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Photoelectron spectroscopy of interhalogens, radicals and transient species Cornford, Alan B. 1971

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PHOTOELECTRON SPECTROSCOPY OF INTERHALOGENS, RADICALS AND TRANSIENT SPECIES BY ALAN B. CORNFORD B.Sc. McMaster U n i v e r s i t y , 1968 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of CHEMISTRY We accept t h i s t h e s i s as conforming to the re q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1971 In present ing t h i s thes is in p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the Un ivers i t y of B r i t i s h Columbia, I agree that the L ib ra ry sha l l make i t f r e e l y a v a i l a b l e for reference and study. I fu r ther agree that permission for extensive copying o f t h i s thes is fo r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s rep resenta t i ves . It i s understood that copying or p u b l i c a t i o n o f th i s thes i s f o r f i n a n c i a l gain sha l l not be allowed without my wr i t ten permiss ion . Department of (^Ju^rtlJ4z^^. The Un ive rs i t y of B r i t i s h Columbia Vancouve r "8 , Canada D a t e te^u^AAi /o^tnz. - i i -ABSTRACT The design and c o n s t r u c t i o n of ph o t o e l e c t r o n spectrometers and h i g h r e s o l u t i o n (20-25 mv) e l e c t r o n energy analysers has enabled d e t a i l e d study of molecular i o n i z a t i o n p o t e n t i a l s . Spectra of the ten diatomic halogens and in t e r h a l o g e n s have e i t h e r been observed by PES. or p r e d i c t e d and a complete account of the P E. data f o r the s e r i e s i s presented. I.P.'s of the polyatomic i n t e r h a l o g e n f l u o r i d e s e r i e s have been c o r r e l a t e d w i t h those of the xenon f l u o r i d e s and evidence f o r MO c o n t r i b u t i o n s to the bonding i s presented. E x p e r i -mental methods of producing f r e e r a d i c a l s and t r a n s i e n t species f o r P-E. d e t e c t i o n are discussed. Evidence f o r P E s p e c t r a of the f o l l o w i n g i s reported: 0 2 > NO, N0 2, NF.2, CK> 2, S0 3F, CH 3 > ( C F ^ N O , atomic 0 and I , CH 3, "hot" CO and HF, BrF and p o s s i b l y XeF. The s p e c t r a are discussed i n r e l a t i o n to s i m i l a r molecules described i n the l i t e r a t u r e , the s p e c t r a f o r which appear i n the Appendix. - i i i -TABLE OF CONTENTS CHAPTER ONE: INTRODUCTION CHAPTER TWO: INTRODUCTION TO THE THEORY OF PHOTOELECTRON SPECTROSCOPY 2.1 R e l a t i o n s h i p Between PES and the E l e c t r o n i c S t r u c t u r e of Molecules 2.1.1 The Franck-Condon P r i n c i p l e and the Born-Oppenheimer Approximation 2.1.2 I o n i z a t i o n P r o b a b i l i t y and P E Band I n t e n s i t i e s 2.1.3 The E i n s t e i n Equation and P.E Band C h a r a c t e r i s t i c s 2.1.4 E f f e c t s Causing S p l i t t i n g s i n P E Bands... 2.1.4.1 J a h n - T e l l e r S p l i t t i n g 2.1.4.2 Spin - O r b i t S p l i t t i n g 2.1.4.3 C o r r e l a t i o n or Exchange S p l i t t i n g i n Open S h e l l Molecules 2.2 L i n e Broadening and L i m i t s to Resolving Power i n PES 2.3 A n a l y s i s of Koopmans' Theorem i n PES 2.4 The Operation of an E l e c t r o s t a t i c Energy Analyser f o r Use i n PES CHAPTER THREE: THE CONSTRUCTION AND OPERATION OF A HIGH RESOLUTION PHOTOELECTRON SPECTROMETER 3.1 I n t r o d u c t i o n 3.2 General C o n s t r u c t i o n C o n s i d e r a t i o n s 3.3 L i g h t Source U n i t 3.4 Sample Handling 3.4.1 Sample Gas Storage and I n l e t Systems - i v -Page 3.4.2 S p e c i a l i z e d Sample and I n l e t Systems 33 3.4.3 Low Temperature P y r o l y s i s U n i t 34 3.4.4 High Temperature P y r o l y s i s U n i t 36 3.4.5 Very High Temperature P y r o l y s i s 36 3.4.5.1 Filament 36 3.4.5.2 E l e c t r o d e Fusion 40 3.4.6 Microwave Discharge U n i t s and P h o t o l y s i s U n i t 40 3.4.7 Molecular Beam and P y r o l y s i s U n i t 41 3.5 Magnetic S h i e l d i n g : Helmholtz C o i l s and Mu-Metal . 42 3.6 E l e c t r o s t a t i c Energy Analysers 43 3.6.1 Double S p h e r i c a l G r i d A n a l y s e r 44 3.6.2 127° E l e c t r o s t a t i c Analysers 45 3.6.3 180° Hemispherical E l e c t r o s t a t i c Energy Analysers 48 3.6.3.1 1" Mean Radius U n i t 48 3.6.3.2 1" Radius, Two-Stage Double Hemispherical Analyser ^ 3.6.3.3 2 1/2" Mean Radius U n i t 5 5 3.6.4 360° S p h e r i c a l E l e c t r o s t a t i c Analyser 5 7 CHAPTER FOUR: PHOTOELECTRON SPECTROSCOPY OF INTERHALOGEN MOLECULES 63 4.1 The Diatomic Interhalogens 63 4.1.1 Experimental 63 4.1.2 P r e l i m i n a r y D i s c u s s i o n and Summary of General Trends 6 5 4.2 I n d i v i d u a l Molecules ^ 4.2.1 Iodine Monobromide (IBr) 7 6 - v -Page 4.2.2 Iodine Monochloride (IC1) 80 4.2.3 C h l o r i n e Monofluoride (C1F) 84 4.2.4 Bromine Monofluoride ( B r F ) : Observed and P r e d i c t e d I.P.'s 86 4.2.5 P r e d i c t e d Iodine Monofluoride (IF) 88 4.2.6 P r e d i c t e d Bromine Monochloride (B r C l ) 88 4.3 Some Trends i n the Diatomic Halogen S e r i e s 89 4.3.1 Energy L e v e l Diagram of the I.P.'s of the Diatomic Halogens 89 4.3.2 The R e l a t i o n s h i p Between Diatomic Halogen I.P.'s and E l e c t r o n e g a t i v i t y 90 4.3.3 I o n i c Frequencies and Bonding Trends 90 4.3.4 The R e l a t i o n s h i p Between Halogen M o l e c u l a r and Atomic I.P.'s 90 4.3.5 ( i T g — I T u ) Energy Separation C o r r e l a t i o n w i t h D i p o l e Moment 91 4.4 Polyatomic Interhalogen F l u o r i d e s and Xenon F l u o r i d e s 91 4.4.1 I n t r o d u c t i o n 91 4.4.2 Experimental: D i s s o c i a t i o n C o n s i d e r a t i o n s 91 4.4.3 Bromine and C h l o r i n e T r i f l u o r i d e (BrF- and C1F 3) f 92 4.4.4 Bromine and Iodine P e n t a f l u o r i d e (BrF,. and I F 5 7 92 4.4.5 Iodine H e p t a f l u o r i d e ( I F ? ) 98 4.4.6 Xenon F l u o r i d e s (XeF 2 and XeF 4) 98 4.5 D i s c u s s i o n and P r e l i m i n a r y I n t e r p r e t a t i o n of the Spectra 98 4.5.1 B r F 3 and C l F 3 98 - v i -Page 4.5.2 B r F 5 and I F 5 1 0 1 4.5.3 I F y 1 0 3 CHAPTER FIVE: THE STUDY OF FREE RADICALS AND TRANSIENT SPECIES BY PES 1 0 8 5.1 I n t r o d u c t i o n 1 0 8 5.2 I n d i v i d u a l Species 1 0 8 5.2.1 Oxygen 0 2, N i t r i c Oxide NO, and N i t r o g e n Dioxide N0 2 1 0 8 5.2.2 Difluoroamino R a d i c a l NF 2 and C h l o r i n e Dioxide C10 2 1 1 0 5.2.3 The Fl u o r o s u l p h a t e R a d i c a l S0 3F H 9 5.2.3.1 I n t r o d u c t i o n and Experimental .... H 9 5.2.3.2 D i s c u s s i o n of I n d i v i d u a l I.P.'s .. 12° 5.2.3.3 Comparisons w i t h Related Molecules -^26 5.2.4 The Methyl R a d i c a l CH 3 130 5.2.5 The B i s t r i f l u o r o n i t r o x i d e Free R a d i c a l (CF 3) 2N0 132 5.2.6 T r a n s i e n t Species 133 5.2.6.1 Atomic Iodine and Atomic Oxygen 0( 3 P ) 133 5.2.6.2 V i b r a t i o n a l l y "hot" CO and HF .... 