1. In the remainder of this thesis. energy in eV are converted to kcal/mole. the conversion factor 23.06 kcalmole-lleV was used. The origin of a particular ion at threshold must be carefully considered. It might be from an impurity. a sequential series Of reactions. or one of several alternative decomposition channels from the parent ion which involve different neutral fragments. As noted in chapter four. it is unlikely that impurities contribute. Mass spectra obtained under .both photoionization and electron impact conditions revealed no detectable impurity in any of the samples. Moreover. except for the parent ion and the dihalogen cations. none of the neutral precursors are stable molecules. Impurities having the same mass as the parent ion are hard to imagine. and there are no geometrical isomers for the halomethanes. The threshold region of each parent ion PIE is sharp and shows no step structure; thus there is no evidence of contribution from impurities. PIE curves of BrCl+ and IF+ were measured; the precursors are clearly not neutral dihalOgen impurities. since the experimental appearance potentials are much higher than the reported ionization potentials. For a given threshold. the assignment of the 121 WAVELENGTH (Angstroms) 1200 1 100 1000 900 800 700 600 IIIIIIIIIIIIIIIIIIIIIIIIIII I I I l l I l 1150 1100 1050 I l l l I I l I I I I T I I PIE (Arbitrary Units) 10.50 10.90 11.30 11.70 ill'lllllllIllllllllllllIIIIIIIIILLLLLIIIIIIIIIIlllllll'lll 10.00 11.00 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5—A-1. PIE of CF2C1+ from CFZCIBr. The solid line represents a smooth fit to the experimental data points. The inset shows the extrapolation method used to obtain the appearance potential. apprOpriate decomposition reaction was made by considering all likely fragmentation pathways. and calculating the fragment ion heat of formation from each reaction equation. The possible contribution of sequential reactions involving first fragmentation into neutral species followed by ionization of one fragment is rather unlikely under PINS experimental conditions. and was similarly evaluated. Comparison Of the calculated heat of formation values for the fragment ion to those given in thermochemical compendia or the chemical literature helped determine the best choice for the dissociation mechanism. The procedure is illustrated in Figure S-Ar2 for the photofragmentation of CF‘ to give + , +F +e- and for its decomposition to give CF,+ and alternative CF neutrals. For each PIE curve measured. the corresponding reaction equation. and the fragment ion heat of formation calculated from the experimental appearance potential and tabulated heats of formation of the neutrals involved. is given in subsection 1-14 below. The heat of formation values of the neutrals which were employed in the ion heat of formation calculations are listed in Table S-Arl. 1. CF3ClBr The photoionization mass spectrum of CFaClBr. obtained under full illumination of the helium continuum. is shown in Figure SArl-l. Relative intensities of the peaks and their assignments are presented in Table SArl-l. All PIE curves of ions emanating from this compound are Shown in Figure 5Ar1-2. and the summary of thermochemical information for these ions is given in Table 5Ar1-2. CF3ClBr+ 123 I. Possible pathways by which CF,+ can be formed from CF‘. 4. ca, [AP 15.44eV] observed. A. C-F bond breakage: CF4 + by ---9 CF,+ + F + e- -221.6 99.8 18.36 APoxpactod-99.8+18.36-(-221.6)-339.76kcal/mole-14.73eV B. Consider a two-step process: CF. + hv or A ---9 CF. + F -221.6 -117.0 18.36 r.‘ctionf-117.0+18.36-(-221.6)-122.97kcal/mele-5.33eV CF. + H _—_, CF". 'I' 0 -117.0 99.8 d-99.8-(-117.0)-218.6kcal/mole-9.40eV AH IPexpecte No CF,+ is detected in the 9.40eV range. Direct ionization/frag- mentation is the correct reaction. II. Different pathways for producing CF,+. or * [AP ob..rv.d-2o.44ov1 A. Fragmentaion to form two F atoms: CF‘ + AP --—-9 CF,+. + F + F + a" '221.6 217.5 18.36 18.36 AP.xp°ct°d‘217.5+18.36+18.36-(‘221.6)‘475.38kcll/I010'20.630v B. Fragmentaion to form one Fa molecule: CF4 + AP -———-9 cr,* + F, + 9' -221.6 217.5 0 AP d-217.5-(221.6)-439.1kcal/mole-l9.04eV expecte “Best" previous literature value:B.'.Jochims. '.Lohr and B. BaumgIrtel. Ber. Bunsenges. Phys. Chem. so. 130 (1976). The excess energy of 1.4 eV accompanying F3 fogmation is unlikely and unprecedented:; with the "best" AB; (CFa )8204.36 kcal/mole from this work that value would be about eV. The decomposition reaction which yields two fluorine atoms is correct; it involves kinetic energy release of about 0.4 eV. Figure 5-Ar2. Thermochemical procedure for determining reaction equations appropriate to experimental appearance thresholds. Example: CF‘ (Beats of formation in kcal/mole from Thble 5-Ar1.) 124 Table 5-Arl. Standard enthalpies of formation (0'!. kcal/mole) - utilized in thermochemical calculations of AB; of fragment ions [From JANAF thermochemical tagles. unless otherwise noted.] 03,013: -107.66ll 0r, -117.00¢1.o 03‘ -221.6110.3 0r,+ . 99.30:2.a 03,01 -168.00t0.8 0113 -43.6011.5 03,3: -152.20¢o.7 01?,+ 223.50 0F,I -139.40t0.8 001, 19.15:2.o 03,01, -116.5012.0 001,+ 56.70t5.0 0301, -6s.24:1.5 F 18.36:0.4 001. -22.42:0.5 01 28.52t0.0 001,3: -11.00b Br 28.19t0.1 cr,a:, -96.39‘ 1 25.6310.1 can, -164.90t0.8 3:01 5.20:0.3 0ar,01 -113.60t3.0 IF -22.19:o.9 carci, -66.36x3.0 0n01, -23.49:0.3 ‘ value from G.Kauschka and L.Kolditz. z. Chem. 19. 377 (1976) b value from J.L.Franklin. J.G.Dillard. B.I.Bosenstock. J.T.Herron and K.Draxl. Iogigatign Potentialg. Appearangg Pgtentiglg 5nd ngtg g; Fromgtion g; Qggggg ngitize Iogg. NSRDS-NBS 26 (1979). RELATIVE INTENSITY 100.0 20.0 0.0 Figure 5A-l-1. 125 p d 111111111]IIIIIJIIIIIILIIIIlllllllllllIlllIII[Ill]llLlIIlllllllllllIllll1111111111!III!ll1IIIILIIIIIIIIIIIIIILIII 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 MASS UNITS (m/ e) (obtained under full illumination of the helium discharge). Photoionization Mass Spectrum of CFZCIBr Table 5A-1-1 . Photoionization Mass Spectrum of C'FZClBra Intensity Relative Percent of Total Probable m/e to CF2C|+ Ion Production Formula 50 18 . 0 9 . 47 (3th 00 2 . 4 1 .20 ‘ cr'cr 68 0 . 7 0 . 38 015701: 79 1 . 2 0 . 60 ”Br’ 81 1 . 1 0 .56 ”31* 00 100.0 02.00 01-2301: 07 32.3 10.90 crzvcr 110. 0.2 0.12 erase 112. 0.2 0.11 CF“Br’ 1 14 0 . 2 0 . 09 ‘Cl‘Br’ 116 0 . 2 0 .12 ”Cl"Br‘,”Cl"Br* 118 0.1 O .03 ”CWBr’ 129 9 . 5 4 . 97 CF ZPBr’ 131 9 . 2 4 .81 CFZ"Br" 145 5 . O 2 . 63 CF'Cl”Br* 147 ‘ 5 .7 3 .00 crecr'emcrvcmsv 14-9 1 . 3 O . 68 CWCI"Br’ 104 1.2 0.02 crzscuasn 166 1 . 5 0 . 76 CF 2”CI“Br’,CF 2”Cl”Br’ 168 O . 4 O .19 CF2”CI"Br* ‘Obtained under full illumination of the helium discharge. 127 WAVELENGTH (Angstroms) 1.300 1100 900 700 1111 Ill! 1111 1111 llll l l I l l l L l I I I W A m .t’ c D ”cart 0 I. :9: . _Q . 1.. < V E . (L "2" Br 0201 apart IIIlll'lljlllllllllllllLLliLLIIIIlIIIILILlJIllll.IIIIIILIlII 10.00 11.00 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 51141-2. Photoionization efficiency curves for parent and daughter ions from CFZClBr. 128 Table 5A-1—2. Summary of 1?. AP and Heat of formation data for all ions from CFZClBr. IP or AP Products of Heats of formation(O°K) ION (eV)a CFZCIBT + “Y (Kcal/mole)a CFZCIBr‘ 11.21 01:20:01» 4 e" 100.04 013011 11. 30 crzcr 4 Br + r 124. 73 CF28!“ 12.00 CFZBr’ ‘1' CI '1' 0' 140.54 CFCIBI" 14.15 CFCIBI” + F + 0" 200.28 0?; 10.99 CFZ’ + 01 4- Br 43- 204.30 CFCI’ 17.02 CFCI’ + F + Br + 0" 237.93 CF'Bf" 17.80 CF31" + F + CI '1' 0" 255.93 BrCI’ 14.03 ClBr’ + crz + 0' 273.31 81" 15.45 81" + CFZCI + 0' 321.80 9see text for uncertainties. PIE (Arbitrary Units) Figure 5A-1 -3. 129 WAVELENGTH (Angstroms) 1000 900 800 700 600 I I 1 l I 1200 1100 IIIIIIIIFITIIITIIIIIIIII I I T I I I ooo oo, 00 O “mooooooooo LlLlJlLllllllllllllllllIIIlllLlllll'lllllllllllllllJFlIII 10.00 11.00 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) PIE of CFzClBr+ from crzcuar. 130 WAVELENGTH (Angstroms) 1180 1160 1140 1120 1100 1080 1060 1040 IIIIIIITIIIIIIIIIIIrIIIIlIIIIlIFWI’IITTIIIIIIIT1WII'IIIIIIIII'IIIITTII 0 A U) .4: C D U L- :1: .0 1 L < . V 1 1 m a E : 1 I I E : : : : ljllllllIllLLLLlllllllIllljllljlll|.1lll|IllIlllllllllllllIl'llllllllllllll 10.50 10.70 10.90 . 11.10 11.30 11.50 11.70 11.90 Figure 5A- 1 -4. ENERGY (Electron Volts) PIE of CFBClBr+ in the threshold region. Circles denote experimental points. The solid line is the best fit of a Gaussian representation of rotational and instru- mental contributions. The dashed line indicates the position of the corrected iOnization potential. 131 The parent ion PIE is shown in Figure 5Ar1-3. where it may be seen that the threshold starts to rise at about 11.10eV. Figure SArl-4 shows the fit of the error function to the threshold region. In the post-threshold region. the PIE curve shows a relatively flat region. about 30V wide; presumably other fragmentation channels compete with direct ionization in this energy range. After this region. the PIE curve rises up again. showing a second threshold. Based on the photoelectron spectrum.90 this threshold corresponds to a transition in which the parent ion is raised to an excited electronic state. The adiabatic IP of the parent ion is 11.21:0.020V. which is slightly lower than and in reasonable agreement with previous literature values: 90 11.83eV34 and 11.51eV. From the reaction equation: CF3ClBr + by -—---9 011,010:+ + e- the value of AH;O(CF,ClBr+) is calculated to be 150.84tl.5kcal/mole. 09,01+ The first. lowest photon energy fragment PIE curve is shown in Figure SArl-S. The PIE starts to rise sharply at threshold(near 11.20eV). and in the post-threshold region it shows a series of peak-like features. which are suspected to be autoionization structure. As for the parent ion. at an energy of about 14.00V the CF3Cl+ PIE shows a second threshold. corresponding to fragmentation of the parent ion in an excited electronic state. The AP value of this fragment ion is ll.3010.04eV; no previous literature value is available for comparison. From the reaction equation: 132 WAVELENGTH (Angstroms) 1200 1 100 1000 900 800 700 600 FIIIIITrrITjIIIFj I I I I I I I l 1 I I I I I PIE (Arbitrary Units) lllll‘llllilljllllLLLLlelIllllllllllllli'llll‘llllllllllll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 01-1-0. PIE of crzcfi' from CFZClBr. The solid line represents a smooth fit to. the experimental data points. WAVELENGTH (Angstroms 12001100 1000 900 800 700 - 600 IIIIIIIIIIIIIIIIT‘IIII I I I I I I I I I r l I PIE (Arbitrary Units) lIIIIlLLUlLLLJJiJJILIIIIlllIIIllll'lllllIILIIJIIIIIIJIIII 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-1-6. PIE of CFzBr+ from CF2ClBr. The solid line represents a smooth fit to the experimental data points. 133 CF3ClBr + hy ---_9 01301+ + Br + e’ the value Of AH;o(CF,Cl+) is calculated to be 124.73i2.0kcal/mole. Canr+ The secOnd fragment ion is CFzBr+. for which the PIE is shown in Figure SArl-G. The PIE shows a relatively sharp threshold at around 120V. and again another threshold is observed near 14eV. The AP value of this fragment ion is 12.00:0.04eV. and by using the reaction equation: CF3ClBr + by --—-9 CF,Br+ + 01 + e— the value Of AE;o(CFzBr+) is calnulated to be 140.54t2.0kcal/mole. 0110113:+ This fragment corresponds to breaking the C-F bond in the parent ion. The PIE curve is shown in Figure 5Arl-7. Since the C-F bond is strong. a great deal of energy must be deposited in the parent ion. and the fragment ion PIE exhibits a slow increase above threshold. The AP value is 14.15r0.07eV. By using the reaction equation: CF3ClBr + by —---—9 0901B:+ + F + e‘ the value 0f AUEOICFCIBI+) is calculated to be 200.28r3.0kcal/mole. + 3 CF The PIE curve for CF,+ is shown in Figure 5Ar1-8. Now two bands are broken. and the onset of the threshold is not so sharp as those for 134 WAVELENGTH (Angstroms) 950 850 750 650 IIIIIII7 I I—I I I I I I I 1 I I PIE (Arbitrary Units) LIIILLIIIlllllLllIllIbillllUllllll'lllLlLl 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-1-7. PIE of 0F013r+ from CF2ClBr. The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 1000 ~ 900 800 700 600 IIFI'TIIfilIIIIIII I I r I I r I I j PIE (Arbitrary Units) LLLIIIIILLIIIIIII'llILIIJII'IIIIIIILLIIIIIIJIIIIIII 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-1-8. PIE of 0172"” from CFZClBr. The solid line represents a smooth fit to the experimental data points. 135 fragment ions where only one band is broken. The AP value is 15.99t0.07ev. and according to the equation: CFIClBr + by -----9 0F,+ + 01 + Br + e’ the value of AH;0(CF3+) is calculated to be 204.36x2.7kcal/mole. 0F01+ This fragment results when the C-Br and one C-F bond in the parent ion are broken. The PIE curve. reproduced in Figure 5Ar1-9. shows a smoothly-rising threshold which leads to an AP value of 17.02t0.leV. According to the reaction equation: CF3ClBr + by ---—9 0F01+ + F +‘Br + e' the value of AEEOICFC1+) is calculated to be 237.9313.9kcal/mole. CFBr+ Rupture of the C-Cl bond and one C-F bond in the parent ion requires slightly higher energy than the previous fragmentation reaction. The PIE curve of CFBr+ is shown in Figure 5Arl-10; the AP value is l7.80:0.1eV. From the reaction equation: CFzClBr + hr -----9 CFBr+ + F + 01 + e' the value of AH;0(CFBr+) is calculated to be 255.93:3.8kcal/mole. BrCl+ This fragment results not from simple bond cleavage. but via a 136 WAVELENGTH (Angstroms) 800 700 600 PIE (Arbitrary Units) llllLLLILLlIJllllILLIJIIL'Illlllll 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-1-9. PIE of 0F01+ from CFzClBr. The solid line represents a smooth fit to- the experimental data points. WAVELENGTH (Angstroms) 750 650 Q I I F I I I I I 1 I I .4: C D C L. :1". .0 L <2 V ELL-I ..".. 0- iiliiiLliiiill'lniliiiiIriiiILii 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-1-10. PIE of CFBr+ from crzcmr. The solid line represents a smooth fit to the experimental data points. 137 WAVELENGTH (Angstroms) 900 800 700 600 I I I I I I I I I I I T r 1 I I I T I I PIE (Arbitrary Units) IJllllIll_LIJIL1I1LII'LLLJJLJJIIIIIIIIIILJLLL 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 22.00 ENERGY (Electron Volts) Figure 5A-1-ll. pm of ClBrT from CFZClBr. The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) ‘ 900 000 700 000 PIE (Arbitrary Units) J_LIJLIIIIIIIIIIIIJIIIIIIIIJIIILIIIIJIIg- 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00- ENERGY (Electron Volts) . Figure 5A-1-12. PIE of Br+ from CFZClBr. The solid line represents a smooth fit to the experimental data points. 138 special mechanism which will be discussed later in this chapter. The AP of BrCl+ obtained from the PIE curve shown in Figure SArl-ll is 14.6310.3eV. The reaction equation is most probably: CF3ClBr + hv -—---9 Br01+ + 0F, + e“ the value of AH;0(BIC1+) is calculated to be 273.00:8.4kcal/mole. Br+ Production of Br+ in the fragmentation process is similar to the production of the first fragment. CF,Cl+; however. in this case the postive charge resides on the bromine. The PIE curve is shown in Figure 5Ar1-12. After careful consideration of alternative photodissociation processes. the AP of Br+ is 15.45r0.1eV and the most likely reaction equation is: CF3ClBr + by ----9 Br+ + CF3CI + e' the value Of AH}O(Br+) is calculated to be 312.80:3.8kcal/mole. 2. CF. The photoionization mass spectrum of CF4. Obtained under full illumination of the helium continuum. is shown in Figure 5Ar2-1. Relative intensities of the peaks and their assignments are presented in Table 5Ar2-1. All PIE curves of ions emanating from this compound are shown in Figure 5Ar2-2. and the summary Of thermochemical information for these ions is given in Table 5Ar2-2. RELATIVE INTENSITY 139 I- '1 100.0 I- '"I L— — 80.0 '- - - -l — -I 60.0 - ._ _ '1 40.0 - _ I— .4 I- -I 20.0 - _ I- _I 000 — A .- " 7 IlllLIlllllIJlllILllLllllIIIllllllllll[IIIIIIIILIJLLIIIILIIIIIJIIIIIlllllllLLllllllllllIll'IllIlIIlIIIIlLILIllIIJ 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 MASS unrrs (m/e) Figure 5A-2—1. Photoionization Mass Spectrum of CF4 (obtained under full illumination of the helium discharge). 140 Table 5A—2-1. Photoionization Mass Spectrum of CF4a Intensity Relative Percent of Total Probable m/e to CF3+ Ion Production Formula 50 0.3 0.25 CF2* 89 100 . 0 99 . 