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MICHIGAN STAT

III IIIII III IIII IIIII IIIIIIIIIIIIIIIII

 

 

 

 

 

 

This is to certify that the

dissertation entitled

A High Performance Ion Beam Instrument for the
Investigation of Ion/Molecule Reaction Energetics

presented by

Paul Jude O'Connor

has been accepted towards fulfillment
of the requirements for

Ph.D. degree“, Analytical Chemistry

 

 

{/QfiAfl/m

Major professor

Date H- 8' 7/

 

MS U is an Affirmative Action/Equal Opportunity Institution 0- 12771

 

 

r’ ‘-
LIBRARY
Michigan State

University I

 

 

\-

PLACE IN RETURN BOX to remove We checkout from your record.
TO AVOID FINES return on or before due due.

  

DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

II I—I

MSU le An Affirmetive ActlorVEquel Opportunity Institution
ammo-pi

 

 

 

 

A HIGH PERFORMANCE ION BEAM INSTRUMENI‘
FOR THE INVESTIGATION OF
ION / MOLECULE REACTION ENERGETICS

by

Paul Jude O'Connor

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1991

ABSTRACT

A HIGH PERFORMANCE ION BEAM INSTRUMENT
FOR THE INVESTIGATION OF
ION/ MOLECULE REACTION ENERGETICS

By

Paul Jude O'Connor

Gas-phase ion / molecule investigations provide a means of probing the
most fundamental of chemical interactions in the absence of solvent or
bulk matrix effects. The experimental approach used in the study of
ion/ molecule systems is dependent on the information desired. One type
of experiment involves focusing a mass selected beam of monoenergetic
ions into a collision zone which contains neutral target atoms or
molecules. These so-called ion beam methods may be used to probe the
intimate mechanistic and energetic details of ion/ neutral interactions.

This dissertation describes the design and performance
characterization of an apparatus designed to provide well-defined
ion/ molecule interactions with respect to the internal and translational
energy of the reactants. The kinetic energy of the primary ion beam can
be varied so that the energy dependent behavior (reactivity) of
ion/molecule reactions, including endothermic processes. may be
studied. Ions may be generated using conventional electron ionization
(E1) or a high pressure drift tube (HPDT) source. The HPDT source
assists in controlling the internal energy of the primary ions. The

primary ion beam is mass (species) selected with a reverse geometry (BE)

I
double-foe

 

ions are st
deceleratn
frequency
cell where I
from the rI

is followed

 

under auI
system.
Each p
Studied m
using be
Pefiorma

Sections f

double-focusing mass spectrometer. After mass analysis, the primary
ions are strongly decelerated using a novel ion optical lens system. The
deceleration lens focuses the energy-retarded beam into a radio
frequency (rfl ocotopole ion beam guide which passes through a collision
cell where 101)] neutral interactions occur. Ionic species are extracted
from the reaction zone and transferred to a quadrupole mass filter which
is followed by a high efficiency detector. Experiments may be performed
under automated control with a PC-based data acquisition/control
system.

Each principle ion-optical component of the ion beam instrument was
studied and designed using computer modeling and then characterized
using beam visualization techniques. The overall operational
performance was determined by measuring energy-dependent cross—

sections for both endothermic and exothermic ion/ neutral reactions.

Ind

Sus

Em

 

In dedicationto

My parents, for their past encouragement and support
Susan. whose present love sustains me
Erin and Alan. who have become my fixture

iv

ACKNOWLEDGMENTS

The focus of this dissertation project was the design. construction and
characterization of a complex instrument. Yet. in all of my passion for
the machine and for the new information it will yield, the most significant
and, indeed, the most satisfying aspects of my graduate experience were
not the hardware. but rather the relationships that were built during my
years at MSU. I hold deep sentiment and gratitude towards my closest
comrades Mark Saba and Jeff Gilbert. Together. we battled the demons
of experimental science and the wiles of the omnipresent Firey Dragon
who inhabited our comer of the world. I have a rich heritage in
remembering Karen, DK. DE. LT, Rich, Spike, Harry (Hairy7), Zippe
Burritos and countless scores of cockroaches who made my life oh-so-
interesting and enjoyable.

This project would not have been successful without the superior
machining/instrument-making skills of Russ Geyer, Deak Watters and
Dick Menke who tolerated my endless and unreasonable requests. I am
indebted to the electronics expertise of Mike Davenport who always went
above and beyond the call of duty in helping to keep the CH5 alive and
kicking. Marty Rabb designed a good portion of the computer interface
and tutored me in rf and digital electronics by answering nearly all of my
stupid questions without making me feel stupid. I am also grateful to
Scooter who proficiently wrote the computer control software and also to
Tony (LT) Romero and Kurt Kneen who assisted in acquiring much of the
early characterization data.

I would
optimism
Finally. I I
eloquently

in addition

 

 

 

 

 

I would like to thank my co-advisor, George Leroi, for his youthful
optimism and for his guidance and support even in the final hour.
Finally. I am grateful to my major professor, John Allison, who so
eloquently conveyed the precepts of chemistry and the scientific method,

in addition to volumes on psychologt and human behavior.

vi

Table of Contents

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Rafi:

list of Tables 3

List of Figures xi

Chapter 1. Introduction 1

1. Mass Spectrometry, The Universe, and Me 1

II. The Significance of Transition-Metal Ion Chemistry 5
III. Experimental Techniques for Gas-Phase Ion / Molecule

Investigations 8

A. Overview 8

B. Conventional Mass Spectrometric Threshold Methods 9

C. ICR Mass Spectrometry 19

D. Swarm Methods g5

E. Ion Beam Methods 29

l . Overview 29

2. Crossed Beam Methods 30

3. Single Beam Methods 34

Chapter 1 List of References 42

Chapter 2. Ion Beam Instrument Design and Construction 45

I. Overview 45

II. Vacuum Chambers and Pumping System 49

III. Ion Generation 53

A. Intensitron Electron Ionizaton Source 53

B. High Pressure Drift Tube Ion Source 56

IV. Primary Mass Analyzer 65

V. Intermediate Off-Axis Detector 69

VI

Deceleration Lens

73

 

vii

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VII. Octopole Ion Beam Guide and Collision Cell 73
A. Multipole Trapping Theory 74
B. Construction 82
C. Octopole Electronics 87
VIII. Transfer Optics and Secondary Mass Analyzer 93
IX. Extraction Optics and Detector System 95
Chapter 2 List of References 100
Chapter 3. Understanding Ion Deceleration Optics 102
l (Janeunviennr 11(12:
11. Introduction 102
III. Deceleration Lens Criteria 106
IV. Ion Optical Modeling 1 13
A. The Over-Focusing Effect 1 14
B. Exponential Deceleration Lens 1 16
1. Theory 1 17
2. Gustafi'son Exponential Lens 1 18
3. Futrell Exponential Lens 121
4. 3000 to 30 eV Exponential Lens 121
C. Fundamental Studies 133
1. Two-Element Lens 133
2. Einzel Focusing Followed by Deceleration 137
V. Construction 142
VI. Characterization 145
A. Alignment 145
B. Angular Divergence 148

(3. 13r321rri ESQVIIlllltitljlefi].
D. Transmission 152
E. Kinetic Energy Spread 156
VII. Conclusion 164
Chapter 3 List of References 167

 

viii

Chapter 4

1. intro
[1. Instr
111. Rear
A
B.

Chapter 4

Appendix

Appendix
APPendix
Appendix
APPCHd'ur
Apperldix
Appendix
Appendix

 

Chapter 4. Ion Beam Instrument Computer Controlled Operation
and Reaction Cross Section Measurements

I. Introduction to Reaction Cross Section

II. Instrument Control

111. Reaction Cross Section Measurements
A. Collision Induced Dissociation of Mn2+
B. Exothermic Reaction of Ar+ with Do

Chapter 4 List of References

Appendix A. Measurements of the CH5-DF Analyzer Energy
Acceptance

Appendix B. Magnetic Hall Probe Calibration
Appendix C. Mass Discrimination in a RF-Only Quadrupole
Appendix D. Retarding Field Energy Analyzers
Appendix E. Mng+ Collision Induced Dissociation
Appendix F. Octopole Characterization ‘
Appendix G. Interference with Mng+ CID

Appendix H. Art/D2 Reaction Instrument Parameters

ix

169
169
171
180
18Q

195

196

 

fable

E.1. Ion
til. Ion

 

LIST OF TABLES

lab}; Rage

E. 1. Ion beam instrument parameters for my CID experiment. 215
H. 1. Ion beam instrument parameters for Art/D2 reaction. 235

 

F1 ure

1.1. Co
1.2. Ma
1-3. Cl
1.4. lon
l~51. Flt
1.6. Bl.
1.7. M;

10

2-2. It

LIST OF FIGURES

 

 

 

 

 

 

 

 

mum: BEES?
1. 1. Conventional mass spectrometer block diagram. 10
1.2. Mass Spectrometer ion source. 12
1.3. Clastogram for Chromium Hexacarbonyl. 14
1.4. lon cyclotron motion. 20
1.5. Flow tube schematic diagram. g5
1.6. Block diagram of a crossed-beam instrument. 31
1.7. MSU ion beam instrument block diagram- 41
2. 1. Ion beam instrument schematic diagram. 46
2.2. Ion beam instrument kinetic energy diagram. 48

 

2.3. Intensitron electron ionization source schematic diagram. 55

 

 

 

 

 

 

 

 

 

2.4. High pressure drift tube source schematic diagram. 58
2.5. High pressure drift tube source circuit diagram. 59
2.6. SIMION model of high pressure drift tube. 63
2.7. Neir double focusing mass spectrometer. 67
2.8. Intermediate off-axis detector. 70
2.9. SIMION model of octopole potential surface. 76
2.10. SIMION model of trajectories in rf octopole. 77
2.1 1. Theoretical rf multipole trapping potential. 79
2.12. Calculated radial ion kinetic energy in rf multipole. 81
2.13. Octopole side view schematic diagram. 83

 

2.14. Octopole top view schematic diagram. 84

xi

Bars
2.15. Rat

 

2.16. 0C1

2.17. 0e:
Opt

2.18. Ext
3.1. M
.1

 

3.3. 0v

3.4. so
1cm

3-5. sn

3'6. F.)
3-7. Si

3'8- Pt

3.9. E
3.10. 4
3.11. 4
3.12_

In

3-13. p
3-14.

rn

3.15. T

3'16. .

2.15.
2.16.
2.17.

2. 18.

3.1.

3.2.
3.3.
3.4.

3.5.

3.6.
3.7.

3.8.

3.9.

3.10.
3.1 l.
3.12.
3.13.
3.14.

3.15.

3.16.

 

 

Page
Radio frequency oscillator circuits. 88
Octopole electronics schematic diagram. 90
Octopole / quadrupole transfer and quadrupole extraction
optics. 94

 

Extraction optics and detector system schematic diagram. g2

MSU ion beam instrument block diagram and electric
potential diagram. 108

Experimental configuration for CH5-BF beam visualization. 111

 

 

 

 

 

 

 

 

Overfocusing effect in two-element deceleration lens. 1 15
SIMION model of Gustafi‘son semi-exponential deceleration

lens with literature electrode potentials. 1 19
SIMION model of Gustaffson semi-exponential deceleration

lens with einzel focusing. 129
Exponential potential distribution. 12,3
SIMION model of Futrell 42-element exponential deceleration

lens. 12:},
Potential surface of Futrell 42-element exponential

deceleration lens. 125
Exponential potential distribution. 123
40-element exponential deceleration lens. 123
40-element exponential deceleration lens potential surface. 139
Simple. two-element deceleration lens. 135

 

Potential surface for simple. two-element deceleration lens. 135
SIMION model of three stage deceleration lens with only einzel

 

 

focusing stage in operation. 138
SIMION model of three stage deceleration lens with

deceleration stage and einzel focusing stage in operation. 139
Potential surface of three stage deceleration lens with

deceleration stage and einzel focusing stage in operation. 140

 

xii

1‘1 ure

3.17.

3.18.

3.19.

3.20.

3.21.

3.22.

3.23,

4.1.
4.2.
4.3.
4.4.
4.5.
4.6.
4.7.
4.8.

4.9.

4.10_
4.11.
4-12.

4.13.

Crc
ion

 

 

1
813
abc
ass

Em,

be

Rel
eV.‘

R61
eV

3.17.

3.18.

3. 19.

3.20.

3.21.

3.22.

3.23.

4. l.
4.2.
4.3.
4.4.
4.5.
4.6.
4.7.
4.8.
4.9.

4.10.
4.11.
4.12.
4.13.

 

 

 

Bag:
Cross-sectional view of the deceleration lens system on the
ion-optical rail. 144
SIMION model of deceleration lens exhibiting severe
aberrations resulting from imperfections in the electrode
assembly. 147
Experimental configuration for measuring the decelerated
beam angular divergence. 149
Relative ion transmission with einzel focusing optimized at 5

154

eV.

 

Relative ion transmission with einzel focusing optimized at 30
eV.

 

 

 

 

 

155
Three grid analyzer and apparent ion kinetic enery
distribution. 159
DC-biased rf octopole and apparent ion kinetic energy
distribution. 163
Ion beam instrument control schematic diagram. 172
Ion beam experimental sequence diagram. 173
Octopole] collision cell DC bias schematic diagram. 175

 

Calibration of digital interface for octopole DC bias supplmeJfi

 

 

 

 

 

 

 

Quadrupole electronics schematic diagram. 178
Detector electronics schematic diagram. 179
my stopping potential. 182
Effect of quadrupole mass filter DC bias for my CID. 184
Energy-dependent Mn2+ CID cross section. 186
Comparison of an+ CID threshold region. 187
Ar+ stopping potential. 190
Ar+/D2 trapping efficiency. 190

 

Ar+/D2 energy-dependent cross-section path-length product. 192

xiii

D8.

13.9.

D10.

 

- Rel
- Rel
- Re]

Re

Re
m2

E igure

Bags:

 

 

 

 

 

 

 

 

 

 

 

 

 

4. l4. Ar+lD2 emery-dependent cross-section path-length product.____193
A. 1. Ion kinetic emery acceptance at 3000 V acceleration. 197
A2. Ion kinetic emery acceptance at 2000 V acceleration. 198
A3. lom kinetic emery acceptance at 1000 V acceleration. 199
B. 1. Hall values vs. m/z values for 3, 2. and 1 kV acceleration. 299
C. 1. Effect of RF amplitude on transmission in the Extrel Quad. 201
D. l. Retarding potential curve for 3 keV ions. 204
D.2. Retarding potential curve for 2 keV and 1 keV ions. 295
D3 Retarding potential curve for 3 keV ions, slits at 8.4. 205
DA. Retarding potential curve for 3 keV ions. magnet only. 292
D5 Retarding potential curve for 3 keV ions. CEMA detector.______208
D.6. Retarding potential curve for 3 keV ions, slits at 9.0. 209
D7 Retarding potential curve for 3 keV ions, CEMA detector.

magnet only. 219
D8. Retarding potential curve for 3 keV ions, slits at 9.0. magnet

only. CEMA detector. 1.5 mm source z-mask. 211
D9. Retarding potential curve for 3 keV ions. slits at open, magnet

only, CEMA detector. 1.5 mm source z-mask. 1 mm field._________212
D. 10. Retarding potential curve for 3 keV ions, slits open, magnet

only, CEMA detector. 1.5 mm source z-mask, 7 mm field. 213
E. 1. Ion current for m/z 28 vs. nominal electron emery. 219‘
E2. Plots of m/z 28 ion current vs. nominal electron emery. 212
F. 1. Effect of octopole rf amplitude on stopping potential. 219
F2. Effect of octopole DC tune bias on stopping potential. 229
F.3. Effect of DC tune bias on high emery transmission. 221
FA. Effect of ion injection emery on stopping potential. 222

 

xiv

Fi ure
F .5.
F .6.
F].
F .8.
F.9.
G1.
(3.2.
G3.
G.4.
(3.5.

 

E11}
E11}
E1117
Sic

Eff

MRI
Ion
911
CO L

10F

 

F.5.
F.6.
F.7.
F.8.
F.9.
G. 1 .
6.2.
6.3.
(3.4.
6.5.

m
Effect of ion injection emery high on emery transmission.______223

Effect of ion injection emery on stopping potential. 224
Effect of ion injection emery high emery transmission. 225

 

 

 

Stopping potential reproducibility. 226
Effect of ion source extraction field on stopping potential. 222
Mm? stopping potential. 229
Iom current for m/z 55 vs. lab collision emery. 239
Quad scan at a nominal collision emery of 2.2 eV. 231

 

Comparison of quad profiles for m/z 55 and CID product.____.2,32
Ion current for m/z 55 and m/z 73 vs. lab collision enery.______233

XV

Chapter 1

Introduction

 

 

niv

There is, perhaps. no other discipline of scientific inquiry that has a
scope of application as broad as that of mass spectrometry.

As a qualitative and quantitative analytical technique. mass
spectrometry embraces many diverse fields of science. Mass
spectrometry is routinely used for structure elucidation and quantitative
analysis of organic and inorganic compounds. Due to its extremely low
limits of detection, mass spectrometry is uniquely applicable to
biochemical problems such as natural products analysis and peptide
sequencing where the analyte of interest may be exist in sub-picogram
quantities.

Mass spectrometers are also employed in nuclear and atomic physics
research. They are used as isotope separators and analyzers in high-
enery particle accelerators. Mass spectrometry is also used as a
diagnostic and research tool in the medical and forensic sciences.
Techniques have been developed to analyze human urine for specific
metabolites which are present in certain disease states, or to check for
steroid abuse in athletes. Mass spectrometers have also been employed

in hospital surgical rooms to provide real-time breath and blood analysis.

Mass 1
extratem
analysis 1
very valu

deposits.

 

structurt
spectrom
miniaturi
balloons
atmosphc
on

athSph

 

associate

Mass
lm'esn'ga
beCQmE.E
moleCUll

depicted

Mass spectrometry has been implemented in both subterranean and
extraterrestrial scientific endeavors. In addition to providing elemental
analysis and carbon dating. mass spectrometric techniques have proven
very valuable to geological science for the analysis of oil and shale
deposits. This has provided a great deal of information on geological
structures and petroleum migration. 0n the other hand. mass
spectrometry has played a key role in astronomical science. Special
miniaturized mass spectrometers have been launched in high-altitude
balloons to study the composition and chemistry of the earth's upper
atmosphere. They have also been used in exploratory space probes and
on unmanned missions to celestial bodies. The recent Mariner mission
to Mars relied heavily on mass spectrometry to study the surface and
atmosphere of the planet and also to quantite chemical compounds
associated with biological activity.

Mass spectrometric methods are also implemented for chemical
investigations of a more fundamental nature. With mass spectrometry. it
becomes possible to study the interactions of individual ions and
molecules. For example, the elementary birmolecular reaction is often

depicted as

A + B —-> C + D (1.1)

This equation represents isolated species A encountering isolated
species B, resulting in the formation of products C and D. This scenario
is accurate only for appropriately low-pressure gas-phase conditions in
which only single-collision processes occur. Under these conditions,
fundamental interactions may be studied in the absence of solvent or

bulk matrix effects. Such experimental environments may be afforded by

a carefully designed mass spectrometer system.

If the conditions of an elementary interaction are carefully controlled.
and if the products can be appropriately characterized, a significant
amount of chemical information may be obtained. This dissertation
details the design. construction and implementation of a new tandem
mass spectrometer designed for such investigations.

The MSU ion beam instrument. described here, enables the study of
gas-phase ion/neutral chemistry under carefully controlled conditions.
The design of the instrument allows for interactions that are well-defined
with respect to the internal and translational enery of the colliding pair.
Ion/molecule reactions and collision induced dissociation (CID) cross-
sections may be studied as a function of ion internal and kinetic enery.
This allows for the accurate determination of the enery required to "turn
on" endothermic processes. This information may then be the related to
bond dissociation energies and ionic heats of formation.

Current theoretical and experimental efforts in our laboratory have
focused on the reactions of transition metal-containing ions with neutral
organic molecules. Theoretical work has involved ab initio calculations at
the electron correlation level for transition-metal ions bonded to small
organic ligands [1]. This has given insight into the nature of the metal
ligand bond as well as the geometry of the ion. Another area of
theoretical study concerns the early dynamic events occurring in
ion/ molecule interactions [2]. Models have been developed to describe
the forces that a neutral molecule experiences in the presence of a
positively charged particle p_r_ig'_tg collision. The electrostatic forces due
to charged-induced polarizability and other interactions influence the

trajectory of the ion as it approaches an organic neutral. This may lead

to favor'
interactie
lliese c2
transitior

Exper
spectrorr
informati
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undertak
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SPECUOSC

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inf(”mail
investjgm
°°mpreh
traI1311101

in the fiel

 

 

to favored ion/neutral orientations and preferred sites of orbital
interaction (chemistry), and influence the final product distributions.
These calculations continue to provide insight into the chemistry of
transition-metal ions.

Experimental efforts have centered on ion cyclotron resonance mass
spectrometry (lCR). A great deal of descriptive and mechanistic
information on transition-metal ion chemistry has been obtained from
these investigations [3]. Another experimental effort has been
undertaken in this lab to investigate the structure of isolated ions [4].
This involves trapping the ions emitted from a quadrupole mass
spectrometer in a low-temperature noble gas matrix. The ions are
subsequently subjected to spectroscopic analysis such laser-induced
fluorescence (LIF) and / or Fourier transform infrared spectroscopy (Fl‘IR).
This emerging technique appears promising for providing direct
spectroscopic analysis of transition metal-containing ions.

The MSU ion beam instrument will provide complimentary
information to the above mentioned theoretical and experimental
investigations of transition-metal ion chemistry. It is anticipated that the
comprehensive and multi-disciplinary approach to investigating
transition-metal ion chemistry will usher in a new era of understanding
in the field.

This chapter first introduces the significance of transition-metal ion
chemistry in Section II. The major experimental techniques implemented
in the study of gas—phase ion chemistry are discussed in Section III with
respect to this research project. This section also gives a general
description of the design and features of the MSU ion beam instrument.
Succeeding chapters of this dissertation are devoted to the detailed

deseripth
componer

of its perf-

11. The S;

 

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catalytic
economic

and alka

 

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highly Te;

 

description and the underlying principles of each of the major
components comprising the ion beam instrument and the demonstration

of its performance.

Ir uni-.1" lg 10- ‘--_ 0.1 9‘19- -1

There are numerous chemical reactions that are enhanced by the
catalytic action of transition-metal compounds. Some of the more
economically important reactions involve the activation of simple alkenes
and alkanes to induce oxidation. reduction, and polymerization. The
low-pressure and low-temperature polymerization of olefins is made
possible only through the implementation of organo-aluminum/ titanium
compounds known as Ziegler-Natta catalysts [5]. The specific oxidation
of alkenes to form aldehydes and ketones can be controlled through the
use of organometallic catalysts [6]. The enery requirements for the
hydrogention of alkenes can also be reduced through the catalytic
activity of transition-metals in various degrees of oxidation [7].
Organometallics have been proposed as intermediates in a wide variety of
catalytic processes [8].

Why do transition-metal species, in particular. exhibit such powerful
catalytic activity? The specific role of the transitiommetals in catalytic
processes is not fully understood. There are. however. unique properties
of transition-metals that may contribute to the ability to act as catalytic
intermediates. ’h'ansition metals have a high density of low-lying atomic
orbitals that provide favorable bonding opportunities with organic
species. This propensity towards bonding may enable the stabilization of

highly reactive intermediates involved in specific reaction pathways.

An example of transition-metal catalytic activity may be found in the
water gas shift reaction [9]. This particular reaction has gained
considerable attention in the search for alternative enery sources since
the species involved in the reaction are derived from bituminous coal.
When coal is heated to moderately high temperatures. an equilibrium
mixture of H20. CO. H2 . and CO2 is produced. The actual process
involves the evolution of water and carbon monoxide which is converted

to molecular hydrogen and carbon dioxide as shown below.

H2O + CO —> H2 + CO2 (12)

There are several compounds with transition-metal centers (M: Fe,
Ru. and Rh) which significantly shift the equilibrium to the product side.
The forward reaction may be facilitated through a transition metal-
carbom monoxide (M-CO) intermediate. Other reactions in which metal
centers behave as catalysts have similar implications in which various
metal-ligand (M-L) species may be the requisite intermediates.

A great deal of progress in understanding the role of transition metals
in catalysis has been gained through the study of the gas-phase
chemistry of transition metal-containing ions with organic molecules.
Under appropriately low pressure conditions, unique species may be
generated and studied using a variety of mass spectrometric techniques.
In the gas phase, bare metal ions , M+ (M: Fe, Ni, Co, etc.), are easily
formed. In the gas phase it is also possible to generate species that
would be impossible to form under condensed-phase conditions due to
high reactivity or instability. For example, ligated metal ions with
varying degrees of coordinative unsaturatiom such as M+-C0n , M+-C0n-

1 , M+-C0n-2 , etc. (M: Fe. Ni, Co, etc.) are easily generated as gas-phase

ions.
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ions.

The reactivity of many such transition metal-containing ions with
neutral organic molecules has been studied extensively in the past
decade [3). Although most of the work in the field to date has been
primarily descriptive in nature. the mechanisms of many transitions-
metal ion/organic molecule reactions have been elucidated. Also, the
effects of various ligands on metal ion reactivity and periodic reactivity
trends have also been investigated. The information obtained from many
gas-phase transition-metal ion studies has been correlated with the
reactivity of transition-metal compounds in condensed phases [10].
Thus, the behavior of transition-metal ion systems observed in the gas
phase may have direct application to understanding bulk catalytic
processes.

Although much has been learned regarding the structure and
reactivity of transition metal-containing ions. many questions remain
unanswered. For example, what is the nature of various M+—L bonds:
are they primarily electrostatic interactions or are they of covalent
character? What orbitals and electronic states are important in metal
ion reactivitf? How strong are the various M+-L bonds?

The MSU ion beam instrument is a valuable tool which can be applied
to probe many of these unanswered questions. Valuable thermochemical
information such as bond dissociation energies and ionic heats of
formation for M+-L species may be obtained by studying translational-
enery dependent ion/ neutral interactions. The ion beam instrument
will enable the investigation of periodic reactivity trends for endothermic
processes. These studies may add insight into the transition-metal

electronic configurations and orbitals involved in many M+-L bonds. In

the MSU ion beam instrument, metal ions may be selectively prepared in
the ground electronic state and/ or in well-defined state distributions.
This allows the state-specific reactivity of transition-metal ions to be
investigated.

Thus, the MSU ion beam instrument will play a role advancing the
current level of understanding of transition-metal ion chemistry. It is
anticipated that a sound relationship between transition-metal ion
structure and reactivity will be developed. This will provide an
unprecedented level of predictive ability with respect to intelligent
catalyst design.

,._iS.‘!-!!'!.-.. ' !!-0-'_ 0 -._' 0o. ' 0! U"
nve n

A. Overview

In the gas phase. the most fundamental of chemical interactions can
be studied. The use of ionic species in the gas phase affords certain
experimental advantages over systems of purely neutral atoms and
molecules. The inherent electrical properties of ions allows for their
facile manipulation and detection through the use of applied electric and
magnetic fields. A variety of mass spectrometric techniques have been
applied to study ion/ molecule reactions. The particular experimental
approach used in the investigation of ion/ molecule systems is dependent
on the information desired. This chapter section will cover the
application of conventional mass spectrometers used in the investigation
of transition-metal ion chemistry. When equipped with variable enery

electron or photon ionization sources, conventional mass spectrometers

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can provide some limited thermochemical and mechanistic information
concerning transition-metal ions. In addition. some of the more
specialized mass spectrometric techniques that are presently available to
study ion/molecule reactions will also be presented. such as ion
cyclotron resonance mass spectrometry. SWARM techniques, and ion
beam methods. The basic operating principles. along with the relative
merits and shortcomings of each technique. will be reviewed with
relevance to the study of transition-metal ion chemistry.

B. Conventional Mass Spectrometric Threshold Methods

Although there is no universal mass spectrometer design. there are
three basic components that are common to most conventional mass
spectrometers. A block diagram for a typical system is shown in Figure
1.1. The four basic components of a mass spectrometer. the sample inlet
system. the ion source. the mass analyzer and the ion detector. are
usually housed in a vacuum chamber which is connected to a pumping
system to provide a low pressure (<10'5 torr) environment. Neutral
atoms or molecules are introduced into the ion source through some type
of sample inlet system. The gas-phase neutrals in the source interact
with high emery electrons or photons and become ionized. The ions
thus formed in the source are extracted with electrostatic optics and
directed into the analyzer region of the instrument. There are many
physical devices which can separate ions on the basis of their mass-to-
charge (m/z) ratio and satisfy the requirements of the analyzer. One of
the most common methods involves injecting the ions into a magnetic

field perpendicular to the principal direction of ion motion. The force

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exerted on the ions by the magnetic field will result in curved ion
trajectories. The ions will be dispersed on the basis of their momentum
in the magnetic analyzer. A description of this type of ion analyzer may
be found in Chapter 2 of this dissertation. Another common ion analyzer
is the quadrupole mass filter. The device consist of four parallel
electrodes to which radio frequency (rt) and direct current (DC) potentials
are applied. Ions traveling through the device experience complex
trajectories. The proper combination of rf and DC potentials may be
applied to enable the device to operate as a mass filter. A brief
description of this type of mass analyzer may also be found in Chapter 2
of this dissertation. The ion detector may be a simple device such as a
Faraday cup to measure ion current. or it may be some sort of particle
(electron) multiplier to amplify small iom currents.