133 5.2.6.3 Bromine Monofluoride BrF 133 5.2.6.4 Evidence f o r the P E D e t e c t i o n of Xenon Monofluoride XeF 135 REFERENCES 137 APPENDIX 14 8 - v i i -17 Plots of the P E Data for the Diatomic Halogens Against Electronegativity, Atomic I.P., Dipole Moment and Vibrational Frequency 77 - v i i i -Figure Page 18 Comparison of Some P E Spectra and ESCA Spectra ... 129 19 P E Spectra of Some Polyatomic Interhalogen Fluorides and Schematic Diagram of Possible Dissociation Products 93 20 Schematic P E.. Spectra of Polyatomic Interhalogens and P E Spectra of XeF„ and XeF. 9 5 2 4 21 Comparison of P E Spectra of XeF 2 and XeF4 and I F 5 with Those of Brundle et al.131 96 22 I.P. Correlation Diagram of the Polyatomic Inter-halogen Fluorides and Xenon Fluorides 97 23 P E Spectra of Some Free Radicals 109 24 P E Spectra of Free Radicals Produced by Pyrolysis of Dimers (N0F, — - NF_ and So0,F„ — _ S0„F) . .. I l l Z H —* Z Z O Z —* j 25 Recorder Tracing of Individual I.P.'s of NF^ 112 26 I.P. Correlation Diagram of Small Polyatomic Molecules 114 27 I.P. Correlation Diagrams of 16 to 20 Valence Electron Triatomic Monoxides, Dioxides and Difluorides and General Orbital Ordering Trends in a Plot of I.P. Against Apex Angle for Oxides 115 28 P E Spectra of the Fluorosulphate Free Radical and Related Compounds 122 29 I.P. Correlation Diagram of the Fluorosulphate Radical and Related Compounds 127 30 Comparison of Discharge Spectrum in Oxygen with Spectrum of Jonathan et al.60 134 - i x -LIST OF FIGURES APPENDIX Figure Page A l P E Spectra of the Methy1amines, Hydrogen and Argon 151 A2 P E Spectra of H 20, C1 20 and F 20 154 A3 I.P. C o r r e l a t i o n Diagram of Some D i f l u o r i d e s 156 A4 I.P. C o r r e l a t i o n Diagram of Some D i c h l o r i d e s 156 A5 P E Spectra of the Hydrogen and Deuterium H a l i d e s . ^57 A6 P E Spectra of the B i s t r i f l u o r o n i t r o x i d e Free R a d i c a l and Related Compounds 158 A7 P E Spectra of Some Small Polyatomic Molecules CDNSO, HNSO and S0 2) 1 6 1 A8 P E Spectra of Some Small Polyatomic Molecules Containing 17 to 20 Valence E l e c t r o n s A9 P E Spectra of Some Microwave Discharge Species and V i b r a t i o n a l l y Hot Diatomics (CO and HF) 1 6 6 A10 P E S p e c t r a l S i m i l a r i t i e s i n T r i p l e Bonded T r i f l u o r o m e t h y l Compounds -1-67 A l l P E Spectra of Methyl S i l o x a n e Polymer Complexes and Related Compounds 168 A12 P E Spectra of S u b s t i t u t e d Benzenes and Methylated S i l i c o n and T i n Complexes -^9 - x -LIST OF TABLES Table Page 1 L i n e Broadening and L i m i t s to Resolving Power i n PES 15 2 P E I.P.'s of the Diatomic Halogens and Interhalogens 68 3 (a) I.P. Data f o r IBr and IC1 70 (b) I.P. Data f o r IF and B r C l . .. 71 (c) I.P. Data f o r BrF and C1F 72 4 Summary of s p e c t r o s c o p i c and P E V i b r a t i o n a l Frequencies and Sp i n - O r b i t Constants f o r the Diatomic Halogens and Interhalogens 78 5 Changes i n Frequency w i t h i n S p e c i f i c Diatomic Halogen S e r i e s 85 6 P E I.P.'s of Some Polyatomic Interhalogen F l u o r i d e s and Xenon F l u o r i d e s 94 7 (a) I.P. Data f o r C1F 3 and BrF^ 100 (b) I.P. Data f o r BrF,., (C1F 5) and I F 5 102 (c) I.P. Data f o r I F ? 104 (d) I.P. Data f o r XeF 2 and XeF 4 105 (e) I.P. Data f o r XeF, 106 o 8 P E Data f o r the F l u o r o s u l p h a t e R a d i c a l 121 9 I.P.'s of the F l u o r o s u l p h a t e R a d i c a l and Related Compounds and Comparison w i t h MO C a l c u l a t i o n s 123 10 Comparison of P E and ESCA I.P.'s w i t h C a l c u l a t i o n s f o r Molecules and Ions S i m i l a r to F l u o r o s u l p h a t e .. 128 11 Spectroscopic and P E Data f o r the Methyl R a d i c a l 131 - x i -LIST OF TABLES APPENDIX Table Page A l P E Data f o r NF^ and an INDO C a l c u l a t i o n 152 A2 P E Data f o r C10 2 153 A3 Comparison of P E. Data f o r F 2 O , CI2O and H 20, CNDO/2 and INDO Calculation of F20and CNDO/2 C a l c u l a t i o n of CI2O 155 A4 I.P.'s of (CF 3 ) 2 N O , (CF3)2NOH, ( C F 3 ) 2 C O , and . CF 3NO, CNDO/2 C a l c u l a t i o n of CF3NO 159 A5 I.P.'s of CH C=CH, CF_C=CH; CH CN, CF CN, and C F 3 C = C C F 3 . . . . T 160 A6 Comparison of P E Data f o r HNSO, DNSO and S 0 2 ... 162 A7 Comparison of P E Data f o r HNSO and SO , HNCO and C0 2, HNCS and COS, and HN 3 and N 20 .... 164 A8 P E Data from Microwave Discharge (see Fi g u r e A9). 166 - x i i -ACKNOWLEDGEMENTS I would l i k e to take t h i s o p p o r t u n i t y to express my deepest a p p r e c i a t i o n to Dr. D.C. F r o s t f o r h i s support, understanding and a s s i s t a n c e throughout the course of t h i s work, and to thank Dr. C.A. McDowell f o r h i s advice and suggestions. I would a l s o l i k e to thank my many c o l l e a g u e s , Dr. C.E. B r i o n , Dr. F.G. H e r r i n g , Dr. G.E. Thomasj Dr. J.S. Sandhu, Dr. G.R. Branton, Dr. I.A. Stenhouse, Dr. R.A.N. McLean, Dr. W.R. Leeder, Dr. J.L. Ragle, Dr. L.A.R. Olsen, W.B. Stewart, D.J. Etches, A. Bain and A. K a t r i b f o r t h e i r u n s e l f i s h a s s i s t a n c e on many occasions. I wish to acknowledge g r a t e f u l l y the s t a f f s of the Mechanical, Illustration, G l a s s , and E l e c t r o n i c Workshops, and e s p e c i a l l y S. Rak, W.B. Henderson and M. Vagg f o r t h e i r a s s i s t a n c e i n design and c o n s t r u c t i o n of the instruments. I thank the N a t i o n a l Research C o u n c i l of Canada f o r generous f i n a n c i a l support. I express my s i n c e r e a p p r e c i a t i o n to my f i a n c ^ l e v a f o r her moral support and understanding. CHAPTER 1 INTRODUCTION Phot o e l e c t r o n spectroscopy (PES) i s the technique of measuring the b i n d i n g energies of e l e c t r o n s i n molecules by determining the k i n e t i c energies of e l e c t r o n s e j e c t e d by the i n t e r a c t i o n s of a monoenergetic beam of X-rays or photons w i t h a molecule. This method i s an e s p e c i a l l y unambiguous one f o r the study of the molecular e l e c t r o n i c s t r u c t u r e s of substances i n the vapour s t a t e . The non-thr e s h o l d behaviour inherent i n the p h o t o e l e c t r o n i o n i z a t i o n process has enabled the gathering of i n f o r m a t i o n normally unobtainable from c o n v e n t i o n a l o p t i c a l techniques. This t h e s i s d e s c r ibes some of the experimental aspects of photo-e l e c t r o n spectroscopy, the e l e c t r o n i c l e v e l o r d e r i n g i n the diatomic and polyatomic i n t e r h a l o g e n s and a p r e l i m i n a r y study of f r e e r a d i c a l s . An i n t r o d u c t i o n to the theory of the pho t o e l e c t r o n process and i t s r e l a t i o n s h i p to r e s o l v