75 CF3" “Obtained under mu illumination of the helium discharge. Table 5A-2-2. Summary of IP. AP and Heat of formation data for all ions from CF4. IP or AP Products of Heats of formation(O°K) ION (eV)a CF4 + hv (KcaI/mole)a cry 13.44 cry 4 r + e- 110.00 01:2: 20.44 . 01-2. 4 r + F + e- 213.02 8see text for uncertainties. 141 WAVELENGTH (Angstroms) 1300 1 1 00 900 700 III'IIII'IIITIIIIT'rIII' I IT I rfl I I I I PIE (Arbitrary Units) IJIIUIIIIIII[IIIIIIIIILIIIILIIIIIIILIllIIIIlllllllllllllllli 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-2-2. Photoionization efficiency curves for daughter ions from CF4 (no parent ion is detectable). 142 0F.+ The parent ion of CF. was not Observed in this work. nor has any previous study successfully detected CF.+. It may be assumed that the parent ion lifetime is too short for mass spectrometric detection. Among factors which influence the lifetime of parent ion are: the symmetries of the neutral and the ion. Franck-Condon factors. breakdown of the Born-Oppenheimer approximation. Jahn-Teller distortions. spin-orbit effects. and accessible decomposition channels. A rather large geometry difference is expected between CF. and CF.+.15 and this most likely contributes strongly to the parent ion instability. (Similar considerations apply to CCl.. for which the parent ion has not been observed. also.) 0F,+ The lowest photon energy fragment PIE curve. shown in Figure 5Ar2-3. is that of the CF.+ fragment. The PIE starts to rise sharply at threshold(near 15.80eV). and the PIE curve shows two steps. The AP value of this fragment ion is 15.44t0.07eV. which is slightly lower than and in a very good agreement with the previous literature value 15.5210.020V.91 From the reaction equation: CF. + hr —----9 CF.+ + F + e' the value Of AR;o(CF.+) is calculated to be 116.08t2.3kcal/mole. + 3 CF The second fragment ion is CF.+. for which the PIE is shown in Figure 5Ar2-4. The PIE shows a relatively sharp threshold at around 143 WAVELENGTH (Angstroms) 850 750 650 PIE (Arbitrary Units) lllLlllllllllLlLllLLlllJlLlLllLJLmLIIIJ 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-2-3. PIE of 0F3+ from CF... The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 650 /('n\ I 1 I r 1 22'. C D O L .‘t’ _ .D L. < V E Q L 4 1 l l l l l I l l l I l J I l I 19.00 20.00 21.00 22. 00 ENERGY (Electron Volts) Figure 5A-2-4. PIE of CFZT from CF... The solid line represents a smooth fit to the experimental data points. 144 20.7eV. The AP value of this fragment ion is 20.44t0.leV. which is slightly higher than and in a good agreement with previous literature value: 20.310.50V.124 From the reaction equation: CF. + by -----9 0F.+ + 2F + e' the value of AH;o(CF.+) is calculated to be 213.02:3.4kcal/mole. 3. CF.Cl The photoionization mass spectrum of CF.Cl. obtained under full illumination of the helium continuum.' is shown in Figure 5Ar3-1. Relative intensities of the peaks and their assignments are presented in Table 5Ar3-1. All PIE curves of ions emanating from this compound are shown in Figure 5Ar3-2. and the summary of thermochemical information for these ions is given in Table 5Ar3-2. 0F,01+ The parent ion PIE is shown in Figure 5Ar3-3. where it may be seen that the threshold starts to rise at about 12.50eV. Figure 5Ar3-4 shows the fit of the error function to the threshold region. The adiabatic IP of the parent ion is 12.6010.020V. which is slightly higher than and in a good agreement with previous literature value: 12.45eV.92 From the reaction equation: RELATIVE INTENSITY 100.0 20.0 0.0 145 b lllllllllIllllllllllllllllilllllllllllIlllllIIIILIJJlIlIllIllllllllLlllIIIIIIIIIIIIIIIIIIlllLlllLIlll'lllIlllIlllI L L411 _I 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 MASS UNITS (m/e) Figure 5A-3—1. Photoionization Mass Spectrum of CF3C1 (obtained under full illumination of the helium discharge). 146 Table 5A-3-1. Photoionization Mass Spectrum of CF3CI° Intensity Relative Percent Of Total Probable m/e to CIR-5+ Ion Production Formula 50 1 .9 1 .62 CF 2‘ 69 100 . O 83 . 87 CF31 85 12.1 10 .21 CF2"CI’ 87 3 . 8 3 .19 CF2”CI’ 104 1 . 0 0 .84 CF3”CI* 106 0 . 3 0 . 27 CFf’CI’ l’Obtained under full illumination of the helium discharge. Table 5A—3-2. Summary of IP. AP and Heat of formation data for all ions from CF3Cl. IP or AP Products of Heats of formation(O°K) ION (eV)a CF30 1' “7 (KcaI/mole)a cr3cr 12 . 00 crgcr + e- 290 . 00 cry,r 12.70 cr3+ + 01 4 e- 90.34 crzcr: 14.44 crzcr + r + e- 140.03 crzr 10.49 crzr + r + 01 + e- 211.50 asee text for uncertainties. 147 WAVELENGTH (Angstroms) 1300 1100 900 700 IIIIIIIFINIIIIFIII'IIII‘lffiI I l I I I I 1 r A (D :t’ C D D n :2: .0 a < V 2:1 0_ ”WWWNWMN 111111111411!IIIIJIIIIIIIIIIIILIIIIJIIIIJLIIIJLLLIIIIIIIIllIIl 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 200021.00 22.00 ENERGY (Electron Volts) Figure 5A-3-2. . Photoionization efficiency curves for parent and daughter ions from CF3C1. 148 WAVELENGTH (Angstroms) 1100 1000 900 800 700 600 III I I I I1 I I I I I I I I j I I r I I I I I I r I 00 c? A (8° (D 3: C D 2‘ E e .":’. “E O :5 o L_I_J o 0- o 0 83900000 a o lJlllLLIIIlIIIIlIIIIIlIlIllJlJlIIIIlILlIlILIIllLLJLlIJ' 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00. ENERGY (Electron Volts) Figure 5A-3-3. PIE of CF3CI+ from CF3C1. 149 WAVELENGTH (Angstroms) 1020 1000 980 960 940 III—VIIIII'FIIIIIIIIIIIIIIIIIFIIIIIIIIIIIITTII PIE (Arbitrary Units) LllllLL'llllLllllllllllIllI‘lllllILULIJJJlLLLl'IIllllllll 12.10 12.30 12.50 12. 70 12.90 1.3.10 ENERGY (Electron Volts) Figure 5A-3-4. PIE of CF3C1+ in the threshold region. Circles denote experimental points. The solid line is the best fit of a Gaussian representation of rotational and instru- mental contributions. The dashed line indicates the position of the corrected ionization potential. 150 CF,C1 + hv -----9 CF,C1+ + e' the value of AH20(CF,C1+) is calculates to be 23U.chl.3kcal/mole. CF3C1+ The PIE of the CF3C1+ fra,-.ut ion is shown in Figure SArB-S: it shows a relatively sharp threshold at approximately 15.5eV. The AP value of this fragment ion is 14.44t0.07eV. which is slightly higher than and in a good agreement with the previous literature value: 14.25eV.91 From the reaction equation: CFJCl + by —---9 CF,C1+ + F + e’ the value of AB;O(CF3C1+) is calculated to be 146.63t2.8kca1/mole. + 3 CF The first, lowest photon energy fragment PIE curve from CF,C1 is shown in Figure 5Ar3-6. The PIE starts to rise sharply at threshold(near 12.70eV). The AP value of this fragment ion is 12.7010.07eV, which is slightly higher than and in a very good agreement with previous literature values: 12.63eV91 and 12.55eV.92 From the reaction equation: CF3C1 + by -----9 CF,+ + c1 + e' the value of AH}O(CF,+) is calculated to be 96.3412.4kcal/mole. CF+ The last fragment ion is CF3+, for which the PIE is shown in 151 WAVELENGTH (Angstroms) 1 100 1000 900 800 700 600 r'IIIr'rIII'IIIIITj I I l I I Ifi I j A..- ‘— pr [IllI'Lllllllll'llillLLLlllllllllllllI1114L11llllllllc 11.00 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-3-5. PIE of CF2C1+ from cr3c1. The solid line represents a smooth fit to the experimental data points. . PIE (Arbitrary Units) WAVELENGTH (Angstroms) 1 100 1000 900 800 700 600 IlIIII'IIIIIIIIIlrIIIl I I T I l I PIE (Arbitrary Units) llllIllIlilllllllllllllllIllLJiLlllllllLLlLlLiLlll‘JIh 11.00 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-3-6. PIE of CF3+ from CF3C1. The solid line represents a smooth fit to the experimental data points. 152 WAVELENGTH (Angstroms) 900 , 800 700 800 I I I I I I r I I I r T I I I W I I . | PIE (Arbitrary Units) LLlLllll'IIIIILIILILLILLLILIIIIIIIlllllll 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-3-7. PIE of CFZ'I‘ from cr3c1. The solid line represents a smooth fit to the experimental data points. 153 Figure 5Ar3-7. The PIE shows a relative sharp threshold at around 18.5eV. The AP value of this fragment ion is 18.49t0.leV, which is slightly lower than and in a satisfactory agreement with the previous literature value: 18.85eV.92 By using the reaction equation: cr,c1 + hr ----9 cr,* + F + c1 + e’ the value of AH;O(CF,+) is calculated to be 211.50:3.5kcal/mole. 4. CF,Br The photoionization mass spectrum of CF,Br. obtained under full illumination .of the helium continuum, is shown in Figure 5Ar4-l. Relative intensities of the peaks and their assignments are presented in Table 5Ar4-1. All PIE curves of ions emanating from this compound are shown in Figure 5Ar4-2. and the summary of thermochemical information for these ions is given in Table 5Ar4-2. CF,Br+ The parent ion PIE is shown in Figure 5Ar4-3. where it may be seen that the threshold starts to rise at about 11.70eV. Figure 5Ar4-4 shows the fit of the error function to the threshold region. The adiabatic IP of the parent ion is 11.76t0.02eV, which is slightly lower than and in a good agreement with the previous literature value: 12.0eV.93 From the reaction equation: RELATIVE INTENSITY 100.0 80.0 40.0 20.0 0.0 154 L. . .. U .l lllllllllllllllllllllllllllllllLLlLlllllljlljllllllllllIllllllllllllllllllllllllllilllllllllLlLLllLllllll llllll 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 MASS UNITS (m/e) Figure 5A-4-1. Photoionization Mass Spectrum of CF33r (obtained under full illumination of the helium discharge). 155 Table 5A-4-1. Photoionization Mass Spectrum of CF3Br’ Intensity Relative Percent of Total Probable m/ e to CF3+ Ion Production Formula 50 1 . 2 0 . 99 CF 2’ 69 100 . O 85 . 09 CF3" 79 O . 6 O . 51 ”Bi" 81 0 . 6 0 .47 l”Br’ 129 3.7 3.14- CF27'Br* 131 3 . 3 2 .73 CFZ"Br* 148 4. 2 3 . 58 CF’3w'Br+ 150 4. 0 3 . 40 CF3"Br’ "Obtained under full illumination of the helium discharge. Table 5A-4—2. Summary of IP. AP and Heat of formation data for all ions from CF3Br. IP or AP Products of Heats of formation(O°K) ION (13V)a CF3B" + “7 (Kcal/mole)° CF38? 11 .76 CF38? + 6’ 118.99 CF3" 11.88 61’3" + Br + e" 93.56 crzer 14 .54 crzsr + F + e‘ 164.73 or; 17.66 crzi + F + Br + e“ 208.49 31* 14.94 31* + or; + e’ 304.01 3see text for uncertainties. 156 WAVELENGTH (Angstroms) 1.300 1100 IIII‘IIIITIIIIFIVIII'OIjrIrI I 1 ”Ii/«TI crsafi PIE (Arbitrary Units) all WWWW) lllllllllLLllllllllLl.l[ILLIJIIIJIIIIIllllllllllllllUJlLll 10.00 11.00 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-4—2. Photoionization efficiency curves for parent and daughter ions from CF3Br. 157 WAVELENGTH (Angstroms) 1 150 1050 950 850 750 650 IIIIIIIIIIIIIIIIIIIIIIITI I r1 rT I )— PIE (Arbitrary Units) ‘Iumoao oo oooooo ooo ooo ooo cm oo 00 . WWW; 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-4-3. PIE of CF3Br+ from CFaBr. PIE (Arbitrary Units) 158 WAVELENGTH (Angstroms) 1 100 1080 1050 1040 1020 1000 IIIIIII'IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII 0W Q°° 0 ° & ago. new“, 0 i i i I I i i I I i i IJLLLILIJJJIlllllLiLllil:JLLlll_lllLLllJlllILJJlllllllLlllll 11.30 11.50 11.70 11.90 12.10 ' 12.30 ENERGY (Electron Volts) Figure 5A-4-4. PIE of CF3Br+ in the threshold region. Circles denote experimental points. The solid line is the best fit of a Gaussian representation of rotational and instru- mental contributions. The dashed line indicates the position of the corrected ionization potential. 159 WAVELENGTH (Angstroms) 1 100 1 000 900 800 700 600 rl—FIIIIIIIIIVTIIIIIIII I r I—fil I PIE (Arbitrary Units) lllllllilllllllllllllllllllllLLLllllllllllllLlJlllllll 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-4-5. PIE of GFzBr+ from CF3Br. The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 1100 1000 900 . 800 700 600 IIIIIIIIIII'IIIIlrfirr1 I I I T I T PIE (Arbitrary Units) llllllLILLllLllllllLlLlllILLlllIIIIILLII'IIIIIIIIIIJJII 11.00 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-4-6. PIE of CF31” from crasr. The solid line represents a smooth fit to the experimental data points. 160 CF,Br + by —----9 CF,Br+ + e' the value of AK;O(CF,BI+) is calculated to be 118.99t1.2kca1/mole. CFzBr+ The PIE of CF33:+ is shown in Figure 5Ar4-5. The PIE shows a relative smooth threshold at around 14.5eV. The AP value of this fragment ion is 14.54t0.07eV; no previous literature value is available for comparison. By using the reaction equation: CF,Br + hy --—-—9 CF,Br+ + F + e' the value of AE;o(CF,Br+) is calculated to be 164.73t2.7kcal/mole. CF,+ The first. lowest photon energy fragment PIE curve from CF,Br is shown in Figure 5Ar4-6. The PIE starts to rise sharply at threshold(near 12.0eV). The AP value of this fragment ion is 11.88i0.04eV. which is slightly higher than and in a good agreement with the previous literature value 11.71t0.02eV.91 From the reaction equation: CF,nr + hr --—-—9 CF,+ + Br + e” the value of AHEOICF,+) is calculated to be 93.56i1.7kcal/mole. + 3 CF The next fragment ion is CF3+, for which the PIE is shown- in Figure 5Ar4-7. The AP value of this fragment ion is 17.6610.1eV. which 161 WAVELENGTH (Angstroms) - 800 700 600 PIE (Arbitrary Units) llllJLLllllllllLlllJLJllllllllllllll 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-4-7. PIE of CF2+ from CF3Br. The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 900 . 800 700 600 PIE (Arbitrary Units) ILLJLIILIIIIIIIlLLlllLLLLllll'llllllllllIll 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-4—8. PIE of Br+ from CF3Br. The solid line represents a smooth fit to the experimental data points. 162 is in excellent agreement with the previous literature value: 17.66eV93. From the reaction equation: CF,nr + hr -----9 CF,+ + F + Br + e“ the value of AEEOICF3+) is calculated to be 208.4713.5kca1/mole. Br+ The last fragment ion is Br+. for which the PIE is shown in Figure 5Ar4-8. The AP value of this fragment ion is 14.90t0.3eV; using the reaction equation: CF,Br + hy ---——9 CF, + Br+ e' the value of AE;0(Br+) is calculated to be 303.09t8.6kcal/mole. 5. CF31 The photoionization mass spectrum of CF,I. obtained under full illumination of the helium continuum. is shown in Figure SArS-l. Relative intensities of the peaks and their assignments are presented in Table SArs-l. All PIE curves of ions emanating from this compound are shown in Figure 5Ar5-2. and the summary of thermochemical information for these ions is given in Table SAPS-2. CF 1* I The parent ion PIE is shown in Figure 5Ar5-3, The threshold starts to rise at about 10.3eV. Figure 5Ar4-4 shows the fit of the error function to the threshold region. The adiabatic IP of parent ion is RELATIVE INTENSITY 100.0 0.0 163 P -1 P .— — —l f q — fl '- 1 Z. . .Iil ‘ - A lllilllilllllllliiiIlliilllllllililLllllillllllliliilllilllliliilllllIll-llllLLllJLLlllililllillllilllllllliLillili 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 MASS UNITS (m/e) Figure 5A—5—1. Photoionization Mass Spectrum of CF31 (obtained under full illumination of the helium discharge). 164 Table 5A-5-1. Photoionization Mass Spectrum of CF31Ill Intensity Relative Percent of Total Probable m/ e to CF3+ Ion Production Formula 50 1 . 0 0 . 68 or; 69 100 . 0 68 . 03 cry 127 8 . 7 5 . 90 I’ 145 4 . O 2 . 72 IF’ 177 7.0 ‘4.76 CF2I* 196 26.3 17.91 CF3I* I'Obtained under full illumination of the helium discharge. Table 5A-5-2. Summary of IP. AP and Heat of formation data for all ions from CF31. IP or AP Products of Heats of formation(O°K) ION (eV)‘ CF3| + “Y (KcaI/mole)a crar 10 .38 cry 4 e‘ 99.96 CF3’ 11.11 CF3T +1 + e’ 91.15 crzr 14.64 crzr + r + s- 180.07 cry 16.94 crze + F + I + e" 207.94 I!” 14.12 IF" 4 crz + e“ 229.81 F 12.93 F + 01:3 + e" 270.47 3see text for uncertainties. PIE (Arbitrary Units) v? 165 WAVELENGTH (Angstroms) 1.300 1 100 900 700 IIIIIIIIIIII‘IIIIIIIIIIjIlI l I 17' T I I l — - l + CF31 1+ CF21 IF «AWPWM lllll'llllllllllllllllllllllllll'llllllllllllllllllilLLlllllll 10.00 11.001200 13.00 14.00 15.00 16.00 17.00 18.00 19.00 200021.00 22.00 ENERGY (Electron Volts) Figure 5A-5-2. Photoionization efficiency curves for parent and daughter ions from CF31. 166 WAVELENGTH (Angstroms) 1.3001200 1 100 1000 900 800 700 600 IllIIIlIIIIlI—IIIIIIIrlTIIIr1II—I I I I I T I r PIE (Arbitrary Units) LllllllLlUJIIlLllllLlllllLJlJJlllllllllllllllIIIIIIIJLLIIIIL] 10.00 11.00 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-5-3. PIE of CF31+ from CF31. 167 WAVELENGTH (Angstroms) 1220 1200 1 180 I I r I I I r I I I I T I I I 1 I I I I I I I O A 0) .‘°:.’ C D O L I .‘t'.’ I .0 I L I < I V l g 5 0. I ° I O o E 0 ° : o o : I o i I l l l l L 1 l l l 1 l 'l L l l_ 1‘ l I. 1 J_ 10.20 10.40 Figure 5A-5-4. ENERGY (Electron Volts) PIE of CF3l+ in the threshold region. Circles denote experimental points. The solid line is the best fit of a Gaussian representation of rotational and instru- mental contributions. The dashed line indicates the position of the corrected ionization potential. 