INFRARED SPECI‘RUM OF HYDROGEN SULFIDE IN THE 3795 CM“1 REGION THESIS FOR THE DEGREE OF M. Sn MICHIGAN STATE COLLEGE CARLETC‘N MCNEAL SAVAGE 1955 0-169 "I” “H" ‘MIiI'H'fiWISiA1lE i'iRIARIES _ 31293 01774 5591 LIBRARY Mlchfgan State I Universlty This is to certify that the thesis entitled Infrared Spectrum of Hydrogen Sulfide in the 3795 cm-1 Region presented by Carleton McNeal Savage has been accepted towards fulfillment of the requirements for M. S. degree in PhYSiCS 8. L9. 74% Major professor Date_A_plil ‘5’ 1955 w- x-)- PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE DATE DUE 1/98 cJCIRCJDateDue.p65-p.14 lNFRARED SPECTRUM OF HYDROGEN SULFIDE 1 IN THE 3795 cm‘ REGION by Carleton MCNeal Savage A THESTS Submitted to the School of Graduate Studies of Michigan State College of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Physics and Astronomy 1955 To Joan 354781 ACKROWLEDGEMENTS The author wishes to express his thanks to Dr. H. C. Allen, Jr. for suggesting the problem. He is also greatly indebted to Dr. C. D. Hause and Dr. T. H. Edwards for their guidance and assistance in carrying out the work. The writer deeply appreciates the financial support of the Research Corporation during the Fall and Winter of 1954—55. WMSa/maé INFRARED SPECQRUM OF HYDROGEN SULFIDE IN THE 3795 our; REGION by Carleton.McNeel Savage .AN'ABSTRAC! Submitted to the School of Graduate Studies oflMichignn State College of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASEER OF SCIENCE Department of Physics and Astronomy 1955 Approved" 51. (S): )§/;L¢4¢1‘£;=1 Carleton M. Savage ABSTRACT The absorption.of hydrogen.sulfide in the region from 3730 to 4030 cm-1 The absorption has been assigned partly to an.A-type.band (n1, n2, n3) 3 (0,1,1) and partly to a.B-type band.(1,l,0). The absorption was measured at pressures between 8 and has been.measured under high resolution . 14 cm of Hg with a path length of 285 cm.using a.mnltiple traverse cell. The spectral slit width was between .15 and .20 cmTl. The spectrograph was a self-recording, vacuum, grating insturment built by R. H. Noble and recently improved by the installation of a new Bausch and Lomb 15,000 line per inch grating and a Baird Associates 450 cycle amplifier and phase-sensitive detector. The source was an argon filled zirconium concentrated arc lamp. The detector was.an.Eastman Kodak lead sulfide photcondueting detector. Calibratfh.was made using‘Fabry-Perot interference fringes to give divisions of equal wave number spacing between two argon lines used as standards. The analysis was made through the use of published energy tables for the rigid rotor. A classical centrifugal distortion correction was applied to the rigid.rotor energy levels. A least squares fit of observed lines to assigned transitions, assuming known ground state energy levels, gives for the excited state inertial parameters (0,1,1) band, A = 10.517 cm'1 1 1 cm'lg (1,1,0) band, A = 10.595 emf 4.603 cm'1,'Vb = 3779.23 cm’l. , a = 9.124 cm‘ , c = 4.619 cm? 1 9 yo = 3789.07 , a = 8.985 cm‘l, c = TABLE E CONTENTS Introduction Apparatus Grating Auxiliary vacuum tank Arc lamp Absorption cell Interferometer Chopper motor Detector Cathode follower Amplifier Experimental Calibration Measurement of lines Ground state energies Calculations and results Trial constants Energy level calculations Centrifugal distortion correction Calculation of d, p, and 6’ . Transition calculation Symmetry and selection rules Intensity Assignment of transitions Page -4 ~J .e TABLE OF CONTENTS (CONT'D) Least squares calculation Calculation of B band Conclusion Tables of results Bibliography Page 36 39 41 42 61 LIST 9; FIGURES H S Molecule 2 Normal vibrations of the H28 molecule The Optical system of the spectrometer The Optical system of the absorption cell in use. The Optical system of the Fabry-Perot interfer— ometer in position. Cathode follower circuit. Block diagram of the 450 cycle amplifier. Comparison of observed and calculated spectra. Page CDUTVJ 13 15 38 LIST OF TAELBS. Table Page I. Constants of the H28 molecule 42 II. Rotational energy levels (cm-l) of the (000), (011) and (110) vibration states 43 III. Comparison of Observed and calculated Spectra in the 2.5}.region 48 PHOTOGRAPHS Photo 1. Typical fringes 10 2. Spectrometer 16 3. Auxiliary tank and absorption cell 17 l 4., 5., 6. H28 Spectra from 3730 to 4030 cm— 19 7., 8. .Typical sections of spectra as measured. 