i n g power, e l e c t r o n i c s t r u c t u r e , and t h e o r e t i c a l MO c a l c u l a t i o n s i s given i n Chapter 2 . The c o n s t r u c t i o n and o p e r a t i o n of a spectrometer, methods of producing t r a n s i e n t s p e c i e s , and an a n a l y s i s of e l e c t r o n energy analysers i s presented i n Chapter 3 . The main body of the experimental r e s u l t s and d i s c u s s i o n are found i n Chapters 4 and 5 . I.P.'s f o r the complete s e r i e s of ten diatomic - 2 -halogens are presented, and where photoelectron data could not be obtained the results have been predicted. In section 4.4 correlation between the orbital energies of the polyatomic interhalogen fluorides and xenon fluorides provides a useful extension to the diatomic data and presents experimental evidence for the orbital ordering in these molecules. Radicals and transient species observed by photoelectron spectro-scopy are reported in Chapter 5 . Electronic level orderings are suggested and correlations made with closed shell molecules. The majority of the molecules not discussed directly in the thesis but analysed during this study and used for comparisons are collected in the Appendix for ease of reference. This includes a l i s t of papers published to date, results presented at conferences and some unpublished spectra. - 3 -CHAPTER 2 INTRODUCTION TO THE THEORY OF PHOTOELECTRON SPECTROSCOPY 2.1 R e l a t i o n s h i p Between PES and the E l e c t r o n i c S t r u c t u r e of Molecules Two groups"'" independently discovered a new spectroscopy i n the e a r l y 1960's based on measurement of energies of photoelectrons emitted from gaseous molecules i n the presence of monochromatic uv r a d i a t i o n . The f i e l d has expanded to such an extent that i n the l a s t few years s e v e r a l extensive reviews have appeared. P h o t o e l e c t r o n spectroscopy i s capable of y i e l d i n g a l l of those i o n i z a t i o n p o t e n t i a l s corresponding to the removal of a s i n g l e e l e c t r o n of energy below t h a t of the l i g h t source, and t h e r e f o r e can t e s t molecular energy l e v e l c a l c u l a t i o n s . PES i s not a " t h r e s h o l d " technique and u s u a l l y i s not complicated by a u t o i o n i z a t i o n , the resonant process by which an e l e c t r o n from a bound s t a t e belonging to one Rydberg s e r i e s i s i o n i z e d by way of a r a d i a t i o n l e s s t r a n s i t i o n to the continuum a s s o c i a t e d w i t h a second Rydberg s e r i e s . 2.1.1 The Franck-Condon P r i n c i p l e and the Bom-Oppenheimer Approximation Consider a t r a n s i t i o n from ty ,, to ty ,ty , ( n e g l e c t i n g r o t a t i o n ) e v e v where ty^ depends on e l e c t r o n i c coordinates q and ty depends on n u c l e a r coordinates Q. T r a n s i t i o n P r o b a b i l i t y P „ , = I / i ! i , i|i , (M +Mrt)iJi „iji „ dqdol v v / e v q Q e v (1) Using the Born-Oppenheimer approximation, the e l e c t r o n i c and v i b r a t i o n a l p a r t s of the t o t a l wavefunction may be separated. P v ' V = l/V Ve" D Q / V V D Q +f*e''K" D?/V M QV D Q | = | P e | 2 | y J v ^ v t l d Q | 2 2 (2) \J^V' ^V"^Q I ^  t^ i e Franck-Condon f a c t o r which gives the r e l a t i v e s trengths of e l e c t r o n i c t r a n s i t i o n s and i n s e r t s the i n t e n s i t y i n t o the appro p r i a t e v i b r a t i o n a l s t a t e s . A p p l i c a t i o n of the Franck-Condon p r i n c i p l e t h e r e f o r e assumes that the p h o t o i o n i z a t i o n process i s r a p i d w i t h respect to the v i b r a t i o n a l p e r i o d . The molecule maintains a f i x e d n u c l e a r geometry during the process, however t h i s i s not t r u e f o r the e l e c t r o n s as discussed i n connection w i t h Koopmans' theorem i n Se c t i o n 2.3. For s t a t e s of the i o n having e q u i l i b r i u m d i s t a n c e s d i f f e r e n t from that i n the n e u t r a l molecule ground s t a t e , maximum wavefunction overlap (Franck-Condon r e g i o n i n Figure 1) occurs at a p o s i t i o n on the p o t e n t i a l energy surface of the i o n d i f f e r e n t from the ground s t a t e . The most commonly observed Franck-Condon overlap c o n d i t i o n s i n PES and t h e i r r e s p e c t i v e p h o t o e l e c t r o n bands are described below and s i m i l a r l y 14 33 numbered i n Figure 1, adapted from Brundle and Robin and F r o s t et a l . (1) An in t e n s e 0-0 t r a n s i t i o n , w i t h l i t t l e other evidence of THE F R A N C K - C O N D O N PRINCIPLE IN PHOTOELECTRON SPECTROSCOPY - 6 -v i b r a t i o n a l e x c i t a t i o n , i n d i c a t e s i o n i z a t i o n of a nonbonding (or "lone p a i r " ) e l e c t r o n , w i t h l i t t l e or no change i n i n t e r n u c l e a r d i s t a n c e or v i b r a t i o n a l i n t e r v a l from that of the n e u t r a l molecule. (2) The removal of an antibonding e l e c t r o n r e s u l t s i n a decrease i n bond le n g t h and maximum Franck-Condon overlap i n the moderate-sloping a t t r a c t i v e p o r t i o n of the i o n i c p o t e n t i a l curve. U s u a l l y only a few wid e l y spaced v i b r a t i o n s occur, v. > v , . i o n molecule (3) Removal of a bonding e l e c t r o n produces t r a n s i t i o n s to the steeper slope of the s t r o n g l y r e p u l s i v e p o r t i o n of the i o n p o t e n t i a l w e l l . The ph o t o e l e c t r o n band u s u a l l y i s composed of a long p r o g r e s s i o n of c l o s e l y spaced peaks i n d i c a t i n g an inc r e a s e d bond l e n g t h (weaker bonding), and a frequency r e d u c t i o n i n the i o n w i t h respect to th a t i n the molecule. C4) I o n i z a t i o n from very s t r o n g l y bonding or antibonding o r b i t a l s may be c h a r a c t e r i z e d by broad bands. In the case where the overlap r e g i o n i n c l u d e s the d i s s o c i a t i o n l i m i t of the i o n , the r e s u l t i s a converging v i b r a t i o n a l s e r i e s merging i n t o a continuum. Lack of f i n e s t r u c t u r e may a l s o i n d i c a t e that v i b r a t i o n a l spacings are beyond the l i m i t of the operating r e s o l u t i o n . (5) I o n i z a t i o n to a r e p u l s i v e i o n i c s t a t e (no p o t e n t i a l w e l l ) r e s u l t s i n a f e a t u r e l e s s band. (6) Merging of f i n e s t r u c t u r e i n t o a continuum may be caused by d i s s o c i a t i o n , p r e d i s s o c i a t i o n , by c r o s s i n g of p o t e n t i a l energy curves, one of which i s r e p u l s i v e , or a more r e c e n t l y p o s t u l a t e d frequency 3 A r e d u c t i o n of the bending mode (observed i n the H^O, H^S, ^ S e , H^Te 35 s e r i e s ) . L i f e t i m e l i m i t a t i o n s of the i o n i c s t a t e may r e s u l t i n