168 10.38t0.02eV, which is slightly lower than previous literature value: 11.11eV.94 From the reaction equation: CF,I + by ----—9 CF,I+ + e“ the value of AE;o(CF,I+) is calculated to be 99.96i1.3kcal/mole. CF,+ The first. lowest photon energy fragment PIE curve is shown in Figure SArS-S. The PIE starts to rise sharply at threshold(near 11.26V). The AP value of this fragment ion is 11.11t0.04eV. which is slightly higher than the lowest previous literature value 10.89eV.91 From the reaction equation: CF31 + hr —--—9 CF,+ + I + e‘ the value of AH}OICF,+) is calculated to be 91.15tl.7kcal/mole. CF31+ The PIE of CF3I+ from CF31 is shown in Figure 5Ar5-6. The PIE shows a relatively sharp threshold at around 14.6eV. The AP value of this fragment ion is 14.64i0.lev. which is slightly lower than previous literature value: 15.3eV,95 and by using the reaction equation: 169 WAVELENGTH (Angstroms) 1250 1150 1050 950 850 750 650 IIIIIIIIIrlII‘rIlIIIIIIiTII I I I I I l I I I PIE (Arbitrary Units) JLllllllllliLlllllll'lllllllllllellllllllllllLlLlllllllll 10.00 11.00 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-5-5. PIE of CF3+ from CF31. The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 1 150 1050 950 850 750 650 IIII'TIII'IIIIITIIIIIIIII I I FI I r I I PIE (Arbitrary Units) Llll'llllillllllllllLlllllllLJllllLlULllllllJlllllllllll 10.00 11.00 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-5-6. PIE of CFZI+ from CF31. The solid line represents a smooth fit to the experimental data points. 170 WAVELENGTH (Angstroms) 1100 1000 900 800 700 600 F—rl—IrrI'IlerllllflllllI I I I [fi PIE (Arbitrary Units) llllllllllLLLllJLLlllllll'llll'lllellLLileullllLllll 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-5-7. PIE of 1"” from CF31. The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 1000 . 900 800 700 600 WiI—IIIIIIIVIIIII—IIIIIwI I I I I PIE (Arbitrary Units) 11'llLl'lllllLlLJIllllllllL'lLillLlIllIIIIIILllllll 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts). Figure 5A—5-8. PIE of Fl+ from CF31. The solid line represents a smooth fit to the experimental data points. 171 CF,I + hy ----9 CF31+ + F + e' the value of AH}O(CF,I+) is calculated to be 180.07i2.8kca1/mole. I+ The PIE of the iodine cation is shown in Figure 5Ar5-7. The AP value of this fragment ion is 12.93t0.3eV, and by using the reaction equation: 4. .. CF,I + by ----9 CF, + I + e the value of AHEOII+I is calculated to be 270.4718.7kca1/mole. IF+ The PIE of the dihalogen cation IF+ is shown in Figure SAPS-8. The AP value for this fragment ion is 14.12t0.3eV: by using the reaction equation: CF,I + hr -----9 CFz + IF+ + e the value of AHEOIIF+) is calculated to be 229.81t9.2kca1/mole. + 3 CF The last fragment ion is CF,+. for which the PIE is shown in Figure 5Ar5-9. The PIE shows a relatively smooth threshold at around 17eV. The AP value of this fragment ion is 16.94t0.leV. From the reaction equation: 172 _/ WAVELENGTH (Angstroms) - 700 LLlllllLLLlllrllglilllllllJlllllLlJllllll PIE (Arbitrary Units) 14.00 15.00 16. 00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-5-9. PIE of CF2+ from CF31. The solid line represents a smooth fit to the experimental data points. 173 CF,I + by -----9 CF,+ + F + I + e' the value of AH;O(CF3+) is calculated to be 207.94t3.5kcal/mole. 6. CF3C13 The photoionization mass spectrum of CF1C13. obtained under full illumination of the helium continuum, is shown in Figure 5Ar6-l. Relative intensities of the peaks and their assignments are presented in Table 5Ar6-1. All PIE curves of ions emanating from this compound are shown in Figure 5Ar6-2. and the summary of thermochemical information for these ions is given in Table 5A96-2. CF,C1,+ The parent ion PIE is shown in Figure 5A96-3. where it may be seen that the threshold starts to rise at about 11.80eV. Figure 5A96-4 shows the fit of the error function to the threshold region. The adiabatic IP of parent ion is 11.87t0.02eV. which is slightly higher than and in a very good agreement with the previous literature value: 11.75eV.92 From the reaction equation: CF,Cia + by -—--—e CF3C13+ + e’ the value of AHEOICF3C13+) is calculated to be 157.2212.5kca1/mole. 01301+ The first, lowest photon energy fragment PIE curve is shown in Figure 5Ar6-5. The PIE starts to rise sharply at threshold(near 11.806V). The AP value of this fragment ion is ll.88r0.04eV, which is RELATIVE INTENSITY 100.0 20.0 0.0 174 fil—l I _- J lllllillillllllllllLJllIllllllIllllllllJillllilLlellill1llilllilllllll‘lljilljllllllilllliljilillliiliLJililli 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 MASS UNITS (m/e) Figure 5A-6- 1. Photoionization Mass Spectrum of CFZCIZ (obtained under full illumination of the helium discharge). Table 5A-6- 1 . 175 Photoionization Mass Spectrum of CFZCIZ" m e... ‘1:?°SI.§L.I%":' E23211: 50 3 .2 2 . 10 cry 66 0.6 0.33 error 66 0 . 2 0 . 1 1 error 85 100.0 66.53 crzmi 87 34.2 -22.78 crzwcr 101 6.3 4.19 CF'CIZ’ 103 4.2 2.78 CF’cr’cr 105 0.8 0.53 0mm; 120 0.6 0.36 crz’mzi 122 0.4 0.23 crzeavcr 124 0 . 1 0 .04 CF2”CI2* ‘Obtained under full illumination of the helium discharge. PIE (Arbitrary Units) 176 WAVELENGTH (Angstroms) 1300 1100900 700 IIIIrWITlIIfilIfiI‘IlIIIM‘I'I I II I MW WWWWWM crzczz cram" WWW lllllllllllllllllllll.llLlLLillllllIllllllllllllllllllllUJ_Ll 10.0011.0012.00130014.0015.0016.0017.0018.0019.0020.0021.00 ENERGY (Electron Volts) Figure 5A-6—2. Photoionization efficiency curves for parent and daughter ions from CFZCIZ. 177 Table 5A-6-2. Summary of IP. AP and Heat of formation data for all ions from CF2C12. IP or AP Products of Heats of formation(O°K) ION (6V)a CFZC'Z + “Y (Kcal/mole)a CFZCIz" 11. 87 0F2c12+ + e“ 157 . 22 cr'zcr 11.88 crzcr + Cl 4 e" 128.93 CFCIZ‘ 14.02 only + F + 6- 166.44 CFCI’ 17.76 CFCI’ + F + Cl + 6" 246.17 Cin 10.51 crzt + Cl + or + 6‘ 207.16 8‘see text for uncertainties. 178 WAVELENGTH (Angstroms) 1200 1100 1000 900 800 700 600 IIIIIIIIIIIIIIIIITIIIIITI I I I I I I I l l PIE (Arbitrary Units) L°°°°°°°O¢no o 00 o 10.00 11.00 12.00 1.3.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-6-3. PIE of CFZCIZT from CFZClz. 179 WAVELENGTH (Angstroms) 1100 1080 1060 1040 1020 IIIlIliiiilelIITIIIIIIII'YIIIIIIIIIITIIIITITTVT co ooo o O A a) :1: c I D : I b 1 I. E :1: . .o " . 2 : o. I e E a. : g : : : : E I : Ll'lLLLllLllLlLlllLllllllil'lllillllllllJLl'lllllllillll 11.30 11.50 11.70 11.90 12.10 12.30 ENERGY (Electron Volts) Figure 5A-6-4. PIE oi CF2C12+ in the threshold region. Circles denote experimental points. The solid line is the best fit of a Gaussian representation of rotational and instru- mental contributions. The dashed line indicates the position of the corrected ionization potential. 180 WAVELENGTH (Angstroms) 1.3001200 1100 1000 900 800 700 600 rlllIIlIrITlIIII'IFIIIIIIIl I I I I I I I I 1 lij llllLlllllLlllllLlllLLlllLlllllllllJillilllllllllllil!lllllIJ 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-6-5. PIE oi Ccml+ from CFZCIZ. The solid line represents a smooth fit to I the experimental data points. PIE (Arbitrary Units) WAVELENGTH (Angstroms) 13001200 1100' 1000 900 800 700 600 IIIIIIIIIII1IIIII rI I I ITrI I I l I I I I I I I I r r I PIE (Arbitrary Units) lllllLlLlLLlLlllllllllLlllllllllllllllilllllllljllIlllllll]lll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-6-6. PIE of CFCIZT from CFZCIZ. The solid line represents a smooth fit to the experimental data points. 181 92 slightly lower than the previous literature value: 12.10:0.026V. From the reaction equation: CF3C1, + hv —-—--+ CF,Cl+ + c1 + e’ the value of AH}0(CF3C1+) is calculated to be 128.93t2.9kcal/mole. + 3 CFCl The second fragment ion is CFC13+, for which the PIE is shown in Figure 5Ar6-6. The PIE shows a threshold at around 14.0eV. The AP value of this fragment ion is 14.02:0.070V, which is slightly lower than and in a good agreement with the previous literature value: 14.15eV.92 By using the reaction equation: CF,Cl, + by ----+ CF01,+ + F + e‘ the value Of AHEOICFC1,+) is calculated to be 188.44:4.0kca1/mole. + 3 CF The fragment ion is CF,+, for which the PIE is shown in Figure 5Ar6-7. The PIE shows a relatively smooth threshold at around 16.5eV. The AP value of this fragment ion is 16.Ser.O7eV, which is slightly lower than and in a satisfactory agreement with the previous literature value: 16.986V.91 From the reaction equation: PIE (Arbitrary Units) 182 WAVELENGTH (Angstroms) 13001200 1 100 1000 900 800 700 600 I'IIIIIIIIIIIIIIIIIrI[III—rrI I I I I I I‘.I I I I a “A “AA! “"v. r 7. IIIIIJIJIUIl'llLLLLlllllllllllllllllllllLl'lllllllllllllllli 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 200021.00 ENERGY (Electron Volts) Figure 5A-6-7. PIE oi CF2+ from CFZClz. The solid PIE (Arbitrary Units) line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 13001200 1 100 1000 900 800 700 600 IIIIIIIIIII'IIII'IIIIIFIIIl II I I I I I I I I I llJlLllilllJllll[I'LLLLILJJLllLllllllllllLJLll11.1llllllil!LI 10.00 11.00 12.00 13.00 14.00 15.00 18.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-6-8. PIE of CFCl+ from CFZCIZ. The solid line represents a smooth fit to the experimental data points. 183 cp,c1, + hr ---579 CF,+ + 2Cl + e‘ the value of An}o(CF,*) is calculated to be 207.18t3.7kca1/mole. CFCI+ The last fragment ion is CFCl+. for which the PIE is shown in Figure 5Ar6-8. The PIE shows a relatively smooth threshold at around 17.7eV. The AP value of this fragment ion is 17.76t0.3eV, and by using the reaction equation: cs,c1, + by ----9 CFCl+ + F + Cl + o“ the value of AH}0(CFC1+) is calculated to be 246.17i8.4kca1/mole. 7. CFCI, The photoionization mass spectrum of CFC1,, obtained under full illumination of the helium continuum.- is shown in Figure 5Ar7-1. Relative intensities of the peaks and their assignments are presented in Table 5Ar7-1. All PIE curves of ions emanating from this compound are shown in Figure 5Ar7-2. and the summary of thermochemical information for these ions is given in Table 5Ar7-2. CFC1,+ The parent ion PIE is shown in Figure 5Ar7-3, where it may be seen that the threshold starts to rise at about 11.4eV. Figure 5Ar4-4 shows the fit of the error function to the threshold region. The adiabatic IP of parent ion is 11.4610.02eV, which is slightly lower than and in a satisfactory agreement with the previous literature value: 11.85eV.92 RELATIVE INTENSI'IY 100.0 20.0 0.0 184 1 llllllllllLll]llllllllllllllllllllllllLLlllllLlLLlJ'llllllllllllllllllllllllllllllllllll]Illullllllllllllhlu 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 MASS UNITS (m/e) ‘ Figure 5A—7-1. Photoionization Mass Spectrum of CFCl3 (obtained under full illumination of the helium discharge). 185 Table 5A-7-i. Photoionization Mass Spectrum of CF013‘ Intensity Relative Percent of Total Probable m/e to CFCI2+ Ion Production Formula 66 19.0 7.92 CF'CP 68 5.5 2.29 CF”CI* 82 5.5 2.71 0150121 84 2.0 0.83 C'CI”CI’ as 0.3 0.13 0302* 101 100.0 41 .72 CF”CI2* 103 55.0 27.12 cra'sclvcu+ 105 20.0 3.34 05'”an1 117 9.0 3.75 030131 110 0 . 0 3 .75 caazvcr 121 3. 0 1 .25 C‘CI”CI2’ 123 0.4 0.17 07013; 136 trace CF“CI3’ 138 trace CFs‘Clz-VCV 140 trace CF'CIVCIZ‘ 142- trace CF”CI3* aObtained under tun illumination of the helium discharge. 186 WAVELENGTH (Angstroms) 1300 1100 900 700 IIIIIIIIITIIIIIIIIIITFIIITI I I l I I l I I I crust ’m‘ ”012* -0-' be I D E‘ 0013 D L. .4: .0 L. < v 1.1.1 cror" 0. IllillllllllllllJlllllllllllllllllllllllll'lililllll'lilllll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-7-2. Photoionization efficiency curves for parent and daughter ions from CFC13. 187 Table 5A-7-2. Summary of IP. AP and Heat of formation data for all ions from CFC13. IP or AP Products of Heats of formation(O°K) ION (eV)° CF03 + “7 (Kcal/mole)a 01-013; 11.46 crc13+ + e‘ 196.03 are; 11.52 crc121 + Cl + 0' 168.89 0013+ 13.59 0013* + F + 0" 226.79 0012* 16.40 cc121 + F + 01 + o- 263.06 CFCI+ 15.50 CFCI" + Cl + Cl + 0' 231.35 I"see text for uncertainties. 188 WAVELENGTH (Angstroms) PIE (Arbitrary Units) 1250 1 1 50 1 050 950 850 I Ifi ] I T I I I F I I I I I T I I I I I I I —l o o 0 o o<§ °3§9 o 5’59) 0 Q) 0 0° 0 cg @§ c’0 §°Qb ° 00 ° 0%“ 06’ o f 630 00 o 0‘98 0% oQ30 5’ 0 go o O O 1 o 000 2 3. ”it 0 o O I o 8 0 0 8 L L I I I #L I I I I I L1 I l I I l I I I L J L I I 10.00 11.00 12.00 1.3.00 14.00 ENERGY (Electron Volts) Figure 5A-7-3. PIE of 0F013+ from CFC13. PIE (Arbitrary Units) Figure 5A-7-4. ~ 189 WAVELENGTH (Angstroms) 1150 1130 1110 1090 1070 1050 1030 IlIIIIIIIIIIIIIIIIIIIIIIITIIIIIrIIIIIIIIIlITIIlIIIIIIITIIIIIIITIIIIIII 09°00 °° 10.80 11.00 11.20 11.40 11.60 11.80 12.00 12.20 ENERGY (Electron Volts) PIE of CFC13+ in the threshold region. Circles denote experimental points. The solid line is the best fit of a Gaussian representation of rotational and instru- mental contributions. The dashed line indicates the position of the corrected ionization potential. PIE (Arbitrary Units) 190 WAVELENGTH (Angstroms) 13001200 1100 1000 900 800 700 600 IIIIIIIII'IIIIIIIII'IIIII I II I l I I I I I I I one, llIllllIJJIIIIIIIIIIILIIIIIILIIIIIIIIIIIIlIllllIIIIIIIiLLILL 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.002000 21.00 ENERGY (Electron Volts) Figure 5A-7-5. PIE of CFC12+ from CFC13. The solid PIE (Arbitrary Units) line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 1 100 1000 900 800 700 800 ' I1IIII'ITII'IIII'II I I I I I I I 1 T IIILLIIIIIIIlllIllIlIIIIlIlIlLIIIIIILIJLLLIILIJILLLIII 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-7-6. PIE of 0013+ from CFC13. The solid line represents a smooth fit to the experimental data points. 191 From the reaction equation: CF01, + by -----9 CF01,+ + e’ the value of AH}O(CFC1,+) is calculated to be 196.03t2.0kca1/mole. cFm,+ The first. lowest photon energy fragment PIE curve is shown in Figure 5Ar7-5. The PIE starts to rise sharply at threshold(near 11.5eV). and the PIE curve shows two steps. The AP value of this fragment ion is 11.52:0.02eV, which is slightly lower than and in a good agreement with the previous literature: value 11.65eV.92 From the reaction equation: CFCl, + hy ----9 CF01,+ + C1 + o' the value of AH;0(CFC13+) is calculated to be 168.89t3.1kcal/m01e.- 001,+ The second fragment ion is CCl,+. for which the PIE is shown in Figure 5Ar7-6. The PIE shows a relatively smooth threshold at around 13.5eV. The AP value of this fragment ion is 13.59t0.1eV. which is slightly higher than and in a very good agreement with the previous literature value: 13.5eV.95 From the reaction equation: 192 WAVELENGTH (Angstroms) 1 100 1000 900 800 700 600 leII'IIIIIIIIIlIIII I I I I I I I PIE (Arbitrary Units) IllllllllllllIJIIIIIILIIIIIIIIIIIIIIIIILIJJJIIIIIIIIIJ 11.00.12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-7-7. PIE of CFCl+ from CFC13. The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 1100 1000 900 . 800 700 600 rlIIIIlIT—FTIIIII'I I I III I T I l I IJLIIILIIILJIIIIIIIIIIIIIILLLJIIILIILLLIIIIIJIIIIIIL 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-7-8. PIE of cc12+ from CFC13. The solid line represents a smooth fit to PIE (Arbitrary Units) the experimental data points. 193 CFCl, + by —---—9 CCl,+ + F + e' the value of AB;o(CCl,+) is calculated to be 226.79t4.2kca1/mole. CF01+ The next fragment ion is CFC1+. for which the PIE is shown in Figure 5Ar7-7. The AP value of this fragment ion is 15.SOiO.3eV. and by. using the reaction equation: CF01, + by --—--9 CF01+ + 201 + e' the value ofAH}O(CF01+) is calculated to be 231.35t8.5kcallmole. + 3 CCl The last fragment ion is CCl,+. for which the PIE is shown in Figure 5Ar7-8. The PIE shows a relatively smooth threshold at around 16.4eV. The AP value of this fragment ion is 16.40t0.1eV, which is slightly lower than and in a reasonable agreement with previous literature value: 17.0eV.92 By using the reaction equation: CF01, + by -----9 001,+ + F + 01 + e‘ the value of AH}o(CCl,+) is calculated to be 263.06:4.2k0al/m01e. 8. CCl, The photoionization mass spectrum of C01,. obtained under full illumination of the helium continuum. is shown in Figure 5A-8-1. No parent ion is detectable. Relative intensities of the peaks and their RELATIVE INTENSITY 100.0 20.0 0.0 194 llllIlIllIilllIlJllIllllIllllIllliIllllIllllIlillIllllIllllIilllIllllIllllIllll'llllIilllIllllIllllIllilIllJlIllll 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 MASS UNITS (m/e) Figure 5A-8-1. Photoionization Mass Spectrum of CCl4 (obtained under full illumination of the helium discharge). Table 5A-B-1 . 