A block diagram of a typical electron ionization (E1) source is found in
Figure 1.2. A detailed description of the principles of El source operation
is given in Chapter 2 of this dissertation. Basically. electrons are emitted
from the resistively heated filament located outside the ion source. An
electric field. created by the potential difference between the filament and
the ion source. accelerates the electrons through the source and towards
an electron collector. If an electron with sufficient enery interacts with
a neutral atom or molecule. the neutral species becomes excited and
emits an electron to form a positively charged molecular ion. Polyatomic
molecular ions may be formed with significant excess internal enery.
This results in rapid unimolecular decomposition of the parent ion to
produce fragment ions. The ion population thus formed in the source is
extracted by an electrostatic lens system and transfered to the analyzer

and detector regions of the mass spectrometer.

 

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12

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The nominal kinetic enery of the ionizing electrons is determined by the
potential difierence between the filament and the source. The electron
kinetic enery may be varied while monitoring the relative production of
molecular and fragment ions. Useful thermodynamic information may be
obtained by monitoring effects of electron kinetic enery on ion
production for a given compound. For example. the ionization enery (IE)
of compound. A. may be measured determining the minimum electron
enery required to detect the formation of the ion. At. This is expressed
below:

A+ e' —) A+ + 2e‘ (1.3)
The the enthalpy change for this process is equal to the ionization

enery.

IE(A) = AI-Irxn. (1.4)

If the heat of formation of the neutral compound is known. then the

heat of formation (AHf) of the ionic species may be determined from the
following relationship.

IE(A) = Aern = AHflAfl - AHf(A) (1.5)

01'

AHf(A+) = IE(A) + AHf(A) (1.6)

Even more information may be derived experimentally by monitoring
the effects of ionizing enery on the production of molecular and
fragment ions. A plot of the relative abundance of the molecular ion and
each fragment ion as a function of electron enery is known as a
clastogram. A clastogram for chromium hexacarbomyl is shown in Figure

1.3 [1 1]. The minimum electron enery required to form the parent ion is

14

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taken as the ionization enery of the molecule. This can be related to the
heat of formation (AHf) of the ionic species if the gas-phase heat of

formation of the neutral precursor is available. PYom the plot shown in
Figure 1.3. the electron energies required to form various ions in the
series shown below can be determined.

Cr(CO)5 + e- —> Cr(CO)5+. Cr(CO)4+. Cr(CO)3+. Cr+ (1.7)
The minimum electron enery which results in the formation of a
particular fragment is taken as the appearance enery (AE) of that

species. For example. the authors determined the appearance enery of
Cr+ from Cr(CO)5 to be 17.7 21:0.3 eV [1 1]. The balanced chemical

equation for this process is shown below.

Cr(CO)6 + e' —-) Cr+ + 6C0 + 2e' (1.8)

This information may then be related to the heat of formation of the
bare metal ion. Cr+. If the heats of formation for the neutral precursor
and neutral products are known the ionic heat of formation may be

derived as follows.
AE(Cr+. Cr(CO)6) = AHrm(l.8) (1.9)
AHl-mug) = AHdCfi) + 6 AHf(CO) - AHdCr(CO)5) (1.10)
combining equations (1.9) and (1 .10)

AHdCfi) = AE(Cr+, Cr(CO)5) - 6AHf(CO) + AHf(CI'(CO)5) (1.1 1)
Although the enery—dependent behavior of ionization processes can
yield important thermochemical information. there are significant
limitations. The difficulty in obtaining accurate thermodynamic

information from experiments such as these arise from two major

 

sources
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16

sources. experimental and phenomenological. Experimental sources of
error are dominated by the uncertainty in the kinetic enery of the
electrons. The nominal kinetic enery is usually taken as the difference
in the voltages applied to the filament and the source which. of course.
ignores potential variations caused by the junction of dissimilar metals
(contact potentials). The electron enery is also strongly influenced by
the penetration of ion repeller and extraction fields inside the source.
Spatial variation of potentials inside the source and the effect of electron
beam space-charge also contribute to electron enery uncertainty. In
addition the electrons are emitted from a high temperature filament with
a Maxwellian enery distribution. This is approximately 0.5 eV (FWHM)
for a filament temperature of 1500 'C [12].

There have been numerous experimental and data manipulation
efforts to circumvent the problems related to electron enery spread.
Experimental improvements include the implementation of sophisticated
ion sources that employ electron momochromators [13] and electron
retarding-field devices [14]. An excellent discussion of the various
methods of extracting threshold energies fi‘om electron enery-dependent
ion production curves is found in a monogragh by Westmore (reference
12). Various data manipulation techniques are discussed such as the
vanishing current method. the linear extrapolation method. the critical
slope method. the semilogarithmic plot method. in addition to derivative
methods.

The great uncertainty in enery of the ionizing medium associated
with electron ionization sources lead, in part. to the development of
radiation sources for ionization. Photoionization mass spectrometers [15)

have been constructed in which photons are used as the ionizing

studies

111161156

 

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17

medium. The enery resolution of the photons is often on the order of a
few milli-electromvolts. However. it is very difficult to produce
monochromatic radiation in the enery range necessary for ionization
studies (5-20 eV). Pseudo-continuum radiation sources may employ
intense hydrogen or rare gas discharges. These are optically coupled to
large vacuum UV momochromators to provide high resolution wavelength
selection on the order of 1 A or less. Internal calibration is accomplished
by analyzing noble gas emission lines. More recently. tunable laser
radiation in the vacuum UV range has been used in photoionization
studies (16). Also. extremely sophisticated experimental techniques have
been developed to induce multi-photom ionization [17). These
experiments require very intense laser sources and may even incorporate
two separate laser sources (two-color ionization)

In addition to the experimental uncertainties encountered in obtaining
thermodynamic information from variable enery ionization sources,
there are severe. and often restricting. phenomenological uncertainties
associated with such threshold methods.

For example Equation (1.3) gives the electron ionization process for a

compound A.

A + e' —) NP 4» 2e“ (1.3)

The expression for the heat of formation of A+ (Equation (1.5))
assumes that all of the reaction products possess no excess internal or
translational enery. It implies that the kinetic enery of the ionizing
(primary) electron is completely utilized by the neutral molecule. Also.
any translational enery of the ejected electron is neglected. Some

sophisticated experimental methods have been devised to measure the

kinetic
mmnne
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Anot
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discuss

18

kinetic enery of the ejected electron when photons are used as the
ionizing medium. These photoionizatiom-photoelectron spectroscopy
techniques explicitly account for the translational enery of ejected
electrons and provide highly accurate ionization energies.with
uncertainties of less than 0.002 eV.

Another more severe phenomenological problem arises when
threshold methods are used to determine the thermodynamics of
fragmentation processes. The production of the bare metal ion. Cr+. from
Cr(CO)6 as shown in Equation (1.8) will serve as an example for this

discussion.

Cr(CO)6 + e’ —> Cr+ +6CO + 2e" (1.8)
In addition to not explicitly accounting for any excess kinetic enery
found in the products. the expression for the heat of formation of Cr”
(Equation (1.10)) assumes that all of the reaction products possess no
excess internal enery. Also. the assumption that Cr+ is produced in its
ground state may not be accurate. When bonded to the 6 carbonyl
ligands. the Cr metal center of the neutral precursor has an electron
configuration associated with a low-spin singlet excited state. This state
lies approximately 3.7 eV above the ground state for free Cr. When
Cr(CO)6 undergoes electron ionization to form free CD”. a high-enery
electron configuration similar to that of the neutral Cr may be retained.
The ionization processes may be viewed as removing the six ligands from
Cr(CO)6 to leave the free metal center and then removing an electron to
form Cr+ in a highly excited state. In fact, information from careful
appearance potential measurements of Cr+ from Cr(CO)6 have correlated
with the formation of excited state Cr+ d5 (21“ 1/2)) (13].

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19

The state of a metal ion formed by electron ionization is therefore
dependent on the state of the metal center in the neutral precursor. This
is supported by other studies in which the heat of formation of Fe+ was
found to range form 14.7 to 16.1 eV depending on the neutral being
ionized [18]. Thus, the thermodynamic information from source
threshold methods is often inaccurate. This limits the applicability of
such thermochemical information to understanding ground state

reactivity of transition-metals in condensed phase catalytic processes.
B. Ion Cyclotron Resonance Mass Spectrometry

If an ion with charge q is moving with a velocity v in a uniform
magnetic field having intensity B the ion will follow a curved path. This
is because of the force exerted on the ion by the magnetic field as

expressed below:

F = qv X B (1.11)

The force constrains the ion to a circular path as shown in Figure 1.4.

In this figure. the ion velocity is restricted to the x-y plane and the
magnetic field is perpendicular to x-y plane. The ion mass and velocity
determine the radius of the curved trajectory. but the frequency, f, of

rotation is dependent on the mass, m, as expressed below.

f = qB/21tm (1.12)

The angular frequency, a) = 21rf. is known as the cyclotron frequency of
the ion and is usually on the order of several hundred kilohertz for
typical magnetic field strengths (~1 tesla). The ion mass/charge value

can thus be related to the frequency of ion orbit. These principles have

20

Analyzer Plates

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Figure 1.4. Ion cyclotron motion.

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21

been applied to develop ion cyclotron resonance (ICR) mass
spectrometers[ 1 9].

Ion manipulation and detection can be accomplished by using one or
more sets of analyzer plates as shown in Figure 1.4. The periodic motion
of the ions may be detected by applying a signal at the ion cyclotron
frequency to the analyzer plates. Ions orbiting at the applied frequency
will absorb power and experience an increased trajectory radius. This
power absorption can be sensed with the electrical circuitry and be
related to the ion population in the ICR. Another powerful method of ion
detection involves briefly applying a complex waveform to the analyzer
plates. This small "chirp" signal usually contains all of the possible ion
cyclotron frequencies (within a given mass range). Ions present inside
the [CR cell will absorb the power and begin to move in coherent ion
"packets" on the basis of their m/q value. The motion of these ion
packets will in turn induce a signal on the analyzer plates. This complex
signal will contain frequencies associated with all of the different ion at] q
values present in the cell. A Fourier transformation may be applied to
convert the complex signal information to the frequency. and hence
mass. domain. This technique of ion manipulation and detection is
known as Fourier transform lCR or simply FI‘MS [20].

Although actual ICR and FTMS instruments are much more complex
than just described. one significant feature is that ions can be trapped in
a magnetic field for a substantial period of time (in the millisecond range
and longer). Trapped ions may be allowed to interact with a low-pressure
neutral gas and the ionic products of such an interaction may be
subsequently analyzed. This makes lCR instruments very useful for the

study of ion / molecule reactions.

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Although other techniques such as laser ionization exist [21], most
ICR instruments utilize high enery electrons (~70eV) to form primary
ions within the analyzer cell. For example. if neutral chromium
hexacarbonyl is subjected to electron ionization, ions in the series shown
below will be formed in the ICR cell.

Cr(CO)5 El —) Cr(CO)5+. Cr(CO)4+. Cr(CO)3+, .... Cr+ (1.13)

If an organic gas such as propane is present. it too will be ionized by the

electrons and produce ions such as those shown below

03113 (El) —+ C3H3+, can/5+, Czrw, etc. (1.14)

Thus. the primary ions in the ICR cell will consist of a mixture of
chromium carbonyls and hydrocarbon fragments. If the pressure is
sufficiently high and the residence time is sufficiently long. these ions
will eventually interact with other neutral molecules such as the propane
and chromium hexacarbonyl present in the ICR cell. The resulting ion
population in the ICR cell from both the primary ions and those resulting
from ion/ molecule reactions, can be very difficult to interpret. Therefore,
special techniques such as double resonance [22] and selective ion
storage have been developed to provide clear precursor ion/product ion
relationships. By monitoring the intensity of precursor ion and product
ion populations as a function of time. while carefully controlling reactant
gas pressure. kinetic studies may be perforrmed. It has been shown that
most reaction rates occur within an order of magnitude of the
ion/neutral collision frequency. This irmplies that most product ions
observed in ICR investigations proceed through exgmermm (or
thermoneutral) channels and that no significant enthalpy barriers exist

for the forward reaction.

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23

A large body of information on the reactivity of transition metal-
containing ions with organic neutrals has been generated using ICR
techniques[23]. Most of the information concerns the product ion
branching ratios resulting from exothermic ion/molecule reactions.
Studies have been performed to determine the relative reactivity of metal
ions with respect to the organic reactant chain length [24] and functional
groups [25]. Also, studies directed towards understanding metal ion
periodic reactivity trends [26]. and the the effects of ligands bound to
metal anions have been conducted [27].

Most of the chemical interpretations from ICR investigations tends to
be descriptive in nature. However, a great deal of information regarding
the actual mechanisms of activation of organic species by transition-
metal ions has been obtained. In addition, some thermochemical
information such as metal ion-ligand (M-L) bond dissociation energies
(BDEs) for ionic species has been inferred from metal ion reactivity. For
example. the bare nickel ion has been found to induce
dehydrohalogenation in i-propylchloride to form Ni(HCl)+ as an
exothermic process. Since the most thermodynamically favorable neutral

product is propene, the reaction can be written as
Ni+ + CH3(CHC1)CH3 -> Ni(HCl)+ + CH3CHCH2 (1.15)

The dehydrohalogenation process for the neutral i-propylchloride

system shown below is an endothermic process.

CH3(CHC1)CH3 -> HCl + CH2(CH)CH3 AHm = +0.83 eV (1.16)
The thermodynamics of reactions 1.15 and 1.16 suggest that the Ni+-

HCI bor

 

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24

HCI bond bond dissociation energy is 20.83 eV.
Ligand displacement reactions can also be used to bracket BDEs in
transition-metal ion systems. An example of this may be found in a

recent article by Stepnowski and Allison [28]. In a study of the reactions
of nickel-containing ions with neutral PF3 the following reaction was

observed.

Nico+ + PF3 —+ NiPF3+ + co AHnm s o (1.17)

From this reaction it can be concluded that the Ni+-PF3 BDE is
greater than the Ni+-CO BDE.

Although relative BDEs can be determined. it is, however. difficult to
get accurate and absolute thermochemical information by implementing
ICR techniques. In addition, for most investigations of transition-metal
ion chemistry using ICR. the most common method of ionization is high
energy (~70 eV) EI. As discussed in the section on source threshold
measurements, this method of preparation results in great uncertainty in
the internal energy of the reactant ions. Thus. many of the reactions and
thermochemical implications derived from ICR work may not be
representative of ground state chemistry.

In the example of a ligand displacement reaction (equation 1.17),.the
NiCO+ reactant ion was prepared by electron ionization of Ni(CO)4. Is the
NiCO+ fragment ion in the ground electronic and vibrational state? Or is
the NiCO+ ion population in the ICR cell a distribution of ground state
and excited state ions? Is the observed chemistry due to ground state
reactivity or is it exclusive to NiCO’r in an excited state? What is the
absolute Ni+-PF3 bond dissociation energy? This illustrates just one

chemical system for which many questions remain unanswered by the

 

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25
limitations of typical ICR methodolog'.

C. Swarm Methods

Low energy interactions between ions and neutral species can also be
investigated using so-called 'swarm' techniques. In these techniques,
ions are injected into a flowing stream of an inert buffer gas such as
helium or argon in a drifl tube. A reactant gas is also introduced into the
flowing stream. The 'swarm' of ions may interact with the neutral
reactants to form new ionic products. A general schematic diagram of a
drift tube instrument is found in Figure 1.5.

The ion source shown at the left side of Figure 1.5 may be one of
several designs. In the flowing afterglow (FA) technique, ions are
generated in the source region by a high enery electrical discharge or
high energy electrons which initially form a large population of excited
state He atoms and ions [29]. The highly-excited plasma traveling with
the flowing buffer gas generates photons through a number of relaxation
processes and creates a bright glowing region — hence the term flowing
afterglow. If another species such as molecular nitrogen is present in
trace quantities, ionic species such as N2+ and N+ may be formed
through Penning ionization and charge transfer precesses. It is obvious
that the flowing afterglow source gives very little control over the
selection of primary ions injected into the drift tube. This limits the type
of ionic systems that may be studied with FA sources. Another, more
versatile. ion source employs a standard E1 or chemical ionization (CI)
region preceding a quadrupole mass fllter. In this manner the ions of

interest may be injected into the flowing stream of buffer gas as a mass-

26

 

Bufl’erGas
Inlet
ReactantGasInlets
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Drift Electrodes

 

Pump

Figure 1.5. Flow tube schematic diagram.

27

selected ion beam. This is commonly referred to as Selected-Ion Flow
’Dube (SIFI‘) methodology [30].

The source region of the instrument is followed by the drift region. The
buffer gas is introduced at one end of the flow tube and pumped away at
the other end. Very high gas flow rates are required to maintain typical
operating pressures of ~0.5 torr. These high gas loads also demand a
very rigorous pumping system. The drift region of the instrument is
usually lined with a series of ring electrodes. By applying an appropriate
bias to the drift electrodes. a linear longitudinal electric field is created
inside the flow tube. The ions traverse the flow tube by a combination of
the bulk motion of the flowing buffer gas and the force exerted by the
electric field. By controlling both the buifer gas flow rate and the electric
field strength. the residence time of the ions in the drift tube may be
varied. Reactant gases are introduced into the flowing stream through a
number of inlet ports. located at various distances from the source end of
the flow tube. The ion/ neutral interaction time may also be varied by
selecting different ports for introducing the neutral reactant gas. After
traveling the length of the drift region. primary and product ions are
extracted into the detector region through a differentially pumped
housing. .

The ion detector may be a simple (non-selective) device such as an
electron multiplier which only provides total ion flux information. Other
detection systems employ quadrupole mass filters or even tandem mass
spectrometers [31]. This type of detection allows the ion population at
the end of the flow tube to be well characterized.

Since the pressure in the flow tube is quite high, the primary ions

experience thousands of collisions prior to encountering any neutral

reacta
YOU“!

l
s sienl
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33

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28

reactant molecules. This results in the collisional relaxation of any
highly excited electronic states present. Thus. swarm methods allow
ground state ion/molecule processes to be studied. The actual ion
kinetic energy is a complex function of the physical temperature of the ~
system, the strength of the applied drift field and the linear velocity of the
flowing stream of buffer gas. The ion/ molecule interaction energies can
cover a range from approximately 0.03 to 3 eV in the best swarm
systems. By observing the relative primary and product ion abundances
under carefully controlled conditions (i.e. well defined interaction times).
the rate constants for various ion/ molecule processes may be accurately
measured. By conducting reaction rate experiments as a function of the
ion/neutral interaction energy, important thermodynamic parameters
such as ionic heats of formation. reaction entropies and proton affinities
may be quantitated [32].

Although swarm methods can provide a great deal of information
concerning fundamental ion/ molecule processes, there are some
restrictions and limitations. The ion/ neutral interactions observed using
swarm methods are characterized by broad (Maxwellian) energy
distributions. And although an interaction energy range of nearly two-
orders of magnitude is often available, the high energy limit of a few eV is
quite restricting. This upper energy limit precludes the application of
swarm methods to the study of highly endothermic ion/molecule
processes which may be important in understanding the chemistry of
transition-metal ions. Also. at high pressures, collisional stabilization
ion clustering processes, instead of reactive processes, may dominate ion

activity for some sytems.

29

D. Ion Beam Methods
1 . Overview

The traditional approach to studying ion/molecule interactions has
involved measuring reaction rates and product branching ratios by
implementing ICR and SWARM techniques. Ion beam methods differ
from ICR and SWARM techniques in that they can be used to probe the
intimate dynamic and energetic details of ion/ molecule interactions. It is
of great importance to understand the exact series of microscopic events
that occur when an ion encounters a neutral molecule and the atoms
involved are transformed into new chemical entities. The goal of modern
research in ion Imolecule reaction dynamics is to fully understand these
elementary reactive encounters in the evolution of products from
reactants. Ion beam methods enable the study of elementary processes
by providing exceedingly well-characterized ion/molecule interactions
and close observation of the results of the interactions. The information
obtained from ion beam methods can be closely tied to theory and can, in
fact. provide a measure of the accuracy of potential energy (PE) surface
calculations.

Ion beam techniques may be subdivided into two related, yet distinctly
different, categories: (a) Crossed Beam and (b) Single Beam. The
particular information desired from a chemical system dictates which
approach is most appropriate. Crossed beam systems are used to
investigate reaction dynamics whereas single beam instruments are more

suitable for acquiring accurate thermochemical information.

3O

2. Crossed Beam Methods
Consider the hypothetical ion/molecule reaction of a mono-atomic

ion/ diatomic neutral system as shown below.

[V + BC —) AB+ + C
There may be numerous questions concerning the reaction dynamics
and energetics of this system. Is the process exothermic or endothermic?
Are there specific orientations of BC with respect to A+ that are required
to allow products to be formed? Is the product AB‘l' formed in excited
electronic, vibrational and/ or rotational states? How does the reaction

cross-section vary as a function of interaction energy? What is the

lifetime of the three body collision complex?

Unfortunately. it is currently impossible to view the motion of
submicrosc0pic atomic particles directly. However, evidence of the
intimate atomic interactions may be obtained by analyzing the
trajectories of the particles before and after the reactive encounter. The
initial velocity and direction of the interacting species may be controlled
by using a crossed beam apparatus. A block diagram of this type of
instrument is shown in Figure 1.6. Ions are formed in the source region
and the ionic species of interest is selected by the first mass analyzer.
Ideally. the ion source could generate ions in specific internal enery
states or at least well-characterized internal energy state distributions.
Electrostatic optics are used to establish the translational energy of the
ions emerging from the mass selection device. This mass-resolved
primary ion beam should be nearly monoenergetic and highly collirnated.
These conditions ensure well defined velocity vectors for the primary ions

in the laboratory frame of reference.

31

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32

The neutral reactant beam is usually generated using a supersonic
nozzle and skimmer in combination with a series of collimating
apertures. If a molecule of interest is 'seeded' in a buffer gas, the
supersonic expansion process also produces a neutral molecular beam
that is vibrationally and rotationally cooled. The kinetic enery (velocity)
of the neutral can also be varied by changing the relative pressures and
masses of the molecular and buffer gases. In addition. exotic neutral
molecular radicals or mono-atomic species such as O, H, or N which are
extremely unstable may be produced with sufficient intensity using
supersonic beam sources.

The neutral beam is oriented perpendicular to the primary ion beam
in the laboratory frame. The characteristics of the primary ion source
and the neutral beam source result in well-defined scattering centers and
highly characterized ion/neutral interaction energies. The flux of the
primary and product ions are measured with a second mass
analyzer/ detector combination. This system is usually mounted on a
rotatable mount to provide angular (directional) resolution of the reactant
and product ions.

In a complete collision analysis, the scattered primary and product
ion flux is measured as function of angle about the collision center. This
differential is supplemented with kinetic enery analysis of the ionic
species. Kinematic treatment of the energy/ flux/ direction data is applied
to produce Cartesian velocity-space contour diagrams in the center-of-
mass frame of the collision pair. This information can be interpreted to
yield the mechanistic details and energy transfer processes involved in
ion/ neutral interactions [33]. Although most crossed ion beam

instruments analyze only the ionic products of the ion/neutral

33

interaction, an ideal instrument would also be capable of ascertaining
the identity and internal/translational energy of the neutral products

generated.

The earliest crossed beam instrument designed to provide both
angular and translational energy resolution for ionic reactions was
developed by Herman and Wolfgang in 1969 [34]. Their apparatus had
an ion kinetic energy distribution of ~0.5 eV and could produce beams
with ion translational energies as low as 3 eV. A more sophisticated
crossed beam instrument was developed by Futrell and coworkers in
1975 [35] which incorporated high angular resolution over an arc of 1 10'
in the laboratory frame. Improved ion optics in this instrument allow
very low energy (~0.5 eV) ion beams to be generated. Mahan and
coworkers also constructed a crossed beam instrument to study low
energy (<5 eV) ion/neutral processes [36]. More recently. Futrell and
coworkers developed a new supersonic beam mass spectrometer for the
study of collision-induced dissociation of ions in the energy range of <1
to 3000 eV [37]. This instrument has exceptional ion kinetic energy
dynamic range. but the novel hemispherical energy analyzer incorporated
in the instrument suffers from poor energy resolution and transmission
at energies below 50 eV.

These and other crossed beam instruments have provided detailed
studies on the dynamics of ion/neutral processes. Because the
instruments are designed to extract so much information (mass, angular,
and energy distributions). the overall sensitivity of the instruments is
often quite low. In addition. poor. and often biased, collection efficiencies
of highly scattered primary and product ions preclude the use of crossed
beam instruments to obtain quantitative integrated (total) reaction cross-

sectim

Sir
accur
accon
produ
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neutr
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34

sections as a function of ion kinetic energy.

3. Single Beam Methods

Single beam instruments have been designed to provide more
accurate total reaction cross-sections measurements. This is often
accomplished in these instruments by limiting angular resolution and
product kinetic energy information. Since directional information is not
necessary for total cross-section measurements. a highly-characterized
neutral beam is not required. Instead. the primary ion beam is directed
through a region of the instrument which contains a neutral gas. The
gas density is kept sufiiciently low (<10’3 torr) to ensure single collision
conditions. The unreacted primary ions and ionic reaction products are
then extracted into a second mass analyzer/ detector for mass-dependent
ion flux measurements.

The velocity and angular distribution of neutrals from molecular beam
sources is very small. In contrast, the neutral collision gas in a single
beam experiments is characterized by an isotropic Maxwellian velocity
distribution. This condition results in a distribution of ion/neutral
interaction energies even if the primary ion beam is truly monoenergetic.
This presents a limit to the absolute energy resolution for single beam
experiments. However, the relative contribution of the neutral gas
thermal energy spread to the overall uncertainty of the ion/neutral
interaction energy becomes critical only at very low collision energies (<2
eV lab). In addition. if the temperature of the collision gas is accurately
measured. the cross-section data can be corrected for this effect.

Because the kinetic energy of the primary ion beam is variable, single

35

beam instruments can provide excellent total reaction cross-section
measurements as a function of ion/ neutral interaction energy.
Threshold experiments may be performed to determine the minimum
kinetic energy required to overcome energy barriers encountered in
endothermic ion/neutral reaction channels. This allows important
thermodynamic information such as bond dissociation energies and ionic
heats of formation to be derived from single beam studies.

One of the first single beam instruments designed to study low energy
ion/ neutral processes was developed by firtrell and coworkers in 1965
[38]. This tandem instrument was comprised of two sector-type mass
spectrometers linked together. Both mass spectrometers consisited of an
electric sector followed by a magnetic sector (EB configuration). A
complex ion optical system was used to establish the kinetic energy of
the ions exiting the the first mass analyzer. The ions were then focused
into a small collision cell containing the static neutral reactant. Primary
and product ions scattered in the forward direction in the laboratory
frame were then analyzed by a second high resolution EB mass
spectrometer. This instrument was able to generate mass-selected
beams of primary ions having very low (<0.5 eV) kinetic energies.

Another single beam instrument based on tandem quadrupole mass
spectrometers was constructed by Beauchamp's group at the California
Institute of Technology in 1977 [39]. This instrument was later modified
by Arrnentrout so that a magnetic sector. instead of a quadrupole. was
utilized as the first mass analyzer [40]. Ions exiting the mass analyzer
region are focused and decelerated to the desired kinetic energy with a
complex deceleration unit before injection into the collision cell

containing the neutral gas. A grid and lens combination serves to extract

the pr
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36

the primary and product ions into the quadrupole mass analyzer. The
instrument was designed primarily to measure onset energies for
endothermic ion/neutral processes. This requires very high sensitivity
since the rate of product ion formation approaches zero at the reaction
threshold energy. To achieve high signal-to-noise ratios for low intesity
ion fluxes, a modulated detection system was employed. The primary ion
beam was pulsed into the reaction chamber and only those ions events
occuring at the modulation frequency were detected with a phase—
sensitive amplifier system. Numerous investigations on the reactivity of
transition-metal ions with small organic neutrals were performed on this
instrument [41]. These experiments yielded the first direct
measurements of BDEs for various transition-metal ions bonded to
organic ligands. Comprehensive studies have resulted in the correlation
of thermochemical properties with reactivity trends in transition metal
systems [42].