broadening of v i b r a t i o n s by the u n c e r t a i n t y p r i n c i p l e . - 7 -2.1.2 I o n i z a t i o n P r o b a b i l i t y and PE Band I n t e n s i t i e s The p r o b a b i l i t y t h a t an e l e c t r o n from a p a r t i c u l a r o r b i t a l w i l l be e j e c t e d i n t o the i o n i z a t i o n continuum i s p r o p o r t i o n a l to i y i v , % v , . d T | 2 (3) where ip „ i s the wavefunction of the molecule i n i t s ground s t a t e , i s the wavefunction of the molecular i o n m u l t i p l i e d by th a t of the e l e c t r o n e j e c t e d i n t o the continuum w i t h energy O - I ) M i s the e l e c t r i c d i p o l e moment operator. The s t r e n g t h of a band i n a p h o t o e l e c t r o n spectrum i s then determined by the product of the f o l l o w i n g : Ci) the occupancy of the ground s t a t e o r b i t a l from which the e l e c t r o n i s removed. Cii) the degeneracy of the r e s u l t i n g s t a t e of the molecular i o n , determining the a v a i l a b l e channels f o r e l e c t r o n escape i n t o the i o n i z a t i o n continuum. C i i i ) the Franck-Condon v i b r a t i o n a l overlap i n t e g r a l s between the ground s t a t e of the molecule and a p a r t i c u l a r v i b r a t i o n a l s t a t e of the molecular i o n . The s t r e n g t h of the PE band i s a l s o g r e a t l y i n f l u e n c e d by the f o l l o w i n g : Ci) the wavefunction of the e j e c t e d e l e c t r o n and t h e r e f o r e i t s k i n e t i c energy Cii) the s i z e of the ground s t a t e o r b i t a l , i . e . , Ca) o r b i t a l s of atoms of higher atomic weight are l a r g e r - 8 -than those of low atomic weight and give stronger bands, (b) Inner s h e l l o r b i t a l s g e n e r a l l y have s m a l l e r e f f e c t i v e c r o s s - s e c t i o n s g i v i n g lower p h o t o e l e c t r o n y i e l d s . ( i i i ) the angular dependence of e j e c t i o n of p h o t o e l e c t r o n s . ( i v ) the t r a n s i t i o n c r o s s - s e c t i o n of d i f f e r e n t type o r b i t a l s w i t h respect to the p r o b a b i l i t y of i o n i z a t i o n by a c e r t a i n wavelength . r a d i a t i o n , see S e c t i o n 2.4. 2.1.3 The E i n s t e i n Equation and PE Band C h a r a c t e r i s t i c s E l e c t r o n s are removed from a l l molecular o r b i t a l s of the sample molecule that have ap p r e c i a b l e i o n i z a t i o n c r o s s - s e c t i o n s f o r s i n g l e i o n i z a t i o n and are more e n e r g e t i c than the i o n i z i n g r a d i a t i o n . These e l e c t r o n s possess k i n e t i c energies given by the E i n s t e i n r e l a t i o n : E. . = hv - ( I . P . + E* + E + „ + E + J (4) Kin trans vib rot where i s the k i n e t i c energy of the e j e c t e d e l e c t r o n hv i s the frequency of the i o n i z i n g r a d i a t i o n I.P. i s the a d i a b a t i c i o n i z a t i o n p o t e n t i a l of the molecule E* i s the t r a n s l a t i o n a l energy of the i o n (which may be trans b J v 7 neglected) E + ^ i s the r o t a t i o n a l energy of the i n d i v i d u a l v i b r a t i o n a l r o t ° J s t a t e s of the i o n and may a l s o u s u a l l y be neglected E ^ b * s t n e v i b r a t i o n a l energy of the a p p r o p r i a t e e l e c t r o n i c s t a t e of the i o n . In g e n e r a l , f o r the present l e v e l of experimental r e s o l u t i o n : - 9 -E. k i n = hv v i b - I.P. (5) Measurement of the k i n e t i c energies of e j e c t e d photoelectrons gives molecular i o n i z a t i o n p o t e n t i a l s . The p h o t o e l e c t r o n bands may c o n t a i n s e v e r a l r e s o l v e d peaks which correspond to t r a n s i t i o n s from the molecule i n i t s ground v i b r a t i o n a l s t a t e to va r i o u s v i b r a t i o n a l l e v e l s of the e l e c t r o n i c s t a t e of the i o n . ( i ) The lowest energy ( f i r s t ) peak u s u a l l y represents the 0 - 0 t r a n s i t i o n or a d i a b a t i c I.P. i f the geometry of the r e s u l t i n g i o n i s l i t t l e d i f f e r e n t from t h a t of the molecule. ( i i ) The most in t e n s e peak r e l a t e s to the Franck-Condon or v e r t i c a l I.P. ( i i i ) The i n t e g r a t e d area of each peak i s r e l a t e d to the p r o b a b i l i t y of producing the i o n i n each v i b r a t i o n a l s t a t e , (the Franck-Condon f a c t o r ) . . ( i v ) The width of the band i s an i n d i c a t i o n of the geometry change i n passing from the molecule to the i o n . (v) Noting observed band shapes, the type of bonding can be deduced from c o n s i d e r a t i o n of the Franck-Condon P r i n c i p l e and Born-Oppenheimer approximation which, together, i n d i c a t e the most l i k e l y t r a n s i t i o n from a ground s t a t e molecule to a molecular i o n , ( S e c t i o n 2.1.1). ( v i ) The v i b r a t i o n a l frequency, V £ Q n > (the spacing between adjacent peaks), when compared w i t h the corresponding n e u t r a l molecule v i b r a t i o n a l i n t e r v a l gives an i n d i c a t i o n of the bonding c h a r a c t e r of the MO from which the e l e c t r o n was removed. - 10 -( v i i ) In some polyatomic molecules one deals w i t h m u l t i d i m e n s i o n a l p o t e n t i a l surfaces and changes i n bond angles and l e n g t h s , both of which may occur simultaneously. S e v e r a l v i b r a t i o n s may be e x c i t e d i n a s i n g l e p h o t o e l e c t r o n band r e s u l t i n g from a shape change along s e v e r a l v i b r a t i o n a l normal coordinates of the r e s u l t a n t i o n as a r e s u l t of a s i n g l e i o n i z a t i o n process. ( v i i i ) In most cases t r a n s i t i o n s o r i g i n a t e from the ground v i b r a t i o n a l s t a t e of the n e u t r a l molecule, although i n cases of a p p r e c i a b l e p o p u l a t i o n of e x c i t e d molecular v i b r a t i o n a l l e v e l s , e.g. diatomic halogens"^ (Appendix A4) v i b r a t i o n a l 'hot band' t r a n s i t i o n s are observed. The Boltzmann -E /kT p o p u l a t i o n f a c t o r , e , may be c a l c u l a t e d to determine the i n t e n s i t y of the 1-0 t r a n s i t i o n which should not be confused w i t h the 0-0 a d i a b a t i c t r a n s i t i o n . Sequence s t r u c t u r e i s a l s o another f e a t u r e which may occur but be unresolved i n p h o t o e l e c t r o n bands. 