195 Photoionization Mass Spectrum of COL],a m/e Intexittcti‘mive Pig'ocegioglflciicotrf' $2333? 47 4.8 1 .59 0301+ 49 1 . 6 0 .53 03701: 82 31 .2 10 .36 0301; 84 14.4 4.78 eraser 86 3.2 1 .06 $012+ 117 100.0 33.20 chuz,+ 1 19 90 . 0 29 .88 cacuzvcr 121 47.0 15.50 030137011 123 9.0 2.98 I 017013: I‘Obtinineti under full illumination of the helium discharge. Table 5A-8-2. Summary of IP. AP and Heat of formation data for all ions from CCl4. IP or AP Products of Heats of formation(O°K) ION (5V)a CC'4 'I' “Y (KcaI/mole)a 0013* 11.46 0013+ + 01 + e- 213.42 0012+ 14.74 0012* + 01 4 c1 + e- 250.45 a see text for uncertainties. 196 WAVELENGTH (Angstroms) 1300 1100 900 700 IIIIIIIIIIIFII‘TIIII’I II r1 I I I I I I I I I I I 0013* PIE (Arbitrary Units) 0012+ LIIIIIIILILIIIIIIIIIIIIIIIIIIJIIIIIIIIIIIIIIIILIIIIIIIILILJll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-8-2. Photoionization efficiency curves for daughter ions from CC14 (no parent ion is detectable). 197 WAVELENGTH (Angstroms) 1250 1150 1050 950 850 750 650 ’0‘? WIIIII'IIII'IIIIIFITIIIIIIl I I I I r1 I I 3.". C :3 C L. 1’: .0 L. < V E & LILIIII'IIIIILLIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII'll 10.00 11.00 12.00 13.00 14.00 15.00 18.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-8-3. PIE of 0013+ from 0014. The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 900 . . 800 700 600 PIE (Arbitrary Units) IIIHIJIILIJIJILLLJIIIIIIJIIILIILIIIJIIIIIII 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-8-4. PIE of 0012+ from 0014. The solid line represents a smooth fit to the experimental data points. 198 assignments are presented in Table SArS-l. All PIE curves of ions emanating from,this compound are shown in Figure 5Ar8-2, and the summary of thermochemical information for these ions is given in Table 5Ar8-2. .1. 3 CCl The first, lowest photon energy fragment PIE curve is shown in Figure 5Ar8-3. The PIE starts to rise sharply at threshold(near 11.46eV). and the PIE curve shows two steps. The AP value of this fragment ion is 11.4610.045V. which is slightly higher than and in a good agreement with the previous literature value 11.28t0.03eV.96 From the reaction equation: 001, + by -—---9 001,+ + e the value of AE;O(CCI,+) is calculated to be 213.4311.5kcal/mole. 001,+ The second fragment ion is CC1,+, for which the PIE is shown in Figure 5A-8-4. The PIE shows a relatively smooth threshold at around 14.7eV. The AP value of this fragment.ion is 14.74t0.07eV, and by using the reaction equation: 001, + by --—-—e 001,+ + 201 + e‘ the value of AH;O(CC1,+) is calculated to be 260.46:2.2kca1/m01e. I 9. CCl,Br The photoionization mass spectrum of CCl,Br. obtained under full RELATIVE INTENSITY 100.0 0.0 199 _ - lllIIllllIllllIllllIlllLIllLlIllLlIllllIllllIllllIllOlIllllIllLiIllllIllliIlllJJJlllIllllIllllIllllI‘lllIlllLIl'li 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 MASS UNITS (m/e) Figure 5A-9-1. Photoionization Mass Spectrum of CCl3Br (obtained under full illumination of the helium discharge). 200 Table 5A-9-1. Photoionization Mass Spectrum of CClaBra Intensity Relative Percent of Total Probable m/e to CCI3+ Ion Production Formula 82 58 . 9 17 .28 0‘02" 84 33 . 8 9 . 89 O‘CI'VCI“ 88 5 . 5 1 . 80 O’CIZ" 117 100.0 29.30 C’Cb’ 119 92 . 2 27 .02 C‘CIZ’TI‘ 121 31 . 2 9 .18 C‘CIVClzt 123 3.1 0.92 O’CI3’. 128 0 . 5 0 .14- C”CI”Br’ 128 0.5 ' 0.18 C’CWBr‘*C”CI”Br’ 130 0 . 2 0 .05 O’CI"Br’ 161 4.5 1.32 O‘Cl2”Br’ 183 7 . 0 2 . 08 C‘Cl2"Br’,C”CI”Cl”Br’ 165 3 . 3 0 . 96 O’CI2”Br*,C”Cl“’Cl"Br* 157 0 . 5 0 .14 ' 017012-185 196 t race (>"’CI3"'Br+ 198 1: race C"‘Cl3"Br*,C"‘CI2”CI"'Br+ 200 t race C“Cl”CI2”Br*,C>”Cl237Cl“Br* 202 1: race C”Cl3”Br’.C"CI”Cl2"Br* 204- t race C37Cl3°‘Br* aObtained under full illumination of the helium discharge. 201 WAVELENGTH (Angstroms) 1300 1100 900 700 IIIIIIIIIIIIIIIIFIIIIIIII I I I I r I I I I i I €0133r+ / 6013+ calzart 0012+ IIIJJIJIIllIIJIIIIIIIIIIIILLIIIIIIIllIlIILIIIIIIIIIIIIIIIIII 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) PIE (Arbitrary Units) Figure 5A-9-2. Photoionization efficiency curves for parent and daughter ions from CClaBr. 202 Table 5A-9-2. Summary of IP. AP and Heat of formatibn data for all ions from CClaBr. IP or AP Products of Heats of formation(O°K) ION (eV)a CCI3Br ‘I' “Y (KcaI/mole)a 00133!" 11.05 CCI3BI” + 0’ 243.81 0013’ 10.71 0013’ + Br + 0" 207.78 CClzar’ 11.39 ccuzsri + 01 + e- 23.13 0012* 13.95 0012* + 01 + Br 4 e- 253.98 aFlee text for uncertainties. PIE (Arbitrary Units) 203 WAVELENGTH (Angstroms) 1300 1200 1100 1000 900 III I I I I I r I I I I I I I l I I I I l 0 ° 0 <2 0 0 0:: o 0 22° .. g’ (90 ’OOOCfloO <> ° °bo ° gcmpald% o %%o 9% 0 $0 0 065’ 0 <9 o 019 0° 0 ° 0 098 o g 0 08° 1153380 9’ o E a o 0 Q 00 ° 8 o 8% egg° ab 8 10.00 11.00 12.00 13.00 14.00 ENERGY (Electron Volts) Figure 5A-9—3. PIE of CCl3Br+ from 001313r. 204 WAVELENGTH (Angstroms) 1180 1160 1140 1120 1100 jI'IIIIlIIIIIIIII‘lIIIIIIIIIIIIII'IIII'IIIIllIrIIIIT O 0 0 e A (I) .4: C D . I b : o l L I 44 l .0 1.. < V 1 1.1.] I a a I I o I I I : LLLLLllIllllIllllIllllIlllILIIIIIIIIIIIIIILIIILJIJ 10.50 10.70 10.90 11.10 11.30 Figure 5A-9-4. ENERGY (Electron Volts) PIE of 0013Br+ in the threshold region. Circles denote experimental points. The solid line is the best fit of a Gaussian representation of rotational and instru- mental contributions. The dashed line indicates the position of the corrected ionization potential. 205 illumination of the helium continuum. is shown in Figure 5Ar9-l. Relative intensities of the peaks and their assignments are presented in Table 5A-9-l. All PIE curves of ions emanating from this compound are shown in.Figure 5Ar9-2. and the summary of thermochemical information _ for these ions is given in Table 5Ar9-2. 001,Br+ The parent ion PIE is shown in Figure 5Ar9-3. where it may be seen that the threshold starts to rise at about 11.005V. Figure 5Ar9-4 shows the fit of the error function to the threshold region. The adiabatic IP of parent ion is ll.0510.02eV. From the reaction equation: CCl,Br + by -----9 001,Br+ + e‘ the value of AH;O(CC1,Br+) is calculated to be 243.81:1.5kcal/m01e. 001,13r+ The PIE of the 001,13r+ fragment is shown in Figure 5Ar9-5. The AP value of this fragment ion is ll.39t0.04eV, and by using the reaction equation: CCl,Er + by -----+ 001,13r+ + 01 + e‘ the value of AH}o(CCl,Br+) is calculated to be 223.1312.0kcal/mole. 001,+ The first. lowest photon energy fragment PIE curve is shown in Figure 5Ar9-6. The PIE starts to rise sharply at threshold(near 10.7eV); the AP value ion is 10.7110.07eV. From the reaction equation: 206 WAVELENGTH (Angstroms) 1300 1200 1 100 1000 900 800 700 600 lIIIIIIIIIlIIIIIIIII'IIIIII I I I rj I I I I I PIE (Arbitrary Units) LILIIIIJ_LIIIIIIIIIIlLIlJLIIIIlIIlIlIILILI LlIlILIIIIIILliIIIL 10.00 11.00 12.00 13.00 14.00 15.00 18.00 17.00 18.00 19.00 2000 21.00 ENERGY (Electron Volts) Figure 5.1-9-5. PIE of CClzBr+ from CCl3Br. The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 13001200 1100 1000 900 800 700 600 ‘TIII'IIIIIIIIIIIIIIIIIIIII I I fl I I I I I I PIE (Arbitrary Units) IIIIIIIIIIIIIIIIIIIILIIIIIIIIIIJIIIIlIllllIllIlIlIllIlllllLL 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 200021.00 ENERGY (Electron Volts) _ Figure 5A-9-8. PIE of 0013+ from CCl3Br. The solid line represents a smooth fit to the experimental data points. PIE (Arbitrary Units) 207 WAVELENGTH (Angstroms) LILLIIIJIIJIIIIIIllllIlIllIlllIIlLIlIlJli 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-9-7. PIE of 0012+ from 00133r. The solid line represents a smooth fit to the experimental data points. 208 + CC1,Br + by -----9 CCl, + Br + e- the value of AH}0(CC1,+) is calculated to be 207.78t2.7kcal/mole. + 3 C01 The second fragment ion is CC1,+. for which the PIE is shown in Figure 5Ar9-7. The PIE shows a relatively smooth threshold at around 13.9eV. The AP value of this fragment ion is 13.95:0.075V. and by using the reaction equation: CCl,Br + hy ----+ 001,+ + 01 + Br + e' the value of AH}O(CCI,+) is calculated to be 253.98t2.7kcal/m01e. 10. CFzBr, The photoionization mass spectrum of CF,Br,. obtained under full illumination of the helium continuum. is shown in Figure SArlO-l. Relative intensities of the peaks and their assignments are presented in Table 5Ar10-1. All PIE curves of ions emanating from this compound are shown in Figure 5Ar10-2. and the summary of thermochemical information for these ions is given in Table 5Ar10-2. CF,Br,+ The parent ion PIE is shown in Figure 5Ar10-3. where it may be seen that the threshold starts to rise at about 10.90eV. Figure 5A910~4 shows the fit of the error function to the threshold region. The adiabatic IP of parent ion is 10.9810.025V, which is slightly lower than and in a good agreement with previous literature value: 11.185V.34 From RELATIVE INTENSITY 100.0 20.0 0.0 209 1- d IIILIJIIIIIJLIIllllI]llLIIlllIllllILlilIlllllllllILlllIllllIllllIllllIllllIllllLlllIllllIllllIllLlIllllIllllIllli MASS UNITS (m/e) 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 Figure 5A-10-1. Photoionization Mass Spectrum of CFzBrz (obtained under full illumination of the helium discharge). 210 Table 5A-10-1. Photoionization Mess Spectrum of CFzBrz“ m/e IntifigFZRBerlg'five fiihcegociiciigii' $3331.15 50 . 15.1 5.52 or; 79 3 . 2 1 .12 "so 81 3. 2 1 .12 "91" 1 10 .7. 1‘ 2 . 47 CF”Br’ 112 7.1 2.47 CF“Br’ 129 100.0 34.83 crzrsri 131 90.3 31 .45 crzssr 189 3.2 1.12 CF”Br2" 191 6.5 . 2.25 CF”Br"Br* 193 :3.2 1 .12 091891 .208 1.3.5 4.72 CFZ”Br2’ 210 21 .9 7.64 CF2"Br"Br* 212 1 1 . 6 4 . 04 CFZ"Br2’ "Obtained under full illumination of the helium discharge. WAVELENGTH (Angstroms) 1300 1100 900 700 I‘III‘I—Il I l l l I l I I I l I l I l I | I l l r r I I I l I I CF23r2+ F231”.- Cine-2+ PIE (Arbitrary Units) IlIllllIlliIIIlIIIllllIllllIllllIlllllllIIIILLIIIIILIJIJILLJ 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-10-2. Photoionization efficiency curves for parent and daughter ions from CFgBrg. 212 Table 5A-10-2. Summary of IP. AP and Heat of formation data for all ions from CFzBrz. IP or AP Products of Heats of formation(O°K) ION (23V)a CFZBrz + hY (Keel/mole)“ ($231.21 10.98 crzar2+ + e' 156 . 31 CF23!“ 11.00 CFZBI" + Br + 0" 128.58 crar2+ 14.24 era-2* + F + 0‘ 213.12 a‘91-”: text for uncertainties. 213 WAVELENGTH (Angstroms) 1.300 1200 1 100 1000 900 J I I I I I r T fT I ' l I I I l —r T I r l 08° 8 0 09 1582, 33 Q) fig 0 g o oo % 9 O 0 c9 °o 39°9§ 0 3%? a» 5&2- 8:0 2, o o o ‘ O 0 o Q 0 A 8 m 0 ."i Q,° C o :3 a? 8 E‘ o E 8 1‘: o .0 L. o < o v o L_'-.-.' 9 0- o O o O 9 o O 8 9 J I l l l l I l I l I l l l I l I I l l I l l l 10.00 11.00 12.00 13.00 14.00 ENERGY (Electron Volts) Figure 5A-10-3. PIE of CFzBr2+ from CFzBrz. 214 WAVELENGTH (Angstroms) 12.30 1210 1190 1170 1150 11.30 1110 1090 1070 IIII'IIII'IIII'IIIU'TTUIIIIIIIIIII'TIIIIUIIIIIIIIIIIIIIIIII'llII'IIIIIIIIUIIIIIIIIIIIIIII PIE (Arbitrary Units) I lllllllllllllllIllI'll1111111111!IlllllllllllllllllllllLJIl'lllLlleJ'llll'lllllllll 10.00 10.20 10.40 10.60 10.80 11.00 11.20 11.40 11.60 ENERGY (Electron Volts) Figure 5A-lO-4. PIE of CFzBr2+ in the threshold region. Circles denote experimental points. The solid line is the best fit of a Gaussian representation of rotational and instru- mental contributions. The dashed line indicates the position of the corrected ionization potential. 215 the reaction equation: CFzBr, + by ----9 CF,Br,* + e' the value of AH;O(CFzBr,+) is calculated to be 156.3111.Skca1/mole. CF,Br+ The first. lowest photon energy fragment PIE curve is shown in Figure SArIO-S. The PIE starts to rise sharply at threshold(near 11.0eV). The AP value of this fragment ion is 11.00t0.04eV, and from the reaction equation: CFzBrz + hy —--—-9 c1138:+ + Br + e- the value of AH;O(CF,Br+) is calculated to be 128.58t2.0kcal/mole. + CFBr3 The second fragment ion is CFBr,+. for which the PIE is shown in Figure 5Ar10-6. The AP value of this fragment ion is 20.44t0.3eV. and by using the reaction equation: crznr, + hy --—--9 CFBr3+ + F + e‘ the value of An;o(crcr,*) is calculated to be 213.12i8.5kcal/mole. 11. cup, The photoionization mass Spectrum of CHF,, obtained under full illumination of the helium continuum, is shown in Figure SArll-l. 216 WAVELENGTH (Angstroms) 13001200 1100 1000 900 800 700 600 I'IIIIIITIIIIFIIIIIII'II I I I I I I I l I I I I I I PIE (Arbitrary Units) llllllll'llLlLLLLlllllllllllllllllLllllLlllllLLllllllllllllll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-10-5. PIE of CFzBrT from cram-'2. The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 1.3001200 1-100 1000 900 800 700 600 I'ITIIIIIIIIIIII'IIIIIrrII1 fiI 1 I I T I I r I I O PIE (Arbitrary Units) lll'iullllllJllllllJllIlllLllllllllllll4JlllllllllLllllllIllJ 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-lO-6. PIE of CFBr2+ from CFzBrz. The solid line represents a smooth fit to the experimental data points. RELATIVE INTENSITY 100.0 20.0 0.0 Figure 5A-11-1. 217 _ l__ _ lLJlIllllL'JlLllLlllLlLllllllIllllliLllllllLlLlll 0.0 20.0 40.0 60.0 80.0 MASS UNITS (m/ e) Photoionization Mass Spectrum of CHF3 (obtained under full illumination of the helium discharge). 218 Table 5A-11-1. Photoionization Mass Spectrum of CHF3a Intensity Relative Percent of Total Probable m/e to CHI";+ Ion Production Formula 51 100.0 57.77 CHI-'2‘ 69 71 .0 41 .02 CF3" 7O 2. 1 1 .21 CHI-'3’ I“Obtained under full illumination of the helium discharge. Table 5A-11-2. Summary of IP. AP and Heat of formation data for all ions from CHF3. IP or AP Products of Heats of formation(O°K) ION (eV)a CHF3 + h)! (Kcal/rnole)a CF3“ 14.14 CF3“ + H '1' 8" 109.54 CHI-2" 14.94 CHI-2' + F '1' C. 161.25 8see text for uncertainties. 219 WAVELENGTH (Angstroms) 1300 1 100 900 700 WIIIIIIIIIIIIIFIIIIIII I I I I I I I I I r I I I cp8+ mmflmkwdb c3r2+ PIE (Arbitrary Units) lIlllllllllllllllLLlllIllLllJlIlllllllllllIlIlllllllllllllll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-11-2. Photoionization efficiency curves for daughter ions from CHF3. 220 Relative intensities of the peaks and their assignments are presented in Table SArll-l. All PIE curves of ions emanating from this compound are shown in Figure 5Ar11-2. and the summary of thermochemical information for these ions is given in Table 5Ar11-2. + 3 CF Under high mass resolution conditions. the CHF,+ parent ion appears weakly in the photoionization mass spectrum obtained with undisperesed helium radiation. However. its intensity is not sufficient to allow a PIE curve to be measured. Therefore. the first PIE recorded from CHF, is that of CF,+, which is shown in Figure 5Ar11-3. The PIE starts to rise at threshold(near 14.1eV). The AP value of this fragment ion is 14.1410.04eV. which is excellent agreement with the previous literature value 14.14t0.02eV.91 From the reaction equation: CHF, + by ----—) CF,+ + H + e- the value of AH;O(CF,+) is calculated to be 109.54tl.7kcal/mole. am,” The second fragment ion is CHF,+. for which the PIE is shown in Figure 5Ar11-4.. The AP value of this fragment ion is 14.94i0.04cV. and by using the reaction equation: 221 WAVELENGTH (Angstroms) 950 850 750 650 I l I T I I I I I I I I I I I I T I I I PIE (Arbitrary Units) llllllllllllllllLlllllLlllllJlllJllllllllllIJ 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-ll-3. PIE of CF3+ from CHF3. The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 900 800 700 600 ..—.— -_-- .—-—.—-.———— -—-——-—-—- -- _- —_—— PIE (Arbitrary Units) JlLlllLllllllllllllLlLJlIlllllllllllllllllil 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-ll-4. PIE of CHF2+ from CHF3. The solid line represents a smooth fit to the experimental data points. 222 cur, + by -----9 cup,+ + F + c' the value of AH}O(CHF3+) is calculated to be 16l.26t2.1kcallmole. 