In the single beam instruments described thus far. the collision cell is
usually constructed as a small metal box or tube with a path length of a
few centimeters. The cell usually has very small apertures on each end
to define the electric field inside the box and to provide minimal gas
conductance to the main vacuum chamber. An appropriate DC bias is
applied to the collision cell to establish the kinetic enery of the primary
ions while they travel through the cell. The beam of primary ions is
usually highly collimated about the central (ion-optical) axis of the
collision cell. When the primary ions collide with neutral species. they
may be scattered at high angles relative to their initial trajectories in the
laboratory frame. Other reactive ion/molecule processes may result in

the production of product ions which have trajectories that are non-

 

DYOQQ

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37

colinear with the ion-optical axis of the collision cell. At very low
interaction energies. long-lived ion/neutral complexes may result in a
symmetric or isotropic distribution of products from the reactive
encounter. Only those ions which have very low-angle trajectories will be
able to exit the collision cell through the small aperture and proceed to
the second mass analyzer and detector. There is. therefore. a high level
of uncertainty in the total collection efficiency in single beam
instruments of this type.

One way to eliminate measured ion flux bias from such dynamic
effects and to improve total ion collection efficiency is to incorporate
some type of ion trapping field within the collision region. This may be
accomplished by using dynamic-field ion beam guides. Details on the
theory and operation of rf-only ion beam guides are covered in Chapter 4.
Ion beam guide methods were pioneered by Teloy and Gerlich in 1974 in
the construction of a tandem mass spectrometer used to study low
energy ion/molecule reactions [43]. In addition to using an ion beam
guide. this single beam instrument incorporated several new innovative
features. First. the ionization source employed a radio-frquency (rt)
trapping field that allowed long ion storage periods on the order of a few
milliseconds. Conventional El sources have ion residence times of ~1 us.
When operated under high-pressure conditions. multiple-collision
processes resulted in electronically cooled primary ions. Second. the
primary mass analyzer implemented a unique two-stage quadrupole
mass filter that allowed both ion mass and velocity selection. The
primary ion beam exiting the mass / energy filter was then focused into an
octopole ion beam guide. This beam guide was perhaps the most

significant feature of the instrument. It was constructed from eight

paral
the o
vdur
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press
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38

parallel rods arranged in an octogonal array. An rf-only signal is applied
the octopole rods to create a radial trapping field within the interior
volume of the beam guide. The rf field does not influence the
longitudinal velocity of the ions traveling in the guide. But an inward
restoring force continually directs the ions toward the central axis of the
device. The beam guide is followed by a magnetic sector mass analyzer
with a highly-efficient detector.

The ion beam guide passes through a collision cell into which a low
pressure gas is introduced. When ion/neutral interactions occur that
produce highly scattered ions. they are effectively contained within the
ion beam guide. The motion of a scattered ion traveling through the
octopole may viewed as a series of specular reflections off the potential
walls created by the rf field. The use of dynamic trapping field ion beam
guides allows very accurate total reaction cross-sections to be measured.

This guided beam technology was implemented in an ion beam
instrument constructed at the University of California at Berkeley where
Gerlich did post-graduate work with Anderson in the laboratory of Y.T.
Lee [44]. They constructed a single beam instrument that utilized a
photoionization source to prepare primary ions in specific vibrational
states. Since the ionization process was so specific. no mass
spectrometer was needed to produce the primary ion beam. Reactant
ions where formed directly in a dodecapole beam guide. The primary
ions were transferred to a collision region and finally to a quadrupole
mass analyzer using a series of mi; separate rf octopole ion beam guides.
This instrument was used to study the effects of translational and
vibrational excitation on ion/ molecule reactions. Anderson subsequently

constructed a similar instrument at (SUNY at Stoney Brook) for the study

dsni

 

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39

of small metal-cluster ions using guided ion beam technology.

Because of the significant advantages prodided by an active trapping
device in the collision region of single beam instruments. Armentrout
incorporated an octopole beam guide in the design of an instrument
constructed at Berkeley in 1983 [45]. A full description of this
instrument can be found in reference 46. The ions may be generated
using EI. surface ionization (SI) or a high pressure drift tube (HPDT)
source. The primary ion beam is mass-selected using a low resolution
magnetic sector mass spectrometer. An exponential lens is used to
decelerate the ion beam before it is injected into an octopole ion beam
guide which passes through a collision gas cell. The primary ion beam is
variable in energy over the range of ~1 to 750 eV. After traversing the
collision region, product and primary ions are analyzed with a
quadrupole mass analyzer followed by a Daly-type detector. Armentrout
and coworkers have utilized this instrument to generate a large body of
information on the kinetic enery dependence of transition-metal ion
reactions with small organic and inorganic neutrals [47]. These
investigations have yielded very accurate BDEs and ionic heats of
formation. By carefully controlling the initial ion formation processes in
the source. state-specific reactivity studies have also been performed.

It is of interest to note that two of the guided ion beam instruments
cited above [44.46] contained design provisions to utilize a supersonic
expansion neutral beam source. This would achieve a greater level of
resolution of the ion/ neutral interaction energy over that obtainable from
a static neutral gas in the collision region. However. because the number
density in a neutral beam is quite low and the interaction volume

(determined by the ion and neutral beam physical cross-sections) is very

40

small. no success from crossed-beams in guided ion beam instruments
been reported to date.

The MSU ion beam instrument is of single beam design and
incorporates an octopole ion beam guide and a static gas ion/neutral
interaction region. This provides a high level of efficiency for the
collection of scattered ions. A schematic diagram of the MSU apparatus
is shown in Figure 1.7 . A general description of the instrument may be
found in the first section of Chapter 2. Details of the vacuum system,
the primary ion sources. and the primary ion analyzer are provided in
succeeding sections of Chapter 2. In addition. the theory and operation
of the octopole ion beam guide. the quadrupole mass analyzer and the
detector system are also covered in Chapter 2. Chapter 3 is devoted
entirely to the theory and design of the ion deceleration optics.
Operational/control concepts of the MSU ion beam instrument and
demonstration of the instrument performance for exothermic and

endothermic chemical processes are given in Chapter 4.

41

 

 

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42

LIST OF REFERENCES
Chapter 1

Mavridis. A.; Harrison. J .F.; Allison. J. Am. Chem. Soc. 1989. 1 1 1.
2482.

Hankinson. D.J.; Miller. C.B.; Allison. J. J. Phys.. Chem. 1989. 93.
3624.

Allison, J .. Frog. Inorg. Chem. 1986. 34. 627.

Sabo. M.S.. Ph.D. Disseration. Michigan State University. 1991.
Ziegler. K. Adv. Organometal. Chem 1988. 6. l.

Aguilo. A. Adv. OrganometaL Chem 1967. 5. 321.

Wilkinson. G. Proc. R.A. Welch Found. Conf. Chem Res. 1966. 9.
139.

Kochi. J .K. "Organometallic Mechanisms and Catalysis". Academic
Press. New York. 1978.

Ungerman. C. J. Am. Chem. Soc. 1979. 101. 5922.
Radecki. B.D.: Allison. J J. Am. Chem. Soc. 1984. 106. 946.
Winters. R.E.; Kiser, R.W. Inorg. Chem. 1965. 4. 157.

Westmore. J .B. in "Mass Spectrometry of Metal Compounds". edited
by Charalambous. J .. Butterworth 81 Co.. London. 1975, p.61.

Nottingham. W.B. Phys. Rev. 1939. 55. 203.

Fox, R.E.; Hickman. W.M.; Grove. D.J.; Kjeldaas. T: Rev. Sci.
Instrum. 1955. 26. 1101.

Darland. E.J.; Ph.D. Dissertation. Michigan State University. 1978.
Rettner. C.; Marinero. E.; Zare. R. Kung, A. in "Excimer Lasers-
1983". edited by Rhodes, 0.. American Istitute of Physics. New York.
1983. p.345.

Berstein R.B. J. Phys.. Chem 1982. 86. 1178.

18.

19.

20.
21.

22.

23.
24.
25.

26.

27.

29.

30.

31.

32.
33.
34.

35.

36.

37_

Allis

Bear
196

Will-
J acI

MCI
471

Cod
Tsa

Cas

Bat
MCI

18.
19.

20.
2 1 .
22.

23.
24.
25.

26.
27.
28.
29.

30.

31.

32.
33.
34.

35.

36.

37.

38.

43
Allison. J .. Ph.D. Disseratation. University of Delaware. 1977.

Beaucharnp. J .; Anders, L.; Baldeschewieler. J. Am Chem. Soc.
1967. 89. 4569.

Wilkins. C.L.: Gross. J .L.: Anal. Chem 1981. 53. 1661A.
Jacobson, D.B.; Freiser. B.S.; J. Am Chem Soc. 1988. 105. 5197.

McIver. R: Dunbar. R. Int. J. Mass Spectrom and Ion Phys. 71, 7,
471.

Cody. R.; Burnier. R.; Freiser. B Anal. Chem 1982. 54, 96.
Tsarbopolous. A.; Allison. J J. Am Chem Soc. 1985. 107. 5085.

Cassady. C.J.; Freiser. B.S.: McElvany. S.W.; Allison. J J. Am Chem
Soc. 1984. 106. 6125.

Babinec, S.J.: Allison. J. J. Am Chem Soc. 1984. 106. 7718.
McElvany. S.W.; Allison, J Organometallics 1986. 5.416.
Stepnowski. R.; Allison. J. J. Am Chem Soc. 1989. 1 1 1. 449.

Fehensenfleld. F.C.: Ferguson. E.E.: Schmeltekopf. A.L. J. Chem
Phys. 1966. 44. 3022.

Adams. N.G.: Smith. D. Int. J. Mass Spectrom and Ian Phys. 1976.
21. 349.

Lane K.R.; Lee. R.E.: Sallans, 1.; Squires, R.R.: J. Am Chem Soc.
1984. 106, 5767.

Graul. S.T.; Squires, RR. J. Am Chem Soc. 1990. 112, 2517.
Wolfgang. R.; Cross. R.J. Jr. J. Phys.. Chem 1969. 73. 743.

Herman, J .D.; Kerstetter, J .D.; Rose. T.L.; Wolfgang R. Rev. Sci.
Instrum. 1969. 40. 538.

Vestal. M.L.; Blakley. C.R.; Ryan, P.W.; Futrell, J .H. Rev. Sci.
Instrum 1976. 47, 15.

Hansen. S.G.; Farrar. J .M., Mahan. B.H. J. Chem Phys. 1980, 73,
3750.

Shukla. A.K.; Anderson. S.G.; Howard, S.L..Sohlberg. KW.; Futrell,
J .H. Int. J. Mass Spectrom and Ion Process. 1988, 86, 61.

Futrell, J.H.; Miller. C.D. Rev. Sci. Instrum 1966. 37. 1521.

39.

40.

41.

42.

43.

45.

46.

47.

Armenu
66. 216

Amient

Armeni
76. 244

Halle. '
963.

Teloy,

- Antler

75. 21

Ervin,
87, 35

Enin.

Amie:

39.

40.
41.

42.

43.
44.

45.

46.
47.

44

Armentrout. P.B.; Halle. L.F.: Beauchamp, J .L. J. Chem. Phys. 1977.
66. 2162.

Armentrout. P.B.; Beauchamp, J .L. J. Chem Phys. 1981. 74, 2819.

Armentrout. P.B.; Halle, L.F.: Beauchamp, J .L. J. Chem. Phys. 1982,
76, 2449.

Halls, L.F.; Armentrout, P.B.; Beauchamp Organometallics 1982. 1,
963.

Teloy. E.; Gerlich, D. Chem. Phys. 1974. 4, 417.

Anderson. S.: Houle. F.; Gerlich. D.; Lee, Y. J. Chem. Phys. 1981,
75. 2153

Ervin. IL; Loh, S. ; Aristov,N.; Armentrout, P. J. Phys Chem. 1983.
87. 3593.

Ervin, KM. ; Armentrout. P.B. J. Chem. Phys.1985. 83. 166.
Armentrout. P.B.; Beauchamp, J .L.: Acc. Chem Res. 1989. 22, 315.

h

 

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45

Chapter 2

Ion Beam Instrument Design and Construction

 

1, mcrvigw

As discussed in Chapter 1. there are very specific design and
operational criteria that are required in an ion beam instrument intended
to perform accurate thermochemical measurements. These criteria
include the ability to generate primary ion beams which are well
characterized / controlled with respect to translational as well as internal
energy. Ideally. the experimental apparatus would be able to monitor
state-specific or state-characterized ion/ molecule interactions. Scattered
primary and secondary ions must be efficiently contained and
transmitted to the secondary mass analyzer. The detection system
should provide high (unit) detection efficiency of ionic species and exhibit
minimal mass-dependent discrimination.

A block diagram of the MSU ion beam instrument is found in Figure
1.7. In this chapter. the design and operational principles of each major
component comprising the ion beam instrument will be detailed. In
addition. a description of the the vacuum chambers and pumping system
will be provided.

A schematic representation of the instrument is shown in Figure 2.1.
The primary ion generation system consists of a MAT CH5-BF double-

focusing mass spectrometer of reverse (BE) geometry. Ions may be

46

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47

produced by a variety of ionization schemes including electron ionization
(El). chemical ionization (CI), High Pressure Drift Tube (HPDT) processes.
and fast atom bombardment (FAB). The primary mass analyzer is
followed by an intermediate continuous dynode. off-axis detector. This
allows operation of the primary mass spectrometer independent of the
complete ion beam optical system.

A novel two-stage deceleration system provides attenuation of the
primary ion energt from a high nominal initial value to the low energt
regime of interest prior to injection into the collision (interaction) region.
Chapter 3 is devoted entirely to the this lens system. It includes a
rigorous theoretical treatment that promotes an understanding of ion
deceleration optics.

The collision region is comprised of an ion beam guide and a static-
gas collision cell. A radio-frequency (rf-only) octopole ion beam guide
passes through a chamber which contains the neutral collision partner
at a number density sufficient for single-collision conditions. A set of
transfer optics extracts ionic species from the collision region and
focuses them into a quadrupole mass filter. This secondary mass
analyzer is used to discriminate product ions from primary ions. A
second set of optics is used to transfer ions into the final detector region.
Two types of charged particle detectors are employed. A continuous
dynode electron multiplier is positioned along the ion optical axis for
measurement of high current ion beams. A sensitive Daly [l] detector is
used to quantitate low intensity ion beams.

To appreciate the complexity of the ion optical system. it is useful to
consider the kinetic energy of an ion as it traverses the various regions of

the ion beam instrument. A diagram of the ion kinetic energy as a

48

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36800 zan—

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deceleratt
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30,000 e‘
The p
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49

function of position in the instrument is plotted in Figure 2.2. When the
El source is used. for example. ions are initially formed with a thermal
distribution of kinetics energies dictated by the source temperature (~300
K). The nascent ions are strongly accelerated to the nominal mass
analysis energy of 1000. 2000. or 3000 eV. The high energy beam is
decelerated and introduced into the collision region. The final interaction
energy may range from near thermal (<1 eV) to several hundred eV in the
laboratory frame. Mass analysis in the quadrupole analyzer occurs at
kinetic energies ranging from <10eV to ~100 eV. When the Daly detector
is employed. ions are accelerated after mass filtering to an additional
30.000 eV.

The performance of the ion beam instrument and the ultimate success
of this project was dependent on the ability to transfer ions efficiently
throughout a wide energy range and an optical path that approaches 4
meters. It must be emphasized that this could not have been
accomplished without the development of a high performance ion optical
system. Therefore. each component constructed for the instrument was
designed using a sound understanding of ion optics based on theoretical
modeling. The PC-based ion trajectory simulation program SIMION (2]
was implemented in the design of each component in the ion beam
instrument.

II V m ham m in m
The vacuum chambers associated with the CH5-DF are constructed

out of welded stainless steel. Flange seals are simple compression type

made with tin/ silver or gold wire gaskets. The entire vacuum housing

may be i
heaters.
spectromi
diffusion
CFM]. (
following
electric s.
(150 1/ s)
mecham
diffusio:
(P01yphe
be isola
different
loads a
Sufficier
preSSun
with PEr
The .
housed
CH5~DF
SectOFlj
SPECtrOJ

dEtectOI

50

may be baked out with the use of externally applied surface-plate
heaters. Three separate pumping regions are maintained on the mass
spectrometer. The ion source region utilizes an Edwards E04 (600 1/ s)
diifusion pump backed by an Edwards E2M8 two stage rotary pump (5.8
CFM). Conductance into the flight tube is limited by an aperture
following the ion source slit. The magnetic sector flight tube and the
electric sector chamber are each separately pumped by an Edwards EM2
(150 US) and diffusion pump. These share a common Alcatel 5 CFM
mechanical pump. To maintain clean high vacuum conditions. the
diffusion pumps are cold trapped at -60 °C and Santovac 5
(polyphenlylether) is used in all three pumps. Each pumping stack may
be isolated from the vacuum chamber by a butterfly valve. This
differential pumping scheme allows the source to accommodate high gas
loads as encountered when using CI and FAB while maintaining
sufficiently low pressures («-10'6 torr) in the mass analysis regions.
Pressures may be monitored in the source chamber and flight regions
with Penning gauges.

The deceleration lens. octopole. quadrupole and detector system are
housed in two rectangular vacuum chambers. Connection between the
CH5-DF and the beam line chambers is accomplished by using a large
sector-linking bellows assembly salvaged from a DuPont CEC-l 10 mass
spectrometer. An adapter flange couples the bellows to the original
detector region of the CH5-DF immediately following the exit slit after the
electric sector. The off-axis detector is situated near the bellows
assembly. The bellows is terminated by an Airco 2" ID pneumatic gate
valve which allows isolation of the primary mass spectrometer from the
remainder of the beam instrument.

The rm
wide. and
aluminum
rectangul:
length of
mounted
are made
access to
vacuum
requirem
deve10prr
aCCOmplj

The ii
the decel
is also 5
dE‘tector
p ”mpin

51

The two beam chambers are each approximately 56 cm long by 30 cm
wide. and 30 cm in height. They are constructed from 1.6 cm thick
aluminum plate stock with inert—gas welds on the internal seams. A
rectangular aluminum ion optical rail is suspended through the entire
length of the beam chambers. The ion optical components are easily
mounted and are interchangable on the rail system. The chamber tops
are made of 1.27 cm thick tempered-glass providing complete visual
access to the chamber interior. Although the large dimensions of the
vacuum chambers are non-ideal from a vacuum system (pumping
requirement) standpoint. the environment is well suited to instrument
development. All seals in the beam chamber tops and side flanges are
accomplished using elastomer o-rings.

The first chamber is divided into two separate pumping regions: (a)
the deceleration region and (b) the octopole region. The second chamber
is also separately pumped and houses the quadrupole mass filter and
detector system. Pressure transmittance is limited between the three
pumping regions by ion optical apertures. which provide the only
conductance paths between regions.

The deceleration region is pumped by a Varian M4 diffusion pump
(800 US) and is backed by a Sargent Welch Duoseal R—1376 mechanical
pump (10.6 CFM). A Bendix water-cooled chevron baffle is employed to
prevent backstreaming of pump fluid. The octopole/ collision cell region
employs a Varian VHS-6 diffusion pump (2400 Us) that is backed by a
Cemco Hyvac 45 mechanical pump (18 CFM). A high speed Varian 362-6
cold trap (2000 Us) tops the diffusion pump and is cooled to -60 C by a
closed-cycle refrigerator. The quadrupole / detector chamber pumping
system consists of a Varian VHS-6 Diffusion pump which is backed by a

11.6 CF 1
all three
diffusion

The 1:
Airco pn
system 1
the tern}:
foreline
Pump fa
COOIing V
Valves a:

The I
”Sing B;
base pn
Pressure
the octo
lowel- ir
preSSUri
usually
even 11111

H0we
high Va<
mange
To mini:
Such as
rapidly 1
ngm

52

1 1.6 CFM Trivac mechanical pump. Due to the large reservoir volumes.
all three beam chamber pumps are filled with silicone based (DC 705)
diffusion pump fluid.

The beam chamber pumps are connected to the chamber housing via
Airco pneumatically-operated gate valves. A relay-logic safety interlock
system provides fail-safe operation [3]. The interlock system monitors
the temperature of each diffusion pump and the foreline pressure. If the
foreline pressure exceeds a preset limit because of a leak or mechanical
pump failure. or if the difi'usion pump temperature goes too high from a
cooling water failure. the diffusion pump power is turned OE and the gate
valves are automatically closed.

The pressure in the individual pumping regions may be monitored
using Bayard-Alpert ionization gauges. The vacuum system achieves a
base pressure in the chambers of ~2 X 10‘7 torr. With a high gas
pressure in the collision cell of ~1 X 10'4 torr. the high pumping speed in
the octopole chamber maintains a pressure ~l-2 orders of magnitude
lower in the chamber immediately outside the collision cell. The
pressures in the deceleration and quadrupole/ detector regions are
usually 3-4 orders of magnitude lower than the collision cell pressure.
even under high gas loads.

However. because the aluminum chambers have not had any special
high vacuum pre-treatment. pumpdown time may be excessively long
because of outgassing of adsorbed water after exposure to atmosphere.
To minimize this effect. venting is normally performed with a dry gas
such as nitrogen. In addition. radiant heaters have been installed to
rapidly heat the interior walls of the chamber. The heaters are fabricated
from 300 watt quartz security lamps and may be procured from the local

K-Martl“ s
111. Ion Ge

There 2
DP mass
combinatl
home-bui
EI/ Fi/ FD
iOilizaliOr
Processes
Sources C

Which m‘l
A- Int:

Elecm
and the
Sl'StEmS.
Covered .
trEmSiUor
proceSS 1
Energy a!
interrial
Operating
thresholc
ability to

53

K—MartTM store for about $10.00 each.

III. Ion Generation

There are currently three ion sources in our laboratory for the CH5-
DF mass spectrometer. These include an Intensitron EI-only source. a
combination EI/Field Ionization(FI)/Field Desorption(FD) unit. and a
home-built high pressure drift tube (HPDT) source. The combination
EI/ FI/ FD source may also be used for fast atom bombardment (FAB)
ionization. Ion formation through conventional chemical ionization
processes may be performed using the HPDT source. The ionization
sources of primary interest to this work are the El and HPDT sources
which will be covered in this section.

A. Intensitron Electron Ionization Source

Electron impact ionization has been practiced since the early 19008
and the phenomenon is well understood at least for simple chemical
systems. An excellent review of the fundamentals of El has been recently
covered by Mark (4). As discussed in Chapter 1. the generation of
transition metal ions by El has found widespread utility. The ionization
process may lead to the production of ions possessing excess internal
energy and hence a wide electronic state distribution. However. excess
internal energy and excited state populations may be minimized by
operating the ionization source with electron kinetic energies near the
threshold required for production of the species of interest. Thus the
ability to finely control the electron kinetic energr is a key feature of a

versatile i
The M
Figure 2.1
by the pi.
end. The
exit the ‘
pusher a
ionizatim
in the V1(
iOilizatio:
PTOduct
kinetic e
Verififid (
the rf~on
long;
The accc
beam al
the slit 1
the aCce
pan“ of
the resis
Dotenua
1mperfeC
The e
the elfict

vOlts.

54

versatile ion source used to studied ion thermochemistry.

The MAT Intensitron source is shown schematically in the top panel of
Figure 2.3. It consists of an ionization box or housing which is bounded
by the pusher on the back end and the extraction plates on the ion exit
end. The split extraction plates form a narrow slit through which ions
exit the ionization box. Potentials applied to the ionization housing.
pusher and extraction plates determine the electric field in the region of
ionization. The source design provides a very homogeneous electric field
in the vicinity of electron/ neutral interactions. A voltage diagram of the
ionization region is found in the center panel of Figure 2.3. According to
product literature. the source is designed to produce ions with an initial
kinetic energy spread of ~0.3 eV. This kinetic energy spread has been
verified experimentally using retarding potential field measurements with
the rf-only octopole.

Ions are accelerated out of the source through a series of electrodes.
The acceleration lens design allows for independent focusing of the ion
beam along the length of the source slit (z-direction) and transverse to
the slit (r-direction). The potentials applied to the various electrodes in
the acceleration system are shown in the voltage diagram in the bottom
panel of Figure 2.3. The source control unit (CH5 Module BHD) houses
the resistive divider network that supplies the accelerating and focusing
potentials. Most of the electrodes are split to provide compensation for
imperfections or asymmetry in the electrode alignment.

The emission control unit (CH5 Module BER) houses the circuitry for
the electron filament. The nominal electron accelerating potential is 70
volts. Emission currents are regulated by monitoring the electron

current incident on the collector. Feedback electronics control the

electron collect

\

Fleur.

55

 

 

 

 

 

 

 

at action In z-dirlctlon bl sacrum m l direction

 

 

I
2
d
I
S
' C
l 2 g
5 2
OJ IV- '
av - mm
”' sum
W
am
i
a
W
. mu

 

 

 

Figure 2.3. Intensitron Electron Ionization Source Schematic Diagram

current th
current. w
electronics
other than
enery to 1
a precisim
the emiss

burnout (

B. Hi5

56

current through the filament to maintain the desired electron emission
current. which is continuously variable from <1 iiA up to 1 mA. The
electronics unit also has provisions for operating at an electron energy
other than 70 eV. An appearance potential (AP) mode allows the electron
energy to be finely adjusted between 4.5 and 29.5 eV through the use of
a precision potentiometer. When the source is in the AP mode. however.
the emission current is fixed at a level of 15 uA to prevent space-charge

burnout of the filament. which is prevalent at low electron energies.

B. High Pressure Drifi Tube Ion Source

Molecular cations created via El mechanisms often have appreciable
excess internal energy which may result in unirnolecular decomposition.
The fragment ions may exist in excited vibrational and/or electronic
states which may be sufficiently long lived (210'6 s) to be extracted from
the source region before relaxation to the ground state. In fact. excited
state lifetimes of bare transition metal ions are calculated to be on the
order of seconds due to parity-forbidden relaxation schemes [5]. Hence it
is often difficult to assess or control the electronic and vibrational states
of species formed via EI. This introduces a great deal of uncertainty in
the true ion/molecule interaction energy and precludes assignment of
relative state reactivity. This is particularly a problem with transition
metal ions since there is a high density of low-lying electronic states
available for population.

For example, the first (4F) and second (4D) excited states of Fe“ lie
0.25 and 0.98 eV above the ground state (6D). respectively [6]. Elkind
and Armentrout [7] have studied the state-specific reaction of Fe“ with

H2 to form

 

orders of n
generated?
translation
high-lying 1'

One me
the excited
drift tube.
uniionn 101

 

 

buffeY/i‘eag
technjque 1
181 to form
I“titrell anc
ground Sta

An ioni
tube (HPD
beam inst
Figute 2.4
The Hpm
1 Cm inte
“33 of ..
Ecrpendi‘
aQCOmmC
housing :‘

plates W]

57

H2 to form FeH+ and found that the first excited state was over two
orders of magnitude more reactive than the ground state. When Fe+ is
generated by El it exhibits some reactivity towards H2 at low
translational energies. This has been attributed to the population of
high-lying (>1 eV) electronic states corresponding to ~44% of the ions.

One method employed to quench excited states involves interacting
the excited ions with a thermal population of inert buffer gas inside a
drift tube. A drift tube is a cylindrically symmetric chamber which has a
uniform longitudinal electric field. The chamber is usually filled with a
buffer/reagent gas to a pressure ranging from 0.01 to 1.00 torr. This
technique has been successfully exploited by Armentrout [6] and Bowers
[8) to form transition metal ions with minimal excited state populations.
Futrell and coworkers have also used high pressure sources to generate
ground state noble-gas ions [9].

An ionization source that incorporates an integral high pressure drift
tube (HPDT) region has been designed and constructed for the MSU ion
beam instrument. A cross-sectional view of the HPDT source is shown in
Figure 2.4. It is similar to the designs of Butrill (10] and Munson [11].
The HPDT source is comprised of an aluminum ionization housing with a
1 cm internal diameter. A circular electron entrance aperture with an
area of ~0.1 mm2 permits the entrance of an ionizing electron beam
perpendicular to the direction of ion motion. The ionization housing
accommodates the standard CH5 filament assembly. The ionization
housing is followed by a series (1-5) of 0.25 inch thick aluminum drift
plates with a 1 cm diameter cylindrical bore. The terminal drift plate
accommodates a 0.005 inch thick stainless steel (SS) circular aperture.
The ion exit plate can be easily interchanged with others that have

Gas [file

T Capa
M(linemeq

FigUre

58

 

Ion Exit Plate

 

Figure 2.4. High Pressure Drift Tube Source schematic diagram.