2.1.4 E f f e c t s Causing S p l i t t i n g s i n PE Bands 2.1.4.1 J a h n - T e l l e r S p l i t t i n g ; 31 Jahn and T e l l e r have shown that i n a degenerate s t a t e of a symmetric or s p h e r i c a l top molecule, u n l i k e a l i n e a r molecule, there i s always at l e a s t one n o n - t o t a l l y symmetric normal coordinate that causes a s p l i t t i n g i n the p o t e n t i a l f u n c t i o n such that the p o t e n t i a l minima are not i n the symmetrical p o s i t i o n . Thus the symmetrical conformation i s not the p o s i t i o n of minimum energy i f v i b r o n i c i n t e r a c t i o n i s taken i n t o - 11 -account; r a t h e r s e v e r a l minima of p o t e n t i a l energy can a r i s e f o r c e r t a i n unsymmetrical conformations. Nonlinear molecules i n degenerate s t a t e s lower t h e i r symmetry and remove the degeneracy as a r e s u l t of asymmetric n u c l e a r displacements. This produces the a p p r o p r i a t e number of components i n the p h o t o e l e c t r o n band w i t h separations up t o 1 eV. The e f f e c t i s l a r g e s t f o r o r b i t a l s s t r o n g l y i n v o l v e d i n the bonding, however, there i s no d i r e c t way of predicting the magnitude that w i l l be observed. There i s doubt that the corresponding e f f e c t f o r degenerate s t a t e s i n l i n e a r molecules, the Renner-Teller e f f e c t , has been observed i n PES. 2.1.4.2 Spin - O r b i t S p l i t t i n g : Since the e l e c t r o n i c s t a t e s of ions are u s u a l l y m u l t i p l e t s , s p i n - o r b i t c o u p l i n g arguments can be a p p l i e d i n a s s i g n i n g molecular 109 o r b i t a l c o n f i g u r a t i o n s to s t a t e s observed i n p h o t o e l e c t r o n s p e c t r a . These s p l i t t i n g s can vary by orders of magnitude from s t a t e to s t a t e of the same i o n whereas other p r o p e r t i e s are of s i m i l a r magnitudes. Development of the theory of s p i n - o r b i t i n t e r a c t i o n i s presented i n a summary by Van Vleck."'"^ For our purposes, however, a very simple model i s a p p r o p r i a t e i n view of the l a c k of precision inherent i n the p h o t o e l e c t r o n data. An appropriate approximation i s reported f o r the diatomic halogens"^ (Appendix A4). N e g l e c t i n g d i f f e r e n t i a l overlap and other two-centre i n t e g r a l s the m u l t i p l e t s p l i t t i n g i s given by: A - 232 < Z E F F / r x 3 > <6> x EFF where £ i s the Bohr magneton, and Z i s a screened n u c l e a r charge. This expression i s j u s t equal to the atomic s p l i t t i n g parameter, that i s , 2/3''"^ ^ of the atomic s p l i t t i n g f o r the same e f f e c t i v e n u c l e a r charge and mean r a d i u s . The e x p e c t a t i o n value v a r i e s approximately as EFF 4 (Z ) , f o r given £. Therefore the f o l l o w i n g g e n e r a l i z a t i o n s apply. (a) The main c o n t r i b u t i o n to the s p i n - o r b i t c o u p l i n g energies - 3 comes from regions c l o s e to the n u c l e i , ( s i n c e A(r) v a r i e s as r and near any nucleus the e l e c t r i c f i e l d i s approximately s p h e r i c a l ) . (b) The s p i n - o r b i t i n t e r a c t i o n i s dominated by c o n t r i b u t i o n s of heavy atoms. (c) The l a r g e s p i n - o r b i t e f f e c t i n heavy-atom molecules quenches 56 p o s s i b l e J a h n - T e l l e r e f f e c t s . (d) I f the s p i n - o r b i t coupled e l e c t r o n on a heavy atom i s ap p r e c i a b l y c o v a l e n t l y bonded to l i g h t atoms, the s p i n - o r b i t i n t e r a c t i o n i s reduced a c c o r d i n g l y . (e) The o r b i t a l c o n t r a c t i o n which accompanies molecule formation from atoms, or p o s i t i v e molecule-ion formation, w i l l i n c r e a s e A , the s p i n - o r b i t s p l i t t i n g . To t h i s approximation, f o r the diatomic i n t e r h a l o g e n s of Chapter 4, the observed s p i n - o r b i t doublet s e p a r a t i o n i s mainly determined by the (n.pir ) o r b i t a l , equation 4 ( 1 ) , and hence i s s i m i l a r t o , but J y j expected to be a l i t t l e l e s s than, the s p i n - o r b i t s p l i t t i n g of the 9 6 - 9 9 l a r g e r ground s t a t e i o n , (Table 4 ) . This d i f f e r e n c e w i l l depend upon the degree of mixing of the outer (ir ) antibonding and (TT ) Y j X i bonding o r b i t a l s , and how much of the i o n i z a t i o n occurs at the expense of the x. atom. x - 13 -Consider a molecular o r b i t a l defined bv: 2 2 2 where N = (C + C, + 2C C, S , ) . Y ,y, are atomic o r b i t a l s centred a D a b ab A a Ab on atoms 'a' and 'b' and the c o e f f i c i e n t s C , C^ are the eigenvectors of the Hartree-Fock m a t r i x , and i s the overlap i n t e g r a l between the atomic o r b i t a l s x a » X^* In the s p i n - o r b i t i n t e r a c t i o n there w i l l be terms: N 2 ) c 2 < a|Z I • s + Z, l,-s |a> 1 a '-a a -b b 1 ' r 3 r 3 (8) + 2C a < a I " lb; a b 1 1 + c b 2 < b | " |b>; N e g l e c t i n g overlap d i s t r i b u t i o n s x X K a n c* <a|l|a> terms the r e s u l t i s a D ^ r b N " 2 j C a 2 < aK V 5 ' a > + S 2 < b ' ? b V S l b > | ( 9 )  r a 3 r b 3 The s p l i t t i n g i n the molecule-ion i s thus of the form"^: E ( C a V 2 ) ' to. i n A?OM ) 4 ' ( 2 / 3 F R E E A T ° M A ) ( 1 0 ) sum over f r a c t i o n a l r a t i o of e f f e c t i v e symmetry a l l atoms c o n t r i b u t i o n atom a charge to r e d u c t i o n w i t h p of atom a f o u r t h power o r b i t a l s - 14 -Consequently i n the I C 1 + i o n , the IT o r b i t a l w i l l be represented as: - ( 5 P T T T - 3pTT ) (11) and thus the i o d i n e p a r t w i l l dominate but be cut back by about 12%. I t i s i n f o r m a t i v e to note t h a t , to t h i s approximation, the IT and ir g u s p l i t t i n g s w i l l be the same s i n c e the magnitude of the s p l i t t i n g only depends on the square of the c o e f f i c i e n t . The diatomic halogen s p i n -o r b i t s p l i t t i n g s observed by PES appear i n Table 4. 2.1.4.3 C o n f i g u r a t i o n or Exchange S p l i t t i n g i n Open S h e l l Molecules I o n i z a t i o n of a s i n g l e e l e c t r o n from an in n e r valence o r b i t a l of an op e n - s h e l l molecule ( r a d i c a l ) leaves the i o n w i t h two unpaired e l e c t r o n s r e s u l t i n g , through exchange i n t e r a c t i o n , i n a low energy t r i p l e t s t a t e (spins p a r a l l e l ) and a higher energy s i n g l e t s t a t e (spins a n t i p a r a l l e l ) . A l l p h o t o e l e c t r o n bands a f t e r the f i r s t are comprised of s i n g l e t - t r i p l e t p a i r s which may be separated by as much as 3 eV and p o s s i b l y by other bands i n the spectrum, e.g. see spectrum of NO^, Fig u r e 23. For a molecule having a t h r e e - f o l d a x i s or higher symmetry i n an e x c i t e d s t a t e , e x c i t e d i o n i c s t a t e s can occur i n which unpaired e l e c t r o n s may r e s i d e i n a degenerate p a i r of MO's having o r b i t a l angular momentum about the p r i n c i p l e a x i s (e.g. T r - o r b i t a l s ) . See f o r example NO, Fi g u r e 23. 