12. an3c1 The photoionization mass spectrum of CHF1C1, obtained under full illumination of the helium continuum, is shown in Figure 5Ar12-1. Relative intensities of the peaks and their assignments are presented in Table 5Ar12-1. All PIE curves of ions emanating from this compound are shown in Figure 5Ar12-2, and the summary of thermochemical information for these ions is given in Table 5Ar12-2. CHF,C1+ The parent ion PIE is shown in Figure 5Ar12-3, where it may be seen that the threshold starts to rise at about 12.2eV. Figure 5Ar12-4 shows the fit of the error function to the threshold region. The adiabatic IP of parent ion is 12.28t0.02eV. which is slightly higher than and in a good agreement with the previous literature value: 12.60eV.93 From the reaction equation: CKF301 + hv -----o CEF3C1+ + e the value of AH}0(CHF,C1+) is calculated to be 169.58i3.5kca1/mole. chc1+ The PIE of CKFC1+ from an301 is shown in Figure 5Ar12-5. The PIE shows a relatively smooth threshold at around 12.0eV. The AP value of this fragment ion is 14.44t0.3eV, and by using the reaction equation: RELATIVE INTENSITY 100.0 20.0 0.0 223 __l ed JllllllllIllllllllllIJlLlIllllllllllllllIlllllllulllllllllllllIlIllUlllllllllllllllllllllllllIllllIljlillll 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 MASS UNITS (m/e) Figure 5A-12-1. Photoionization Mass Spectrum of CHFZCI (obtained under full illumination of the helium discharge). 224 Table 5A-12-1. Photoionization Mass Spectrum of Garza“ Intensity Relative Percent of Total Probable m/e to CHFZT Ion Production Formula 51 100.0 66.67 CHF2* 67 32.0 21 .33 CHF”CI* 69 14.0 9.33 CHFVCI’ 86 3 .. O 2 . 00 CHF2”CI* 88 1 . O O . 67 CHFZVCP aObtained under full illumination of the helium discharge. Table 5A-12-2. Summary of IP. AP and Heat of formation data for all ions from CHFZCl. IP or AP Products of Heats of formation(O°K) ION (eV)°‘ CHFZCI + “V (Kcal/mole)a CHsz 12 .28 CHFZCI" + 0" 189 .58 CHFz" 11.98 CHI-'2’ + Cl + 0’ 134.14 CHFCI" 14.44- CHFCI‘ + F + 0" 201.13 I“see text for uncertainties. 225 WAVELENGTH (Angstroms) 1300 1 100 900 700 IIIIIIIII‘ITrllIIIIIIIrIFl I I I I I r I j I I cgpzcrI-m PIE (Arbitrary Units) lll'lllIlLlllLLLlllllLLl111'lllllllLllllllIlllllllllllllllllll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 200021.00 22.00 ENERGY (Electron Volts) Figure 5A-12-2. Photoionization efficiency curves for parent and daughter ions from CHFZCI. 226 WAVELENGTH (Angstroms) 1100 1000 900 800 700 600 IIIIIIIIIFII'IIIIIII I I l I I I r I I PIE (Arbitrary Units) IllIllllllLlllLllllllllllllllllJlllLlllllIllllllIlllll 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-12-3. PIE of CHF2C1+ from CHFZCI. The solid line represents a smooth fit to the experimental data points. PIE (Arbitrary Units) 227 WAVELENGTH (Angstroms) I lkllllLlLikllLlllllllllllLlIlLllllJ 1040 1020 1000 [Tllilrlllirllrrrllrlrrllit] O O 11.90 12.10 12.30 12.50 ENERGY (Electron Volts) Figure 5A-12-4. PIE of CHF2C1+ in the threshold region. Circles denote experimental points. The solid line is the best fit of a Gaussian representation of rotational and instru- mental contributions. The dashed line indicates the position of the corrected ionization potential. 228 WAVELENGTH (Angstroms) 1200 1 100 1000 900 800 700 600 IIII'IIII'IIj—I—rTIIII I I I I I I I I I l I PIE (Arbitrary Units) llli_LlLLlllIlIllllllIllllllllllllllllllllllllIIILJJJIIII 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-12-5. PIE of CHFCIT from CHFZCI. The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 1 1 00 1 000 900 800 700 600 IIIIIIIIIII1IIrIIII I IT I I I I l I lllLllJlllllllllllllllllllllllllJ4lellllllllllLlLlll 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-12-6. PIE of CHF2+ from CHFZCI. The solid line represents a smooth fit to the experimental data points. PIE (Arbitrary Units) 229 CHF3C1 + by —----9 CIIFC1+ + F + e’ the value of AH}O(CHFC1+) is calculated to be 201.03:10.4kcal/mole. CIzF,+ The first, lowest photon energy fragment PIE curve is shown in Figure 5A-12-6. The PIE starts to rise at threshold(near 12.0eV). The AP value of this fragment ion is 11.98t0.04eV. From the reaction equation: CHF3C1 + by —---—9 an,+ + C1 + e‘ the value of AH}O(CHF3+) is calculated to be 134.14t4.0kca1/mole. 13. CHFCIa The photoionization mass spectrum of CHFClz, obtained under full illumination of the helium continuum, is shown in Figure 5Ar13-1. Relative intensities of the peaks and their assignments are presented in Table 5A913-1. All PIE curves of ions emanating from this compound are shown in Figure 5Ar13-2, and the summary of thermochemical information for these ions is given in Table 5Ar13-2. anc1,+ The parent ion PIE curve is shown in Figure 5A713-3. The PIE starts to rise sharply at threshold near 11.50eV. Figure 5Ar13-4 shows the fit of the error function to the threshold region. The small disagreement between the fit curve and experimental PIE curve is attributed to the charge exchange reaction between the neutral parent RELATIVE INTENSITY 100.0 20.0 0.0 230 LIJIIIIIIIJILIILIIIIIIliLIlLlJlllllllillIlIIIIILLLLIJ' 0.0 20.0 40.0 60.0 80.0 100.0 MASS UNITS (m/e) Figure 5A—13-1. Photoionization Mass Spectrum of CI-lFClz (obtained under full illumination of the helium discharge). 231 Table 5A-13-l. Photoionization Mass Spectrum of CHFCI‘?a Intensity Relative Percent of Total Probable m/e to cnrcu+ Ion Production Formula 67 100.0 55.71 CHF"CI* 69 36 . 1 20 . 09 CHF”CI* 82 2.7 1 .37 C’Clz’ 83 12.7 7.08 CH“CI2' 84 1 .6 0.91 O‘CIVCI” 85 8 . 2 4 . 57 CH”CI”CI’ 86 0.4 0.23 CVCIZ" 87 1 .2 0.68 CH”CI;’ 101 2.7 1 .37 CF“CI2* 102 7. 4 4.11 CHF"CI2’ 103 1. 6 0 .91 CF'CI”CI* 104- 4.1 2.28 CHF"CI”CI* 105 0.4- 0.23 CWCIZ' 106 0.8 0.46 CHF”Cl2* aObtained under full illumination of the helium discharge. 232 WAVELENGTH (Angstroms) 1.300 1100 900 700 TIII'IIII'IIrr‘IIIIlI I I II T I I I T I I I I I I carolz‘t chum+ PIE (Arbitrary Units) carat,t lllllLllIlJlllllllllLLlLLllll114111llllllllllLlIlJllllllllll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-13-2. Photoionization efficiency curves for _parent and daughter ions from CHFClz. 233 Table 5A-13-2. Summary of IP. AP and Heat of formation data for all ions from CHFClz. IP or AP Products of Heats of formation(O°K) ION (eV)a CHFCIZ + by (Kcal/rnole)a CHFCIZ" 11 . 75 cnrc12+ + e- 204. 49 CHFCI" 11.87 CHFCI" + 01+ 0' 174.23 CHCIZ" 13.84 cucnzr 4 F + o- 234.83 8see text for uncertainties. 234 WAVELENGTH (Angstroms) 1200 1100 1000 900 800 700 600 IIIITTIIYUIIIIIIFIIIIUIrrl I l I I I I l I (903% PIE (Arbitrary Units) L“mmmficmcoo o oo o lllll'llllllllllllllllllLllllllllLJlLllLlllllllLllllllI'll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-13-3. PIE of CHFC12+ from CHFClz. PIE (Arbitrary Units) 235 WAVELENGTH (Angstroms) 1 120 1 100 1080 1060 1040 1020 TIIIIIITIIIUIIITTI'IUIIIITIFI—rrIjlIIIIIIIIIIII'llIIIIIT lllllIllI'JUI'IIII'ILLIIJIIIIIlllllhll'lULlelelllllllll 11.00 11.20 11.40 11.60 11.80 12.00 12.20 ENERGY (Electron Volts) Figure 5A-13-4. PIE of CHF012+ in the threshold region. Circles denote experimental points. The solid line is the best fit of a Gaussian representation of rotational and instru- mental contributions. The dashed line indicates the position of the corrected ionization potential. 236 and first fragment ion, which has a lower AP than the IP. The adiabatic IP value of parent ion is 11.74t0.02eV. which is lower than and in a good agreement with the previous literature value: 12.0eV.93 From the reaction equation: enrol, + by -----9 anc1,+ + e- the value of AH}0(CHFC13+) is calculated to be 204.49t3.5kca1/mole. anc1+ The first fragment PIE curve is shown in Figure 5Arl3-5. The PIE shows a sharp threshold at at around 11.6eV. In the post-threshold of this fragment ion PIE curve, autoionization structures are observed. The AP value is 11.67r0.04eV, which is 0.07eV lower than the IP of the parent milecule. It is interesting to comparethis with the previous literature value: 12.69eVi0.15eV,93 one electron volt higher. Loss of a chlorine atom is easily distinguished in the mass spectrum, and there is no chance that the parent ion interfered with the measurement of the CEFCl+ PIE curve. Because it is lower than the previous determination. the current value for the AP should be accepted as more reliable. From the reaction equation cxzrm3 + by --——-9 CIIFC1+ + c1 + e’ the value of AH;0(CKFC1+) is calculated to be l74.23i4.0kcal/mole. cxc1 + 3 The PIE curve for CHClz+ is shown in Figure 5Ar13-6. The PIE 237 WAVELENGTH. (Angstroms) 13001200 1100 1000 900 800 700 600 IIIWIIIIIIIIIrrlrIIIlII TI | I I 1 I I r I I I I I PIE (Arbitrary Units) llllllllLllllllLllllllllJJLlJlJllllllllllllillllllllllILLJIJI 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-13-5. PIE of cnrcr“ from CHFCIZ. The solid line represents a smooth fit to the experimental data points. WAVELENGTH (Angstroms) 130012001100 1000 900. 300 700 600 IIIIIIIIITITIWIII—FIIII I I I l I I I I 1 I I I I I I 0*... . C o...- ’- PIE (Arbitrary Units) lllllilJJJlLlllLLllllilllllllllllllllI|IILJILLJILLJIIJIlllIll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-l3-6. PIE oi CHC12+ from CHFClz. The solid line represents a smooth fit to the experimental data points. 238 shown a smooth threshold around 13.5eV. The AP value of this fragment ion is 13.84ro.leV. No previous literature value is available for comparison. From the reaction equation: CHFCI, + hv ----9 CECI,+ + F + e‘ the value of AH;O(CECI,+) is calculated to be 234.83r5.7kca1/m01e. 14. CHCl, The photoionization mass spectrum of CHC1,, obtained under full illumination of the helium continuum. is shown in Figure 5Ar14-1. Relative intensities of the peaks and their assignments are presented in Table 5Ar14-1. All PIE curves of ions emanating from this compound are shown in Figure 5Arl4-2, and the summary of thermochemical information for these ions is given in Table 5Arl4-2. + CECI, The parent ion PIE curve is shown in Figure 5Ar14-3. where it may be seen that the threshold includes two steps. Figure 5Arl4-4 shows the fit of the error function to the threshold region. The adiabatic IP of parent ion is ll.4lt0.02eV, which is slightly higher than and in a very good agreement with previous literature values: 11.3910.12eV96 and 11.37zt0.02eV.97 From the reaction equation: RELATIVE INTENSITY 100.0 40.0 20.0 0.0 239 lllll'lllllllll]LilllllllllllllllllllllllujlllllllllllllllllllIll[Ill]llllllllllllllllllllflllllllllLLlJllllllll 1 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 220.0 MASS UNITS (m/e) Figure 5A-14-1. Photoionization Mass Spectrum of CHC13 (obtained under full illumination of the helium discharge). ' 240 Table 5A-l4-1. Photoionization Mass Spectrum of CI-lCl3a Intensity Relative Percent of Total Probable m/e t0 CHCI2+ Ion Production Formula 83 100 . 0 50 . 70 CH”CI2* 85 68.3 34.61 CH”CI"’CI* 87 14.3 7.24 CH”CI2’ 118 6.3 3.22 CH”CI3* 120 6.0 3.06 CH3Cl237CI" 122 2.1 1 .05 (3H”Cl"’CI2+ 124 0.3 0.12 CH”CI3* ‘Obtained under full illumination of the helium discharge. Table 5A-14-2. Summary of IP. AP and Heat of formation data for all ions from CHCla. IP or AP Products of Heats of formation(O°K) ION (eV)a CHC'3 + W - (KcaI/mole)a CHCI3‘ 11.41 CHCI3” + 6' 239.71 c1402: 11.42 01-1012: + c1 4 e- 211.34 asee text for uncertainties. 241 WAVELENGTH (Angstroms) 1300 1100 900 700 IIIII'TIIIIITFIIIIjIIIII ITI I I I l I I r I l j A (I) .4: c I) D L. P: .0 L < v I._l_J 0. 0110121' [ILLIlllJllILlLllllllllleLIllLlIllllllllllllll'llllllllllll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5A-l4-2. Photoionization efficiency curves for parent and daughter ions from CHCla. 242 WAVELENGTH) (Angstroms) 13001200 1 100 1000 900 800 700 500 I I I I I I I I I IIIIIIIIIT'IIIIIIIIIIIIIII I I PIE (Arbitrary Units) lllllllllllLlLllllJllLlLlllllllllLllLlllllllllllllLlJuLlllll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.002100 ENERGY (Electron Volts) Pigure 5A-14-3. PIE of cnc13+ from 011013. 243 WAVELENGTH (Angstroms) 1140 1120 1100 1080 1060 IIIIIITIIITIIIIIIIIIflI‘FIijITIIIIrI'rIII PIE (Arbitrary Units) I LJilJllllllllllJllanLLlllLLlll[lllllgllLLL 10.90 11.10 11.30 11.50 11.70 ENERGY (Electron Volts) Figure 5A-l4-4. PIE of CHCI3+ in the threshold region. Circles denote experimental points. The solid line is the best fit of a Gaussian representation of rotational and instru- mental contributions. The dashed line indicates the position of the corrected ionization potential. 244 WAVELENGTH (Angstroms) 1.3001200 1 100 1000 900 800 700 600 lIIII'IIrI'rIrIlIIIIII I1 I I I T I I l I I T 1 r I PIE (Arbitrary Units) lLlLLlLLllllllLllllllllLLlllllll'Illllllllllllllllllllllllll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.002000 21.00 ENERGY (Electron Volts) Figure 5A—l4—5. PIE of CHC12+ from CHC13. The solid line represents a smooth fit to the experimental data points. 245 CECI, + hv —----s CECI,+ + e‘ the value of AH}o(CHCl,+) is calculated to be 239.7110.8kcal/mole. CIICI,+ The only fragment PIE curve measured from CECl, is shown in Figure 5Ar14-5. The PIE shows steps at threshold and autoionization structure in the post-threshold region. The experimental AP obtained from the PIE curve is 11.4210.04eV. which is slightly lower than and in a good agreement with previous literature values: 11.6410.02eV97 and 11.49r0.02ev.96 From the reaction equation: CECI, + hr --—-—9 01101,+ + C1 + e' the value of AH;O(CHC1,+) is calculated to be 211.34t1.3kcal/mole. B. Discussion 1. The Beats of Formation of Fragment Ions Because of the factors described in chapter four, experimental appearance potentials are upper limits to the desired thermochemical threshold values. Thus the heat of formation of a given fragment ion calculated from an experimental AP is also an upper limit. The lowest value obtained for a given ion is thus considered "best". (1) CF,+ Figure 58-1-1 shows the photoionization efficiency curves of CF,+ from all the precursors studied in this work. Table 53-1-1 lists the heats of formation (OoK) of CF,+ derived from the measured appearence 246 WAVELENGTH (Angstroms) 1300 1 100 900 700 IIIIlIfiIlIIrI'IIIIIIIIIII I I I rI I I r I from CFaCl from CF4 PIE of cr3+ (Arbitrary Units) LL.ULllllLLllllllLLllllllllllIllllllllllllllll'llllllllLIll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 5B-1-l. Photoionization efficiency curves for CF3+ formed by fragmentation of various molecular percursors. 247 Table 5B-1-l. Estimates of AH?O(O°K) for CF3+. PARENT OTHER AH?O(O°K) TECHNIQUE NEUTRAL PRODUCT (kcaI/mole) c1=4 F 110nm P 1 . CHF3 H 10954:“? P 1 a cr301 01 ' 00.34:“ P 1 . CF3Br Br - Gasser: P1. CF3I 1 91.15:” P I a 0.2-31 I 37.3. PI c1301 Cl Mancini) PI CF3CI ' cacao ICR 01:31 1 cal-7.1.1.2 PIPECO 021-} cr 09223.2 . Pl CF3I.CF3 I .- 90.8:h2.8 PI 01-}, laments El CF3OF OF mason-7.0 El *this work. 248 potentials. It is clear from the Table that the stronger the bond which is broken, the greater is the apparent excess energy deposited into the CF,+ ion. The lowest AH}O(CF,+) obtained in this investigation is calculated from the fragmentationi of CF11: the value is 91.1511.7kcal/mole. Table SB-l-l also lists previous literature values, and the techniques used to obtain the primary data. Ajello et al.98 used the PIMS technique. and found AH21300°K)=93.Skcal/mole. Lies and 99 Ausloos used the results of an ICR investigation of 005* + CF,C1 --+ CF,+ + 001 + co. to get the AE}0(CF,*) value 93.8:2.0kcal/mole. Bombach et al..94 from a PIPEco study of 03,1. found 1.100 AH;OICF,+)=96.2r1.2kcal/mole. Walter et a studied photoionization and fragmentation of CzF‘. Their value for the heat of formation of + (99.213.2kca1/m01e) is higher than those obtained from direct bond CF, rupture processes, probably because rearrangement of the parent ion prior to fragmentation into CF,+ + CF is required. Most likely this requires promotion to an excited state of CaF‘+. with the excess energy being deposited in the fragments. The JANAF101 thermochemical tables list AH;O(CF3+)=99.8r2.Skcal/mole: that value was chosen from the average of photoionization determinations from the CF,I and CF, radical. 1.102 studied the IP of CF,, and obtained AH}O(CF,*) equal Syrvatka et a to 107.3711.0kcal/mole. Thyme and Macneil103 investigated CF,OF by the E1 technique; their value for AH}0(CF,+) is almost double all the others reported in the Table. From.an early photoionization study of halomethanes, Noutary123 derived AH;o(CF,+)=87.3kcal/mole. 