59

Repeller Plate

 

 

 

ionization Housing

a<
All
(EU

:3
l0
AVA A
A'

 

Drift Electrodes

:o
E

 

 

Ion Eidt Plate

 

 

_<2
I
I'l
.2U

 

 

 

 

"'"" V
Accelerating ‘ Switch 3- e- High Bias

I Filament

DC Bias

 

v

Figure 2.5. High Pressure Drift Tube Source circuit diagram.

various ex

center-lint

 

plate. An
opening tr
thickness
rear plate.
clamped t
hole cerar
the Stand
exiting (h
lens Syste

Care 1
SOUrce. 1
PM in t
construe
Inanornet
0.001», :

maintain

60

various exit apertures with either circular or rectangular dimensions. A
center-line gas inlet is provided in the 0.0625" thick SS rear repeller
plate. An optically flat SS screen has been spot-welded over the gas inlet
opening to maintain a uniform electric field. Teflon gaskets of 1 mm
thickness provide electrical insulation and a gas tight seal between the
rear plate. ionization housing. drift rings and ion exit plate. The unit is
clamped together with four SS threaded rods that run through single-
hole ceramic rods for electrical isolation. The entire HPDT unit replaces
the standard ionization housing on the Intensitron El source. Ions
exiting the HPDT source are accelerated and focused by the El source
lens system.

Care has been taken to minimize the conductance out of the HPDT
source. Pressure inside the source may be measured via a second gas
port in the rear repeller plate. Alternatively. a special drift ring was
constructed with an US" ID conduit to connect to a capacitance
manometer. With an ion exit slit having a height of 4 mm and a width of
0.001". a pressure differential of ~4 orders of magnitude may be
maintained between the interior of the drift tube and the surrounding
source vacuum manifold. This is sufficient to provide high pressure drift
tube conditions of >0. 1 torr while minimizing ion losses due to scattering
collisions outside of the drift tube.

The standard EI electronics have been modified for operation of the
HPDT source. The circuit diagram for the source is shown in Figure 2.5.
One modification has been implemented to increase the kinetic enery of
the ionizing electrons entering the drift tube to >200 eV. This enables
better penetration of the electron beam into the high pressure gas and

also allows for higher emission currents since the space-charge limit

increase
the elec
housing
potenti:
current
two cur

The
lFariablt
in this
divider
lhe vat
the file
nomin:
Valiabl
hOUSir
magnn
Energy

The

elecu-C

61

increases with electron energy. The electron kinetic energy is dictated by
the electric potential difference between the filament and the ionization
housing. The filament is normally biased relative to the accelerating
potential. Thus. by simply lowering the bias voltage of the filament
current power supply the electron energy may be increased. There are
two currently available circuit schemes that accomplish this.

The first scheme involves a voltage divider circuit that allows a
variable potential to be used as the filament current power supply bias.

In this mode the accelerating potential is connected to a three-resistor
divider network. The center resistor. R4. is a precision variable resistor.

The value of the resistors are selected such that the reference potential to
the filament supply is approximately 100 to 200 volts lower than the
nominal accelerating potential. The normal 70 eV electron energy is then
variable between 170 and 270 eV. In drift tube operation the ionization
housing is biased higher than the accelerating potential by the
magnitude of the applied drift voltage. Thus to calculate the electron
energy in this mode. the drift potential must be taken into account.

The second mode of high electron energy operation provides a fixed
electron energy of ~270 eV. This is accomplished not by changing the
reference potential to the filament electronics. but rather by internally
changing the voltage offset to the filament. The normal offset is changed
from 70 volts to 270 volts through a voltage regulator circuit inside of the
emission control unit. This mode of high energy operation has the
advantage of utilizing the emission control circuits.

The electron emission may also be regulated by the conventional EI
circuits if a few wiring changes are employed. Normally. the electron

current on the collector is used as the feedback for filament current

control.
current i
filament
that norr
A uni
creating
The total
be variec
series (1:
evenly a
neton-k
resistors
adJUStec
Potentjo
source a
consignI
used to
potentia
kinetic E
adeStjn
conditim
Under H]
SUCC:
generaut
Volume C
describer

62

control. Since the HPDT source does not utilize a collector. the electron
current to the ionization housing may by monitored and used as the
filament current feedback. This is accomplished by connecting the lead
that normally goes to the collector directly to the ionization housing.

A uniform potential gradient is maintained in the drift region by

creating a uniform voltage drop between each of the drift ring electrodes.
The total drift field voltage is determined by the potential V2. which may

be varied by a precision potentiometer R2. A pair of 67 volt batteries in
series (134 V) supplies the voltage source for V2. The drift field voltage is
evenly applied to each of the drift electrodes via an integral divider
network formed by linking the electrodes with 1 Ma high-precision

resistors. The potential of the rear repeller plate may be independently
adjusted using the voltage source V3 (normally 9 volts) and the

potentiometer R3. The ion exit plate is usually biased at the nominal
source accelerating potential. However. an additional circuit feature
consisting of the voltage source V1 and the potentiometer R1 may be
used to trim the ion exit plate potential relative to the accelerating
potential. This is useful when ions exiting the source have a slight
kinetic energy component along the axis of ion beam propagation. By
adjusting the potential to the ion exit plate. the standard operating
conditions of the accelerating lenses and mass analyzer may be utilized
under HPDT source operation.

Successful operation of the high pressure source implicates the
generation of a highly uniform electric field throughout the internal
volume of the source. The ion optical properties of the drift tube source
described here have been thoroughly modeled using SIMION. Figure 2.6

shows a cross-sectional view of the source. In this model a total drift

....... .::.:..lEEEi-EEEEEEEE
notebooi 5.5

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63

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01 Partic
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InOTIOatr
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Mason

64

potential of 100 V was applied and the equipotential lines are plotted for
5 volt increments. It can be observed that a highly parallel electric field
is present for greater than 70% of the internal diameter.

The theory of ion motion in a drift tube has received extensive
treatment in the literature [12.13]. The salient points relavent to the
state characterization of transition metal ions are found in reference 14.
In addition to the therrnalization of excited electronic states. the HPDT
source can be applied to vary the internal energy of polyatomic species.
Of particular interest is the excitation of vibrational modes in molecular
ions. The kinetic energy gained by ions under high-electric field
acceleration is attenuated by collisions with the neutral bufi‘er gas. A
portion of the kinetic energy is converted into translational energy of the
monoatomic neutral collision partner. The internal energy of the ion is
also increased as a consequence of collisions. The steady—state kinetic

energy of the ions in a high pressure drift field is often viewed as an
effective translational temperature. Terr. This has been quantitated by

Mason [11] as a function of the drift velocity. Vdnfi, as expressed below

Terr = T + Mdeft12/3kb

Where is kb the Boltzmann constant. and the parameters T and M are
the temperature and mass of the buffer gas respectively. Under high
pressure and drift field conditions (>100 volts/cm-torr) the effective ion
temperature can achieve several thousand kelvins. The effective internal
energy of the ions can also be on the order of the effective translational
temperature. Thus. molecular ions can be made to populate elevated

vibrational levels and possibly higher electronic states. By varying the

field stre:
Excess vi
internal 1
energy in
byimplei

The p
from che
t0 gener
collision
tnvestig;
field cor
the Wor
HOWCVe
t.Vpicall
Produc.
the Ver
conch1C
%

The
Sheet“
SectOr
and f0
focusu

In '

magne

65

field strength. the extent of ion vibrational energy can be controlled.
Excess vibrational energy can. of course. lead to ion fragmentation when
internal energies exceed bond dissociation values. Thus. the effects of
energy in vibrational as well as translational modes may be investigated
by implementation of the HPDT source.

The properties of the HPDT source allow for the production of ions
from chemical ionization processes. In addition. the source may be used
to generate species that arise from endothermic processes such as
collision induced dissociation. This source has been characterized by
investigating high pressure (>0.l torr) methane under high and low drift
field conditions. The ion populations observed agree quantitatively with
the work on methane chemical ionization by Munson and Field [15].
However. one drawback of the HPDT source is that the ion output is
typically one or more orders of magnitude lower than ion currents
produced by the high efficiency Intensitron El source. This is because of
the very small ion exit slits employed in the source to minimize gas

conductance.

IV. Primary Mass Analyzer

The primary mass analyzer is a MAT CH5-DP double focusing mass
spectrometer of reverse (BE) geometry. It consists of a 90° magnetic
sector followed by a 90° electric sector. The principals of ion dispersion
and focusing in sector fields as well as the important concept of double
focusing will be briefly discussed.

In Chapter 1. the force experienced by an ion moving in a uniform

magnetic field was described. The trajectory of an ion with mass M and

be

66

charge q moving with velocity v perpendicular to a magnetic field, B. will

have a radius rB given by

['3 = MV/qB

Thus ions will be dispersed in a magnetic field as a function of their
mass and velocity. Magnetic sectors may be used as ion momentum
filters. In addition to dispersion. a uniform magnetic field also provides
focusing of ion trajectories. A trajectory diagram for a 60° magnetic
sector is shown in the top panel of Figure 2.7 . Consider a beam of ions
originating at the same point source and having identical velocities but
having different initial directions to form a distribution of initial
trajectory angles (divergent beam). If the ions are directed into a
magnetic sector of the the proper geometry. the magnetic field will
provide refocusing of the ion beam at a distant point in space. This type
of focusing is referred to as angular or direction focusing.

In a uniform electric field. E. an ion will have a trajectory of radius rE,
given by

r5; = 2KE/qE

Where KE is the kinetic enery and q is the charge on the ion. Thus
an electric field will provide radial dispersion of ions on the basis of
kinetic energy. In addition. directional focusing of angularly diverging
ions may also be obtained in a field created by concentric parallel plates
similar to the phenomenon in a magnetic field.

Under perfect magnetic focusing conditions, the dispersion in the ion

beam at the angular focal point in a magnetic sector system will be

67

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2.7. Neir Double Focusing Mass Spectrometer.

68

limited only by the monochromaticity of ion kinetic energy. In the early
19305, Dempster recognized that the resolution of a magnetic mass
analyzer could be improved by using an electric sector to provide kinetic
energy filtering of the ion beam preceding the momentum analyzer.
However. this type of tandem sector arrangement was not considered
true double focusing. On the other hand. if a magnetic sector is
combined with an electric sector such that the dispersion due to velocity
in the magnetic sector focus is equal and opposite to the velocity
dispersion in the electric sector. perfect double focusing may be achieved.
In this configuration a sizable ion kinetic energy spread may not preclude
high-resolution operation.

Neir has described a forward geometry (EB) mass spectrometer
configuration that also provides double focusing [16]. The schematic
diagram of a Neir double focusing instrument is shown in the lower
portion of Figure 2.7. In this design. the intermediate slit between the
two sectors is quite wide. The electric sector effectively energy-disperses
divergent ions of different energies into a plane of energy-resolved beams
at the entrance to the magnetic sector. In this way. the ions are
presented to the magnetic sector at the proper spatial positions to be
momentum-focused to a single point in space after traversing the
magnetic sector. This geometry tolerates an appreciable ion kinetic
energy distribution while maintaining high mass resolution.

Thus the CH5-DF mass spectrometer of BE design is capable of
transmitting ion populations with a kinetic energy spread on the order
1% or more of the nominal accelerating energy. The energy acceptance
for our instrument as a function of accelerating potential has been

characterized and may be found Appendix A.

hi

69

The instrument has a mass range of up to m/z 3.600 when operated
at an acceleration potential 1000 V. The magnetic field strength is
measured using a Hall effect probe. The field strength is displayed in
Hall m/z units normalized for operating at a 3000 V accelerating
potential. The relationship between Hall values and ion m/z values for
operation at 1000. 2000. and 3000 volt accelerating potentials is
documneted in Appendix B.

The mass resolution is dependent on the source (object) and detector
(image) slit dimensions. The nominal resolution is ~250 with both slits
wide open. The maximum attainable resolution is ~20.000 according to
the instrument specifications, but has a practical limit of <3.000. For
ion/molecule investigations in the beam instrument. the slits are usually
maintained at full width to provide maximum ion current.

V rmi ff-

In the CH5-DF stock configuration. an in-line Faraday cup and
secondary electron multiplier were located immediately following the
electric sector exit (image) slit in a small vacuum housing. The detector
housing has been replaced with a flexible bellows assembly which
provides a vacuum interface to the beam chambers. (The bellows
assembly was originally used as the electric/ magnetic sector union in a
DuPont CEC-l 10 mass spectrometer.) An off-axis detector was designed
and implemented in the transfer region. This provides for detection of
ions, immediately after the electric sector. independent of the terminal
detector system.

A schematic diagram of the interface bellows and off-axis detector

70

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71

system is shown in Figure 2.8. The bellows assembly was modified by
welding a 2.5 " ID tube perpendicular to the central axis. A continuous
dynode detector and beam deflection system was mounted onto the
flange capping the tube. The detector was constructed from components
of a Galileo 4770 series Channeltron detector. The inlet horn of the
detector is positioned at an angle of approximately 70° with respect to the
ion beam optical axis. An integral pair of deflection electrodes with a
spacing of 1.3 cm is centered on the ion optical axis. The upper
deflection electrode has a 1 cm diameter aperture through which ions
pass to reach the input of the electron multiplier. A high transmission
SS grid has been spot welded over the aperture to minimize penetration
of the detector dynode bias potential. Additional shielding of the dynode
has been achieved by a connecting a SS plate to the aperture electrode.
This shield runs parallel along the length of detector.

If no potentials are applied to the detector system. the ion beam
exiting the CH5-DF continues on to the ion beam optical system
uninterrupted. Potentials applied to the aperture and repeller are
dictated by the ion beam energy. For a nominal ion energy of 3000 eV.
the repeller is normally held at ground potential. The detector is
normally biased at around ~2900 volts and the aperture is set at ~-2800
volts. The potential applied to the aperture may be varied through the
use of a voltage divider circuit that is powered from the detector bias
voltage supply.

The detector current generated by the off-axis detector is
approximately two orders of magnitude lower than that recorded at the
terminal in-line detector for the same ion beam current. There are two
main factors that contribute to the relatively low sensitivity. First, the

72

4770 Channeltron used in the off-axis system was salvaged from another
mass spectrometer. It had been removed from service since its useful
current-gain lifetime had been expended. Second, the terminal in-line
detector is a 4800 series high gain model from Galileo which has an
inherent gain two orders of magnitude greater than the 4770 series
detector.

One problem encountered in operating the off-axis detector involves
charge build up on insulating surfaces inside the interface vacuum
housing when the highest energy ion beams were in use (3000 eV).
During detector power-up. the deflection optics direct the primary ion
beam onto surfaces which are non-conducting. The primary ion beam
may also strike (conducting) surfaces from which charged species are
sputtered onto non-conducting surfaces. As a result, when the off-axis
detector is turned off and the ion beam proceeds into the beam chamber.
it travels through a region which contains residual charge (stray fields).
This causes the ion beam to be deflected in a nonsyrnmetric manner.
This beam distortion phenomenon was observed with the use of a
Channel Electron Multiplier Array (CEMA) detector. which provides a
visual representation of the ion beam profile. Under surface charge
conditions the normally rectangular beam profile would become highly
asymmetric and take on the appearance of a Normal Smiley Face (NSF).
In fact. under certain conditions the author has observed the beam
profile to strangely resemble various celebrities such as Vanilla Ice. Milli
Vanilli. and, of course. Elvis (from an era prior to his final pathetic and
obese state). When beam distortion occurred. it would gradually
diminish over the course of several hours as the static charge dissipated.

However. be must be noted that beam distortion was prevalent only when

73

the ion energy was >>1000 eV.

VI. Deceleration Lens

The deceleration lens utilized in the beam instrument is fully described
and characterized in Chapter 3. A complete analysis of ion deceleration
optics from both a theoretical and a practical approach is included. The
ion deceleration lens employed in the MSU ion beam instrument presents
an ion beam with minimal angular divergence (<5°) to the octopole beam
guide. Ions may be decelerated from the initial accelerating energy to to
<1 eV with excellent transmission efficiency.

The energy/mass selected ion beam is directed into a reaction zone
containing a neutral reagent gas. Depending on the reaction dynamics
and the interaction energy. secondary ions can be widely scattered about
the collision center in the laboratory frame. Quantitative threshold
measurements require product ions from the ion/ neutral interaction to
be efficiently collected for subsquent mass analysis. The MSU ion beam
instrument employs an ion beam guide that provides dynamic ion
trapping in the ion / neutral interaction region.

There are several stringent requirements for an ion beam guide
suitable for use in an instrument intended for thermochemical
measurements. It must provide highly efficient trapping of ions scattered
off-axis in the laboratory frame. Ions of different mass-to-charge ratios

should be transmitted with equally high. efficiency he no mass

74

discrimination effects). This is especially important for reactive collision
processes where the charged species may instantaneously have a very
large positive or negative change in mass. Quantitative measurements
require that the ion population in the collision region be effectively
transferred into the second mass analysis region. The ion beam guide
must thus possess strong focusing properties. Finally. and perhaps most
importantly. the dynamic trapping process associated with the ion beam
guide must not greatly perturb the initial ion kinetic energy distribution
so that the ion/neutral interaction enery may be known with a high
degree of certainty.

The octOpole ion beam guide first employed for ion/ molecule reaction
containment by Teloy and Gerlich (17] fulfills the above operational
criteria. The ion optical principles that govern dynamic trapping in an
octopole will be briefly discussed. A detailed description covering the
design and construction as well as the the electronics associated with

our octopole device will be presented.

A. Multipole Trapping Theory

An exhaustive theoretical perspective of oscillatory electric field
multipole devices has recently appeared in the literature as a series of
articles by Szabo (18-21]. A comparative study on use of quadrupole.
hexapole. and octopole ion traps concluded that the higher order
multipole devices are far better suited for rf-only ion beam guides.

An octopole ion beam guide consists of eight parallel cylindrical rods
equally spaced in an octagonal array. Circular rods maybe used to

approximate an ideal eight-fold hyperbolic potential surface. Radio

75

frequency (rt) potentials. 180° out of phase. are applied to adjacent rods.
The inhomogeneous electric field creates an efi’ective trapping potential in
the interior tubular volume of the array. However. there is no net force
exerted on an ion in the longitudinal direction inside of an rf multipole
device.

This concept of dynamic trapping may be demonstrated by the
SIMION model of an octopole cross-section shown in Figure 2.9. The top
panel shows the potential surface created by eight circular rods at a
point in time such that the potential between adjacent rods is very small.
The lower panel shows the same system at a phase in which the potential
between adjacent rods is much greater. In times of high potential
difference. the octopole electric field creates a radial force towards the
center of the array for regions of the surface of higher potential. such as
indicated by the arrow. As the phase changes. alternate regions of the
internal volume will exhibit centrally restoring forces. If the angular
frequency of operation exceeds the transverse velocity of ions travelling
longitudinally through the device. there will be a net radial force directed
towards the central axis of the array. For example. a hydrogen ion with a
transverse velocity component of 1 eV will require an operating frequency
of >2 MHz.

A modified program for ion optical simulation using time-varying
potentials was developed with SIMION to calculate ion trajectories in rf
multipole devices. Figure 2.10 shows a cross-sectional model of an
octopole trapping device. The system is modeled at a 300 Vp-p potential
operating at an angular frequency of 1 MHz. Trajectories are calculated
for a m/z 100 ion with an initial transverse kinetic energy of 0.026 eV
starting at an initial position. r, of 0.30 of the distance from the center of

76

       
       
 

       
      

   

  

       
       

                    

           

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77

ION THAJECTORY: 0.025 eV. 0.3 Filo)

 

Figure 2.10. SIMION model of trajectories in rf octopole.

78

the array to the rods.

The effective radial potential well for a multipole trap is dependent on
the multiplicity of poles and the physical dimensions of the device. The
potential restoring force. U611. may be expressed a function of the radial

distance from the center of the array as shown below [22].

2
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ion. respectively: ro is the inner radius of the array. The rf potential
applied to the rods is Vocoszknt). Thus it can be seen that the relative
potential is proportional to the number of poles minus 2.

From the above expression. the relative trapping potential was
calculated for a series of multipole devices. The plot of trapping potential
as a function of relative radial distance is shown in Figure 2.11. It can
be seen that the devices with higher orders of poles exhibit radial
potentials that better approximate square-well potentials. Thus, an
octopole provides a more ideal trapping environment compared to lower
order multipoles.

Another point of octopole superiority concerns the transverse ion
energy. The longitudinal kinetic energy is not affected inside the ion
beam guide. However, ions inside the rf field will possess a time-
dependent transverse energy component as they oscillate through the
beam guide. This transverse energy must be considered in the total
kinetic energy of ion/molecule reactions. The transverse motion is

mainly influenced by three initial factors as an ion enters the trapping

79

 

 

 

 

 

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80

field: (a) off-axis spatial orientation. (b) phase of the rf—field and (c) the
initial transverse velocity component.

The various factors that influence transverse kinetic energy for both
quadrupole and octopole beam guides were studied. SIMION models
with time-varying potentials were used to determine the relative effects of
rf phase, initial position. and initial angle of entry. An ion was subjected
to oscillatory forces for a typical residence time of 20 ms and the
transverse kinetic energy was then plotted as a function of time. One
such result is shown if Figure 2.12. In this model. an ion with an m/z
value of 100 and nearly thermal initial kinetic energy (0.02 eV) was
started at a position of 0.2 r0. Both the quadrupole and the octopole
were operated at an rf amplitude of 300 volts and a frequency of 1 MHz.
It can be seen that the energy of the ion in the quadrupole varies greatly
as a function of time and the value may exceed 1.5 eV at times. The
time-weighted average of the transverse energy in the quadrupole is
0.530 eV. In contrast. the transverse energy of the ion in the octopole
did not exceed 0.1 eV. The time-weighted average kinetic energy of the
ion in the octopole is 0.022 eV. which is very near the initial value.
Numerous trajectory calculations have suggested that the transverse
kinetic energy perturbation by an octopole electric field is usually
minimal when the ion enters the octopole at a radial position of $0.2 r0 .

From the above result. it may be concluded that quadrupole beam
guides would introduce a high level of uncertainty in an ion/neutral
interaction energy. In addition. the initial rf phase relative to the point of
ion injection has an extreme effect on the transverse kinetic energy in
quadrupole devices. but is of little importance in octopole systems.

Another important point involves the mass range over which stability

81

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82

(trapping) is achieved in an rf multipole. The ion stability is determined
by the operational values of the potential and frequency. In a quadrupole
device there is a narrow mass range that may be trapped under constant
voltage/frequency conditions. This introduces mass discrimination for
transmission through an rf-only quadrupole which is deleterious to
quantitative cross-section measurements. An experimental example of
mass-dependent transmission in a quad is documented in Appendix C.
In contrast. the octopole ion beam guide does not exhibit significant
mass-dependent behavior over a wide operational frequency and voltage

range.

B. Construction

A scaled sideview schematic diagram of the octopole ion beam guide
and collision cell is shown in Figure 2.13. The octopole is constructed
from eight 3.18 mm diameter stainless steel precision ground shafts from
Small Parts Inc. The rods are fixed in an octagonal array with an
inscribed radius of 8.73 mm. Four mounts machined from DuPont SP—l
Vespel are used to secure the back-tapped rods with 0-80 screws. The
ion guide has a total length of 33.6 cm. The design yielded a very
mechanically-sound array with a parallel deviation estimated at <0.03
mm.

The central region of the ion guide is surrounded by collision cell
constructed from a 5.1 cm ID SS cylinder which is 7.62 cm long. To limit
conductance between the main chamber and the collision cell interior,
each end of the collision cell is terminated by an extension tube with an

ID of 2.22 cm and a length of 6.35 cm. The diameter is slightly larger

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than the OD of the octopole rod array. A high transmission grid
surrounds the length of the octopole to provide electrical shielding. This
prevents penetration of stray fields and maintains a high degree of radial
symmetry throughout the length of the device. The octopole assembly is
constructed on a single 4" long horizontal mount for the ion optical rail.
It is electrically insulated from the rail by two vertical rods fabricated
from SP-l Vespel.

A schematic diagram of the vacuum chamber containing the
octopole/ collision cell is shown in Figure 2. 14. The collision cell has two
1/4" Cajon Ultratorr fittings positioned at the center of the cell to provide
reagent gas introduction and cell pressure measurement. Teflon tubing
provides electrically-insulating gas transfer lines between the collision
cell and the Cajon gas feedthroughs on the chamber flange.
Reagent/collision gas may be introduced directly to the collision cell
through a Granville-Phillips precision leak valve. Alternatively. the
collision gas may be diverted into the main chamber to make corrective
background cross-section measurements. The pressure in the center of
the collision cell is monitored by an MKS Baratron 310 capacitance
manometer.

The inability to precisely define the effective path length and the gas
density inside the collision cell may. in fact. be the most significant
contribution to the uncertainty of absolute cross section measurements.
Absolute pressure measurements are difficult to make with high
precision and accuracy in the O. l mtorr region. Although hot cathode
ion gauges (Bayard-Alpert type) may be used in this pressure range. they
sufler from gas dependent response because the technique relies upon a

secondary property of the gas such as ionization. The species dependent

86

response is because electron ionization cross sections. and hence the
detector response. varies greatly from species to species.

Thus. it is advantageous to use a pressure measurement technique
that utilizes a primary effect of gas density and temperature, such as
force. Absolute pressure measurements are obtainable using a
capacitance manometer. The mechanism of operation measures the
force exerted by a gas which is independent of the nature of the gas.
However. the uncertainty in of pressure measurement at 0.1 mtorr or
lower may be more than i250/o even with the best capacitance manometer
currently available. Also. the measured pressure must be corrected for
thermal transpiration effects [23].

In addition to gas number density, the effective path length must also
be precisely known for absolute cross section determinations. The
collision cell ends present a relatively high conductance path to the
surrounding vacuum manifold The effective length of the gas cloud at
the measured pressure inside the collision cell is thus somewhat less
than the physical length of the collision cell. Approximations for the
pressure profile through the length of the cell can be applied to give an
approximate effective collision cell length. One model results in a
trapezoidal pressure profile through the length of the collision cell [24].
The effective length of our collision cell can be calculated from the cell
body and extension tube dimensions. We can assume that pressure
inside the 7.62 cm long collision cell body is constant. If the pressure
decreases linearly from the end of the cell body along the extension tubes
then the effective length of the extension tube is one-half of the physical
length (6.35 cm). Thus. the overall effective length of the collision cell
may be approximated as sum of the cell body length plus the effective

87

extension tube lengths which results in a value of 13.97 cm.

C. Octopole Electronics

The oct0pole ion beam guide must be operated at a frequency of 1 to
10 MHz with potentials in the 200 to 2000 Vp-p range in order to provide
efficient trapping and strong focusing. A brute-force method to supply
these power levels would be to simply connect the device to an extremely
high power rf generator. Unfortunately. such a generator costs in excess
of $20,000 and presents a very inefficient and unsophisticated approach
to the operational requirements.

A more appealing solution applied here involves the use of rf
transformers and tuned oscillator circuits. Figure 2. 15 shows the
fundamental circuitry associated with the transformers and oscillating
circuits pertinent to this project. An effective method of stepping up ac
potentials in the rf range is through the use of an air gap transformer.
As shown in the top portion of Figure 2. 15. a magnetic field is produced
by applying an ac current to a primary inductor. If a secondary inductor
is aligned properly in close proximity, the magnetic field from the primary
coil will induce a current in the secondary coil. A voltage gain may be
produced by having a greater number of turns in the secondary coil than
in the primary coil.

A highly efficient means of producing high power ac oscillations at a
fixed frequency is by employing a tuned inductor] capacitor circuit. An
inductor (L) coupled in series to a capacitor (C) is called an LC tank
circuit as shown in Figure 2.15. Under the proper resonance frequency

conditions. power is alternately stored in the capacitor and in the

88

 

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bCTank

 

Octopole rf circuit

Figure 2.15. Radio Frequency Oscillator Circuits.

89

magnetic field of the inductor. An LC circuit has a resonance frequency
of oscillation that is dictated by the value of the inductance. L. and the
capacitance. C. The resonance frequency in cylces/second. f, is given as

r: 1
2MB

 

If no resistive losses were present the capacitor could charge and
discharge itself indefinitely without additional power input. Thus. an ac
potential could be maintained across the capacitor plates. Practical LC
circuits have a finite resistance that is mostly dominated by the length of
wire used in the inductor. The resistance of the inductor is inversely
related to a value referred to as the quality or Q of the inductor. The
resistance of the inductror and attached circuitry determines the amount
of power required to maintain oscillations of a given amplitude. In
addition. internal circuit resistance determines the frequency range over
which resonance occurs. Hence. for high amplitude oscillations at a very
specific frequency. a high quality inductor should be used.