15 -2.2 Li n e Broadening and L i m i t s to Resolving Power i n PES To date a best working r e s o l u t i o n of between 4 to 7 mv has been o 40 41 0 42 u t i l i z e d by Asbrink and R a b a l a i s , R a b a l a i s et a l . , A s b r i n k , and 43 Ed q v i s t et a l . The fundamental l i m i t s of the r e s o l v i n g power i n 36 3 7 phot o e l e c t r o n spectroscopy have been discussed by Turner. ' The magnitudes of the l i n e broadening i n v a r i o u s l i g h t sources and due to the thermal motion of molecules to be i o n i z e d , are d e a l t w i t h 38 i n d e t a i l by Samson. A r a t h e r complete review of these f a c t o r s , and those a t t r i b u t a b l e to experimental analyser l i m i t a t i o n s , are 27 reported by Brundle and Kuebler. Only a t a b u l a t e d summary of the same i s presented here. TABLE 1 L i n e Broadening and L i m i t s to Resolving Power L i m i t i n g F a c t o r Approximate Magnitude of the E f f e c t 1. Neglect of conservation of momentum between the i o n and the e l e c t r o n : 2. C o n t r i b u t i o n from the v e l o c i t y of motion of the ta r g e t molecule: 3. L i g h t source l i n e broadening ( l i n e width of the i o n i z i n g f l u x ) Lorentz l i n e w idth: 38 ( i ) ( l i ) N a t u r a l broadening ( l i f e t i m e of resonant s t a t e ) : l e s s than 1 p a r t i n 10 ( e r r o r = 1/1837 E f o r Hydrogen) 0.732 (ET/M) A/2 1/2 mV ( p r o p o r t i o n a l to v E*'*") ( f o r M = 100, E = 10 eV, m -3 broadening i s 1.76 10 eV) o h a l f widths f o r H„ produced by 584 A, o Z. and 304 A are 21 and 44 mV 38 r e s p e c t i v e l y . approximately 10 ^ eV 3.3 x 10~ 5 A - 16 -( i i i ) Pressure (resonance) broadening: ( i v ) (v) Doppler broadening: Stark broadening: 6 0 7.5 x 10 A/Torr -3 ° 3.6 x 10 A (too s m a l l to c o n t r i b u t e to observed broadening)38 (a microwave discharge operated at lowest p o s s i b l e pressure produces the sharpest 584 A resonance l i n e , h a l f - w i d t h approx. 1 mV, but not the l a r g e s t r a d i a n t energy f l u x ) 3 8 ( v i ) S e l f r e v e r s a l i n the lamp: ( s e l f a b s o r p t i o n by unexcited helium i n the path of the photon f l u x ) -3 (not l i k e l y to exceed 5 x 10 eV) 4. The n a t u r a l l i f e t i m e of the  i o n i c s t a t e s produced: ( i ) R a d i a t i v e t r a n s i t i o n s to lower i o n i c l e v e l s : (fluorescence) ( i i ) Rapid d i s s o c i a t i o n from the i o n i c s t a t e i n t o fragments, i . e . , to a r e p u l s i v e s t a t e : ( i i i ) A n o n - r a d i a t i v e t r a n s i -t i o n t o a s t a t e of s i m i l a r energy (e.g. curve c r o s s i n g ) : approx. 10 ^ eV up to 0.4 eV up to 10 eV 5. The r o t a t i o n a l envelope h a l f - width w i t h i n a s i n g l e  e l e c t r o n i c v i b r a t i o n : 6. T h e o r e t i c a l r e s o l u t i o n of the  e l e c t r o n energy analyser: of the order of kT = 0.025 eV -3 (can be e s t a b l i s h e d to 10 eV or b e t t e r ) 7. Experimental f a c t o r s : ( i ) The r e s u l t a n t magnetic f i e l d environment i n the analyser region a f t e r s h i e l d i n g : ( i i ) E l e c t r o d e s u r f a c e p o t e n t i a l s : ( i i i ) E l e c t r o d e and aperture end e f f e c t s ( f r i n g i n g f i e l d s ) : (Magnetic s h i e l d i n g has reduced r e s u l t a n t f i e l d s to between 30 to 1 m i l l i g a u s s ) approx. 2 x 10 ^ eV but may be reduced by coatings to 2 x 1 0 - ^ eV. 39 Reduced by Herzog c o r r e c t i o n curves - 17 -( i v ) Length of the discharge column and the helium p r e s s u r e 3 8 R e s o l u t i o n i s maximized by using an a p p r o p r i a t e e x c i t a t i o n source which produces low energy p h o t o e l e c t r o n s . 8. The nature of the molecule under i n v e s t i g a t i o n : ( i ) Molecules w i t h l a r g e d i p o l e moments tend t o i n c r e a s e s u r f a c e charges i n the c o l l i s i o n chamber, r e s u l t i n g i n s p e c t r a of lower r e s o l u t i o n , e.g., see reference 40 f o r R^O, Ar mixture. ( i i ) Molecules w i t h many degrees of freedom e x h i b i t broad s p e c t r a because of overlap of v i b r a t i o n a l and/or r o t a t i o n a l l i n e s , as noted i n 5 above. 2.3 A n a l y s i s of Koopmans' Theorem i n PES 44 Koopmans showed that i f a p a r t i c u l a r set of molecular o r b i t a l s , {<t>^ } i s chosen: * ' 1 ^ ( 1 ) 0 ( 1 ) ^ ( 2 ) 3 ( 2 ) . . . . | (12) and an e l e c t r o n removed from one of them, <j> , w h i l e the others are unchanged: * - 1 ^ ( 1 ) 0 0 1 ) ^ ( 2 ) 6 ( 2 ) . . . * k_ 1( )a( ) < j) k l(l ) B(l) 4 > k _ 1 ( l ) a ( l ) ? k _ 1 ( )B( ) (13) Then <f>k,, ( f o r which the energy has been minimized) M > N (14) must be the e i g e n f u n c t i o n w i t h eigenvalue of the s m a l l e s t magnitude - 18 -( w i t h i n each symmetry type). Therefore, w i t h i n the given c o n d i t i o n s , the most app r o p r i a t e s e t , {<J>^ }j f o r c o n s t r u c t i n g the singly i o n i z e d c o n f i g u r a t i o n s appears to be that set which corresponds to the given p i c t u r e of the s h e l l s t r u c t u r e . That i s , i o n i z a t i o n preserves the s h e l l s t r u c t u r e of the parent system. For a molecule w i t h 'n' doubly occupied molecular o r b i t a l s , <f> , the energy i s : n N n E = 2 E e, + E (2J -K ) (15) i = l 1 i , j = l 1 J 1 J N Consequently the energy i s a sum of , (energy of each e l e c t r o n alone i n the n u c l e a r environment; k i n e t i c plus n u c l e a r a t t r a c t i o n energy); e i = / i i t - 1 / 2 ^ + E IT" ] < f , i d x i ( 1 6 ) a i a J_j_ , (coulombic r e p u l s i o n between a l l e l e c t r o n p a i r s ) : J i j = ^ i * C 1 ) * / ( 2 ) r ^ * i C l ) * j ( 2 ) d x 1 d x 2 (17) and K. , (an exchange i n t e r a c t i o n between every p a i r of e l e c t r o n s of the same spin) K i j -Jfi±*™*fa • 1 ( 2 ) * J ( l ) d T 1 d x 2 (18) Assuming these i n t e g r a l s to be the same i n the i o n ( s i n g l y i o n i z e d ) , the energy d i f f e r e n c e (I.P.) i s : N n _ 1 e + E (2J -K. .) + J (19) n . . n i n i nn ' i = l - 19 -which i s e x a c t l y the same sum of i n t e g r a l s f o r the SCF o r b i t a l energy. Therefore, t h i s e x p r e s s i o n assumes no o r b i t a l change from molecule to i o n and t h e r e f o r e no r e o r i e n t a t i o n . A l s o , the Hartree-Fock theory n e g l e c t s r e l a t i v i s t i c e f f e c t s , whereas the v i r i a l theorem shows the i n n e r e l e c t r o n s to have massive k i n e t i c energies and important r e l a t i v i s t i c e f f e c t s . I t i s not v a l i d to assume that the r e l a t i v i s t i c energy i s the same i n both the molecule and the i o n . The f oregoing d i s c u s s i o n presents a f o r m u l a t i o n of Koopmans' 44 theorem : For a c l