'Inspection of the original paper reveals that the extrapolation method employed to determine appearance 249 potentials is suspect. and the compilers of the JANAF Table101 ignored Noutary's result in their critical evaluation. From the data at hand, it seems safe to say that the heat of formation of CF,+ lies in the range 90-95kcal/mole. It is probably close to the lowest upper limit calculated from the PIMS experiments in this‘ work: i.e. AH}0(CF,+)$91.15:1.7teil/mole. (2) CF,CI+ Figure 53-1-2 shows the photoionization efficiency curves of CF,Cl+ from all the precursors studied in this work. Table 5B-1-2 lists the heats of formation (Ooh) of CF3Cl+ derived from the measured appearance potentials. It is clear from the Table that the stronger the bond which is broken. the greater is the apparent excess energy deposited into the CF,Cl+ ion. The lowest AH}o(CF,Cl+) obtained in this investigation is calculated from the fragmentation of CF,ClBr; the value is 124.73i2.0kca1/m01e. Table 5B—1-2 also lists previous literature values, and the technique used to obtained the primary data. Ajello98 studied cr,c1,, gave AH}(300°E)(CF,C1+)=130.oreal/uole. Lias and Ausloos99 and found the results of an ICE investigation of the reaction 0,115+ + cr,c1, -+ CF,Cl+ +czn5c1 to get the AH}O(CF,CI+) value l30.0r4.0kcal/mole.- Jochims et a1.92 studied CF3C13. and obtained AH;O(CF3C1+)=132.45kcal/mole. Leyand et a1.104 studied CFIClCF3Cl by electron impact and provided the AH;O(CF3C1+) value 133.712.0kcal/mole. al.1°2 studied cr,=cc1r. by EI: they give 92 Syrvatka et AH;(298°K)(CF3C1+)=130.05kca1/mole. Jochims et al. also studied CF,Cl, and obtained A320(CFzCl+)=l4O.6kcal/mole. From the data at hand. it appears that the heat of formation of CF3C1+ is probably close to the 250 WAVELENGTH (Angstroms) 1.300 1100 900 700 IIII'IIII'IITI'IIIIIITIfrrI I II I I I I r I from CFZClBr from CF21”: from CF30! PIE of CFZCI+ (Arbitrary Units) llllllJllllllllllllllIJhllilLllJlllllllLulIlllllllllllllLJ 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 53-1-2. Photoionization efficiency curves for CF2C1+ formed by fragmentation of various molecular percursors. 251 Table 58-1-2. Estimates of Al-I?O(O°K) for CF2C1+. PARENT OTHER - AH?O(0°I<) TECHNIQUE NEUTRAL' PRODUCT (kcaI/mole) CF30 F 146.631:2.6 ' P I . c1=2CI2 CI 12323120 Pla CFZCIBr a r 12433220 ' P I . CF2CI2 CI - 130.0(st PI . CFZCIZ lso.0sd.o ICR 0.72012 CI 1s2.45 PI CFZCICFZCI CFZCI 1:33:20 :1 CFz-CFCI or ise.os(2se°s) EI CF3CI F 140.0 PI ‘this work. 252 lowest upper limit calculated from the PIMS experiments in this work: i.e. AH}OICF,CI+)$124.73s2.OEcsl/uole. (3) CFCI,+ Figure SD~1-3 shows the photoionization efficiency curves of CFC13+ from all the precursors studied in this work. Table 53-1-3 lists the heats of formation (0°K) of CFCl,+ derived from the measured appearance potentials. It is clear from the Table that the stronger the bond which is broken, the greater is the apparent excess energy deposited into the CFCl,+ ion. The lowest Afl;o(CFCl,+) obtained in this investigation is calculated from the fragmentation of CFCl,; the value. is 168.89:3.1kcal/mole. Table 53-1-3 also lists previous literature values, and the technique used to obtain the primary data. Lisa and Ausloosgg note that no reaction was observed in an ICR investigation of the system sec-C317+ + CFC1., which lead them to determine 93 used the PIES 1.92 AH}0(CPCI,+)=155.0s5.02csl/mole. Ajello et a1. technique to obtain AH}(300°K)(CFClz+)=168.6kcal/mole. Jochims et a used the same technique. and gave AH;0(CFC13+)8170.6kcal/mole. Syrvatka 1.102 used the E1 technique, to provide a heat of formation value at a 170.6kca1/mole at 298°K. From CP,CI,. Ajello et al. obtained AB;o(CFC13+)=18l.9kcal/mole. Jochims et al. also investigated CF1Clz. which gave AH;O(CFCl3+)=189.9kcal/mole. From the data at hand. it seems safe to say that the heat of formation of CFCl,+ lies in the range l67-170kcal/mole. It is probably close to the lowest upper limit calculated from the PIMS experiments in this work; i.e. AH;o(CFCl,+)5168.89t3.lkcal/mole. + 3 (4) CCl 253 WAVELENGTH (Angstroms) 1300 1 100 900 700 IIIIrIIII'IIIIIITrI'IIIIrIj I I r] I r I I I from C’zCIz from 00131? PIE of CFC|2+ (Arbitrary Units) lllllilllllllIllllllllllLLllLJj-llllllllllllljllllLllllllllll 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 53-1-3. Photoionization efficiency curves for CFC12+ formed by fragmentation of various molecular percursors. 254 Table 53-1-3. Estimates of AH§O(0°K) for crc12+ PARENT OTHER -’ AH?O(O°I<) TECHNIQUE NEUTRAL PRODUCT (kcaI/mole) . CFZCIZ F 103.44120 ‘ P I s CFCI3 c1 “8.89:3.1 P I s CPCIZ'I' 153.0ss.0 ICR CFCI3 CI 1sa.e(soo°z) PI CFCI3 c1 170.0 PI CFCI-CFCI CF 17o.ed(2ila°x) E l CPZCIZ . F 101.9 PI CFZCIZ F 100.0 P I 'this work. PIE 0f CCI3+ (Arbitrary Units) 255 WAVELENGTH (Angstroms) 1300 1 100 900 700 IIIIIITIIIrIIIIIIII'IIIII I I I I I T I I I 1 I I from C'ClsBr from 0014 from 6013! WI?" . ‘ IIIlllllllllllllllLllllll[I'LULLIIJIIIIIlIlllJllLLlIlLJl'lU 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 58-1-4. Photoionization efficiency curves for CC13+ formed by fragmentation of various molecular percursors. 256 Table 53-1-4. Estimates of AH?0(O°K) for CCI3+. PARENT OTHER " AH?O(O°I<) TECHNIQUE NEUTRAL PRODUCT (kcoI/mole) CFCI3 F massaz ' P I . c014 c I 213.43.21.15 P I a CCI33r Br scams? P I e 10:10:13.0 10R cc14 c1 208.8(295°K) PI CFCI3 F 217.5(3000x) PI eras F all.“ ' PI ‘this work. 257 Figure 58-1-4 shows the photoionization efficiency curves of CCl,+ from all the precursors studied in this work. Table 58-1-4 lists the heats of formation (0°K) of CCl,+ derived from the measured appearance potentials. It is clear from the Table that the stronger the bond which is broken, the greater is.the appearant excess energy deposited into the CCl,+ ion. The lowest AH}O(CCl,+) obtained in this investigation is calculated from the fragmentation of CC1.Br; the value is 207.78:2.7kcal/mole. Table 5B-l-4 also lists previous literature values, and the technique used to obtain the primary data. Lias and Ausloos used the results from an ICR study to provide 96 used the PINS technique AH;0(CC1,+)=198.027.Okcal/mole. Werner et al. to investigate CCl‘, and gave AH}(298°K)(CCl,+) equal to 208.8kcal/mole. Ajello et al.98 used the same technique. and found. CFCl, obtained AH}(300°E)(cc1,+)=217.5keal/uole. Jochims et al.92 also studied CFCI,, and found AB;0(CFCl,+)=223.54kcal/mole. From the data at hand, it seems safe to say that the heat of formation of CCl,+ lies in the range 206-210kcal/mole. It is probably close to the lowest upper limit calculated from the PIMS experiments in this work: i.e. AE}OICCI,+)$207.7812.7keal/mole. (5) CFzBr+ Figure 58-1-5 shows the photoionization efficiency curves of CFzBr+ from all the precursors studied in this work. Table 53-1-5 lists the heats of formation (0°K) of CFzBr+ derived from the measured appearance potentials. It is clear from the Table that the stronger the bond which is broken, the greater is the apparant excess energy deposited into the CFzBr+ ion. The lowest AH;0(CFzBr+) obtained in this 258 WAVELENGTH (Angstroms) 1300 1 100 900 700 IIIIIIIIIIIIII'IIII'IIII]II I II I I I I l I WWW m. . I mm, from £72618? from 01387 PIE of CFZBr+ (Arbitrary Units) lLlllllllllllllllllLJJLLlLlllLllllllllllllllllllllllilLlLlLl 10.00 11.00 12.00 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 53-1-5. Photoionization efficiency curves for CFzBr+ formed by fragmentation of various molecular percursors. 259 Table 53-1-5. Estimates of AH?O(O°K) for CF2Br+. PARENT OTHER AH?O(O°K) TECHNIQUE NEUTRAL PRODUCT (kcal/mole) CF3Br F 104.7as2.7 P I e CFZCIBr c1 14054120 P I a crzsrz s r Izssasao P I . CF38r F 17:53:20 El . 'this work. 260 investigation is calculated from the fragmentation of CFzBrz: the value is 128.58r2.0kcal/mole. Table 58-1-5 also lists previous literature values, and the technique used to obtained the primary data. Marriott and Craggs105 used EI technique on CF,Br and obtained AH;O(CF,Br+)=175.8t2.9kcal/mole. From the data at hand, it seems safe to say that the heat of formation of CFzBr+ lies in the range 125-140kcal/mole. It is probably close to the lowest upper limit calculated from the PIMS experiments in this work: i.e. au}o(crc1,*)$128.sss2.0tcal/nole. (6) CF,+ Figure 53-1-6 shows the photoionization efficiency curves of CF,+ from all the precursors studied in this work. Table 58-1-6 lists the heats of formation (0°K) of CF,+ derived from the measured appearance potentials. The lowest AB}o(CF,+) obtained in this investigation is calculated from the fragmentation of CF3ClBr; the value is 204.38t2.7kcal/mole. Table 58-1-6 also lists previous literature values, and the technique used to obtain the primary data. Ajello et al.98 used the PIMS technique to investigate both CF,Cla and CF,C1; he obtained the heat of formation values 216.2kcal/mole and 217.4kcal/molc at 300°K. Jochimsg2 used the same technique, and from CF,Cl and CF,Clz obtained AH}0(CF,+)=217.5keal/nole and AHEOICF3+)=221.4kcal/mole. Hildenbrand106 investigated the IP of CF,, and gave the heat of formation as 219.28i1.5kcal/mole. Steele107 studied -CE3F, by EI and obtained AH}O(CF3+)=235.39t9.6kcal/mole. From the data at hand. it seems safe to say that the heat of formation of CF,+ lies in the range ZOO-210 kcal/mole. It is probably close to the lowest upper limit 261 WAVELENGTH (Angstroms) - 900 800 700 600 I I I I r l I T I I 1 I I I T I I from crzczar from CFZC’lz from CF31 from 67331- W i from CF30! W from €74 PIE of CF24” (Arbitrary Units) llllllILLLIlllllllllllllllklllllLlllll_ll 14.00 15.00 16. 00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 53-1-6. Photoionization efficiency curves for CF2+ formed by fragmentation of various molecular percursors. 262 Table 5B-1-6. Estimates of AH?O(0°I<) for 0172+. PARENT OTHER AH?O(O°K) TECHNIQUE NEUTRAL PRODUCT (kcaI/mole) CF4 F . F 213.02234 P I a CF3CI F.CI 2115*” Fla CF3Br F , B r ”8.40:3.5 P I a CF3| F . I 207.041535 P I e- CFZCIZ C I . CI 207.16t3.7 P l e CFZCIBr Cl .Br 204.36%? P I e CFZCIZ CI .01 21s.2(soo'°x) P I CF3CI F .01 217.4(300°K) P l CF 301 F , C! 217.5 P I CF2 21036215 E I CF2012 Cl . C I 221.4 P I CI-IZFZ "2 235391293 E I l"this work. 263 calculated from the PIMS experiments in this work: i.e. AH}OICP,*)$204.36s2.7kcal/mole. (7) CPCI+ Figure 58-1-7 shows the photoionization efficiency curves of CFCl+ from all the precursors studied in this work. Table 58-1-7 lists the heats of formation (0°E) of CFCl+ derived from the measured appearance potentials. The lowest AH;o(CFCl+) obtained in this investigation is calculated from the fragmentation of CFCl,; the value is 231.3518.5kcal/mole. Table 58-1-7 also lists previous literature values, and the technique used to obtain the primary data. AJello et al.98 used the PIMS technique to investigate both CFCl, and CF,Clz and obtained the heat of formation values 242.5kcal/mole and 242.2kcal/mole at 3000K. Syrvatka et a1.102 studied CFz-CFCl and CFClsCFCl: they calculated heat of formation values 256.0:7.0kcal/mole and 262.0:6.0kcal/mole. Horbock and Kisen.97 from 81 on C8F,Cl, supplied the heat of formation value 264.0kcal/mole. From the data at hand. it seems safe to say that the heat of formation of CFCl+ lies in the range 230-240kcal/mole. It is probably close to the lowest upper limit calculated from the PIMS experiments reported in this work: i.e. an}O(CPc1+)$231.35ss.5kcal/nole. + 3 (8) CCl Figure 58-1-8 shows the photoionization efficiency curves of CC13+ from all the precursors studied in this work. Table 58-1-8 lists the heats of formation (0°K) of CCla+ derived from the measured appearance potentials. The lowest AH}O(CCl,+) obtained in this investigation is calculated from the fragmentation of CCl,8r; the value is 264 WAVELENGTH (Angstroms) 900 800 700 600 from €013? from CFzCIBr PIE of CFCI+ (Arbitrary Units) from CFzCIz MVP LlllllllllllllllllllllLllll'llllllllll_Ll 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 58-1-7. Photoionization efficiency curves for CFCl+ formed by fragmentation of various molecular percursors. 265 Table 58-1-7. Estimates of AH?O(0°K) for CFCl+ PARENT OTHER -~ AH?O(O°K) TECHNIQUE NEUTRAL - PRODUCT (kcaI/mole) CF2C|2 F.CI 246.192“ ‘ PI. CcmIBr F . Br masses P I a CFCI3 CI .Cl fil.3&85 PI. CFCI3 c I . c I- 242.5(st P l CFZCIZ F .CI 244.2(soo°x) ‘ PI CFz-CFCI CF2 250.0:73 E I CFCI-CF'CI CFC l 382.0163 E I CHFZCI H.F" 204.0 EI ’this work. 266 WAVELENGTH (Angstroms) 900 800 700 600 from 001331- from €014 from €013! PIE of CCI2+ (Arbitrary Units) llllJJLlILlllllll'lLlLlLLllJllllllilllll 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 ENERGY (Electron Volts) Figure 58-1-8. Photoionization efficiency curves for C015" formed by fragmentation of various molecular percursors. 267 Table 58-1-8. Estimates of AH?O(O°K) for CC12+. PARENT OTHER - AH?O(O°I<) TECHNIQUE NEUTRAL PRODUCT (kcaI/mole) ' CCI3F F .Cl cosmos-.42 . P I . CCI4 CI .CI 211040.122 PI. CCI3Br c I . Br atlases” P I e cal4 Cl .CI 275.ces0.e EI . CFCI3 F.Cl 270.0(3000x) Pl CCIz-CCIZ cc I 2 270.4453 El CI-ICI3 HCI 27036:“ El c012 calms” El CPCI-CI-‘CI CF2 203mm EI ‘this work. 268 253.9812.7kcal/mole. Table 58-1-8 also lists previous literature values. and the technique used to obtain the primary data. Ajello et al.98 studied CFCl, by PIMS and obtained AK;(300°K)(CCl,+)=278kcal/mole. 102 Syrvatka investigated CFCl=CFCl by the BI technique and provided the heat of formation value 283.83t6.8kcal/mole at 298°K. Shapiro and Lossing108 studied CClz, CCl‘, CHCl, and €3Cl‘ by 81. They obtained the heat of formation values: 281.7715.0kcal/mole, 275.6610.56kcal/mole, 279.86r0.35kcal/mole and 279.44rS.7kcal/mole. respectively. From the data at hand. it seems safe to say that the heats of formation of CClz+ lies in the range 250-260kcal/mole. It is probably close to the lowest upper limit calculated from the PIES experiments reported in this work: i.e. AE}0(CCI,*)$253.9ss2.7kcal/nole. 2. Qualitative Consideration of Parent Ion Fragmentation Energetics In chapter three it was noted that three factors which influence the fragment threshold are: (l) the activation energy and heat of reaction, (2) the kinetic shift and (3) competitive reactions. The fragmentation processes of all halomethanes investigated in this work are all highly endothermic. Under these circumstances the activation energy for the dissociation reaction and the heat of the reaction are expected to be similar. In other words, the activation energy for the reverse. highly exothermic recombination reaction is likely to be small. This generalization almost certainly pertains to the ion-radical reactions of interest here.109 The difference between the activation energy and the heat of the forward reaction is distributed to the 269 fragments as translational and internal energy: that difference is probably negligible or small, and thus contributions to the fragmentation threshold energy from factor (1) above will be ignored in the remaining discussion. From the experimental results. it is clear that the excess energy deposited in the decomposition products when only one bond of the parent ion breaks depends strongly on the nature of the bond which is broken. For example, C-F bond scission is accompanied by about 20-30kcal/mole excess energy deposited as either translational or internal energy of the fragments, according to the experimental heats of formation of a given ion from various precursors. For the C-Cl bond, a somewhat smaller comparison indicates an excess energy of about 10-15kcal/mole, while for a C-Br bond rupture the corresponding value is only about 5 kcal/mole. Since the rupture of one bond is generally the lowest or second-lowest energy fragmentation process, there will be few or no competitive reactions. Thus the kinetic shift must be the factor of primary influence in these one-bond fragmentations. The kinetic shift is itself an imprecise term which encompasses many contributions. including the geometry and the lifetime of the parent ion, its complexity and the coupling among its normal modes of vibration, and the nature of the bond(s) broken in the decomposition reaction. The parent ions of the halogenated methanes investigated in this work show highly variable stabilities under uniform experimental conditions (see Table 58-2-1). One can view photoionization as a two-step process involving (vertical) absorption to quasi-discrete states of the neutral. followed by radiationless transfer into the Table 58-2-1. 270 Relative parent ion stability. 