The DC circuit used to generate the octopole rf potentials is shown in
simplified form in Figure 2.15. The inherent rod-to-rod capacitance is
used as the capacitor in the LC tank. The capacitance of the octopole
assembly has a measured value of ~50 picofarads.

A detailed schematic of the entire octopole electronics is shown in
Figure 2.16. It consists of a frequency synthesizer, an rf power amplifier,
a power meter. a tuning circuit and the transformer/ LC tank circuit. A

modified Extrel Model E High-Q head comprises the transformer portion
of the circuit. The primary coil is shown as L1 and the secondary coil,

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91

122, is part of the LC tank circuit. A balance capacitor, Cb, is used to trim
capacitance from one half of the octopole rods to the other in order to
maintain a symmetric push-pull (balanced) waveform on the rods. A DC
bias may be applied to the octopole through a center tap on the
secondary coil. The resonant frequency of the LC (octopole) system is
~2.5 MHz.

A high precision (sub-ppm) Schomandl ND 3OM-B frequency
synthesizer is used to produce the primary signal. The synthesizer was
salvaged from a Bruker NMR and has a continuously variable frequency
range from 0.1 Hz to 30 MHz. The output amplitude is adjustable from ~
0.1 to 1.0 volts at 50 ohms. The signal is fed into a ENI Model A-150
power amplifier. Broadband amplification of up to 150 watts into 50
ohms (85 Vans) may be achieved in the 0.3 to 35 MHz range.

Since tuned LC circuits oscillate over a specific frequency range, the
tank circuit power absorption is frequency dependent. If rf power is not
absorbed by the circuit it is reflected back to the signal source. Reflected
power represents wasted energy that must be absorbed by the amplifier
and should be minimized for efficient operation. In addition, most of the
line circuitry between the secondary inductor and the amplifier exhibit
frequency-dependent impedance and, hence, transmission. General
discussions on rf circuitry and transmission concepts may be found in
any edition of the American Radio Relay League (ARRL) handbook.

To maximize forward power and minimize reflected power. a radio
antenna tuner is inserted in the transmission line between the power
meter and the primary coil. The Heathkit Model HFF-QA antenna tuner
consists of a variable capacitor, C 1, a variable inductor, L. and a second

variable capacitor. C2, in series. By carefully adjusting the values of the

92

capacitors and inductor, a 50 ohm impedance match between the
amplifier and the high Q head may be made. This results in maximum
power transfer and minimized reflected losses.

A relationship exists between forward and reflected power that is
quantitated as the standing wave ratio or SWR. In a perfect reactance
match the SWR approaches a value of unity. The Heathkit Model HM-9
QRP Wattmeter measures and displays the forward power and the SWR
directly. The SWR may be monitored while adjustments are made in the
tuner circuitry until the lowest SWR value is achieved. In addition. the
source frequency may be adjusted until maximum power absorption is
attained. The antenna tuner and source frequency adjustments may be
made in an iterative manner until the best settings are reached.

The tuned circuit described above has achieved phenomenal
efficiency. At a frequency of ~2.5 MHz. the system can produce
potentials of 1000 Vp-p between adjacent octopole rods while consuming
< 1 watt of rf power. For conditions when the SWR approaches unity, an
empirical relationship has been established between the forward output
voltage displayed on the amplifier and the octopole peak-to-peak voltage

as shown below.

Vp-p (octopole) = ~200 Vuns(amplifier)

The octopole has been successfully operated at potentials as high as
5000 Vp-p without dielectric breakdown or electrical discharge in the

vacuum chamber. The octopole system has proven to be very stable over

time and effective at providing high ion trapping/ transmission efficiency.

93

VIII, 'h'ansfer Qpflcs and ficondary Mass Analjger

Ions are extracted from the exit aperture of the octopole beam guide
and transferred to an Extrel quadrupole mass filter for subsequent mass
analysis. The principles of quadrupole mass filters have been well
documented in the literature [25]. The Extrel operator's manual provides
an excellent resource for the practical aspects of the theory, electronics.
and operation of quadrupole mass spectrometers.

The transfer optics must be able to effectively focus ions of a wide
energy range into the quadrupole. Depending on the ion interaction
energy in the octopole and the final ion analysis energy in the quad, the
optics may have to perform both acceleration and deceleration in the
same experiment.

Figure 2.17 shows the geometry of the transfer lens system. which is
similar to the optical arrangement of the Extrel Model 041-1 Axial
Molecular Beam Ionizer. The lens stack consists of 5 independently
controlled electrodes which are isolated by sapphire bearings. The
internal apertures are all 13 mm. The optimal electrode potentials are
determined by the energy range encountered in the octopole/ collision
region and the final analysis energy desired in the quadrupole. Extensive
SIMION modeling has shown that this lens configuration can provide
excellent transmission over a broad energy range.

The voltages for the transfer optics are generated by a single high
voltage power supply and a resistive divider network with eight outputs
at two different voltage ranges. For an initial nominal ion energy of 1000
eV and a quad analysis energy of 50 eV. typical operating potentials are
520, 920, 1026, 907 , and 975 volts for electrodes 1 through 5,

94

  

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respectively. Ion transmission through the transfer optics is nearly
100%.

The quadrupole mass filter is a high transmission Extrel #4-324-9
model combined with a Model 15 High-Q head. The unit has an upper
m/z limit of ~350. To increase the acceptance area of the mass filter, the
ELFS leaky dielectric element has been removed. The standard high Q
head circuitry has been modified to provide electric isolation at DC-bias
potentials up to 5 IN. The modifications are identical to those found in
the Special HV Head offered by Extrel. Details of the operation and
computer control of the mass filter are covered in Chapter 4.

The quadrupole mass filter is an ideal mass analyzer for ion beam
work since it performs mass discrimination independent of ion enery.
Typical analysis energies may range from <5 to >100 eV depending on
the requirements of the experiment. Also. when it is operated in a low
resolution mode, the quadrupole transmission efficiency can be >80%.
In fact, the overall transmission of the beam instrument is limited by the
resolution setting of the quadrupole. Thus the highest ion currents are
obtained when the filter is operated at the lowest resolution compatible

with the chemistry of the experiment.

[XE n D tr m

After mass analysis in the quadrupole, ions are transferred to the
terminal detector region of the instrument for flux measurement. A set of
extraction optics has been developed to direct a focused ion beam to
either of the two detectors. The focusing system is a modified einzel lens
design as shown in Figure 2.17. The first element of the lens is the exit

96

aperture of the quadrupole housing. The center element (7) is a circular
plate electrode. A split tube electrode (8.9) with an extension tube
comprises the final element of the einzel lens. The electrodes are
separated by 4mm ceramic insulators. Potentials for the extraction
optics are obtained from the same power supply/ divider network as used
for the octopole transfer optics.

The extraction optics and the detector system are shown in Figure
2.18. When the Daly detector is in operation. the split tube lens may be
asymmetrically biased so that the ion beam is directed toward the center
of the conversion dynode. This allows high efficiency at any ion energy or
conversion dynode potential. For an ion enery of 1000 eV and a quad
DC bias potential of 950 volts, the extraction elements 7, 8, and 9 are
nominally set at 0. 750 and 700 volts. respectively. when the conversion
dynode is biased at -30.000 volts.

Ions may be detected by either a continuous dynode electron
multiplier or a Daly detector. A Galileo 4800 series Channeltron detector
and a shielding enclosure are mounted on the ion optical rail. The input
horn of the detector is coaxial with the ion optical axis. The detector is
normally operated in an analog current mode for measurement of
relatively high ion currents. When coupled to a electrometer/picoameter,
the Channeltron can quantitate ion currents that are less than 10‘17 to
more than 10'10 amps in the analog mode. This corresponds to count
rates of ~100 ions/ second to 109 ions/ second. The electron multiplier
also has the capabilities of fast response time for high ion currents. This
feature facilitates beam instrument diagnostics and rapid scanning for
tuning of the quadrupole mass filter.

For quantitating very low ion cun'ents such as are encountered in

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threshold measurements, ion counting techniques must be employed.
The Channeltron may be configured for pulse counting, but it sufl‘ers
from poor initial conversion efficiency and serious mass discrimination.
Therefore we have developed a highly efficient Daly detector as shown in
Figure 2.18. The concepts and operation of this detector have been
described and characterized elsewhere [26]. An excellent perspective on
the measurement of low ion fluxes is found the literature [27].

In this design. ions are accelerated through a potential of up to
30,000 V by the conversion dynode bias. The conversion dynode is
constructed from a 1.25" diameter SS disk with a thickness of 0.25". The
dynode has been polished optically flat on the top surface by using 3 pm
grit on an electrochemical electrode polisher. Secondary electrons
sputtered by ion impact are accelerated through the same field to a
plastic scintillator disk with a 3 A thick aluminum coating. The
scintillator is optically coupled to a Hamamatsu R-425 photomultiplier
tube. The high energy electrons stimulate photon emission in the
scintillator which is detected by the phototube. The operation of the
computer-controlled electronics associated with the Daly detector system
are covered in Chapter 4.

The Daly detector system provides nearly unit detection efficiency with
minimal mass discrimination effects. The dark count appears to be most
strongly dominated by field emission of electrons from the negatively
biased conversion dynode surface. In fact, a highly polished aluminum
conversion dynode was found to yield dark counts in excess of 104
counts/second under high field conditions. This was significantly
reduced by employing a stainless steel conversion dynode. The raw

undiscriminated dark count rate of our system is typically <20

99

counts/ second under a standard conversion dynode potential of -30,000
volts and a photomultiplier bias around 1000 volts. The background

may be reduced to fewer than 5 counts/second by utilizing the

discrimination circuitry.

P

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ll.
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13.
14.

15.
16.
17.
18.

100

LIST OF REFERENCES
Chapter 2

Daly. N. R.: Rev. Sci. Instrum. 1960. 31. 264.

Dahl.D.A.;Delmore. J .E. ,SIIVHON PC/PSZ Version 4.0. EGG-CS-7233
Rev.2. April 1988.

Darland. E.J.: Ph.D. Dissertation. Michigan State University. 1978.

"Electron Impact Ionization". T.D. Mark in "Gaseous Ion Chemistry
and Mass Spectrometry" ed. J. Futrell . John Wiley and Sons.
New York. 1988.

Garstang.R.H.: Mon. Nat. R. Astron. Soc. 1982.124. 321.

Corliss. C.; Sugar.J. J. Chem Phys. Ref Data. 1982.1 1. 135.

Elkind.J. ; Armentrout. P. J Chem Phys.. 1988.90. 5736.

Van Koppen. P.; Kemper. P.; Illies. A.; Bowers.M. . Int. J. Mass
Spectrom and Ion Proc.. 1983. 74. 263.

Birkinsaw. K; Shukla. A. :Howard. S. :Biggerstaff. J. :Ritrell. J. Int.
J. Mass Spectrom. and Ion Proc.. 1988.84 . 283.

Price, P.; Swofford. H.; Butrill. S. Anal Chem. 1977. 49. 1497.
Blom. K.: B. Munson J. Am. Chem Soc.. 1983. 105. 3793.

McDaniel. E.W.. "Collision Phenomena in Ionized Gases". Wiley. New
York. Ch. 9. 1987.

HE. Riveracome and EA. Mason. Anal Chem..1975. 47. 970.

Elkind.J. L.; Armentrout. P.B. Int. J. Mass Spectrum and Ian
Processes. 1988. 83, 259.

Munson. M.; Field. F. J. Am. Chem. Soc 1966. 88 . 2621.
Johnson. E.G.; Neir.A.0. Phys. Rev. 1953. 91 . 10

Teloy. E.; Gerlich. D. Chem. Phys. 1974. 4, 417.

Szabo. 1. Int. J. Mass Spectrom. and Ion Proc.. 1986. 73. 197.

19.

20.

21.

22.

23.
24.
26.
27.
28.

101

Hagg, C.; I. Szabo. Int. J. Mass Spectrom. and Ion Proc.. 1986, 73,
237

Hagg. C.; I. Szabo. Int. J. Mass Spectrom. and Ian Proc.. 1986. 73.
277

Hagg. C.; I. Szabo,1nt. J. Mass Spectrom. and Ion Proc.. 1986. 73.
295

Landau L.; Lifschitz, E. "Mechanics", Permagon Press. Oxford p. 93
1980

Loriot. (3.; Moran. T.; Rev. Sci. Instrum. 1975. 46. 140.
Amdur.l.; Jordan. J .: Adv. Chem. Phys. 1966. 10. 29.

Dawson. P. Mass Spec. Rev.. 1986. 5. l

Romero. L. A.: Masters Thesis. Michigans State University. 1989.
Wetner. H. Int J Mass Spectrom and Ion Physics. 1972. 8, 459.

102

Chapter 3

Understanding Ion Deceleration Optics

 

 

I. Overview

One of the most critical components of the MSU ion beam instrument
is the ion deceleration lens. The traditional approach for obtaining
highly-collimated low energy (<200 eV) beams from high energt (1-10
keV) sector mass spectrometers involves the use of a complex exponential
deceleration lens assembly. Through ion optical modeling of the
exponential lens and other less-complex lens designs. the fundamental
features that contribute to deceleration lens performance were
investigated. From this evaluation. a simple high-performance lens was
designed. constructed and characterized. The novel lens described here
decelerates a 3 keV ion beam down to the 3-200 eV range while providing

low angular divergence. excellent focusing and excellent transmission.

1] In n

There are various applications in mass spectrometry in which it is
necessary to decelerate ions having an initial translational energy of l to
10 kiloelectron volts. to a final translational energy of 1-200 electron
volts in the laboratory frame-of—reference. Ion deceleration is

accomplished by passing the high energy ions through an energy-

103

retarding field. Historically. the ion optical device selected to create the
decelerating field has depended on the specific requirements of the
experiment. The deceleration lens may often prove to be the the most
critical ion optical component in determining the overall performance of
an instrument that bridges the gap between high translational energy
(keV) and low translational energy (eV) domains.

Recently there has been a great deal of effort directed towards
developing versatile mass spectrometric instrumentation that combines
both sector and quadrupole or quistor mass analyzers [1-4]. In some so-
called "hybrid" instrument configurations the first mass spectrometer
consists of a magnetic sector (B) or a combination of magnetic and
electric (E) analyzers preceding a radio frequency-only quadrupole
collision cell (q) in tandem with a quadrupole mass analyzer (Q). Other
hybrid configurations incorporate sector mass spectrometers followed by
various combinations of quistor ion traps and quadrupole mass
analyzers. In a hybrid mass spectrometer of BEqQ geometry. for
example. ions are extracted from the source and accelerated through a
nominal potential of 1 to 10 kV. If low energy (1-200 eV) ion/neutral
collisions are desired in the rf-only quadrupole. ions exiting the electric
sector must be decelerated and focused into the first quadrupole. The
ion optical device required for this task may consist of as few as two or
three electrodes that establish the enery retarding field and provide
focusing. The angular and energy acceptance of rf-only quadrupoles and
quistor devices is often quite broad [5]. Therefore the decelerated beam
of ions may have a half-angle of divergence as great as 25° and may
exhibit a broad kinetic energy distribution (>5 eV FWHM). This is

sufficient to provide reasonable transmission through the collision region

104

for typical analytical applications where the kinetic energy of the ion need
not be accurately known.

There is a significant field of gaseous ion chemistry that is interfaced
with surface science and materials research. This has lead to the
development of numerous experimental procedures to study low energy
ion/surface interactions. These include surface induced dissociation
(SID) (6]. "soft landing" of polyatomic ions for surface modification [7].
atomic ion implantation studies (8.9] and ion/ surface scattering
investigations [10]. In many of these applications. ion production occurs
in an electron ionization source and the ion of interest is selected by
accelerating the ions to high translational energy and dispersing them in
a magnetic field or other mass separation device such as a Wein filter.
The mass selected ion beam exiting the mass selection device must be
decelerated to the desired energy and directed toward the surface. Here.
the ion beam is typically tightly focused to a small region of the surface.
This type of spatial focusing can often be accomplished with a
deceleration assembly consisting of a small number of ion optical
elements.

Another developing area of mass spectrometric instrumentation
requiring strong ion deceleration optics is high-pressure external-source
Fourier transform mass spectrometry [l 1). In this technique. the high-
pressure ion formation region is remotely located from the the low-
pressure. high magnetic field region where mass analysis is performed.
Ions are extracted from the source and accelerated to high kinetic energy
(~2 keV) for transport through several stages of differential pumping.
High ion kinetic energy also facilitates penetration into the solenoid

magnetic field without significant scattering losses (magnetic mirror

105

effect). Prior to injection into the analysis cell. the ions must be
attenuated in energy to approximately 2 eV or less to be compatible with
the applied trapping fields. The ion deceleration optics for such an
instrument have been constructed with as few as two elements.

Another recent application of ion deceleration is found in the work of
Kemper and Bowers [12) in which a high pressure drift tube is coupled to
a reverse geometry (BE) mass spectrometer. Ions are initially accelerated
to 5 keV and then mass analyzed. The high enery ion beam emerging
from the electric sector is decelerated in two stages by an assembly of five
cylindrical elements. Final ion translational energies between 0.5 and 10
eV were obtained with this deceleration lens design.

In contrast. for hybrid ion-beam instruments designed to accurately
probe the energetics and dynamics of ion/ molecule interactions there are
greater demands on the deceleration system. The deceleration lens must
manipulate a high energy ion beam from a sector mass analyzer and
(ideally) produce a highly collimated. monoenergetic beam of ions having
low translational energy. Several types of deceleration systems have
beam designed to create highly-collimated ion beams. Armentrout and
Beauchamp [13) developed a hybrid mass spectrometer of B9 geometry
which utilized a complex lens assembly known as a "Menzier
Deceleration Unit". Recently. Futrell and coworkers developed a hybrid
tandem supersonic beam mass spectrometer for the study of CID
processes in the energy range of <1 to 3000 eV [14]. Their ion
deceleration system followed a magnetic sector and implemented a total
of six separate stages of deceleration. Each stage consisted of a two-
element cylindrical (tube) lens.

However. the traditional design for such high performance

106

deceleration system involves creating an exponentially-decaying retarding
potential field. This system is considered to be the "state-of-the-art" in
deceleration lens design but its implementation normally requires a
complex lens assembly. often incorporating 40 or more electrodes [15].

In determining the deceleration optics for the MSU ion beam
instrument. the following question was addressed: can a high
performance deceleration lens suitable for ion beam applications be
designed without implementing a complex exponential retardation field?
The design approach was to first evaluate the performance of a state-of-
the—art exponential lens system using the ion optical simulation program
SIMION PC/P82 [16]. A simple two-electrode deceleration lens design
was also modeled to investigate ion deceleration from a fundamental
perspective. From these studies. a high performance deceleration lens
incorporating as few as two active elements was proposed (an active
element is defined here as an ion-optical electrode to which some
nonzero potential is applied). This versatile design has been constructed
and its ion transmission and angular divergence characteristics

determined.

III. Deceleration Lens Criteria

The MSU hybrid ion beam instrument of BEoQ geometry (0 = rf—only
octopole collision cell) has been constructed to study ion/ molecule
reactions and collision induced dissociation cross—sections as a function
of translational energy. Since the ultimate goal of this project is to obtain
accurate thermodynamic values for the onset of endothermic processes

(e.g. ionic heats of formation). the instrument design emphasis is on

107

establishing a well-defined interaction enery between the ion and the
neutral collision partner. To attain this goal. the deceleration system
must satisfy the following criteria:

1. Decelerate a 1000. 2000. or 3000 eV beam to <2-200 eV

Provide good focusing (final beam diameter <4.25 mm)

Convert the beam symmetry from planer to cylindrical

Produce a highly-collimated beam (divergence half-angle <5°)
Maintain high ion transmission

. Produce minimal kinetic energy perturbation (KE spread <O.5 eV)
. Mechanical simplicity for precise alignment (minimize aberrations)

«@9199.»

A block diagram of the MSU ion beam instrument is shown in Figure
3.1a. As discussed in preceding chapters, the primary ion beam is
generated using a Varian MAT CH5-DP reverse geometry double focusing
mass spectrometer which utilizes a 90° magnetic sector followed by a 90'
electric sector. The nominal accelerating potential of the instrument is
selectable at 1000, 2000 and 3000 volts with an upper mass limit of
1.200 daltons at the full accelerating potential. Ions exiting the final
focusing slit after the electric sector travel through a 50 cm field-free
region containing an off-axis detector. and then enter the deceleration
assembly. After deceleration to the desired kinetic energy. ions are
introduced into an rf-only octopole ion beam guide which passes through
a collision cell containing the neutral target gas. The octopole and
collision cell are DC-biased and referenced to the source accelerating
potential to establish the ion kinetic energy in the collision region.

Unreacted primary ions and ionic reaction products are extracted into a

108

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109

quadrupole mass filter and the intensity measured with a Daly-type
scintillation detector operated in the pulse counting mode.
The ion acceleration/deceleration sequence for our instrument is

graphically presented in Figure 3.1b. In this diagram. the ion source is
positively biased at the accelerating potential. U0. with respect to

instrument ground. U1. Ignoring the initial thermal energy spread of the
gas being ionized and spatially-varying ion source potentials (AUG). ions
are formed initially at rest (E0 = zero) in space at the potential U0 and are
then accelerated through the potential given by U0 - U) as shown in the
figure. The kinetic enery. E1. gained from the accelerating field by ions
carrying a single charge is simply e(Uo - U1). where e is the elementary
unit of charge. After mass analysis in the B/E region. the ions are
decelerated to the appropriate energy by the decelerating potential. Uf.
which is applied to electrodes in the deceleration region relative to U1.
The final ion kinetic energy after deceleration. Ef. may be taken
nominally as the difference in electric potential between the ionization
region and the deceleration region. Ef = e(Uo - Uf).

In order to properly design the ion optics for a deceleration system for
this instrument. the dimensions and angular divergence of the ion beam
emerging from the CH5-DF was measured. This was done for 1000. 2000
and 3000 volt nominal accelerating potentials. A Galileo 3040 channel
electron multiplier array (CEMA) equipped with a P20 phosphor screen
deposited on a fiber optic bundle was placed in the path of ions exiting
the electric sector. Ions incident on the detector produce an image on
the phosphor screen which may be observed through the fiber optic .

The experiment required the removal of the stock CH5-DF detector

housing and installation of a suitable vacuum enclosure to house the

110

CEMA. A discarded detector housing from a CEC-l 10 mass
spectrometer with an ID of ~9 cm and a length of ~15cm and an ample
number of high voltage feed-throughs was attached to the electric sector
with an adapter flange. The experimental configuration is shown in
Figure 3.2. Since the CEMA entrance plate is biased at a high negative
potential (-l400 V). a high transparency stainless steel grid was placed a
few millimeters in front of the CEMA and held at ground potential to
maintain a field-free ion flight path prior to detection. The beam exiting
the electric sector was visualized at two difi'erent distances.(9 and 20 cm)
from the the exit slit. Beam images were recorded on slide film using a
35 mm camera. After development of the slide film. the beam images
were projected onto a large section of paper. traced and measured. For
all three source accelerating potentials. the beam cross-section
immediately after the exit slit of the electric sector has an approximate
height of 4 mm and a width of 0.75 mm with the source and exit slits
fully open. For the 3000 eV ion beam. the half-angle of divergence in the
plane perpendicular to the length of the slit is ~0.25'. The 1000 and
2000 eV beams possessed slightly higher angular divergence in this
plane. There was no measurable angular divergence in the plane parallel
to the slit length for all three ion beam energies. Thus. the ion beam
exiting the electric sector can be described. to the first approximation. as
paraxial .

After traversing the 72 cm field-free region between the electric sector
slit and the entrance to the deceleration region. the ion beam profile is
approximately 4 mm high by 3 mm wide. The 3000 eV ions must be
decelerated and injected into the rf-only octopole which has an inner

radius. r0. of 8.5 mm. The deceleration lens should be capable of

111

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producing an ion beam variable in energy from ~1 to 300 eV in the
laboratory frame. In addition. it would be advantageous to convert the
planar symmetric (rectangular profile) beam from the sector region to a
beam having a circular cross-section to match the axially symmetric
octopole/ quadrupole regions of the instrument.

Although rf-only multipole beam guides have no longitudinal electric
field components. the oscillatory radial trapping field can have a
profound effect on the transverse ion motion. In order to provide
accurately known ion kinetic energies in the octopole beam guide. the
time-dependent component of kinetic energy transverse to the principal
axis of motion must be minimized. Computer modeling of the ion-optical
properties of rf-only multipoles performed in this laboratory (see Chapter
2). and also reported in a recent article by Davies and Wright [17].
suggest that there are particular characteristics of an ion beam entering
the multipole beam guide that preserve minimal transverse enery
components as the ions travel through the oscillatory field. This requires
that ions entering the rf-only device must have trajectories nearly parallel
to the main axis of motion (highly collimated) and that the ion beam
must have a small diameter (tightly focused). Our modeling suggests
that an ion beam should have a cross-sectional radius of less than 0.25
r0 (r0: oct0pole inner radius) when entering an rf~only octopole field.
For our system (r0: 8.5 mm) this requires a decelerated beam diameter of
54.25 mm. A small initial beam at the entrance of the ion beam guide
also insures the highest transmission of the primary ion beam through
the device. More importantly for our application. a nearly paraxial beam
(divergence half-angle <5°) entering the octopole minimizes ion losses

during collision processes such as dissociation or ion/ molecule reactions

113

that result in an instantaneous change in the mass of the charged
species while in the oscillating field [18]. This eliminates the need to
correct for dynamically-dependent transmission features of the octopole
when measuring total reaction cross-sections.

Ideally. manipulation of the ion beam by the deceleration system
should not perturb the kinetic energy distribution the ions found at
formation. Much effort has been directed towards the development of
ionization sources that provide beams with low energy spread.
Conventional electron impact ionization sources produce beams with
translational energy variations of <05 eV FWHM. A small ion kinetic
energy spread after deceleration will provide well-defined ion/neutral

interactions.

Also. having a minimum number of elements in the deceleration
assembly is very advantageous. Aberrations or imperfections in the focal
properties of the lens can arise from very slight imperfections in the
construction and alignment of ion-optical components. Ellipsoidal
aberrations. in particular. can be very pronounced in deceleration
systems because of the strong retarding fields involved. In addition.
when the deceleration occurs in a series of stages or involves several
internal cross-over points. aberrations due to misalignment are
compounded within the lens system. Therefore. it is best if a lens system
can be devised that utilizes a minimum number of active elements.

IV M lln

The design process utilized the ion trajectory simulation program
SIMION PC/ PS2 to first model the typical high performance deceleration

114

lens systems of the exponential type currently in use for ion beam
instruments. They were evaluated with respect to their basic ion-optical
properties and suitability for our specific application. The next step was
to gain a more fundamental understanding of ion deceleration by
modeling a simple, two-electrode deceleration lens. The final design of

our deceleration system evolved from this simple model.

A. The ”Over-Focusing" Effect

The difficulties associated with producing a highly collimated low
energy ion beam from a high energy beam have been discussed in an
early monograph by Hasted [18). When the kinetic energy of an ion beam
is reduced to less than about 5% of its original value in a lens system.
the resulting beam will be highly divergent. That is to say the lens
exhibits an extremely short focal length.

An illustrative example of this effect is found in Figure 3.3. This
figure shows the cylindrically symmetric SIMION model of a two-element
tube-type deceleration lens with left electrode at ground potential (0 V)
and the right electrode at 2990 V. Equipotential contour lines are plotted
along with trajectories for a group of paraxial ions entering the lens with
an initial kinetic energy of 3000 eV (principal ion motion is from left to
right). The ions are decelerated by the retarding field to a final energr of
10 eV. a 99.7% decrease in energy.

As shown in Figure 3.3. The ions enter the lens gap with trajectories
parallel to the central axis. but diverge wildly after traversing the
decelerating field. The region of minimum beam diameter is not

characterized by a global axis cross-over (global focal point). Rather. the

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ion trajectories cross over the center axis over a fairly broad region and
then diverge greatly. In fact. the outermost ions which entered the lens
system closest to the electrode have a final divergence half-angle
approaching 90‘ (i.e. final trajectories are perpendicular to the desired
direction of motion)! This display of focal points which are strongly
dependent on initial beam position followed by high angular divergence
may be categorized as severe spherical aberration.

B. Exponential Deceleration Lens

One approach to circumvent this so-called ”over-focusing effect" is to
create a potential field which internally corrects for the short focal length.
This correction may be approached by implementation of an
exponentially-decreasing retarding field.