o s e d s h e l l molecule, the negative of the o r b i t a l energy computed i n an ab_ i n i t i o SCF c a l c u l a t i o n i s approximately equal to the i o n i z a t i o n p o t e n t i a l of an e l e c t r o n from that o r b i t a l . I t i s 46 cautioned that Koopmans1 v a r i a t i o n of energy of a s i n g l e c o n f i g u r a t i o n i s r e l e v a n t only to the ground i o n i c s t a t e of each symmetry. In these cases the eigenvalues are upper bounds to the Hartree-Fock c o n t r i b u t i o n to i o n i z a t i o n p o t e n t i a l s . There a l s o remains the c o r r e l a t i o n energy e r r o r which a r i s e s from the f a c t that the theory assumes that each e l e c t r o n experiences e f f e c t s of the others only by i n t e r a c t i o n w i t h the smoothed-out average found by squaring the one e l e c t r o n wavefunctions. These c o r r e l a t i o n e f f e c t s a r i s e from p a i r i n t e r a c t i o n s between e l e c t r o n s and hence the c o r r e l a t i o n energy w i l l be d i f f e r e n t and probably l e s s i n the i o n than i n the parent molecule. Therefore, separate HF c a l c u l a t i o n s give a d i f f e r e n c e which i s too s m a l l an I.P., whereas simple Koopmans' theorem c a l c u l a t i o n s , (|E| = I . P . ) , give values too b i g . However, the f o r t u i t o u s c a n c e l l a t i o n of e r r o r s , that i s , the combined neglect of both r e o r g a n i z a t i o n and c o r r e l a t i o n energy c o r r e c t i o n s , r e s u l t s i n a r a t h e r 47 reasonable estimate i n the end. Experience has shown that the a p p l i c a -t i o n of Koopmans' theorem to ab_ i n i t i o c a l c u l a t i o n s of the valence - 20 -s h e l l energy l e v e l s ^ o f t e n gives energies too high by 8%, p a r t i c u l a r l y where only f i r s t row elements are i n v o l v e d . The agreement of the "92% estimate of the c a l c u l a t e d v a l u e s " i s not p e r f e c t , although i t i s o f t e n s u f f i c i e n t to unambiguously a s s i g n the p h o t o e l e c t r o n bands to i o n i z a t i o n s from s p e c i f i c MO's i n the molecules. The c o r r e l a t i o n energy need not be the same f o r every s t a t e of the i o n and, as a r e s u l t , i n 54 the case of Koopmans' theorem gives the i o n i z a t i o n s i n the wrong order. C a l c u l a t i o n s to date show that d e v i a t i o n s from Koopmans' theorem values are not h i g h l y v a r i a b l e from one o r b i t a l to another w i t h i n a p a r t i c u l a r system, although the f i r s t p r e d i c t e d I.P. may be i n e r r o r by up to 4 eV. Comments on the v a l i d i t y of using Koopmans' theorem are 45 46 15 discussed among others by R i c h a r d s , Newton, Turner, Hoyland and Goodman,^8 and Ehrenson.^^ For open s h e l l molecules, there are f u r t h e r p i t f a l l s . The o f f -d i a g o n a l m u l t i p l i e r s i n the SCF c a l c u l a t i o n no longer are e l i m i n a t e d as i n the c l o s e d s h e l l case, a l l o w i n g no p h y s i c a l i n t e r p r e t a t i o n to Koopmans' theorem, which then no longer a p p l i e s d e s p i t e approximately c o r r e c t answers. A l s o , o p e n - s h e l l s t a t e s o f t e n r e q u i r e wavefunctions that are not s i n g l e S l a t e r determinants, r e s u l t i n g i n an energy d i f f e r e n c e not being representable by an o r b i t a l energy t h a t i s w e l l d e f i n e d . The i n t e r p r e t a t i o n of the p h o t o e l e c t r o n spectrum of a f r e e r a d i c a l i s best accommodated by an "energy d i f f e r e n c e " c a l c u l a t i o n , the computation of the energy of the ground s t a t e r e l a t i v e to the energy of the u n i p o s i t i v e i o n and a l l i t s e x c i t e d s t a t e s . The removal of an e l e c t r o n from a doublet s t a t e system can l e a d to both s i n g l e t and t r i p l e t i o n i c s t a t e s . With the a i d of the r e s u l t s of - 21 -c a l c u l a t i o n s f o r the i o n i t i s p o s s i b l e to estimate, i n a " f r o z e n o r b i t a l " approximation, ( i . e . , e s s e n t i a l l y the same approximations as Koopmans' theorem) the r e l a t i v e energies of the i o n e l e c t r o n i c s t a t e s . The f i r s t e x c i t e d s t a t e of the i o n w i l l have an energy r e l a t i v e to the ground s t a t e of AE 1 = e . - e . - J . . (20) 1 J IJ where, e., e. are the o r b i t a l energies of the v i r t u a l i o r b i t a l and the highest occupied j o r b i t a l , r e s p e c t i v e l y , and J i s the molecular coulomb i n t e g r a l between these o r b i t a l s . The second e x c i t e d s t a t e of the i o n w i l l have an energy r e l a t i v e to the ground s t a t e o f : AE 3 = e - e . - J . . + 2K,. (21) 1 3 i j i j where K ^ i s the molecular exchange i n t e g r a l between the i and j o r b i t a l s . T his type of CNDO/2 c a l c u l a t i o n was performed by Dr. F.G. He r r i n g f o r NF^ and CIO2 i n the appendicized references of the p u b l i s h e d 5 1 5 2 53 papers. ' Brundle et a l . attempted to perform SCF c a l c u l a t i o n s on the t r i p l e t e x c i t e d s t a t e s of NO^ "*" using Roothaan's open s h e l l f o r m u l a t i o n w i t h very l i m i t e d success f o r convergence on most o r b i t a l s . L i m i t e d s i n g l e e x c i t a t i o n c o n f i g u r a t i o n i n t e r a c t i o n c a l c u l a t i o n s using v i r t u a l o r b i t a l energies of N0^ + were a l s o only p a r t i a l l y s u c c e s s f u l . A s i m i l a r u n s u c c e s s f u l attempt i n c a l c u l a t i n g the lowest l y i n g s t a t e s of the SO^F r a d i c a l by H i l l i e r and Saunders"'"' d i d not show very favourable agreement w i t h our photoe l e c t r o n r e s u l t s . - 22 -In summary, the broad fe a t u r e s of a p h o t o e l e c t r o n spectrum of a cl o s e d s h e l l molecule, such as the rough p o s i t i o n s of peaks and t h e i r assignment to the removal of e l e c t r o n s from p a r t i c u l a r o r b i t a l s , can be i n t e r p r e t e d merely from the o r b i t a l energies found from ab_ i n i t i o c a l c u l a t i o n s on the n e u t r a l molecule. This i s g e n e r a l l y true s i n c e the p h o t o e l e c t r o n spectrum represents the removal of e l e c t r o n s only one at a time and hence Koopmans' theorem should be c l o s e l y f o l l o w e d f o r v e r t i c a l e x c i t a t i o n even i f not f o r a d i a b a t i c i o n i z a t i o n . Such v e r t i c a l I.P.'s are more n a t u r a l l y r e l a t e d to o r b i t a l energies than are a d i a b a t i c ones, because both the c a l c u l a t e d energies and v e r t i c a l I.P.'s i n v o l v e the geometry of the ground s t a t e n e u t r a l molecule only. 2.4 The Operation of an E l e c t r o s t a t i c Energy Analyser f o r use i n PES The major design and o p e r a t i o n s p e c i f i c a t i o n s f o r a low energy e l e c t r o s t a t i c e l e c t r o n monochromator are o u t l i n e d by Kuyatt and 57 58 Simpson and Simpson. Of primary importance f o r optimum r e s o l u t i o n and decreased s e n s i t i v i t y to s t r a y magnetic f i e l d s i s o p e r a t i o n of the a n a l y s e r at the lowest p o s s i b l e energy. An equation of the e l e c t r o s t a t i c p o t e n t i a l f o r the general case of a t o r o i d a l c o n d e n s e r ^ i s : 2 2 s?