1221111122 221221111 Relativg stability Ion This work Literature value [CF, .100] [th0.0ZeV] [8V] cr,c18r 7 11.21 11.83 5.13 CE,CI 12.60 12.39 11.54 CF,Bt 11.76 12.00 51.28 CF31 10.38 10.45t0105 100.00 cr,c1, 11.87 11.7510.04 5.13 CPCI, 11.46 11.7710.01 0.04 cc1,8r 11.05 10.911’ 0.03 cr,sr, 10.98 11.18 6.41 cnP,c1 12.28 12.60 8.33 carol, 11.13 12.00 23.08 0301 11.41 11.37t0.02 69.23 ‘ R.D.Levin and S.G.Lias. Ignizagion gag Appsaraggg‘ggggggigl ggggggggggg, 1971-1981. NSRDS-NBS 71 (1981). b I.Vovak, T.Cvitas and L.Ilasinc. J. Chem. Soc. Faraday Trans. 2. 11. 2049 (1981). 271 'ionization continuum via preionization or predissociation.26 The ionization rate will depend on the symmetries of the states involved (including possible Jahn-Teller distortion), spin-orbit coupling. the applicability of the Born-Oppenheimer approximation, and the stability of the parent ion with respect to fragmentation. If a vertical transition from the ground electronic state of the neutral intersects the repulsive portion of a potential surface describing the reaction coordinate in the (ground or excited state) parent ion, then direct photoionization with dissociation may occur before quasi-equilibrium can be established. This is almost certainl the case for CF‘, where parent ions are not detected and CF,+ + F + e- is the lowest-energy ionization process observed. This mechanism has also been involved to account for the kinetic energy releases measured upon fragmentation of CF,Cl+ and CF,Br+ ions prepared in excited electronic states.110 However. most of the halomethane cation ground states are sufficiently stable that complete PIE curves could be measured. Because fragmentation generally involves significant distortion of the most stable ionic configuration. at threshold it is likely to be a rather slow process. Thus the closer is the geometry of the parent ion to that of the activated complex for the decomposition, the more likely will the products be formed with little excess energy.86 Alternatively. a more stable parent ion is more likely to live long enough to obtain the proper, near-threshold fragmentation geometry which releases little kinetic energy to the products. The CF,+ heats of formation obtained from photoionization of CF3X (X=F.Cl,8r.I) follow this trend: the more 272 stable is the parent ion, the lower is the A820 upper limit calculated from the measured CF,+ appearance potential. For parent ions with similar lifetimes, the kinetic shift from the desired fragment threshold appearance energy will depend on the nature of the bond which is broken. According to quasi-equilibrium theory. excess vibrational energy in the parent ground ionic state is randomly distributed among thefvibrational normal modes. among which it flows in the usual(RRKM) unimolecular decay formulism. For a given number of normal modes, the probability that sufficient vibrational energy flows into a given stretching mode to rupture the bond clearly depends on the bond energy: for a given total internal energy, strong bonds are much less likely to break than are weak bonds. In other words. in order to produce a detectable ion signal in the mass spectrometer. the stronger bond fragmentation process will require more excess energy deposited into original parent ion. This leads to a larger kinetic shift in the threshold for the corresponding fragment ion. and part of this excess energy remains in the dissociation products. Thus CF3Clz and CFzClBr have similar stabilities, but 8r loss shows smaller kinetic energy release. The various contributions to the .kinetic shift cannot be considered independently. For example, CF3Cl+ is more stable than CF3C13 (Table 58-2-1), yet the heat of formation of CF3C1+ calculated from the appearance potential from the dichloro precursor is lower. The larger kinetic shift in the CF3C1+ threshold from the more stable CF,Cl+ parent ion arises from (a) the larger excess energy required to break the more stable C-F bond. and (b) competition from the lower-energy 273 Cl-loss fragmentation process. For fragmentation processes involving breakage of two bonds, for example the production of CF,+ from several different precursors, the calculated values of the heat of formation are rather independent of the composition of the parent ion. That is, unlike one-bond rupture. the excess energy does not depend significantly on which bonds are broken in the fragmentation process. In this case the kinetic shift in its usual sense is not the predominant factor controlling the experimental appearance potential: rather. the shape of the fragment PIE threshold region is governed by the effects of competitive reactions. Since the energy required to produce fragments with two bonds broken is much higher than that needed for any one-bond cleavage, when the former process occurs. the majority of parent ions still dis8ociate through the lower-energy pathways in which only one bond breaks. To produce a detectable amount of two-bond-broken fragment ions enough excess energy has to be deposited into the parent ion to overcome the low rate of production, and the photofragmentation cross section increases only slowly with increasing photon energy above the threshold. This is also a kind of kinetic shift. caused here primarily by the competitive reactions, rather than by vibrational energy redistribution effects. Now the excess energy contained in the excited parent ion is so high that the small differences in the traditional kinetic shift involved in breaking single C-X (X=halogens) bonds are insignificant. In all cases where dihalomethylenc cations are formed upon photoionization of the halomethane precursors studied in this work. thermochemical calculations indicate that the neutral products are two 274 halogen atoms, rather than the dihalogen molecules which would require less energy (lower AP). The same, rather striking behavior has been observed by the Baungrtel group when two halogen bonds are broken in the halogenated ethylenes.111 It is possible that these fragmentation occur directly on an excited electronic state surface of the parent ion under kinetic control. rather than by the GET mechanism described in the proceeding paragraph. Or perhaps the two-bond fragmentation is sequential, with a short-lived trihalomethyl cation being formed first. followed by dissociation of a second C-X bond. One might expect to see evidence of these mechanisms in the PIE curves of the parent or primary fragmentation ions: 8 step corresponding to a new electronic state or a dip in the photoionization cross-section concomitant with the rise of the dihalomethylenc fragment. No obvious correlation of this type is observed (see composite PIE curves for each parent). although the intensity of the dihalomethylene cation is so much lower than that of the trihalomethyl fragments that the effect might not be discernible. Also, the energetics of these fragmentation pathways are likely to be parent-specific, and this is not observed. Thus there is no direct evidence of non-statistical behavior in the observed multi-bond fragmentations. 3. Discussion of the Beats of Formation of Trihalomethyl and Dihalomethylene Cations. The values obtained in this work for the lowest upper limit of the heats of formation of the trihalomethyl and dihalomethylenc cations form a coherent pattern. One can tabulate these data together and compare 275 them with corresponding values for the analogous. neutral trihaloboron and dihaloboron compounds, which are assumed to have the same geometric configuration. Table 58-3-1 lists the heats of formation of all the fluoro-chloro methyl and methylene cations calculated from the photoionization measurements and the values for the corresponding boron halocompounds. obtained from the thermochemical literature. It is clear that the heat of formation values of these cations depend mainly on which kinds and how many halogens are attached to the central carbon atom. For example, a change of one fluorine atom to a chlorine atom in the trihalomethyl cations will increase the heat of formation by about 20-30 keel/mole; in the dihalomethylenc cations this substitution will increase the value of the heat of formation by about 38 kcal/mole. Since other factors such as symmetry. steric effects, local charge density, etc.. also influence the heats of formation. the difference accompanying substitution of chlorine for fluorine is not exactly the same between series. An examination of the haloboron compounds in Table 58-3-1 reveals that they also follow the same trend, although the numerical differences are not the same. The different magnitude is attributed to the different central atom; it is interesting to note (middle column of Table 58-3-1) that the ratio of the difference in the heat of formation of successive members of a given series upon substitution of chlorine for fluorine is approximately constant within each series. Table 58-3-2 gives the heats of formation of the fluoro-bromo methyl cations, methylene cations, and corresponding haloboron compounds. Based on the trends shown by the fluoro-chloro series, one 276 Table 58-3-1. Systematic trends in the heats of formation of chloro- fluorocarbon cations and their isoelectronic boron analogs. ION at,“ Clt,c1" 11201,“ cm," ango“'b’ 91.1511.7 124.7312.o 168.8913.1 207.7812.7 0117mm“) 33.5813.7 44.1615.1 38.8915.8 RATIO or limos 1.48 1.52 1.48 cinnamon“) 49.7015.4 67.0110.o 58.0025.5 1111;0“'°’ -27o.710.4 -221.o15.0 -154.015.0 -96.081o.5 0011mm an, m,c1 8801, 801, ION 1:3,” CFCl+ cm,” 1150“”) 204.3612.7 231.3518.5 253.9812.7 limos") 33.5813.7 44.1625.1 mm or 01mm: 2.42 2.47 DIFFERENCE“) . 65.4110.o 55.9110.0 “10“”) -141.113.o -75.717.o -91.313.o comm an, BFCI 8C1, ' keel/mole b Lowest upper limit. this work ° From JANAF Thermochemical Thbles 277 Table 58-3-2. Systematic trends in the heats of formation of bromo- fluorocarbon cations and their isoelectronic boron analogs. low at,” CF,Br+ Clair,+ CBr,+ da;o“’ 91.1511.7d [139.5] [186.2] 207.7812.7f nrmmcs“'b’ [48.316.51 146.7112.) [47.516.41 2171002011717an 1.59° animation“) 77.0110.4 74.5120.0 75.7110.2 AII;O("°’ -27o.710.4 -193.7110. -119.2110. -43.510.22 comm 82, 811,13.- altar, has, ION cm,“ CFBr+ our,+ Aldo“) 204.3612.7 [257.9] (311.5110)1 DIFFERENCB“'h) 53.6018.1 53.60116. mm on DIFFERENCE 1.49 cinnamon“) 79.5112.o 79.5124.o an;o(‘r°) -141.113.0 -61.5519.h 18.2115. 0011200101 as, Mr lillr3 heal/mole - Estimate value. from different ratio (middle row) From JANAF Thermochemical Tables This work + + Obtained from (BBr,-BF,)/(CBr, -CF, ) A.S.'erner, B.P.Thsi and T.8ae£, J.+Chem. Phys. g9. 3650 (1974). Obtained from (BBra-BF,)/(CBr, -CF, ) Calculated under the assumption that BF, + BBrz is thermoneutral. Ayerage of two values reported by R.I.Reed and '.Sneeden, Trans. Faraday Soc. 51, 301 (1958). WB‘UHOOIOU‘fl 278 can roughly predict the heats of formation of CF38r+, CFBr,+ and CFBr+; these values are not available in the previous literature or thermochemical tables. The procedure by which these three values are estimated is shown in Figure 58-3-1. The predicted values of AHIO for CF38r+, CFBr,+ and CFBr+ are: 139.45 kcal/mole .186.2 kcal/mole and 257.93 heal/mole. respectively. Not surprisingly, the heat of formation change upon substitution of bromine for fluorine is rather different than that observed upon chlorine substitution. There are many differences between chlorine and bromine atoms. such as size, reactivity, electronegativity. etc, which can account for the different ratios observed. The heat of formation values of CFzBr+, CFBr,+ and CFBr+ predicted by the constant-ratio procedure are compared with the values obtained in this work in Table 58-3-3; the agreement is quite good. This implies that the assumptions upon which the predictions were made are sound. The boron/carbon ratioing procedure proposed here has been satisfactorily tested, and it might be expected to yield good qualitative results when applied to other isoelectronic series for which thermochemical data are incomplete. 4. Mechanism for the Formation of Dihalogen Cations Several dihalogen cations were detected in the photoionization mass spectra of the fourteen halomethanes investigated in this work, two of them in sufficient intensity that photoionization efficiency curves could be obtained. Appearance potentials for BrCl+ from CF3ClBr and IF+ from CF31 were extracted from the corresponding PIE curves. Neither 279 From the CF" C13,; and CF" C12,; thermochemical data one may expect: AH:o(BFzBr)‘AH,°é 813); AF“; (BFBrzl-Al-IfQIBI-‘zarlg AH,?,IBBr:)-AH,3 (BFBrz) aH,°.ICstFl-AH,°°ICP;) AHf‘b (crap - 411,310; 811 811,1, (CBr;)'AHf% ICPErp : am; (88%) ~8H,318F,) 8H,; . (cargl - 8H,; (or-'3’) 77. D g- 74.5 g 75.7 AHfgICFzBr‘) - 91.15 .AHagICFBrp-AHfg Icgar”) 233.7 - 811.3 (CF org“) ._.._ 227.2 _ — 142.35 ‘.' '-59 Then AHfgimgsr“)g I395 "“‘..... AHf; (CF81; )g 1862 mKele By The same proesdtlre for CF" 85:, one would find: oH,;ICP81) =5 257.9 "°°%m Figure 58—3-1. Procedure used to estimate the heats of formation of CFzBr+, Clair2 . CFBr list in the Table 58-3-2. 280 Table 58-3-3. Beats of formation (keel/mole) of mixed CF'xBry cations. ION 4. CFBr CP,8r+ + CFBr2 ESTIMATED FROM PREVIOUS TABLE 257.9 139.5 186.2 THIS WORK . 255.93 (from CF CEBr) 2 128.58 (from CF Br 2 2) 140.54 (from CF2C28r) 213.12* (from CFzBrZ) *An activation energy of 20-30 Real/mole is typical in cases involving C-F bond breakage. 281 BrCl+ nor IF+ include the central carbon atom, and the neutral parents have no direct bonding between halogen atoms: thus the mechanism which produces such fragments seems particularly interesting. BrCl+ will be used as an example in the following discussion: a similar reaction mechanism and reaction equation apply to IF+, although the energetic details are of course different. Figure 58-4-1 shows calculations of the heat of formation of BrCl+ according to all possible reaction equations. Comparison with literature values of the heat of formation of BrCl+ from other reliable sources clearly reveals which pathway is correct. The overall reaction equation is: CFICIBr + hr -----> CF, + lit-Cl+ + a“. Possible reaction mechanisms are illustrated in Figure 58-4-2. Mechanism (1) involves rearrangement prior to decomposition. Since the rearrangement requires considerable nuclear motion, parent ions must be produced with significant excess energy. It is postulated (and supported by the step near 14eV in the experimental PIE curves for CFzClBr) that the parent ion is first placed in an excited electronic state which facilitates formation of an incipient three-membered ring, which upon appropriate distribution of vibrational energy can decompose to form BrCl+ and CFz directly. (Whether the three membered ring ion is a true reaction intermediate or an activated complex is not important in this discussion.) This mechanism takes place entirely on the surface of the excited electronic state, in competition with the expected QET 282 AH:(BrCt) a 5.28 kcaI/mole lP(BrC£) = 11.1 eV BrCi + energy - BrCIL+ + e 5.28 kcal/mole II.I eV AH$(8rCI*) - 11.1 x 23.06 + 5.28 . 261.25 (keel/mole) crzczar + hv + [crzj + BrCt+ + e' -I07.66 kcaI/mole 14.63 eV -43.6 kcaI/mole AH:(BrC£+) = 14.63 x 23.06 + (-IO7.66) - (-43.6) = 273.31 (kcaI/mole) ] = cr+r AH$IBrcz*) = 151.25 (kcaI/mole) [—11—1 I—J = c+r2 AH$(8rc1*) - 58.82(kcaI/mole) ("I L—J ll C+F+F AH:(BPC£+) = 22.10 (keel/mole) Figure 58-4-1. 'l'hermochemistry of BrCl+ formation from CFZClBr. Beats of formation of neutral species from Table S-A-l. IP(8rCl) from 1.1.. Franklin. I. 0.Dillard H.M. Rosenstock, J.T. Eerron and K.Draxl, Ioniz- ation Potential, Appearance Potential, g4 Beats 2;, Formation _9_f; Gasous Pogitive long. NSRDS-NBS 26. (1969). an f F I C-Br +hv—- C-Br@ 4-e6 F/ F/\ a ' a F F /$;J~.--Br BrCI F I 9’ F CI' II. Ion-Molecule Reaction 1.- 0501866 I /C-Br+ lur 05%: 3' + e9 F ’ \ CF: 81' +CI CI + 9?. a... CFZCIBr BrCI + Products O 3'2 Figure 58-4-2. flechanisms of BrCl+ formation from CcmlBr. 