An early exponential lens was demonstrated by Gustafsson and
Lindholm in 1958 for the deceleration of an ion beam of 2000 or 3000 eV
in energy emerging from a sector instrument [19]. The lens design
consisted of a series of slit electrodes of varying thickness designed to the
attenuate the ion energy to ~3% of its original value. A semi-empirical
exponentially-decreasing field was created by applying the appropriate
potentials to the electrodes. Their findings indicated that much more
efficient operation of the lens was obtained by significantly deviating from
electrode potentials associated with a pure exponential field.

Futrell and coworkers also developed a crossed ion beam/neutral
beam instrument that utilizes a high performance deceleration lens [15].
The lens was designed to produce a pure. axially-symmetric

exponentially-decaying field. This sophisticated device required a stack of

117

more than 40 separate electrodes with 12.5 mm ID circular apertures. A
complex resistor divider network was utilized to develop the necessary
exponentially-decaying voltages on the electrodes. IMtrell and coworkers
reported excellent performance of the lens when decelerating a 0.5 mm
diameter beam of 750 eV ions down to the 0.2 to 10 eV range.
Transmission of very low enery ions in this system was reported to
approach the theoretical space-charge limit. This particular design has
gained considerable acceptance as the standard in attenuating the
energy of ion beams by two or more orders of magnitude [20-22]. More
recently. an exponential retarding field deceleration lens was used by
Kofel and McMahon [1 1] to transfer an ion beam from an extemal high
pressure source into a Fourier transform ion cyclotron resonance mass

spectrometer.

B. 1 . Theory

For an axially symmetric exponentially-decaying decelerating field. if
the major axis of motion is in the positive x direction, the electric

potential. U. along the central axis. should be of the form

U(x) = -an e-fix (3.1)

In this expression. U0 is the ion stopping potential of the ions
entering the deceleration field (assuming U] = 0). The constants a and 15
may be empirical or derived from solutions that satisfy Laplace's
equation. Equation 3.1 also represents one of the few potential

distributions that is amenable to ion trajectory calculations by direct

118

integration using the paraxial ray expression. In order to calculate ion
trajectories in fields that differ from theoretical exponential potential
distributions. we have implemented the ion-optical simulation program
SIMION PC/ P82. This modeling approach also allows the calculation of
ion trajectories through lens systems with realistic electrode geometries.
To a first approximation the electric field between two parallel plates
with circular apertures (a single lens gap) is linear. Therefore. creating a
smooth exponential field requires a large assembly of electrodes with

exponentially-decreasing voltages applied to successive elements.

8.2. Gustafsson and Lindholm Exponential Lens

A SIMION model of the semi-empirical exponential lens of Gustafsson
and Lindholm is shown in Figure 3.4. The ten-element lens is designed
to decelerate 2000 eV ions emerging from the exit slit of the sector
instrument represented on the left-hand side of the figure. The ions are
decelerated to a final energy of 30 eV as they enter the collision region
represented on the right-hand side of the figure. The system is modeled
with semi-empirical exponential electrode potentials as described in the
reference. Potentials on the second and third electrodes are slightly
lower than those required to form a pure exponentially-decaying field.
This results in some focusing action in regions of the lens where the
decelerating field is the strongest. As shown from the ion trajectories of
Figure 3.4. the lens produces a smooth beam cross-over after traversing
approximately one-third of the lens system. Ion angular divergence is
also very low after the focal point. However, it can be seen that only
those ions with initial trajectories which begin very near the central axis

of the lens are successful at entering the collision region.

119

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121

The lens of Figure 3.4 exhibits a transmission efficiency of ~50% for
the trajectories modeled. Ideally. a lens system should provide 100%
transmission. An attempt was made to improve the number of ions that
can be decelerated and injected into the collision region by using the final
three electrodes of the deceleration assembly as an einzel lens. Figure
3.5 shows the SIMION model of the Gustafsson and Lindholm
deceleration lens with the additional einzel focusing near the exit of the
assembly. The potential on the second-to-last electrode is higher than
electrodes on either side of it, thereby forming a decelerate/accelerate
einzel lens. The efiect of this einzel focusing significantly enhances the
transmission of decelerated ions into the collision region. This example
also supports the conclusion of Gustaffson and Lindholm that the
performance of an exponential ion deceleration lens may be improved by

deviating from a pure exponentially-decaying field.

8.3. Futrell Exponential Deceleration Lens

The next modeling study explored an ion deceleration lens that
employed a pure. axially symmetric exponentially-decaying field. This
lens was implemented by Futrell and coworkers [15) to decelerate a beam
of 750 eV ions to ~12 eV. The electrode assembly consists of 42 parallel
plates with circular apertures equally spaced over a distance of 22 cm.
Since the principal direction of motion is in the positive x-dirnension. the
electrode potentials were calculated from the following exponential-decay

expression.

122

U(x) = U0 - an e-BX
The constants a and 13 were calculated using the method described by
Futrell et al. which allows the exponential field to have a fixed focal
length for a given final energy. If the first focus is to occur at a specific
position along the x coordinate. x'. the constant 13 may be calculated from
the relationship:

 

For our model, a focal plane is desired 1.63 cm beyond the last
electrode of the deceleration lens. This provides a first focus at a position
corresponding to the entrance of the collision region. Therefore. x' =
23.63 em if the entrance electrode is considered to be the located at the
origin (x=O) and 13 is calculated as 0.307/cm'1. The constant a is
normally taken as unity.

The final expression for the axial electric potential. in units of volts.
for the deceleration of 750 eV ions to 1.2 eV is given as

U(x.cm) = 750 - 750 expl-(O.3O7/cm'1)x]

From the above expression. the potential for each electrode was
calculated. A plot of the electrode potential as a function of the x-axis
displacement is found in Figure 3.6. The points on the graph correspond
to the calculated electrode potentials. As can be seen from the curve. the
electric potential asymptotically approaches. but does not attain. 750 eV.

Figure 3.7 shows the cylindrically symmetric SIMION model of the 42-

element deceleration lens preceding a DC-biased collision region. The

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126

model represents a stack of electrodes electrodes equally spaced over a
distance of 22 cm along the x-dimension. Electrode aperture diameters
correspond to 12.5 mm. The exponential lens is terminated with a
collision region 37 mm long with an inner diameter of 17 mm. The
collision cell potential is held common to the potential on the last
element of the deceleration lens. Equipotential contour lines are plotted
in 50 V increments. Trajectories are calculated for ions corresponding to
a 0.5 mm diameter paraxial beam having an initial kinetic energy of 750
eV. as characterized in the article by Futrell et al [15].

A more conceptual presentation of the lens system is shown in Figure
3.8. In this SIMION model. the electrodes are plotted in the x- and the y-
dirnensions. and the electric potential is plotted in the z-(relief)
dimension. This view clearly shows the exponentially-decaying profile of
the decelerating field in the x-z plane.

As the ions travel (from left to right) through the lens system. a
nominal beam cross-over occurs about midway through the deceleration
assembly. After the focal point, the ions proceed towards the collision
region with a divergence half-angle of approximately 1’. The final ion
kinetic energy is 1.2 eV which represents a 99.8% reduction in
translational energy.

These modeling studies of the Futrell lens demonstrate that. indeed. a
low-energy highly—collimated ion beam may be generated with a pure
exponentially-decaying deceleration field. But many questions regarding
ion deceleration remain unanswered. What qualities of the exponential
lens enable high performance? Also. are these properties unique to pure.

axially symmetric exponentially-decaying fields?

127

8.4. 3000 eV to 30 eV Exponential Deceleration Lens

Since our interest is directed towards gaining a fundamental
understanding of ion deceleration, further modeling studies were
performed on exponential deceleration lenses. A primary exponential
deceleration lens model was developed with respect to our particular
application. This model was critically analyzed in order to uncover the
underlying lens qualities that provide high performance operation.

A SIMION model of an exponential lens designed to decelerate an ion
beam with an initial energy of 3000 eV to a final enery of 30 eV was
developed. The cylindrically symmetric model consists of 40 individual
electrodes, each with an aperture diameter corresponding to 16 mm and
equally spaced over a distance of 20 cm. The deceleration lens is
followed by a collision cell with an 8 mm diameter opening. The end of
the collision cell and the deceleration entrance lens are terminated with a
thin element simulating a grid. This was done to establish boundary
conditions which reduce calculation time during potential array
refinement. and to provide more uniform fields.

By following the method of discussed by Futrell et a1. [15]. the
potentials for all electrodes in the deceleration lens were calculated from
the equation given below with a=l and b=0.363/cm-1 to theoretically
yield a first focus at the entrance of the collision cell.

U(x.cm) = 2970 - 2970 expl-(O.363/cm'1)x]

The final ion stopping potential applied is 2970 V since the entrance

128

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Collision Cell

       

Exponential Lens Elements

otential

X

Z
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40-element exponential deceleration lens potential surface.

Figure 3.11.

131

aperture of the decelerating field is assigned a potential of zero volts. The
electric potential of the exponential field asymptotically approaches 2970
volts. This electric potential distribution is plotted in Figure 3.9.

In the SIMION model found in Figure 3.10. the DC-bias of the collision
cell is 2970 volts. which results in the ion beam being reduced in energy
to a final value of 30 eV. Equipotential contour lines are shown for 250
volt increments. Figure 3.1 1 displays a three-dimensional view of the
exponential model with the electric potential plotted in the z-(relief)
dimension. In this view the smooth exponentially-decreasing field is
clearly displayed.

Ions emerging from a plane source provide a useful test for
determining the fundamental ion optical properties of the lens.
Representative ion trajectories are plotted for an monochromatic ion
beam having an initial energy of 3000 eV. a divergence half-angle of 0'
(paraxial), and an initial cross-section of 5 mm and 4 mm in Figure 10
and Figurel 1, respectively. The exponential field produces a beam cross-
over about one-third of the distance through the lens. After this cross-
over the beam diverges with a half-angle of approximately 2". Although
the final angular divergence is quite low, because of the short focal length
of the lens. only those ions with initial trajectories which begin very near
the central axis enter the collision region.

Close examination of the equipotential contours in Figure 3.10 and of
the potential surface in 3.11 reveal the electric field characteristics which
result in the inherently short focal length. Near the central axis of the
lens. the equipotential lines exhibit very little curvature in contrast to
regions in space very near the lens electrodes. Under high magnification,
the three-dimensional plot reveals a concave or trough-like topography of

132

the potential surface for the majority of the axial dimension of the lens.
With the exception of the lens entrance region (first one or two
electrodes). the potential surface concavity is most pronounced in regions
of high potential gradient (dU/dx) along the longitudinal direction. This
curvature is thus most distinct in the steepest regions of the retarding
potential hill. which occurs within the first ten electrodes for this model.
Also. the degree of curvature (dU/dy) increases as. y approaches the edge
of the electrode. This causes ions (with initial trajectories removed from
the central axis) to be forced in the y-dimension to produce an early
cross-over approximately midway through the lens at an energy of ~200
eV.

Since dU/dy increases with increasing y values, ions experience very
little force in the y-direction if the initial beam diameter, Db. is quite
small relative to the lens aperture diameter, Da. The relative ratio
(Da:Db) for the model shown in Figure 3.10 is 7.2:1 . Successful
trajectories (ions which pass through the 8 mm aperture and enter the
collision cell) are found only for ions corresponding to a Dasz ratio of
15:1 or greater. Other trajectory calculations show that the focal
properties of the exponential lens are very sensitive to Dasz. In fact, as
this ratio is decreased, the nominal beam crossover point moves to lower
x values and the angular divergence of the decelerated ion beam
increases.

lon transmission in the exponential lens may actually be improved by
having a non-paraxial initial beam. Increasing the angular divergence of
the incoming beam can compensate for the inherently short focal length.
For the model shown, an ion beam with an initial divergence half-angle of

4-5' reduces the Dasz ratio in which successful trajectories may be

133

found (increase in lens acceptance area).

The SIMION model reveals that reasonable transmission and low
angular divergence may be obtained for a low energy ion beam when it is
decelerated in an axially symmetric exponentially—decaying field under
certain geometrical constraints which provides a Dasz ratio of at least
15: 1. In fact. further studies suggest that the successful performance of
the exponential lens design is more dependent on the Da:Db ratio than on
the exponential nature of the decelerating field. Therefore, the excellent
operation of the lens design of Futrell et al. [15] may be attributed to the
small initial beam diameter (0.5 mm) relative to the electrode aperture
diameter (12.5 mm): here Dasz has a value of 25:1. Since the initial
beam diameter for our application is approximately 4 mm, an
exponential deceleration lens would require a lens aperture diameter of
at least 60 mm (Da:Db > 15:1) to maintain high transmission and low
angular divergence. If a 60 mm diameter exponential lens precedes an
ion-optical element with an entrance diameter of 17 mm, as in our
instrument. the deceleration field would be dominated by the potential on
the small diameter element penetrating into the region of large open area.

Also. an exponential lens has focal properties which are dependent on
the final energy of the beam. The inflexibility of a purely exponential field
would place limitations on the final beam characteristics when the

experiment requires that ions be decelerated to a range of final energies.

C. Fundamental Studies:
1. A Simple, Two-Element Deceleration Lens
After modeling ion behavior in an exponential deceleration system, the

ion optical properties of the other extreme approach. a simple two-

134

element deceleration lens, were investigated. Figure 3.12 shows the
SIMION model for a lens consisting of two parallel plates with circular
apertures having a diameter of 15 mm and a separation distance of 4
mm. The extreme boundaries of the potential array were set at the
potential of the nearest element to minimize calculation time and provide
a more accurate field. as in the exponential model.

Ion trajectories simulating the deceleration of a 3000 eV ion beam
from a 5 mm diameter planar source having a 0' divergence half—angle
are plotted in Figure 3.12 With a final kinetic enery of 10 eV the system
yields an ion beam with a nominal divergence half-angle exceeding 25°.
This lens may be characterized as having severe spherical aberrations.

The three-dimensional view of the simple lens potential surface shown
in Figure 3.13 provides a powerful conceptual model for understanding
the ion optical properties of this deceleration system. This model
provides an enhanced view of the similar potential surface features
discussed in the exponential lens section. As ions with initial trajectories
non-coincident with the center axis approach the lens gap the convex
nature of the surface (dU/dy) exerts an outward (radial) force in the y-
dimension. This results in increasing divergence of the beam as it moves
up the retarding hill in the first half of the lens assembly. The beam
broadening from this effect is minimal due to the relatively high velocity
of the ions in this region. Approximately half-way through the lens gap
the potential surface attains a concave or trough-like structure. Thus
ions in the outer part of the beam experience an inward force as they
traverse this region of the lens.

Having been attenuated in energy by the retarding field. the ions move
at relatively low velocity through the second half of lens, the exit channel.

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137

with reduced longitudinal velocity, the inward radial force dominates the
forward momentum resulting in a sharp cross-over and a highly
divergent ion beam after deceleration. This over-focusing effect, a severe
spherical aberration. is an inherent property of both simple and complex
deceleration systems and is more pronounced as the ion beam diameter

approaches the element aperture dimensions.

C.2 Einzel Focusing Followed by Deceleration

From SIMION modeling of the exponential and two-element
deceleration systems it was noted for both cases that angular divergence
is minimized when trajectories are restrained to regions near the central
axis of the electrode assembly. This may be accomplished by having a
relatively high Da:Db ratio as in the exponential design of Futrell and
coworkers. Alternatively, we have proposed the use of an einzel focusing
lens preceding the decelerating field of a simple decelerating lens of
moderate aperture diameter. The einzel lens is used to focus ions prior
to the deceleration stage, effectively decreasing the diameter of the ion
beam at the deceleration stage of the lens.

A SIMION model for a simple deceleration lens which included a 15
mm ID einzel focusing stage preceding the deceleration stage was
developed. In this system, the decelerating field is established by
applying the desired decelerating potential to a single tube-type injection
electrode terminated with a circular plate. The 15 mm ID injection
electrode is adjacent to two other entrance electrodes of slightly larger
diameter which are normally held at ground potential. The injection

element is used to decelerate and, for our experiment, transfer the ions

138

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and deceleration stage in operation.

141

into the octopole collision region which has an entrance aperture of 8
mm. For the modelling with DC potentials only, the octopole region is
biased with same potential as the injection electrode of the deceleration
stage. The full design also includes horizontal and vertical deflection
fields for alignment of the high energy ion beam with the einzel focusing
and deceleration stages.

Figure 3.14 shows the SIMION model of the system with only the
einzel focusing stage in operation. The horizontal aligning plates, as well
as the deceleration injection and octopole collision region electrodes, are
held at ground potential. The einzel lens is operated in the decelerate-
accelerate mode with a center element potential of 4300 V. The
trajectories plotted in Figure 3.14 were calculated for 3000 eV ions
characteristic of our sector instrument (4 mm diameter, divergence half-
angle < 0.250) which results in a nominal beam cross-over midway
through the decelerating stage. The einzel focusing causes the 4 mm
initial beam diameter to be reduced to less than 1 mm near the beam
cross-over. This maintains a very small beam profile as ions traverse the
most critical high-field region of the retarding stage.

The combination of einzel lens focusing preceding a simple
deceleration stage is modeled in Figure 3.15. The simulated trajectories
for 3000 eV ions characteristic of our sector instrument, decelerated to
final energy of 30 eV, are plotted. The only active electrodes in the
assembly are the center element of the einzel lens (4300 V), the final
injection element of the deceleration lens (2970 V), and the octopole
collision region (2970 V). The potential surface view of this model,
displayed in Figure 3.16, reveals the saddle-type topology of the einzel

lens which focuses the high energy ion beam into the deceleration zone.

142

As clearly shown in Figures 3.15 and 3.16, the focusing stage reduces
radial excursions of the ions as they pass through the convex surface of
the exit channel. This enables ions to avoid regions of the deceleration
potential surface having high radial fields (dU/dy). Thus, the ion
trajectories favor the formation of a highly collimated, low energy beam.
The ion beam, having a kinetic energy of 30 eV, exhibits a divergence
half-angle of less than 3‘ at the entrance to the octopole beam guide
region. In this system, low angular divergence is achieved with only 2
active elements in the lens assembly.

More than 20 different deceleration systems which involved the
principle of einzel focusing preceding a deceleration stage were studied.
Several different deceleration stage variations were modeled. Some
lenses implemented a simple two—element deceleration gap. Some
models involved creating a linear deceleration field with a series of 3 to
10 electrodes. Other models also included some focusing electrodes
within the deceleration stage. This is similar to the "zoom" type
deceleration lens utilized by Cooks and coworkers [2]. However, for most
models studied, the general lens performance was determined primarily
by the relative ion beam diameter through the critical regions of the
deceleration field. Thus, we decided to implement a simple deceleration

lens in which the deceleration field is produced by a single lens gap.
V. Construction

The final SIMION model was translated into a working drawing for
construction. For versatility and ease of mechanical alignment, the lens

components were designed around a universal horizontal mount that

 

143

affixes to an ion optical rail as detailed in a previous chapter. The rail is
housed in a differentially-pumped vacuum chamber that connects to the
sector instrument with a 2.88 inch ID flexible bellows. A pneumatic gate
valve is used to isolate this beam chamber, which is located 72 cm from
the exit slit of the CH5-DF.

A cross-sectional schematic drawing of the axially symmetric lens
assembly is shown in Figure 3.17. The design incorporates an einzel
focusing stage followed by the deceleration stage integrated on a single
100 mm aluminum mount for permanent mechanical alignment. In
addition, a separate 50 mm mount containing a set of horizontal and
vertical deflection plates was made to align the incoming ion beam with
the focusing and deceleration stages.

As in the final SIMION model, the einzel lens has an aperture
diameter of 15 mm and a spacing of 4 mm between elements. All plate
elements in the assembly are constructed from 0.8 mm thick stainless
steel sheet. The deceleration stage consists of a series of one or two
plates with increasingly smaller apertures separated by 5 mm ceramic
spacers followed by a tube-type injection element which was machined
from a single rod of stainless steel. The 15 mm inner diameter injection
element is 28 mm long and protrudes through a wall in the vacuum
chamber that separates the deceleration and octopole collision regions.
This injection electrode geometry provides a minimal conductance path
between the differentially-pumped chambers.

In addition, a 53 mm i.d. stainless steel tube with a wall thickness of
1.6 mm and a length of 42 mm is situated between the einzel lens stage
and the deceleration stage. The tube is in held in physical and electrical

contact with last element of the einzel lens and the first element of the

144

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145

deceleration lens. This tube is used to guard against stray potentials as
well as to maintain cylindrically symmetric fields in the regions between
the einzel focusing and deceleration stages. Another similar tube is
placed between the alignment stage and the einzel focusing stage. The
entire electrode assembly is clamped together onto vertical mounts using
2-56 threaded rod insulated with one-hole alumina tubes. The alumina
tubes have an CD of ~5 mm and an ID of ~2.5 mm and were obtained

from Scientific Instrument Services.

VI. Characterization

This section details the deceleration lens performance and operation
in the following areas:

A) Alignment; B) Angular Divergence; C) Beam Symmetry;

D) Transmission; and E) Kinetic Energy Spread

A. Alignment

When strong electric fields are used in a deceleration lens, mechanical
alignment of the beam with the lens assembly is extremely crucial.
Ellipsoidal aberrations arise from imperfections in the electrodes and
from mechanical misalignment of the ion beam axis with the central axis
of the lens assembly. This may be a limiting factor in determining overall
lens performance.

Alignment is especially critical for a system with utilizes very high
electric fields such as the two-element deceleration lens. These lenses

typically possess inherently short focal lengths and severe spherical

aberration coefficients. Cs. The extent of elliptical aberration due to

1

146

improper alignment of the beam and apertures has been quantitated by
the relationship [23] expressed below.

 

in this expression, C3 is the ellipsoidal aberration coefficient, f is the
focal length. and Ae is the off-center displacement or mis-alignment.
From the equation, it can be seen that when Ae is nonzero, a short focal
length (as is the case for most deceleration lenses) highly exaggerates
aberrations due to alignment imperfections.

We have performed ion optical modeling studies to determine the
effect of mechanical imperfections and beam misalignment on
decelerated beam properties such as angular divergence and focal
position. Figure 3.18 shows an example deceleration lens model for
decelerating 3000 eV ions to 10 eV prior to injection into a collision
region. The nearly paraxial ion beam enters the lens assembly from the
left-hand side of the figure. In this model. there is a slight misalignment
of the guard cylinder with the ion-optical axis of the system. This causes
asymmetry in a critical region of the decelerating field. The trajectories
shown exhibit a focal position that is skewed with respect to the central
axis of the system. These and other results suggest that the central axis
of the incoming beam must be within 51 mm of the center of the
deceleration assembly to minimize ellipsoidal aberrations.

Alignment of the deceleration assembly with the sector instrument to
extremely close tolerances was facilitated by constructing a set of

decelerations lens electrodes with 0.5 mm apertures. A He-Ne laser was

 

147

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148

used to create an Optical axis between the collision region and the exit
slit of the CH5-DF. The ion optical rail was then adjusted to set the
deceleration assembly co-axial with the optical axis of the laser.

During operation, further alignments were made with the horizontal
and vertical deflection plates preceding the entrance of the deceleration
system. By placing the CEMA/ phosphor screen detector directly after
the injection lens, the ion beam profile could be visualized. Exact
alignment of the incoming beam was performed by applying a potential to
the einzel lens to create a beam cross-over approximately 1 cm in front of
the detector. In this manner the aberrations of the beam exiting the
einzel lens could be visualized. The horizontal and vertical deflection
plate potentials were then adjusted until the most symmetric beam

profile was obtained.
B. Angular Divergence

The angular divergence of the ion beam exiting the deceleration lens
was determined by measurement of the ion beam profile as depicted in
Figure 3.19. The CEMA/ phosphor screen combination for beam
visualization includes a fine stainless steel screen positioned 2 mm in
front of the CEMA entrance plate. This was normally held at the
potential of the deceleration lens injection element, thereby shielding the
beam from the high negative potential applied to the CEMA entrance
plate (- 1400 V).

The detector was first positioned on the ion optical rail immediately
after the injection element of the deceleration lens. A video camera was

used to record the ion beam profile of Ar+, formed by E1 in the source of

 

149

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150

the CH5-BF. as a function of both the final ion energy and the central
einzel element potential. A second set of beam profiles were recorded by
placing the CEMA at the end of a 135 mm long flight tube which was
held at the potential of the injection element.

A resistive divider network originating from the source accelerating
potential was used to supply the voltage to the injection element of the
deceleration lens. The nominal final ion energy was taken as the
difference between the potential applied to the source ionization housing
and the potential applied to the injection element of the deceleration lens
as measured with a floating digital voltmeter. This method of ion enery
determination includes several sources of uncertainty in the true electric
potentials such as surface charging. space charging and contact
potentials from the junction of dissimilar metals. Most significantly, the
potential applied to the source ionization housing may not accurately
reflect the electric potential in space where ionization occurs. Because of
the ion source extraction fleld. the electric potential at which the ions are
formed may be as much as several volts lower than the potential applied
to the source. Thus, the nominal final ion energy represents an upper
limit of the true kinetic energy. Further discussion of ion beam energy
considerations follows. A recent article provides a quantitative study on
the effect of source operating parameters reflected in ion translational
energy measurements [24].

As predicted by the SIMION model, the optimal potential applied to
the einzel lens for minimum angular divergence was dependent on the
final ion enery selected. It was found. however, that a static einzel lens
potential of 5 kV provided excellent focusing for final ion energies in the
10-140 eV range. With an initial beam energy of 3000 V the decelerated

151

beam divergence half—angle was less than 5° in this energy range. This
suggests that the beam energy may be selected with control of only the
potential applied to the injection element of the deceleration lens. When
the einzel lens potential was optimized for various final energies, the
decelerated beam divergence half—angle was less than 2° for ion energies
between 30 and 300 eV. and less than 5' for ion energies between below
30 eV.

It was observed that the vertical and horizontal position of the
decelerated beam varied with the potential applied to the deceleration
injection element. For example, as the final ion energy was scanned from
100 to 30 eV. the beam image on the CEMA would exhibit a lateral shift
in position of ~2 mm. This type of behavior has been modeled with
SIMION as shown in Figure 3.18. The extent of this energy-dependent
displacement was dependent on the potentials applied to the horizontal
and vertical steering electrodes. This beam displacement is indicative of
mechanical imperfections and beam misalignment. In fact, the extent of
image displacement. Ay, as a function of relative lens potential, AV. is
directly proportional the relative beam misalignment, Ae, as found in the
expression below [23]

Ay a Ae(AV/V)
C. Beam Symmetry

One significant aspect of the deceleration lens performance is the
conversion of the beam from the rectangular slit profile to a circular
cross-section as displayed in Figure 3.19. Due to the circular apertures,
the high electric field in the deceleration zone is cylindrically symmetric.

a

152

This forces the rectangular beam into an axially symmetric potential well.
thereby converting the beam to the desired circular cross-section. This
effect. to the best of our knowledge. has not been previously discussed in
the literature. Beam symmetry conversions such as this normally
require the implementation of complex ion-optical components such as
DC quadrupole lenses.

The ability to change the ion beam profile from rectangular to circular
with just the plate-type electrodes further reduces the complexity of our
deceleration assembly in comparison to previous lens designs. The
deceleration lens employed by Cooks and coworkers [2] consisted of a
two-stage dual "zoom" deceleration lens system for enery retardation
with an intermediate DC quadrupole singlet lens for beam shaping.
Armentrout and coworkers also employ DC quadrupole lens pairs for ion

beam shaping prior to injection into an exponential lens assembly

[2 l .22].
D. Transmission

Ideally. one would like to obtain absolute values of ion transmission
through the deceleration lens. In concept. the measurement is easily
performed by placing a Faraday plate in front of and immediately
following the lens to record ion current. However there are numerous
experimental difficulties in such a procedure. Measurement of high
energy (keV range) beams with faraday plates is often complicated by
ion / surface interactions. which result in ion or electron sputtering. The
detection efficiency of faraday plates is also dependent on the geometry

and materials of construction. Also. a means of moving the faraday plate

153

into and out of the beam path while maintaining vacuum is required.
Furthermore it is difficult to measure ion beam intensities below 10'12
amps with conventional faraday plate/electrometer systems. Ion beam
currents from our sector instruments are typically less than 10‘13 amps.

Therefore. transmission measurements of the ion beam through our
deceleration lens were performed using a single. continuous dynode
electron multiplier placed immediately after the final injection element.
’I\vo high transmission stainless steel grids were situated in front of the
detector. The grid nearest the deceleration lens was held at the injection
lens potential and the second grid was held at ground. This minimizes
penetration of the detector potential (~2900 V) into the deceleration
region.

The intensity of a 3000 eV Ar+ beam entering the deceleration lens
was measured by setting the einzel lens and injection lens at ground
potential and trimming the horizontal and vertical fields for maximum
signal. Potentials were then applied to the einzel lens and deceleration
lens injection electrode while again adjusting the horizontal and vertical
deflection elements to achieve the maximum signal.