23 INTRODUCTION Hydrogen sulfide (H28) is an asymmetric top rotator simi- lar to the H20 molecule. Much work has been done with both low and high resolution in the photographic infrared and the nearinfrared region1 to 7. The absorption near 2.5 microns was observed and re— ported in 1931 by H. H. Nielsen.l Since no analysis has been reported in the literature, the purpose of this paper is to present an analysis of the rotational fine structure in this region. The absorption has been assigned to the combination bands (011) and (110). This notation represents the vibra- tional quantum numbers of the upper state involved in the transitions.- The lower state would be designated (000). Thus the (011) band consists of transitions from the ground vibrational state to a vibration of theV2 andifs modes. An alternate method of labeling found in some papers gives the vibrations in the order (V5,L’1,1’é) and the vibrational quantum numbers are designated by Ina, n”, n5) following the vearly notation used in the studies of H20. The accepted molecular model for H28 is shown in Figure l. The three fundamental modes of vibration are shown in Figure 2. The modesif andtfé are symmetric and)!”3 is antisymmetric with 1 respect to rotation about the symmetry axis (B axis). B (Symmetry axis) 2 Constants for ground state . e = 92° 20' r = 1.345 A IA = 2.667 x 10"40 g cm2 IB = 3.076 x 10’40 g cmz IC = 5.845 x 10-40 g cm2 Figure 1. H28 Molecule. \\/ \/ V’Or ‘W VE or é, / 1 2635 cm- 1189 cm— / \ Vora— 2651 cm- Figure 2. Normal vibrations of the H23 molecule (not to scale) APPARATUS A self-recording, vacuum, prism-grating spectrometer constructed by R. H. Noble8 and described in a thesis by N. L. Nichols9 was used for this work. The portion of the Optical system enclosed in the principal vacuum tank is shown in Figure 3. It includes a foreoptic section, monochromhter and detector. The foreprism was not used, being replaced by a plane, front surfaced mirror. Under these circumstances, the second order argon emission lines used in calibration appeared directly on all the records. The new Bausch and Lomb 15,000 line per inch grating, purchased by the Research Corporation, was used for this work. The grating has a ruled area of about 6" x 8" and is blazed at about 27°. The first order wave length at the blaze is 1.5 microns. At first it was not expected that this grating could be used in the region of 2.5 microns (an angle of 490 to 55°); however, it was found to give better results than the 7,200 line per inch grating originally tried. (The 7,200 line grating is blazed at 15° and has a useable ruled area of 4” x 5".) Although the vacuum feature of the spectrograph greatly reduced the atmOSpheric water vapOr absorption inside the tank, much absorption was found to take place in the Optical path before entering the Spectrograph proper. A cardboard box was constructed around the external Optical section, and parabaloidal mirrors t F’ , ‘ grating :::> l ' l chOpper ‘ \ motor / ‘ ii! . flat % (mirrors ~./‘ ‘» . exit entrancgiiiis::3' C::;;ig;$Lfilit , fix} _' detector \ J 4 ' ‘(nri‘s'ni' parabaloida I. ¢ collimati , mirror’ l ;’ Figure 3. The Optical system / of the spectrometer. a solution of CO2 dissolved in acetanewas put into a copper vessel designed to freeze out the H20 in this part of the appa- ratus. This was found to reduce the water absorption greatly and showed that most of the absorption took place in the Optical path outside the tank. An auxiliary steel tank was constructed to house the absorption cell and its necessary lenses. Two quartz windows were placed in the tank and were so designed that the concentrated zirconium arc lamp used as a source could be placed in contact with the window. This allowed the lamp to be Operated outside the vacuum system where it might be cooled easily and moved for slight optical adjust- ments without introducing additional atmOSpheric absorption. Provision was made so that the absorption cell might be filled without removing it from the spectrOgraph, thus making it unnecessary to let air into the tank whenever the pressure or material in the cell was to be changed. The auxiliary tank was connected to the main spectrograph at the