284 behavior. A necessary condition for any postulated unimolecular step is that it be linearly dependent on the reactant pressure. Thus, mechanism (1) requires that the BrCl+ production rate be directly proportional to the parent neutral pressure. Mechanism (2) involves an ion-molecule reaction: From this mechanism, unlike the first one, other dihalogen positive ions (8r3+, Cl,+) also should be produced. The probability of their formation depends on the probability of producing primary (one broken bond) fragment ions and the relative rate of ion-molecule reaction between those ions and neutral parents. This in turn is governed primarily by thermodynamic factors. Moreover, the rate of production of BrCl+ or other dihalOgen cations will not be linearly dependent on the parent neutral pressure. The results of parent pressure dependence measurements are shown in Figure 58-4-3 for an1+ and 58-4-4 for IF+. An excellent linear relationship between the parent neutral pressure- and the dihalogen cation count rate is observed in each case. These results provide direct and strong evidence Ithat this ion not the product of an ion-molecule reaction. Although only a few dihalogen cations were detected in the PIMS studies reported here, examination of the normal (generally 708V) electron impact mass spectra of the halomethanes reveals that many of them contain dihalogen positive ions.112 The photon energy (<218V) of the PINS excitation is not high enough to produce several of the dihalogen cations, and the intensities of others are too low to obtain PIE curves. Nevertheless, the general observation of products similar 285 150.0 r 140.0 - 130.0 — B 1‘ C 1 + 120.0 — 110.0 — 100.0 - 90.0 - ION COUNTS/MINUTE 80.0 - "I 70.0 - 60.0 - 50.0 - J 0.5 1.0 PRESSURE (MICRONS) Dependence of BrCl+ ion count rate on CFZClBr pressure. Figure 58-4-3. 2.0 286 210.0 - I F+ 200.0 - 190.0 _- 180.0 - 170.0 - 160.0 - 150.0 _ 140.0 - ION COUNTS/MINUTE 130.0 - 120.0 - 110.0 '- 100.0 - I l I l 1 l J I ' l J L ' ' l I I ' ' ' I 1 0.0 0.5 1.0 1.5 2.0 PRESSURE (MICRON) Figure 58-4-4. Dependence of IF+ ion count rate on CF31 pressure. 287 to those predicted by mechanism (1), even for simple molecules such as methane,suggeSt that (providing sufficient energy is available) reaction pathways involving rearrangement and dissociation are common. Rearrangement-d1ssociation mechanisms involving small-ring intermediate are often inkaed in organic mass spectrometry interpretations. There are at least two research groups113'114 using multiphoton dissociation techniques to investigate halogen molecules and their direct elimination from halomethanes. Although these studies concentrate only on the neutral species, it is interesting to note that the proposed elimination mechanism for the analogous dissociations also involve a three-membered ring intermediate. 5. Rationalization of Observed Fragmentation Channels The absence of some possible fragment ions from the halomethane photoionization mass spectra, and the observation of experimental appearance potential values for some other fragment ions which are higher than might be expected deserve comment. Also worth consideration is the identity of which partner in a given dissociation may be expected to carry the positive charge. Both thermodynamic and kinetic effects are important in the interpretation of the experimental results. The tap portion of Figure 58-5-1 gives the appearance potentials observed for alternate charge disposition upon single bond fragmentation of CF3ClBr, CFzI and CF18r. The relative values for Br+ production from CF,Br and CF3ClBr indicate that the C-Br bond is about half an electron volt stronger in the latter molecule. The trend is consistent with the electronegativity difference between Cl and F, which leads to a lower 288 --—-9 CF,C1+ + Br + e‘ AP-11.3oev CF,Clsr + AP --— --—-9 CF,C1 + 8r+ + e‘ AP-15.45ev I-.--9 CF,+ + I + e“ AP-11.11ev CF,I + AP ~ --—-9 CF, + 1* + e’ AP-15.45av ---—9 CF,+ + Br + e' AF-11.18ev CF,Br + AP L---9 CF, + 8r+ + e' AP-14.90eV ---9 CF,+ + BrCl + e“ AF-[13.76av1 CF,ClBr + AP —; CF, + Bret+ + e' AP-14.63ev ---e CF,+ + Br + C1 + e‘ AP-15.99ev F""’ CF,+ + IF + e' AP-[13.94ev1 CF,I + AP —5 CF, + IF+ + e‘ 12-14.12ev P—-——e CF,+ + I + F + e‘ AP-16.94ev Note: numbers in [ ] are calculated values. Figure 58-5-1. Appearance potentials for competitive fragmentations of selected halomethanes. Top: Alternative charge disposition upon single bond breakage. Bottom: Possible reactions involving two broken bonds. 289 electron density at carbon in CF,Br, and therefore a weaker C-Br bond. As noted earlier, the experimental results for decomposition reactions which involve breaking one bond of the parent ion are consistent with the expectations of quasi-equilibrium theory. That is, a photoexcited parent ion in an excited electronic state will, by internal conversion, become a (highly) vibrationally excited ion on the ground electronic state surface and then follow fragmentation channels which are determined by the thermodynamic stability of the products. under thermodynamic control, the lowest energy dissociation path should predominate. Both theoretically and experimentally it is expected that the halocarbonium ion should be more stable than the halogen cation, and the experimental results for the three molecules cited substantiate this prediction. It is interesting to note that Br+ was not detected in. an earlier photoionization study of CF,Br.115 The situation is more complicated when two bands of the parent ion are broken upon fragmentation. As illustrated for CF,ClBr and CF,I in the lower portion of Figure‘ 58-5-1, three possible fragmentation reactions merit consideration in each case. (Rupture of two C-F bonds in CF,I, or one C-F bond plus a C-Br or C-Cl bond in CF,ClBr is unlikely on energetic grounds. For example, about 76 kcal/mole more energy is required to break two C-F bonds in CF,I than is needed to break one C-F bond and one C-I band.116 A similar energy difference would apply to CF,C18r.) The approximate appearance potential for each of these competitive processes can be calculated from tabulated thermochemical data; the expected values fall in the 14-l7eV range, with fragmentation to form of CF,+ plus a neutral dihalogen molecule being the lowest 290 energy path in both cases. Yet CF,+ formed by this mechanism was not detected from either CF,ClBr or CF,I. Rather, formation of the dihalogen cation is the lowest energy fragmentation involving the rupture of two bonds. CF,+ is observed only at higher photon excitation energies, under conditions where two halogen atoms are the likely neutral fragments. These results can be rationalized by the model that follows. It is assumed that all reactions leading to scission of two bonds require photon energy at least equal to the excitation energy of an excited electronic state of the ion, an electronic state (above 148V) which facilitates formation of the three-membered ring described in the previous subsection. According to GET, parent ions in this electronic state should form vibrationally-excited ground state ions, which dissociate under thermodynamic Control as noted in the previous paragraph. However, if a competing fragmentation process can occur prior to the establishment of the "quasi-equilibrium", i.e. within a very few vibrational periods, then the products of the alternate mechanism might be observed. This is an example of kinetic control, and the ion count rates from such fragmentation processes are expected to be very much lower than those of products of the thermodynamically favored competitive pathways. Consider CFzClBr as an example. The positive charge on the electronically-excited parent ion most probably resides on the bromine site. If the C-Cl and C-Br bonds are broken during the first few vibrational periods following photoexcitation to the excited electronic State of the ion near l4eV, then neither QET nor charge redistribution 291 will be established, and fragmentation to form DrCl+ may occur. A schematic sequence depicting this mechanism is shown in Figure 58-5-2. (The lower energy path by which CF,+ plus a neutral dihalogen are formed is temporally not competitive with the GET fragmentations, which involve breaking only one band, with the excess energy being deposited in the fragments.) Why then is CF,+ observed at high appearance potentials? Possibly a new excited state of the ion is reached which involves nuclear rearrangement conducive to formation of CF,+ and two halogen atoms under kinetic Control. Or perhaps QET is Operative, and the fragmentation to form CF,+ occurs on the ground electronic state surface.' At these photon energies (16-178V), so much excess energy(on the order of 68V) is deposited in the fragments that sufficient internal energy resides in the putative dihalogen neutral to cause dissociation into neutral atomic partners, or perhaps the two-step fragmentation mentioned in section 5.8.2. takes place. In summary, a reaction can occur on the potential surface of an excited electronic state of an ion, provided that the products form before internal conversion is established. The details of the nuclear dynamics are very important under these kinetic control conditions. On the other hand, most decompositions occur on the ground electronic state surface of the parent ion. Then the predictions of QET are valid, and the competition among alternative fragmentation channels will the determined by energetic considerations, i.e. thermodynamic control. Thus CF,+ is not observed from the lower-energy neutral dihalogen pathway because the dynamic requirements on the excited-state surface 292 F/ c l + \ 7m F>C A CI F\C/Br F'efl"" I“\~1:I-~g‘~“-. :} ‘+ F’-..-(:',,——i?r P/ \9CI ‘ ‘+ Fr“-..c, +' 13? I F/ CI Figure 58-5-2. Schematic reepregentation of vibration:l motion leading to BrCl formation on CFZClBr excited electronic state potential surface. 293 cannot compete efficiently with internal conversion, whereas lower energy fragmentations will dominate the decomposition reactions in the ionic ground state. 294 CHAPTER SIX SUMMARY AND SUGGESTIONS FOR FURTHER WORK A. Summary Fourteen halogenated methanes have been investigated by photoionization mass spectrometry. Ionization potentials for eleven parent ions and appearance potentials for all abundent fragment cations were obtained. These data, combined with other necessary thermochemical information, were used to calculate the heats of formation of the ions. These are direct, short-range applications. The experimental data are good: they are reliable and fairly precise. Undoubtedly other scientists will use these results in broader contexts. From comparison of heats of formation of a given ion from a series of precursors, the "best" value (i.e. lowest upper limit) has been determined for several fragment ions. These heat of formation values are sensibly related to those of the isoelectronic neutral boron compounds. The threshold behavior of fragment PIE curves has been described in terms of quasi-equilibrium theory. Where one bond is broken the kinetic shift governs the value of the extrapolated appearance potential. Competitive reactions involving lower-energy channels dominate the thresholds for fragment ions involving cleavage of two bonds in the parent ion. A Special fragmentation pathway has been prOposed for the formation of dihalogen cations, and a discussion of alternative decomposition pathways for a given pair of dissociation 295 partners, in which the location of the positive charge is controlled by either thermodynamic or kinetic considerations, has been presented. 8. Suggestions for Further Work 1. Without Change in the Instrument (1) Theoretical Investigation An almost complete experimental investigation of the ionization and fragmentation of fourteen different halomethanes has been carried out: the results have been presented and discussed in chapter five. The spectroscopic study most likely provides more accurate transition energies and thermodynamic information for these ions than one could currently calculate theoretically. However, the understanding of the relative stabilities of the ions is less well founded, and a theoretical study would prove most useful. Theoretical predictions similar to those advanced on experimental grounds would enhance the reliability of the interpretations postulated here, and would provide more detailed understanding of the factors which influence the experimental results. A possible theoretical project would:(1) find the lowest energy of the neutral and ionic compounds: (2) determine the corresponding molebular electronic structure and nuclear geometry: and (3) evaluate possible excited state potential surfaces, in order to substantiate or negate the conclusions drawn here on the basis of the experimental observations. (2) Beats of formation The primary advantage of PIM is the precise determination it provides of IPs and APs. If thermochemical information for a class of 296 compounds of interest is incomplete or imprecise, PIMS remains a useful tool, although for a given fragment ion only an upper limit to the desired heat of formation can be established. Among the compounds of choice in new studies are those of aeronomical interest, since VUV radiation is particularly important in atmospheric chemistry. (3) Parent Ion Stability Only a qualitative discussion of parent ion stability is possible at this time because all fourteen molecules studied in this work contain a central carbon atom. A natural extension would involve molecules contining Si, Ge, and/or Ti as the central atom. Stability trends could than be followed as a function of the central atom, and as the halogen atom distribution is altered. More complete experimental results would be quite useful, and if they were combined with theoretical predictions of parent and fragment ion stabilities, more quantitative conclusions regarding halomethane ion chemistry could be drawn. 2. With a Small Change to the Instrument (1) Temperature Control of the Sample Inlet and Ion Source Systems The PINS instrument currently can accept only samples in the gas phase or those of liquids with moderate vapor pressure at room temperature. If the sample handling system and the ion source were modified to have temperature flexibility, then the capability would be extended to include the study of almost all compounds. In addition, with the availability of a temperature control system, the temperature dependence of ionization and fragmentation processes could be followed. 297 The most obvious application is the determination of the hot band contribution to the threshold region of experimental PIEs. In the investigation of some reaction mechanisms to determine the thermodynamic or kinetic contral of the reaction rate, a temperature dependence study could also play an important role. (2) Negative Ion Detection With only slight changes in the electronics which bias the ion detector, the MSU PIMS apparatus would have the capability to detect negative ions.89 Ion-pair formation following photoexcitation of the halomethanes is an interesting possibility,117 especially since the relative values of the pertinent electron affinities and bond energies are such that this reaction might occur at an energy below the molecular ionization potential. Compared with dirhct ionization, the intensity of ion-pair formation reactions is rather low, and contributions from this mechanism are difficult to discern in a normal (cation) PIE curve. However, if negative ions are monitored directly, the contribution from ion-pair formation will be readily apparent, since other sources of negative ions (e.g. electron attachment) are not important in photoionization experiments. A study of ion-pair formation in the halomethanes would make this investigation complete. (3) Addition of a Second Reaction Chamber. One advantage of PIES is that precise control of the source energy is provided; thus for simple atoms or molecules, PIES can produce predictable ions with relatively well-defined internal energy. Addition of a second chamber to the interaction region would allow ion-molecule reactions to be studied in which the products of the reaction of 298 "state-selected” ions from the first chamber of the ion source with neutral molecules in the second chamber could be monitored mass spectrometrically. An important application is to reactions which might occur in the troposphere or stratosphere. This kind of study would be of great help to the understanding of ion-molecule reaction theory. (4) Laser-Induced Fluorescence of Ions . Laser induced fluorescence measurements, which provide information about excited electronic states, could be carried out on the ions produced by photoionization--provided that the ion production rate is sufficiently high. 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