When the einzel lens potential is optimized for a final nominal beam
enery of ~5 eV. approximately 90% transmission of the incoming beam
is achieved. Figure 3.20 shows the relative ion intensity (a value of 1.0
equals the decelerated signal maximum) plotted as a function of the
nominal final beam energy. This was obtained by scanning the potential
of the injection element of the deceleration lens while the potential
applied to the einzel lens was held constant at 4860 V. This results in a
sharp local intensity maximum centered around 5 eV and approximately
50% attenuation of the beam for final energies up to ~130 eV. The

154

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156

energy-dependent transmission is due to the change in focal length of the
decelerating portion of the lens as a function of final beam energi.

If the einzel lens potential is optimized for an energy of ~30 eV. then a
fairly flat transmission profile. approaching 80 % transmission for the
energy range <5-l30 eV, is obtained as shown in Figure 3.21. This
demonstrates that reasonable transmission over a broad energy range
may be attained by varying the potential of a single electrode in the lens
assembly. In actual practice. when low collision energies (<10 eV) are
desired. it is more effective to inject ions into the rf-only octopole with
somewaht higher energy (>50 eV) and perform the final deceleration
using the DC-bias of the ion beam guide. This is due to the rf fringing
fields that can easily deflect low energy ions at the entrance to the
octopole. resulting in lowered overall transmission. It is also
advantageous to form very low energy beams (< leV) within the radial
trapping field of ion beam guide. This allows effective ion currents that
exceed the densities imposed by space-charge limitations.

For each final beam energy there exists an optimum einzel lens focal
length that yields maximum transmission. Since the einzel lens power
supply is programmable. it could be adapted to computer-controlled
operation. Scanning the einzel lens potential in conjunction with the
decelerating potential would provide the highest ion transmission as a

function of beam energy.
E. Kinetic Energy Spread

Ideally. an ion beam instrument designed to investigate the

translational energy dependence of ion/neutral interactions would

157

posses a monochromatic primary beam source variable in enery. but
having no inherent kinetic energy spread. This reduces uncertainty in
the ion/neutral interaction energy. Typical electron impact ionization
sources for double focusing instruments are reported to produce ion
beams with energy spreads of <O.5 eV (FWHM)[25]. At best. all of the
ion-optical components of the primary ion source/mass analyzer and of
the deceleration assembly. in particular. should not greatly contribute to
the ion energy spread. However. if the kinetic energy spread of the ion
beam can be accurately characterized. this information may be used for
data deconvolution.

There are numerous approaches found in the literature for
determining the spread of ion kinetic energy. The simplest experimental
approach involves the use of a series of three (or more) parallel grids
placed in the ion beam path normal to the principal axis of motion and
preceding a detectorl26]. This configuration is shown in Figure 3.22a.
With grid 1 at ground potential (U1). a retarding potential applied to grid
2 (Uf) establishes a linear energy-retarding field between the grids l and
2. The transmitted ion beam is measured by the detector. Grid 3 serves
to shield the deceleration field from the detector bias. When the
retarding potential is swept through the highest electric potential at
which ions were formed (U0 max). the ion transmission vanishes. The
fall-off profile of the ion current versus applied retarding potential plot
should reveal the variation in the ion beam energy distributed around the
mean potential at which the ions were formed. The most significant
origin of kinetic spread in El sources is the spatial variation of the source
potential (AUG) within the ionizing electron beam dimensions from the ion

extraction field. For such an achromatic ion source, the mean potential

158

at which ion formation occurs can be considered to be U0. This is the
potential at which the mean ion kinetic energy equals zero.

Other methods of ion energy analysis require a higher level of
instrumental sophistication. These include the use of hemispherical
energy analyzersl27]. tandem parallel-plate electric sectors[28]. or
deceleration within a dynamic-field trapping device such as rf-only
multipoles [29]. We attempted to measure the KB spread of a high
energy ion beam exiting the electric sector of the CH5-DF using the
three-grid retarding potential analyzer situated immediately after the
injection electrode of the deceleration lens as shown in Figure 3.22a. A
(3000 eV) Ar+ beam. formed by El with the stock MAT CH5 Intensitron
ion source. was directed onto the analyzer after traversing the field-free
region between the exit slit and detector. All of the electrodes in the
deceleration assembly were held at ground potential. The potential on
grid 2 was scanned up to and beyond the source accelerating voltage
while the transmitted ion intensity was measured to produce the

stopping potential curve shown in Figure 3.22b. Here. the potential
difference {V(source) - V(retarding grid)} is defined as Vstop- The ion

transmission begins to decline at a Vstop value of ~30 V and makes a
nearly linear drop in intensity between Vstop values of to 20 V and 5V.
respectively. The ion current reaches a minimum level at a Vstop value
of ~3.5 V. This implies that highest electric potential at which ions were
formed and transmitted through the analyzer is approximately 3.5 V
below the applied source potential for the source parameters used in the
experiment.

It is also of interest to note that the minimum signal found in the

retarding potential experiment was higher than the base noise level of the

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160

detector/electrometer combination. The minimum signal always
approached a level approximately two orders of magnitude lower than the
signal associated with the maximum beam current. The minimum signal
level was found to persist even when the potential applied to grid 2
exceeded the source accelerating potential by >66 V. It also scaled
linearly with ion current exiting the source as measured by varying the
electron emission level of the source filament. When the grid 2 voltage
exceeded the source accelerating potential. biasing grid 3 at potentials
between --5000 V and +5000 V had no appreciable effect on this
minimum signal level. We therefore concluded that this signal was due
to neutral particles or x-rays striking the electron multiplier. This may
be expected since the detector is co-linear with the ion-optical axis of the
instrument and fast secondary neutral particles may be sputtered at the
exit slit (a limiting aperture) with low-angle trajectories.

The data in Figure 3.22b also suggest a very broad kinetic energy
distribution in the ion beam of approximately 25 eV. This energy range
is significantly higher than the specifications of the Intensitron El source
(FWHM < 0.3 eV) and much higher than typical ion energy spreads
reported for other double-focusing instruments when using an El source.

A large number of similar retarding potential experiments were
performed using a variety of retarding field analyzer configurations (see
Appendix D). The grid number. grid type (thickness. pore size and
percent open area). grid spacing. and detector type (single continuous
dynode and CEMA) were varied as well as the orientation of the analyzer
relative to the optical axis of the beam. Both the energy-zero and the
kinetic energy spread obtained in these experiments were strongly
dependent on the configuration of the retarding field analyzer. The use

161

of different El sources and accelerating potentials (1000. 2000 and 3000
V) did not yield the expected energy spread result (FWHM <0.3 eV) with
any energy analyzer configuration. For most cases. the apparent ion
energy range corresponded to approximately 1% of the accelerating
potential. This was quite disturbing since ion kinetic energy spreads in
the range of those measured (~25 eV for a 3 keV beam) would preclude
the use of the El source/CHS-DF as the primary ion source in a high
performance ion beam instrument due to unacceptable energy
resolution.

Retarding potential energy analysis using parallel grids suffers from
severe experimental uncertainties and difficulties in proper
implementation. The majority of these may be explained by considering
the actual requirements of the retarding field. First. the electric field
established by the grids must be perfectly perpendicular to the principal
component of ion velocity to ensure an accurate measurement of the
energy-zero (U0). if perfect alignment is not achieved. then only a
fractional component of the applied stopping potential is effective
towards attenuating the forward motion of the ion beam. Second. the
retarding field between the grids should be spatially uniform to minimize
ion losses from divergence. This requires that all of the grids used be
perfectly flat. which is difficult to achieve considering the mechanical
stability typical of grids having a thickness of ~0. 1 mm. Field uniformity
is also compromised in a grid-type energy analyzer since the grids do not
truly establish a unipotential boundary due to the penetration of electric
potentials into the deceleration zone from nearby sources. such as the
high potential applied to the detector. Most. however. is the fact that

grids can be considered to be a series of apertures of square or circular

162

cross—section uniformly distributed in a plane. Therefore. two grids in
parallel represent. ion-optically. a large array of misaligned lenses. When
used to provide a deceleration field. the very non-ideal lens system yields
poor transmission and thus a very distorted profile of the ion kinetic
energy distribution.

The retarding potential analysis method which employed parallel grids
was henceforth abandoned as a means of obtaining accurate ion enery
spreads. A more accurate method of energy analysis involves the
implementation of a hemispherical energy analyzer. tandem parallel-plate
electric sectors. or deceleration within a dynamic-field trapping device
such as a rf-only multipoles. We have used the octopole ion beam guide
of our instrument to measure the ion kinetic energy spread of ions
exiting the deceleration assembly. When the final ion energy retardation
is performed inside the rf-only octopole. ion losses normally associated
with space-charging and high angular divergence are eliminated. In this
experimental configuration. the octopole beam guide is aligned co-aidally
to and directly following the injection element of the deceleration stage.
as shown schematically in Figure 3.23a. An off-axis Daly-type detector
employing a conversion dynode biased at ~30 kV is used to detect ions
exiting the rf-only octopole. The retarding potential (Ufl was applied as a
DC—bias to the octopole radio frequency signal using a computer-
controlled power supply referenced to and floating at the source
accelerating potential.

The retarding potential profile of a decelerated NiPF3+ ion beam from
the CH5-DP is shown in Figure 3.23b. The ions were formed with the
Intensitron El source. at an nominal source accelerating potential of

1000 V. mass analyzed and decelerated to approximately 200 eV before

 

163

 

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164

injection into the rf-only octopole. The ion intensity is plotted as a
function of the difference in electric potential between the ion source and
the octopole DC-bias. This quantity, (Vsource - Voctopole). is defined as
Vstop- As this potential difference decreases below Vstop = 2.75 volts
the level signal exhibits a rapid decrease in intensity. The ion current
vanishes at an approximate Vstop value of 2.15 volts. Also given in
Figure 3.23b is the first derivative of ion current with respect to octopole
DC-bias. The mean potential of ion formation. the source potential (Uo).
is found at the maximum of the first derivative curve which appears at
Vstop = 2.54 volts. The mean potential of ion formation is thus 2.54
volts below the potential applied to the ion source housing. The ion
kinetic energy spread appears as a distribution around this potential.
On the plot shown. the ion energy spread is ~0.3 eV FWHM, which agrees
quite favorably with the design specifications of the Varian MAT
Intensitron El source of 0.3 eV FWHM (as reported in the product
literature). Similar stopping potentials are obtained for much lower
octopole injection energies. These data indicate that the simple ion
deceleration lens design allows significant ion energt attenuation without
adding any additional ion kinetic energy spread to that found in the

nascent ion beam.
VII. Conclusion

Through ion optical modeling of the complex exponential and simple
two-electrode deceleration lenses. the fundamental features that govern
lens performance have been elucidated. When creating an energy

retarding field with electrodes having circular apertures, the electric field

 

165

exhibits steep gradients perpendicular to the central axis. These
transverse gradients lead to the over-focusing of ions (high angular
divergence) at the exit of the deceleration region. This effect is most
pronounced for ion beams with initial diameters that approach the
electrode aperture diameters. It is suggested that the ratio of the initial
beam diameter to the electrode aperture diameter (Dasz) is the most
significant criterion in determining the performance of ion deceleration
lenses. and not the complexity of the retarding field. In order to avoid
excessive force on the ions perpendicular to the main direction of motion
it is necessary to constrain the ion beam near the central axis of the lens
assembly. This may be accomplished by using very large electrode
apertures relative to the initial beam diameter as in the conventional
exponential decelerating lenses.

We have proposed the use of a simple two— or three-electrode
assembly with moderate aperture diameters to establish the retarding
field. Ions are constrained near the central axis through the critical
portion of the decelerating field by a an einzel lens that precedes the
deceleration stage. This new deceleration system is able to decelerate a
3000 eV ion beam down to <3-200 eV while maintaining low angular
divergence, minimal kinetic energy perturbation, and good transmission.
In addition. the lens converts the high energy ion beam with a
rectangular profile (plane symmetry) to a low energy ion beam with a
circular profile (axial symmetry). The combination of these features
makes the deceleration lens described suitable for implementation in an
ion beam instrument designed to probe the energetic and dynamic
details of ion/ neutral interactions. Thus, it has been demonstrated that

a simple deceleration lens does not compromise high performance versus

 

166

a complex deceleration lens when the fundamental ion-optical aspects of

ion deceleration are well understood.

 

10.

ll.

12.
13.
14.

15.

167

LIST OF REFERENCES
Chapter 3

Glish. G.L.; McLuckey. S.A. Anal. Instrum. 1986.15, 1.

Schoen, AE; Amy. J .W.: Ciupek. J .D.; Cooks. R.G; Dobberstein,
P.: Jung. G.Int. J. Mass Spectrom. and Ion Proc. 1985. 65, 125.

Harrison. A.G.: Meecer, RS: Reiner. E.J.; Young. A.B.: Boyd. R.K.;
March. RE; Porter.C.J. Int. J. Mass Spectrom and Ion Proc. 1986.
74 , 13.

Suter, M.J.-F.; Gfeller, H.; Schlunegger. U.P. Rapid Commun. Mass
Spectrom. 1989, 3. 89.

Dawson. P.H. Mass Spectrom. Rev. 1986.5. 1.

Laue. H-J; Amy. J .W.; Winger. B.; Cooks. R.G. Proceedings of the
36th Conference on Mass Spectrometry and Allied Topics 1988. p
958.

Franchetti. V.; Solka. B.H.; Baitinger, W.B.; Amy. J .W.; Cooks, R.G.
Int. J. Mass Spectrom. and Ion Phys. 1977. 23. 29.

Pohlit. H.M.; Erwin. W.R.: Reynolds. F.L.; Lemmon. RM.; Calvin. M.
Rev. Sci. Instrum. 1970. 41, 1012.

Youchision, D.L.; Nahemow. M.D. Rev. Sci. Instrum. 1990. 61.
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Liebl. H.; Bohdansky, J .; Roth. J .; Dose, V. Rev. Sci. Instrum. 1987,
58. 1830.

Kofel, P.; McMahon.T.B. Int. J. Mass Spectrom and Ion Proc.
1990.98. 1.

Kemper. P.R.; Bowers. M.T. J. Am. Soc. Mass Specrtrom. 1990.1. 197
Armentrout. P.B.; Beauchamp. J .L. J. Chem Phys. 1981. 74. 2819.

Shukla. A.; Anderson 5.; Howard. S.L.; Sohlberg. K.W.. Futrell, J .H.
Int. J. Mass Spectrom. and Ion Phys. 1988. 86. 61.

Vestal. M.L.; Blakley. C.R.: Ryan. P.W.; Futrell, J .H. Rev. Sci.
Instrum. 1976. 47, 15.

16.

17.

18.

19.

20.

21.
22.

23.
24.
25.

26.

27.
28.

29.

168

DA. Dahl; J .E. Delmore SIMION PC/PSZ Version 4.0. EGG-CS-7233
Rev.2. April 1988.

Davies. S.C.; Wright. B. Rapid Commun. Mass Spectrom. 1990. 4,
186.

Hasted. J B. In Atomic and Molecular Processes ; Vol. 13; DR. Bates,
Ed.; Academic Press: New York, 1962; p 696-720.

Hasted. J B. In Atomic and Molecular Processes : Vol. 13; DR. Bates.
Ed.: Academic Press: New York. 1962; p p 705.

Hansen. S.G.: Farrar. J .M.; Mahan. 8.11. J. Chem. Phys. 1980. 73. i

3750. I‘—
Ervin. K.; Armentrout. P.B. J. Chem. Phys. 1985.83. 166. L

Loh, SK: Hales. D.A.; Pan . L.; Armentrout, P.B. J. Chem. Phys.
1989. 90. 5466.

Brewer. G.R.; Wilson. R.G. Ion Beams. John Wiley and Sons 1973
Quian. K.; Shukla. A.; Futrell. J. Anal. Chem. 1990. 62. 1547.

Beynon.J .11.; Baitinger, W.B.; Amy. J .W.; Komatsu.l. Int. J. Mass
Spectrom. and Ion Phys. 1969. 3. 47.

Hodges. RV.: Armentrout. P.B.; Beauchamp, J .L. Int. J. Mass
Spectrom. and Ion Phys. 1979, 29. 375.

Siegal. M.W.: Vasile. M.J. Rev. Sci. Instrum. 1981. 52.1803.

Gentry. W.R.: McClure. D.J.: Douglas. C.H. Rev. Sci. Instmm.
1975.46. 367.

Teloy. E.: Gerlich. D. Chem. Phys. 1974. 4. 417.

169

Chapter 4

Ion Beam Instrument Computer Controlled Operation
and
Reaction Cross-Section Measurements

 

 

I. Intr u on n ross- ections

The purpose of the ion beam instrument is to measure the enery
dependence of ion/neutral reactions. The most important energy
contribution for reactions and that which can be carefully controlled in
the beam apparatus is the kinetic energy of the reactant ions. Because
of the conservation of momentum. the energy which is utilized by the
colliding pair for chemical transformation is not the motion in the
laboratory frame. but the relative kinetic energy in the center-of-mass
(CM) reference frame. If the motion of the neutral gas is neglected. the

kinetic energy in the CM frame (KEcm) may be calculated from the

relative masses of the collision partners as shown below.

KEcm= KEiab (m/ (m + M1)

Here KElab is the nominal kinetic energy in the laboratory frame. in is
the mass of the stationary neutral. and M is the mass of the reactant ion.
The reaction cross-section refers to the extent to which a particular

process occurs as a result of a collision. This is a quantitative measure

170

of the probability that a collision will lead to specified products. The total

reaction cross-section. 0. can be viewed in terms of attenuation of the
primary ion beam. 10, as it travels a distance. 1. through a static collision

gas with a number density. n. This relationship is given in an integrated

Beer's law form below.

IR =10 exp(-Gnl)

The term IR is the intensity of the primary ion beam transmitted
through the collision cell length. The initial primary ion intensity can be
considered to be equal to the sum of the transmitted intensity and the

product ion intensity, Ip. as shown below.

10 =1R + 21p

Thus cross-section measurements only require the determination of
the transmitted primary and product ions. the effective length of the
interaction region. and the density of the collision gas. The number
density can be derived from the gas temperature and pressure.

If cross-section measurements are performed at different nominal
kinetic energies. the energy-dependent behavior of a reaction system may
by studied. The details of data interpretation. including deconvolution
and determination of threshold energy as well as extraction of
thermodynamic information has been documented elsewhere in the
literature I 1 .2).

This chapter describes the measurements of reaction cross-sections

on the MSU ion beam instrument utilizing computer-controlled data

 

 

171

acquisition. Two chemical systems are studied to characterize the

instrument. The first system involves the endothennic collision-induced

dissociation (CID) of my with argon as shown below in reaction 4.1.

Mng+ 4» Ar —-> Mn+ + Mn + Ar (4.1)

The second system presented here is the exothermic reaction of argon
ion with molecular deuterium to form ArD+ as shown in reaction 4.2.

Ar+ + D2 -—> ArD+ + D (4.2)

II In mn n l

Energy-dependent collision cross-sections are measured by
introducing a neutral reactant gas into the collision cell. The pressure is
maintained at 0.1 mtorr or less in order to minimize the probability that
an ion experiences more than one collision while traveling the length of
the collision cell. The nominal interaction energy is established by the
DC potential between the ionization source and the octopole/collision
cell. The ion current is measured at the detector for the primary ion and
each product ion of interest. The interaction energy. quadrupole mass
filter setting and detector signal are all controlled/monitored by a
computer system as shown in Figure 4.1. The system is comprised of a
12 MHz 80286 PC running MS DOS. No IBM DACA interface boards
are controlled using custom programs developed in the ASYST 2.0
platform.

The timing sequence for a typical experiment is shown in Figure 4.2.
The octopole/ collision cell potential is stepped through the desired range

172

 

 

 

 

 

 

 

 

IBM DACA Board |

 

 

Primary Ion Collision
Source Cell

 

 

 

 

Figure 4.1. Ion Beam Instrument Control Schematic Diagram.

173

 

 

Octopole] Collision Cell DC Potential

 

Collision Energy ——->

 

 

 

 

 

 

 

Quadrupole
Mass Value

N

\

E':
Detector Signal

Intensity ——->

 

 

 

 

 

 

____/\__l.

 

 

 

Time —->

 

 

Figure 4.2. Ion Beam Experimental Sequence Diag'am.

 

 

 

174

of interaction energies. At each energy setting, the quadrupole may be
scanned over the mass range of interest while the detector signal is
monitored. An alternative, and more efficient method involves stepping
the quadrupole filter to the primary and product ion m/z values instead
of scanning over a wide mass range. In this mode. the optimum mass
values should be determined by scanning the quad over the primary and
product mass profiles in a preliminary experiment.

The interface control scheme for biasing the octopole collision cell is
shown in Figure 4.3. A zero to -500 V floating power supply is used to
generate the offset potential. The supply is referenced to the ion source
accelerating potential to compensate for any drift in the source
electronics. A +5V power supply is connected in series to the
accelerating potential. This five volt over-potential ensures that ions with
energies greater than the accelerating potential will be covered in a
stopping potential analysis. It also allows the DC supply to be operated
in a linear region of output.

The DC power supply may be programmed using a 0-5 volt driver.
The driver potential is generated using a 12 bit DAC. Since the power
supply is floating at the source potential of 1000 to 3000 volts. it cannot
be directly connected to the computer interface. A circuit was designed
to eletrically isolate the DACA interface from the floating DAC and power
supply utilizing high voltage opto-isolators.

The 12 bit DAC allows the octopole supply to produce a voltage
controllable in 4095 increments. To provide a highly accurate and
precise value of the interaction energy. the output of the octopole power
supply was measured as a function of the digital number sent to the DAC

from the computer. The voltage calibration curve is shown in Figure 4.4.

 

 

175

 

 

 

 

 

 

 

IBMDACABoard 1

 

 

 

 

 

 

 

 

 

Vol
at we

l 12 Bit DAC

 

 

 

 

 

O to +5
Volts

 

 

 

+5 Volt DC Ref ee DC Power Supply

_l_ o to -500 Volt

 

 

 

 

 

 

 

 

 

 

Ion Source Accelerating— _ O
PW mm -=- Octopole/Collision Cell
1 DC Bias

Figure 4.3. Octopole] Collision Cell DC Bias Schematic Diagram.

176

Octane». DC Bias vs Diqiu Nam

 

 

 

 

 

 

 

y - .uflu - 5.030“. m: m
12
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at
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.2 f . . - - .
40 so so 70 no so too - n'o - 150 - 130
Digs-m

 

 

Figure 4.4. Calibration of digital interface for octopole DC bias supply.

177

The regression equation fit to the line is displayed at the top of the plot
has a high degree of linearity (R=0.999998). This equation is thus used
in data analysis to determine the relative ion energy in the collision cell
which may be incremented in units of 0.128 volts. This limits the
uncertainty in the relative interaction energy to ~:l:0.064 eV in the
laboratory frame.

The interface electronics associated with the quadrupole mass filter is
shown in Figure 4.5. The m/z stability (pass) value is determined by the
amplitude of the rf and DC potentials generated on the rods. The rf
potential is generated by a high power rf amplifier and the DC voltages
are obtained by rectification circuitry in the High-Q Head. There is a
linear relationship between the rf/DC amplitude and the magnitude of
the m/z value which is stable in the filter. The quadrupole control
electronics and rf amplifier are driven by a 0-10V DC signal. Thus a
simple linear relationship exists between the drive voltage and the mass
axis. An analog output on one of the DACA boards is dedicated to the
quad driver. It allows the m/z value to be set to ~i0.1 unit.

In addition. a DC offset may be independently applied to the
quadrupole as shown in Figure 4.5. The difference between the quad DC
bias and the ionization source accelerating potential establishes the
nominal ion kinetic energy through the quadrupole mass filter.

A conceptual diagram of ion signal generation and measurement with
the Daly detector is depicted in Figure 4.6. The PMT produces a
temporally narrow (<50 ns) negative pulse generated by a single ion
event. The pulse is amplified by an MIT F-IOOT amplifier with an integral
discriminator circuit. A narrow TI‘L level output pulse is generated by

the preamplifier for each input signal that exceeds a certain threshold.

 

178

 

 

 

 

IBMDACABoard I

 

Oto 10Voltl

 

 

guadaramuna ‘_fl’ mwm‘

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

High-Q Head
L1 14 .
%.Dc
I+Dc Cb
T
ti and DC
i Quadrupole DC Bias
1

Figure 4.5. Quadrupole Electronics Schanaiic Diagram.

179

Photomultiplier 10!“

photo elect-mo\
/
W Cm
Dynode

 

 

i MSU!“

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ortec Pulse Counter

Pre-Amp/ k ' Time -—" J
Discriminator A
J

 

 

 

 

 

 

 

Reset Start/ Stop

 

 

 

 

 

 

 

 

 

 

 

IBM DACA Board J

 

 

 

Figure 4.6. Detector Electronics Schematic Diagram.

180

By judicious setting of the threshold level. signals originating from
spurious events may be discriminated from signals related to ion events.

Pulses from the preamplifier are detected by a six digit ORTEC 775
pulse counter. The counter has been modified in-house for digital
interfacing. This is accomplished by parallel readout of the binary coded
decimal (BCD) value in each of the six LED displays. The start count and
stop count states are also controlled by computer digital output signals.
The counter control software has been rigorously tested and includes
software corrections to establish exact counting time intervals.

The overall bandwidth of the Daly detector system electronics exceeds
1 MHz. This system provides the requisite sensitivity and broad dynamic
range for the measurement of small ion/ molecule reaction cross-sections

encountered in threshold measurements.

11. Reaction Cross-Section Measurements
A. Collision Induced Dissociation of an+

The collision induced dissociation (CID) of singly charged manganese
dimer ion was studied in the ion beam instrument using Ar as a collision
gas. This is shown in reaction 4.1. All of the operating parameters used
in the CID experiment are documented in Appendix E. The reactant
Mn2+ primary ions were formed by electron ionization of Mn2(CO)1o.
Solid Mn2(CO)10 was introduced to the source through the direct probe
inlet. Since the vapor pressure of the compound is quite low at room
temperature. the butterfly valve was used to throttle the source diffusion
pump until reasonably high primary ion currents (~106 counts/ second)

could be attained. A range of ionizing electron energies were used to

181

generate Mn2+. However. excess internal energy in the primary ion
results in slow metastable decomposition of the dimer ion at energies
much greater than the appearance energy. Therefore. the results
presented here utilized 18.13 eV electrons. which minimized excessive

internal energy effects. The electron accelerating voltage circuitry was
calibrated by measuring the ionization potential of N2 as a standard. The

threshold plot for nitrogen ionization is shown in Appendix E.

As discussed in Chapter 3, because of a variety of effects in the ion
source, the nominal ion energy may differ significantly from the source
accelerating potential. The true ion energy is determined by performing a
stopping potential experiment. This is done by monitoring the primary
ion current while sweeping the octopole DC bias through the accelerating
potential. A plot of the Mn2+ (m/z 110) ion count rate versus the
octopole DC bias relative to the ion source potential is shown in Figure
4.7. The ion current exhibits a very rapid rise to the maximum value of
approximately 5.4 x 105 counts/ second after the threshold near 1 volt.
The first derivative plot may be used to determine the enery zero and
the inherent kinetic energy profile in the primary ion beam. From the
first derivative plot, an energy spread of about 0.3 eV (FWHM) is
measured. This value is in agreement with El source specifications. For
an approximately Gaussian energy distribution. the apex of the first
derivative plot may be used to determine the average ion energy or
potential of energy zero. Since the energy spread is narrow relative to the
octopole voltage step size. it is more accurate to use the extrapolated
potential required to stop 50% of the ion population. This value is 1.488
volts as shown Figure 4.7. The energy zero potential is implemented in
the data analysis to correct the nominal octopole DC bias potential.

.15:ch mango; +352. .3. 2%:

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183

The effect of various instrumental parameters (such as source
potentials. deceleration (injection) energy, and octpopole operational
values) on ion stopping potentials were studied using Ar+ as a model ion.
This stopping potential data is compiled in Appendix F.

The collision cell was filled with high purity argon to a typical
pressure of ~0.1 mtorr. Interfering reactions of my with background
water were observed at low interaction energies when technical grade Ar
was employed. Cross-section data exhibiting this low energy interference
feature is detailed in Appendix G. These interferences were not present
when either a gas scrubber or high purity argon were used.

Since the product and primary ions differ significantly in mass. the
quadrupole could be operated at a very low resolution setting. This
enabled nearly 100% transmission of mass selected ions. The
quadrupole was stepped between m/z 1 10 and m/z 55 to monitor my
and Mn+. respectively. The product ion transmission was highly
dependent on the quadrupole DC bias as shown in Figure 4.8. This
results presumably because the product ion varies greatly in kinetic
energy with respect to the primary ion. The data suggest that the Mn+
species is formed with very little forward momentum in the laboratory
frame. Therefore, the quadrupole DC bias was set at a potential lower
than the minimum octopole DC bias to maintain complete product ion
transmission.