light entrance port of the spectrograph. The quartz window was removed, thus making it possible to evacuate the auxiliary tank with the same pump as that used to evacuate the spectrograph. A pressure Of about .2 mm of Hg could be maintained throughout the system when pumping steadily. A zirconium concentrated arc lamp was used to obtain the spectrum. This lamp is argon filled, and the argon emission lines were used for calibration. An attempt was made to use a carbon are as a source but it was found to be too unsteady for this type of instrument. The absorption cell, a multiple reflection system, is also described by Nicholsg, The absorption cell is made of brass and the inside is readily attacked by the H S. To 2 minimize this, the inside was painted with clear glyptal and baked to give a hard surface. Several coats of glyptal were applied and baked. The absorption cell had been previously used with D01 and the mirrors were badly corroded. It was therefore necessary to re-aluminize the mirrors before this work could be carried out. Figure 4 shows the absorption cell as used in the work. The source is shown at the side window of the auxiliary tank. For calibration of the spectrograph, interference fringes produced by a Fabry-Perot interferometer were used. The Optical setup of the interferometer is shown in Figure 5. The source is shown at the end window of the auxiliary tank. Typical fringes are shown in Photograph 1. The plates of the interferometer were adjusted for parallelism by use of Haidinger fringes. That is, the interferometer is placed in front of an extended monochromatic source, and by looking toward the source through the interferometer a ring pattern of fringes is observed. If the plates Of the interferometer are parallel, this pattern will not change as the eye is moved vertically or horizontally across the field of view. If the plates are not parallel, the fringes seem to "expand out of" or "collapse into" the center of the pattern as the eye is moved across the field. mirrors 2 \m (mm (8 traversals) 12.5 cm . uartZ‘s Qb-K q p \ 7 lenses kmirror concentra -d arc lamp \ i -[fore slit of c::rc::i spectrOgraph Figure 4. The Optical system of the absorption cell in use. concentrated arc lamp R~ 7.5 cm quartz lens g Fabry-Perot ,’ I. etalon 0‘ K20 cm quartz I / lens fore slit Of Spectrograph The optical system of the Fabry-Berot interferometer in position. Figure 5. mmozEu 29.25340 Adoit The 2.88. _ I. 10 For proper alignment in the tank, the interferometer is placed in position without the lenses. By looking at the source through a circular hole at the spectrograph slit, multiple images of the source can be seen if the interferometer is at an angle with the line of light (five images could be seen with this setup). The interferometer is rotated about a vertical axis until the multiple images are in a vertical line. Then by rotating about a horizontal axis, the images may be made to overlap. The lenses are then put into place. The light is collimated for visible light. This does not colli- mate it for the infrared however. Therefore, the lenses are moved by amounts calculated from the indices of refraction of quartz for visible and infrared. Although the chopper motor was not designed to operate in a vacuum, it was decided to install it inside the evacuated spectrograph. The grease was removed from the bearings and replaced with Dow Corning high vacuum grease (a silicone lubricant). At one stage of the work considerable trouble was experienced with the fuses blowing for the circuits connected to the apparatus in the evacuated tank. Photographic film was placed at various places in the tank near motors, termi— nals, etc. where are discharges might occur. A number of the film strips were blackened when the spectrograph was evacuated and the fuses purposely blown. A complete rewiring job was done on the spectrograph. The electrical equipment inside 11 the tank consists of a selsyn1x>adjust the slit widths, a selsyn to rotate the prism table, a selsyn to rotate the grating, the chopper motor and the cathode follower circuit. Although it helped, the rewiring did not completely eliminate the fuse blowing. The chOpper motor was suspected and removed, and the area near the starter coil switch was cleaned and painted with clear glyptal. The apparatus has worked satis- factorily thereafter. From previous work on the instrument, it was found to ihave a nonlinearity of about .4 percent between the marker signals placed every .10 on the record. The transmission on the grating drive was dismantled and a badly worn fiber gear was replaced. The cone bearing on which the grating rotates was also removed and relapped. This gave an improve- ment by a factor of four. The detector, amplifier and recorder now in use con- sists of an Eastman Kodak lead sulfide cell with a cross- sectional area of 1/2 by 10 mm, together with a new 450 cycle amplifier which replaced the original 10 cycle equipment 9 and a Speedomax recorder. The detector has two matched sections of about 5 megohms impedance each, one to be used as a photoconductive detector and the other section as a load resistor. This has the advantage that when the cell is cooled the two sections re- main matched. The impedance of the PbS cell did not match the impedance of the amplifier and it was therefore necessary 12 45V+ PbS cell oC4 '001/ifd 33 K 45 V + .25 llfd Meg. 50 Meg. 680 K “AAAAAAL V'V‘VV .3ilpfd out put ..|‘. Figure 6. Cathode follower circuit. 13 a 45v- to construct a cathode follower circuit as shown in Figure 6. This was constructed to be Operated inside the vacuum tank as near as possible to the PbS cell. Tests were run on the effects of cooling the lead sulfide cell, and it was found that cooling did not help in this particular installation. The signal was increased by about 50 percent during the initial stages of cooling, but when equilibrium was reached the energy was actually less than at room temperature. It was there- fore decided when the cathode follower was constructed not to cool the cell and to use only half of the PbS cell as a detector and to use a White resistor as a load resistor. This was done in an attempt to minimize inherent noise from the PbS Cell. The cathode follower feeds into a 450 cycle amplifier and phase-sensitive detector, Model CM 2—3, made by Baird Associates, Inc. A block diagram of this instrument is shown in Figure 7. Its input impedance is on the order of 0.68 megohms. Included but not shown in the diagram of the amplifier is a voltage regulated power supply and a high voltage power supply for a photomultiplier tube. The reference input signal is generated by a small synchronous generator mounted on the shaft of the light chopper motor such that by rotating the generator with respect to the chopper the phase between the signal and reference voltages can be adjusted for best rectification of the signal. The output of the amplifier is fed into the Speedomax recorder. 14 Twin "T“ filter ' A Signal 6SJ7 6SJ7 G 68J7 in ah _,ya .J amplifier__. amplifierp_.i mg amplifier L i n G 6SJ7 a amplifier i n 6SN7 h Phase R c pus fie pull a» COHIPB~IVEUSOI‘—1 filter jfower to recorder amplifij?" 6SH7 reference amplifier H reference signal in Figure 7. Block diagram of 450 cycle amplifier. 15 Spectrometer 15 Auxiliary tank and Absorption cell 17 EXPERIMENTAL The hydrogen sulfide was obtained from the Matheson Company. As the cylinder was nearly empty, we were limited in the amount of H23 that could be put into the absorption cell. The maximum pressure that could be obtained was 14 cm of Hg. As the H23 contained a considerable amount of water vapor, the shipping container was cooled in an acetone and dry ice bath; and the H28 was passed through a similarly cooled U-shaped trap filled with glass beads. Even with these pre- cautions an appreciable amount of water absorption was recorded. For the absorption expected in the region of 2.5 microns, little energy was available with this apparatus. There are several reasons for this. First, the black body radiation of the arc is rapidly falling off. Second, the glass envelope of the arc lamp, although thin, transmits less in this region. Third, the angle of the grating used (490 to 550) results in less energy. Fourth, the sensitivity of the PbS cell is decreasing. As mentioned before, cooling did not result in the expected increase in signal. The total effect of the aforementioned resulted in a spectrum of inn; ”downhill" type as shown in Photographs4 , EBandéS. For the narrow slit widths necessary to resolve these 1 lines, the energy was so low below 3730 cm- that a suitable spectrum could not be recorded. In addition, water absorption 18 a NO. v.02 0 ”k.” - q u 7.6%.. 