Primary and product ion intensities were measured for incident ion
translational energies ranging from zero to 20 eV in the laboratory frame.
Raw ion counts were background corrected by subtracting the average

dark count rate from the ion intensities. Dark count measurements were

184

8/25/90 EffeaonuadmpoleDCBiuonDanghterIonTranmiaim
Accelerating Voltage = 1.010 V

 

 

 

 

 

 

 

 

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Octopole DC Bias (V,Fit)

Figure 4.8. Effect of quadrupole mass filter DC bias for my CID.

185

performed for each experiment at all collision energies by tuning the
qudrupole mass filter to an m/z value where no ion transmission is
obtained. The laboratory frame ion energy was converted to CM collision

energy. Reaction cross-sections were calculated from the equation

shown below.

-In

 

IMna
IMn.+IMn’

nl

 

Diffusion of the collision gas cloud into the vacuum chamber results
in an uncertainty in the length of the interaction region. Therefore.
relative cross-sections (O/Gmax) were calculated as opposed to absolute
cross-sections.

Figure 4.9 shows a plot of the relative Mn2+ CID cross-section as a
function of the CM collision energy. Because of the background
subtraction, some relative cross-section values are less than zero at
energies below the CID threshold. In these cases. negative cross-section
values have been truncated at zero. The data points for the two runs
plotted exhibit a high degree of precision considering the complexity of
the experiment. The run-to-run cross—section variation is usually around
3%.

The threshold region of the reaction cross-section is shown in an
expanded plot in the top panel of Figure 4.10. The enery accuracy of
our cross-section data may be determined by comparison with a previous
study on Mn2+ by Armentrout (PBA) and coworkers [3]. The lower panel
of Figure 4.10 shows the PBA absolute cross-section data for my

186

 

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0.0 1.0 2,0 3.0

Figure 4. 10. Comparison of an+ CID threshold region.

9361' (eV. 00

 

188

formed by ~18 eV electron ionization. It can be seen that our data is in
quantitative agreement with the PBA energy axis. Through sophisticated
curve-fitting and deconvolution techniques. the PBA data yield a
translational energy threshold of 0.85 eV for the Mn2+ dissociation
process. From error analysis and internal energy considerations. the
Mn2+ bond dissociation energy, D°(Mn2+), was determined by PBA as
0.85 i 0.2 eV.

The an+ CID study demonstrates that the MSU ion beam instrument
can be used to obtain reproducible. energy—accurate realative cross-

section measurements for endothermic processes.

B. Exothermic Reaction of Art with Dz

The reaction of singly charged argon ions with molecular hydrogen
and deuterium has received extensive treatment in the literature [1).
Reaction 4.2, shown below. is exothermic by 1.50 eV for ground state

reactants and products.

Ar+ + D2 —> ArD+ + D AH=-l.50 i003 eV (4.2)

This presents an excellent candidate system to study exothermic
ion/ molecule reactions in the ion beam apparatus.

The complete operational parameters utilized in this experiment are
documented in Appendix H. The primary ion beam was generated by 70
eV electron ionization of argon at a source pressure of around 5 x 10'6
torr. Primary ion currents on the order order of 2 x 106 ions/ second

could be easily produced. Stopping potential analysis were performed

189

using the octopole DC bias to determine the energy axis zero and the
initial ion kinetic spread. A typical stopping potential curve is shown in
Figure 4.1 1. In this experiment the collision cell was at base pressure
and the quadrupole mass filter was operated in the rf-only mode at an
amplitude corresponding to m / z 20. From the first derivative plot of the
stopping curve it can be seen that the initial ion kinetic enery spread is
<O.4 eV (FWHM).

For reaction studies. the collision cell was filled with technical grade
deuterium to a pressure of approximately 0.1 mtorr as measured with a
capacitance manometer. The chamber background pressure could be
maintained at ~l x 10'5 torr as measured by a Bayard-Alpert ionization
gauge. Reactions were monitored from zero to 20 eV in the lab frame.
which corresponds to center-of-mass collision energies up to 1.8 eV. The
quadrupole mass filter was operated at the lowest resolution capable of
separating the primary and product ions. Since the mass difference is
only 2 amu. the resolution setting limited the total ion transmission
through the quad to <50%.

Exothermic ion/molecule reactions exhibit maximum cross-sections
at the lowest translational energies. In addition. the negative enthalpy
change may be manifested in the production of translationally hot
reaction products. Both of these factors contribute to primary and
product ions highly scattered about the collision center. This presents a
high degree of difficulty for monitoring low energt interactions in a
longitudinal ion beam instrument. Thus. by monitoring the transmitted
primary and product ion flux. the performance of the ion beam
instrument can be rigorously examined.

Figure 4.12 shows the results of a set of experiments in which the

 

Comm/Seamd

Counts/Second

190

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Figure 4.11. Ar+ stopping potential.

'l'rappmg liffieciency: Ar+ + DZ --> ArD+ + D
Colllslilll Cell Empty vs. Collision Cell 0.] mtorr

’
n
‘ -

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—._ l(ArD+)
—"_ NA")
- - - v vvvvvvvvv r vvvvvvvvv r vvvvvvvvv t vvvvvvvvv i
0 l 2 3 4

Ion Kinetic Energy (eV. lab)

Figure 4.12. Ar+/D2 trapping efficiency.

 

191

instrument trapping/transmission efficiency is measured. The argon
primary ion current was monitored at a function of kinetic energy with
the collision cell empty. The collision cell was filled with D2 to a pressure
of 0.10 mtorr. The transmitted primary ion. Ari: and product ion. ArD+.
currents were then measured. As shown in Figure 4. 12. the sum of the
transmitted primary and product ion current compares quite well to the
intensity of the primary ion beam measured in the absence of collision
gas. These data verify that the dynamic trapping process in the octopole
is effective collecting highly scattered primary and secondary ions even at
very low interaction energies. In addition. the quadrupole mass filter
exhibits very little mass dependent transmission over this particular
energy and mass range.

Data were collected for the argon/ deuterium reaction and converted
to CM collision energies as described previously. The energy dependent
relative cross-section is plotted in Figure 4.13 as an average of four
separate experiments. The data exhibit a reaction maximum at the
lowest energies and an exponential decay towards higher interaction
energies. This is consistent with Longevin-Gioumousis-Stevenson (LGS)
rate theories based on ion/ neutral captures driven by ion/dipole and
ion-induced / dipole attractive forces [4].

The greatest uncertainty in obtaining absolute values of the reaction
cross-section comes from the inability to precisely measure the effective
length of the interaction region and the absolute number density of the
collision gas. To determine the effective cell length. the reaction data are
plotted as a product of the cross-section and the path length (0'1) as a
function of the collision energy. The effective path length may be derived

by comparison with accepted absolute cross-section values found in

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194

reference 3. The enery average effective path length is calculated at
approximately 6 :5 cm. The high level of imprecision may explained by
examining the data from the four individual runs plotted in Figure 4.14.
The curves for the individual runs have similar shape and run.
approximately parallel. but are of slightly different magnitude.

The extent to which the curves are off-set may be explained in terms
of the limited precision and accuracy of the gas number density value. It
is very difficult to measure absolute pressures in the 10'4 torr range. as
required in a beam experiment. This pressure is slightly outside the
range where a capacitance manometer can make accurate and precise
measurements. In addition. the capacitance manometer used in the ion
beam instrument is. like many of the ion beam components. salvaged
from the REsource For Unwanted Scientific Equipment (REFUSE).
Therefore. a better method for pressure measurement and control should
be implemented to obtain absolute cross-section measurements.

The argon/deuterium reaction demonstrated the high
trapping/ transmission efficiency for exothermic ion/molecule reactions
attainable with the MSU ion beam instrument. Because of imprecise
determinations of the neutral gas pressure. high precision absolute
cross-section values were not obtained. However. it must be emphasized
that the inability to obtain absolute cross-section values does not
preclude the measurement of accurate endothermic reaction threshold
energies.

An effective instrument for the energy-dependent mesurement of both
endothermic and exothermic relative reaction cross-sections has been
constructed and demonstrated. The instrument should now be employed

in the energy-dependent study of new ion/molucule reactions.

195

LIST OF REFERENCES
Chapter 4

Ervin. K.; Armentrout. P.B. J. Chem. Phys. 1985.83. 166.
Armentrout. P.B.; Beauchamp. J .L. .J. Chem. Phys.. 1981. 74. 2819.

Ervin. PL; Loh. S. ; Aristov.N.: Armentrout. P. J. Phys Chem. 1983.
87. 3593.

Gioumousis. G.; Stevenson. D.J. Chem. Phys.. 1958. 29. 294.

 

196

Appendix A

Measurements of the CH5-DF Analyzer Energy Acceptance

The range of ion kinetic energies from an ion source that can be
accepted and effectively transmitted by an analyzer is known as the
energy acceptance. The energy acceptance for the CH5-DF was
measured by using the using the Combination EI/FD/FI ion source
operated in the El mode. Ar+ was used as the test ion. Since the El
source produces a narrow kinetic energy distribution ion beam. the ion
source accelerating potential was varied to present a range of ion kinetic
energies to the BE analyzer while the transmitted ion intensity was

recorded.

 

In Current

197

CH5 Kinetic Energy Acceptance

Como. El Source. Sliis at 8.4

 

 

 

 

 

 

 

 

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3 l o
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2980 2990 3000 3010
Source Potential (Volts)
Como. E! Source. Sliis Fullv Ooen
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6.00e-lO-l /— 4.
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4.00e-l0 J / 5.
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2.00e-l0-J /
-7.S7e-29 ~-——¥-=/. . K‘s—c
2980 2990 3000 3010 3020

Figure A. 1. Ion kinetic energy acceptance at 3000 V acceleration

Source Potential (Volts)

198

(2115 Kinetic Energy Acceptance 2kV
Como. E! Source, Sllts Fully Open, pg. 86

 

 

 

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Accelerating V

Figure A.2. Ion kinetic enery acceptance at 2000 V acceleration.

 

199

CH5 Kinetic Energy Acceptance lkV

Como. El Source, Sllts Fully Ooen, pg. 86

 

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2 i I! \\
l i E
O ' l .
" 2.00e-l l i f \

.. g \
l l
l .OOe-l l ~ .’ \

a

3 ,x \O

i ./

«17312-30 % w’ . .
996 998 1000 1002 1004 1006 1008

Accelerating V

 

Figure A3. Ion kinetic energy acceptance at 1000 V acceleration.

200

Appendix B
Magnetic Hall Probe Calibration

Hall Values vs m/z Values for Acceleration
Potentials of 3 kV. 2 kV. and 1 kV

1 31V
55 1 [A
50 j /
45 1 /

40 2kV

 

 

 

 

 

 

 

 

 

lldll Value

 

 

 

 

:54 f . /
4 // 112V
154 l

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0 5 IO 15 20 25 3O 35 40 45 50 55 60

Figure 3.1. Hall values vs. m/z values for 3.2. and 1 kV acceleration.

201

Appendix C
Mass Discrimination in a RF-Only Quadrupole

| m/z 16

m/z 15

 

 

 

 

144 vpp 216 Vpp

 

288 Vpp 350 Vpp

Figure C.1. Effect of RF amplitude on transmission in Extrel Quad.

 

202

Appendix D
Retarding Field Energy Analyzers

Parallel grid retarding field energy analyzers were used to evaluate
ion energy spreads as described in Chapter 3. Most of the data in this

appendix were generated using argon gas in the Intensitron EI source
or in the combination EI/FD/FI source operated in the E1 mode.
Although various grid configurations were employed, the general
configuration may be found in Chapter 3.

Figures D.1 and D.2 show the stopping potential curves obtained
using a three-grid analyzer with both sectors (BE) in operation and the
object] image slits fully open. and ions generated in the combination
source. The stopping potential curves for 1000, 2000, and 3000 V
acceleration is shown. The analyzer was constructed from moderate
transmission (<80%) grids which were spaced 3.5 mm and 7 mm
apart. respectively. Detection was accomplished using a 4800 series
Channeltron biased at -2900 volts. Figure D.3 shows the stopping
potential obtained using with the source potential at 3000 V and the
object/image slits at 8.4.

Figure D.4 shows the stopping potential obtained with a three-grid
analyzer placed at the entrance to the electric sector approximately
30 mm following the intermediate 3 slit. The grids were spaced 3.3
and 6.3 mm apart, respectively. In this experiment the image
(source) slit was fully open and the combination source was used.

Figure D.5 shows the stopping potential after the magnetic sector for

the Intensitron source with the image slit wide open. Figure D.6

shows the curve with the source slit at 9.0.

 

 

203

A CEMA detector was used in place of the Channeltron in the
retarding potential analyzer immediately following the magnetic
sector. The CEMA input was biased at ~1500 V and was located 7 mm
behind the stopping grid. The retarding field distance was 3.5 mm.
Figure D.7 shows the stOpping potential obtained with this device.
when the Intensitron source was used with the slit wide open.

A mask was installed on the last element in the source acceleration
lens to restrain the z-dimension (length) of the ion beam profile. With
the source (image) slit at 9.0, the beam profile dimension presented
to the magnet was approximately 1.5 mm by 0.1 mm. The stopping
potential obtained with the CEMA analyzer is shown in Figure D.8.

The grid spacings were changed to produce a stopping field 1 mm

long with the CEMA detector. The stopping potential obtained using

the masked source with the slit fully open is shown in Figure D.9.
The retarding potential analyzer was configured with the retarding
field 7 mm long and a distance of 7 mm to the input face of the CEMA.

The analyzer was placed at the end of the deceleration lens following

the B and E sectors. The stopping potential obtained using this

configuration is shown in Figure D.10.

 

 

204

CH5 Kinetic Energy Distribution
Comb. E! Source, Slits Fully Open, pg 83

 

 

 

 

1000-6
J
O
8 2.00e-8 «
b
t
u /
i /
3 m
'/
’ I
LOOe-B -1 f/
l
l
i / '
I
g 3,5 .
l .528‘27 i-h-i .
O 10 2O 30

Visnurce) - Vlnrid)

Figure D.l. Retarding potential curve for 3 keV ions.

 

205

CH5 Kinetic Energy Distributfon‘, 21W
Comp. E! Source, Sllts Fully Open, pg. 88

 

I 002-9

1

8.009-I0 "

6.009-10 “

1

I0. Current

 

I /
2.00e-IO-I

I
J in
I
I
I
T

 

Ya

 

-7 5712-29 . .
IC 20 30 40
Kinetic Energy

 

 

 

 

”1.589-30 r v -
0 IO 20 30

 

VtS) - W.)

Figure D.2. Retarding potential curve for 2 keV and 1 keV ions.

206

CH5 Kinetic Energy Distribution
Comp. El Source, Sllts at 8.4, pg 82

 

 

 

 

8.00e—ll
I
6.00e-I I 'I
O
I I
C 1 I
s I.
3 l /
1 4.00e-ll j I)”
.. /
= l
I
2.00e-II 1 /
i
J
. /
I [5'3
-3.Ioe-30 C 3 3 s
0 IO 20 30 4O

V(source) - V(grid)

Figure D.3. Retarding potential curve for 3 keV ions. slits at 8.4.

207

KE Distribution, Magnet Only
Comb. EI Source, 3 kv, Slits Full Open

 

 

 

 

 

3.00e-9
«5 20013-94
: a
_ ,
8 ~j ,/
c f /
-°- :
1.00e-9 —; f
f P
/
: ,5”
-2.o2e-2a : _ 2“”5 - .
-‘0 0 IO 20 so 40
V(s) - V (9)

Figure 0.4. Retarding potential curve for 3 keV ions. magnet onlv.

 

new

208

KB DISTRIBUTION, "AGNETIC SECTOR ONLY
El sounds, CEMA, sms wo, 3w, pg 95

 

6.00e—ll

 

S.GOe-ll d

J

4.00e-II <
3.00e-ll ‘

I
2.00e-l l . p

4

It)
1.00e-l l -I f
U
“1.739-30 ' .' *
-I00 0 IOO 200
INS) - WGRID)

 

 

 

Figure D.5. Retarding potential curve for 3 keV ions. CEMA detector.

IOII CURRENT

209

KE DISTRIBUTION, MAGNETIC SECTOR ONLY
El SOURCE, 3 Kv, SLITS 9.0, PG 93

 

l .00e-9

8.009'I0 .4 /Z/&—-9

 

 

 

6.00e-lO-I /
4 OOe-IO ‘1 Fl
/,
I,
/
2.00e-l0 'I ,1
7S7e-29 . f , 2
.10 o lo 20 30
V(S) - «6)

Figure D.6. Retarding potential curve for 3keV ions. slits at 9.0.

 

210

 

KE DISTRIBUTION, "AGNETIC SECTOR ONLY
El SOURCE, CEMA, surs wo, 3kV, pg 95

 

6.00e-Il

 

S.GOe—ll -

4&009-Ils

I
3.00e-ll <

mm"

2.00e-I I 1

LOOe-IIJ

Cl

 

 

.4 .738’30 .7 r
- I 00 0 I 00 200

V(S) - V(6RID)

 

Figure 0.7. Retarding potential curve for 3keV ions. CEMA detector,

magnet only.

211

KE DISTRIBUTION MAGNETIC SECTOR ONLY
El SOURCE, CEMA, 3KV, 1.5 mm Z-proflle, slits 9.0

 

0

DR CURRENT

 

 

-I0 0 to 20 30 4o so
VlS) - «6)

Figure 0.8. Retarding potential curve for 3keV ions. slits at 9.0. magnet

only. CEMA detector. 1.5 mm source z-mask.

212

KE DISTRIBUTION I‘IAGNETIC SECTOR ONLY

EI SOURCE, 3 KV, I.S m Z-PROFILE, SLITS W0
CEMA, STOPPING FIELD I m, PG 97

 

 

1.00940

d

8.00e-ll 4

6009-)! 4

III (HIRED?

4.00e-ll '1
2.00e—ll 4

I
-9.47e-30
- l 00 0 I 00 200
V(S) - V(6RID)

 

 

 

Figure D.9. Retarding potential curve for 3keV ions. slits at open.
magnet only. CEMA detector. 1.5 mm source z-mask. 1 mm field.

213

 

 

 

 

 

 

 

KE Ditribution, BE, CEMA
El SOURCE, SLITS WO, RETARDING FIELD 7mm, pg 98
J K\
‘c'
a 4
3
9 -I
3
I I
I
1
l .
—loo 6 loo zoo
l
I ,e—e
4. [,z’
/"
'3' I /D
2 ‘ /
3 7
U I /
3 . /-'3
f
I /”
-10 v o lo 2o 30

W3) - W.)

Figure D. 10. Retarding potential curve for 3keV ions. slits open. magnet

only. CEMA detector. 1.5 mm source z-mask. 7 mm field.

 

214

Appendix E
Mn2+ Collision Induced Dissociation

The operational parameters used in the CID study are listed in
Table E. l . The ionization energy (IE) of nitrogen was used to calibrate
the electron energy in the Intensitron source under field conditions
identical to those used for the production of my. The m/z 28 ion
current is plotted as a function of nominal electron enery in Figure
E.l. The nominal electron energy is calculated from the energy
setting of the emission control unit of the CH5. The extrapolated
onset for ion production is approximately 14.2 eV. The accepted
literature value for the nitrogen IE is 15.57 eV. Therefore. a 1.37 ev
correction must be applied to the nominal electron energy.
Interesting low energy features in the m/z 28 ion current as a function

of nominal electron energy are found in Figure E.2.

215

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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90 +852. EoEmem

 

 

 

 

 

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.HQ 22mg.

 

216

8/17/90 Ionization Potential of Nitrogen

8.0c+5 "

6.0e+5 "

4.0e+5 "

(‘llllllls/Sccullll

2.0c+5 '1

 

0.0e+0‘
012 34 5 6 7891011121314151617181920

Nominal e- Energy

Figure 12.1. Ion current for m /z 28 vs. nominal electron energy.

217

8/17/90 Ionization Potential of Nitrogen

I.0c+6 ‘
8.0c+5 "
6064-5 "

4.0e+5 " l

Counts/Second

A

2.0e+5 'I

A

 

 

0&4'0 ""I' "'v ""v "" I"- '1'" 'v" "7""? "'1 "" I "'1'-

o l 2 3 4 5 6 7 8 9 lOlll2I3l4151617181920
Nominal e- Energy

8/17/90 Ionization Potential of Nitrogen

(‘Olllllh/SCL‘UIHI

012 3 4 5 6 7 8 91011121314151617181920
Nominal e- Energy

Figure E.2. Plots of m/z 28 ion current vs. nominal electron energy.

 

218

Appendix F

Octopole Characterization

Ion stopping curves were generateed by ramping the octopole DC bias
through the ion source accelerating potential. Many operational
parameters. such as rf amplitude. were varied to characterize the
octopole. Art at a nominal energy of 1000 eV was used as a probe ion for
these studies. Figure F.l. shows the efiect of the rf amplitude. Figures
E2 and R3 show the effect of tune bias. The tune bias is the octopole
DC potential at which the deceleration optics were optimized for
maximum transmission prior to the octopole DC ramp. Figure F.4.
Figures F.5-F.7 show the effect of the deceleration lens injection element
DC bias. The stopping potential reproducibility is shown in Figure F8.

The effect of the source extraction element potential is shown in
Figure F.9. Curves for runs 20 and 27 were obtained with an extraction
element setting of 0.0. whereas the intermediate run 24 was performed

with an extraction element setting of 2.0.

 

BEOQ Ar+ Stopping Potentials

219

Dependence on OctOpole RF Amplitude

 

4&0“
3e+0‘
4
>
E 2““
In
so
2
E I
lco-O"
Oewoj

64-3-2401

 

 

—0— 7490412

1000 vpp

20 vpp

_.'- 7490a

—"— 74%- I4

1500 vppi

 

 

—I— 7490-18

23456789IOI11213I415

Oct DC Flt

Figure F. 1. Effect of oct0pole rf amplitude on stopping potential.

 

220

BEOQ Ar+ Stopping Potential
Effect of Octopole DC Tune Bias

 

 

 

 

 

 

4.
3.1
3
‘5
t:
.29
m
5:0
E 2*
< I
-.-— TunoTV
l —o— Tunelov
t” —O— TUMZOV
I.
1-1 5] —.— TUM‘OV
1,
‘1 --0— Tumwv
I!
Ill
0d
-5-4-3~2-1012 3 4 5 6 7 8 9101112131415
Octopole DC Bias (V,Fit)

Figure F.2. Effect of octopole DC tune bias on stopping potential.

Allulllg Signal (V)

3'1

Z‘I

 

0'1
-10

Figure F.3. Effect of DC tune bias on high energy transmission.

0

 

10

BEOQ Ar+ Stopping Potential

221

Effect of Octopole DC Tune Bias

20

3O

40 SO 60 70

Octopole DC Bias (v, Fit)

—

 

 

Tut-N

Tune 10V

TumZOV

Tun-40V

Tune DOV

 

 

 

90

vvj—VVTV‘VTv—VIVVf"Vvvvtvvvv

100

I
ITO

222

 

 

 

 

 

 

 

 

 

 

 

BEOQ An» Stopping Potential —o— Iqalaooov
Effect of Injecrion Lens Bias

*Iqabmv
4o-

‘ +IniBiu976V

. —o— lush-«2v
3.0-4
2’ 1
15 .

:20
m 1
2°
7‘; 204
2 .
4
q
#01
I
D
3.0" vvvrvxwrrvnrrrr"'r"Vrr'rrfi""r""r""r""r""t
.10 o 10 20 3o 40 so so 70 so so 100 no

Octopole DC Bias (V,Fit)

Figure F.4. Effect of ion injection energy on stopping potential.

 

223

BEOQ Ar+ Stopping Potential
Effect of Injection Lens Bias

 

 

 

 

 

 

 

 

4.0-
3.0-1
3‘ .
.59
m 1
2°
3 2.0"
<
lnjalassoov
Inlaiumv
1.0 'I
InjBias 975v
< ImBhsflEV
4
0-0J "" I""T""v'"'1

121314 15

Octopole DC Bias (Vfit)

Figure F.5. Effect of ion injection energy high energy transmission.

 

224

BEOQ Ar+ Stopping Potential
Effect of Injection Lens Bias

31

 

 

 

 

 

 

 

 

 

>
:20
m I
:o
.2 I
53 I
< I
I —O— lniBiawOV
I I —O— lnjBiu198V
—O— InjBiulOOV
. —-O— InlBiasGOOV
I
I —I— lnlBiasDOOV
I
Q
3 vyxwrvvvrr"V'r'rr‘r'rrrrrr'*r*"*r""r""I"'Wfi"'1
-‘3 0 10 20 30 40 50 60 70 80 90 100 110

Octopole DC Bias (V,Fit)

Figure F.6. Effect of ion injection energy on stopping potential.

225

 

BEOQ Ar+ Stopping Potential

 

 

 

 

 

 

, , , —.— usuaoov
Effect of Injection Lens Bias
-0— InjBiuiOOV
4-1
--— walnuoov
—0— Iniaiafioov
--0— when”
34 . . ,
Z,
.520
i0
.2 2.1
:3
<
1
.I
< .I
II
“I
:".I
0 “I ""1'“'I'"'t""r""I""I""r""l""I""I""r""v""1
-s-a-3-2-1012 3 4 s 6 7 a 9101112131415

Octopole DC Bias (V,Flt)

Figure F.7. Effect of ion injection energy high energy transmission.

Allllltlg Slglllll (V)

226

BEOQ Ar+ Stopping Potential
OctOpole RF Amplitude lO V(rms)

 

 

 

 

 

 

3.01

I

I —o— 12
2.01 .

I ‘—’_ l3

‘ I

‘ I —c-— 14
IO'I ‘

L——J
‘--—.___

~54-3-2-IOI23456789101ll213l415

Octopole DC Bias (V,Fit)

Figure F .8. Stopping potential reproducibility.

227

BEOQ Ar+ Stopping Potential
Effect of Source Extractor

 

 

 

 

 

Analog Slglllll (V)

 

 

 

-5 -~l -3 -Z -1 O 1 2 3 4 S 6 7 8 9 101112131415

Octopole DC Bias (V,Fit)

Figure F9. Effect of ion source extraction field on stopping potential.

228

Appendix G
Interference With Mn2+ CID

As discussed in Chapter 4. low energy features were observed in

the an+ CID reaction when contaminated argon was used as a
collision gas. This feature was eliminated when either high purity
(99.9990/0) argon or getter-scrubbed argon was used. The low energy

reactions may be attributed to processes related to water.

 

('nllllls/Sct‘llllll

ZCO¢5

..J

3995

3 3600

 

229

Experiment: 822_01: Mn2+ Stopping Potential

AJ+AAJAL¥ALJAA

A. L A A A A__l

 

I
I

 

Octopole DC Bias (V, Fit)

Figure (3.1. Pliny stopping potential.

 

 

l(Mlll)

822-12 : Mn+ vs Nomial KE Lab
Cell Pressure = 0.14 mtorr : CE = 29.5 eV

3000 1 a

 

ln
0
O

 

t)
o
o

 

”l
L)
O

. .4 . . A A J A A A A_ J L A.__A_ A L _A_ .

Ul

 

 

U

 

Octopole DC Bias (V, Fit)

Figure (3.2. Ion current for m/z 55 vs. lab collision enery.

231

822-14: Mn2+ CID Quad Scan ; Oct DC: 7.2V
Cell Pressure = 0.014 mtorr Ar

(5
o
o

500

(-‘nllllls/Sccollll

 

 

 

Quad Mass

Figure G3. Quad scan at a nominal collision enery of 2.2 eV.

232

8/22/90 : Comparison of Mn2+ CID product and Mn+ Quad Profill

3000-1

 

—o— MnZ+CD

2000-I

 

 

 

(‘llllllls/Sccnlltl

1000-1

 

 

 

45 50

Quad Mass

Figure (3.4. Companson of quad profiles for m/z 55 and C1D product.

233

822-18: Mn2+ CID ; e- E = 29.5 eV
Cell Pressure = 0.014 mtorr

 

 

 

 

 

3000 ~

1

2500 -I

I

I

I

2000 <

E I
'J

‘J I
7:

7. I

'_E_ 500 -I

g I

L. I

".300 '1

q

1

500 -1

I

I

 

 

 

 

Octopole DC Bias (V,Fit)

Figure 0.5. Ion current for m/z 55 and m/z 73 vs. lab collision energy.

234

Appendix H
Instrument Parameters for the Ar+ / D2 Reaction

The operational parameters for the ion beam instrument used in

the argon/ deuterium experiments is shown in Table H.l.

235

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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