1.5.3 .EJm Admhomam SommN Ea ZOEQm—Ommd or 3v. wmnmmmma zoCamomme mu: 19 who» n~mn 21 in this region is especially intense. For these reasons, much of the low frequency side of the bands could not be observed. The spectrum was obtained under the following conditions. Eight traversals in the absorption cell gave a path length of 285 em. Five runs were made with pressures from a to 14 cm of Hg (14 was the total pressure obtainable). The slit widths were from 95 to 135 microns. The grating was turned at a speed of about 1.25 degrees of are per hour. The chart paper_was fed at 120 inches per hour. A run from 5750 to 4030 cm"1 took about five hours and resulted in a record fifty feet long. Sections of a typical run as they were used for measurements are shown in PhotOgraphs 7 and a. 22 -- was? :55, 5m gum—am : iuao aha TI:...L u4 (4) where T is the sublevel of the J quantum state. There are 2J + 1 values of T running from.T = -J to 1': +J. We can find the values for positive K as follows. Since K is positive, we use the value of 4_3. For this level the energies from the table are given as E (K) = E (—.4) = -15.63481 E (-.5) = ~15.98528 E (-.6) = -l6.35224 so that by relation (4) s (.4) = 15.65481 E (.5) = 15.98528 E (.6) = 16.55224 Using quadratic interpolation between these three points, we get 43 = a(x) + Mac)2 (5) where.e,is the difference between two given E (K) values. x is 10 times the difference between the K's. To find a and b A]. = a + b (X z 1) A2 = 2a 4» 4b (x = 2) Al is the difference between E (.4) and E (.5); A2 is the 30 difference between E (.4) and E (.6). Hence .35047 = a + b Solving these two equations one gets a = +.342225 b = +.008245 s (K) = r (.4) + a(x) + b(x)2 (6) ForK = .5348, x = 1.348 = 10(.5348- .4) E (.5348) = 16.111 cm'l From equation (2) E = (——-—A‘2*C)(J2+J) + (———1A'2'C E(K) E 198.747 cm.1 (for 4 level) 3 At this point a classical centrifugal distortion correc- tion is applied. Using the method of Cross 2 , it is assumed that the molecule is rotating as would a classical rotator having the same values of o< , p and K ;where 0< , f3 and X represent the component of the square of the angular momentum along the A, B and 0 axis respectively. Hence o<+ fl+ X = (J2+J) Thus the energy may be divided into three components. E = o(A + p B + X'C and ' _ «BE - as -ee °<" 311' (3" 3B and 'ac Since the center of gravity is so near the sulfur atom, the further assumption is made that the rotation takes place about the S atom. Using these assumptions, the total change 31 in energy due to rotational distortion is shown to be .2 _ E («A tan 4:— p B cot ck )2 ~16 where -16 —1) 1.963 x 10 = ho (to reduce to dimensions cm E = energy as calculated from equation (2) PT ll 1 4.2 x 105 dynes cm.1 (molecular force constant) k3 = 4,1 x 104 dynes cm-l (molecular force constant) r = 1.361 A ([3 =460 30' <+ bp+ OX (10) was set up, where ZXE is the difference betwaalthe Observed and calculated value of the transition energy in cm-1. .AL/o is a change in the band center, and SE: _ ép _ «53 SA fi‘ 513 X‘S'é (for explicit expressions, see page 32 ). a, b, and c are (X: changes in the rotational constants. (‘3VO’ a, b, and c are undetermined coefficients.) To bring the Observed and calculated frequencies into agreement, the set Of equations Obtained from equation (10) was solved for zavb, a, b, and c by the least squares method. These increments were added to the original assumed values and new rotational energy levels were calculated. A new calculated spectrum was prepared and compared with the Observed spectrum. Many lines in the calculated band were shifted in 35 location so that more transitions could now be assigned and another least squares fit carried out. The process should be repeated until the change in the increments is negligible. The number of times depends on the original estimate of the rotational constants and on the correct assignment Of lines. In each case here it was done twice. For a molecule for which the ground states were unknown, equation (10)becomes AE=£wb+a}u' +bfl3'+c'K'-ad—bfi—OX where the primed values are for the excited states and the unprimed for the ground state. Least Squares To solve a series of equation Of the type AE =AV0+O