(u,v) = v» + E r [-u + \\- (1+c) - c + ...] (22) o o eo 2. Z where -e = the charge of the e l e c t r o n U q = energy of e l e c t r o n e = charge number r = rad i u s of c i r c u l a r mean path i n r a d i a l d i r e c t i o n eo R = rad i u s of c i r c u l a r mean path i n a x i a l d i r e c t i o n eo re'^e' ® = c v l i n d r i c a l coordinates r - r u = = r a d i a l coordinate r eo Z v = — = a x i a l coordinate r eo w = 0 = azimuthal coordinate r R eo eo E = E^G^—) (j£— ) = f i e l d s t r e n g t h on e q u i p o t e n t i a l s u r f a c e e e R = R + ( r - r ) = R (1+cu) where c = r /R e eo e eo eo eo eo Se v e r a l s p e c i f i c cases may be defi n e d : u E r ( i ) V ? Q = - —— = — ^ — f ° r a n e l e c t r o n of zero v e l o c i t y moving eo on the main o r b i t path, r eo ( i i ) c = — — = 0 gives the p o t e n t i a l d i s t r i b u t i o n f o r a c y l i n d r i c a l condenser: <P_ = E r [0.5-£n(r / r ) ] (23) J c o eo e eo r eo ( i i i ) c = — — = 1 gives the p o t e n t i a l i n a s p h e r i c a l condenser: K eo r >P = E r C — - 0.5) (24) s o eo r ' e A schematic of the analyzer e l e c t r i c a l system, scan u n i t , and app r o p r i a t e o p e r a t i n g parameters i s i l l u s t r a t e d i n Figure 10. In t h i s - 24 -work, f o r f i x e d e l e c t r o d e p o t e n t i a l s and grounded mean e l e c t r o n o r b i t , the analyser transmits only those e l e c t r o n s of f i x e d k i n e t i c energy ( u s u a l l y 1-2 eV) (band pass width E - 25 mv). A ramp v o l t a g e a p p l i e d to the t a r g e t chamber provides a v a r i a b l e r e t a r d i n g p o t e n t i a l between i t and the grounded analyser s l i t to a l t e r the k i n e t i c energy of the photoelectrons to the e s t a b l i s h e d analyser t r a n s m i s s i o n v a l u e . In a second mode of ope r a t i o n no a c c e l e r a t i n g or d e c e l e r a t i n g v o l t a g e r e t a r d s the photoejected e l e c t r o n s , but each e l e c t r o n energy i s focussed i n t u r n by sweeping the condenser e l e c t r o d e v o l t a g e s . However, the t r a n s -m i s s i o n of the analyser f a l l s o f f as e l e c t r o n energy decreases, being most d i s c r i m i n a t o r y f o r near-zero energy e l e c t r o n s . Furthermore, the analyser operates w i t h continuously v a r y i n g r e s o l u t i o n , (FWHM). S l i g h t m o d i f i c a t i o n s of t h i s method employ an a c c e l e r a t i n g e l e c t r o d e i n the t a r g e t chamber but t h i s creates some minor problems t h a t are 27 discussed by Brundle and Keubler. An approximate t h e o r e t i c a l r e s o l u t i o n f o r 0.020" apertures (w) and t r a n s m i s s i o n of 1 eV e l e c t r o n s f o r a mean e l e c t r o n o r b i t (R ) of o 1", 2 1/2", and 5" (see Chapter 3) i s 100, 250 and 500 r e s p e c t i v e l y from a p p l y i n g the f o l l o w i n g equation: ' A W 2 R r- " -r (25> The best r e s o l u t i o n achieved w i t h the 5" u n i t was approximately 12 mV (FWHM)fhowever, a nominal working r e s o l u t i o n was between 20 and 25 mV. T y p i c a l s i n g l e scan count r a t e s f o r sample pressure of a few microns ranged from 10,000 cps f o r the atomic rare gases to 200 and l e s s cps - 25 -f o r pentatomics and l a r g e r polyatomic molecules. For low count r a t e s r e s o l u t i o n may be s a c r i f i c e d by a n a l y s i n g e l e c t r o n s w i t h higher k i n e t i c energy to g a i n s i g n a l i n t e n s i t y . I o n i z a t i o n p r o b a b i l i t y and PE band i n t e n s i t i e s , and l i n e broadening and l i m i t s to r e s o l v i n g power have been o u t l i n e d i n Sections 2.1.2. and 2.2. S e v e r a l other i n t e n s i t y determining f a c t o r s i n c l u d e the t h e o r e t i c a l and experimental l u m i n o s i t y 57 58 of the condenser type employed ' (see S e c t i o n 3.6.4), and the i o n i z a t i o n c r o s s - s e c t i o n dependence on i o n i z i n g wavelength ( S e c t i o n 2.1.2). 35 P r i c e has shown that h i g h energy (ESCA X-ray) l i g h t sources enhance the r e l a t i v e i o n i z a t i o n c r o s s - s e c t i o n s of s- and c-type o r b i t a l s w i t h respect to p - and Tr-type o r b i t a l s compared w i t h r e s u l t s obtained w i t h ° ° 185 H e l 584 A r a d i a t i o n . H e l l 304 A observations f o l l o w a s i m i l a r t r end. CHAPTER I I I . THE CONSTRUCTION AND OPERATION OF A HIGH RESOLUTION PHOTOELECTRON SPECTROMETER 3.1 I n t r o d u c t i o n One of the purposes of t h i s study was to design and c o n s t r u c t a high r e s o l u t i o n spectrometer that could be used f o r the r o u t i n e a n a l y s i s of noncorrosive gaseous molecules at room temperature. Two such instruments have been cons t r u c t e d here w i t h i n the l a s t three years employing s e v e r a l d i f f e r e n t a n a l y s e r designs subsequently d e s c r i b e d i n S e c t i o n 3.6. The f i r s t system was adapted f o r the production of f r e e r a d i c a l s by p y r o l y s i s and microwave di s c h a r g e , and f o r the study of c o r r o s i v e substances. The design s p e c i f i c a t i o n s f o r the general spectrometer system r e p r e s e n t i n g the second u n i t are described below, and only those m o d i f i c a t i o n s and u t i l i t i e s unique to the f i r s t w i l l be f u l l y d i s c u s s e d . 3.2 General C o n s t r u c t i o n Considerations A schematic diagram of the spectrometer and pumping system appear i n Figure 2. The main frame i s constructed of 1 1/2" square aluminum angle dexion w i t h a 32" square t a b l e s u r f a c e , i n the centre of which i s p o s i t i o n e d the vacuum chamber base p l a t e . The f i g u r e i l l u s t r a t e s the b o l t l e s s O-ring vacuum s e a l c o n s t r u c t i o n designed f o r easy removal of P H O T O E L E C T R O N SPECTROMETER D E S I G N VACUUM CHAMBER Light Source ..Electronics Roughing Line Energy Analyser Boltless O-Ring Seal A. Helmholtz Coils B- Vacuum Chamber C. Sample System D. Scan Unit & Ramp Amplifier E. Ratemeter, Discriminator, Amplifier F. Digital Voltmeter G. Mains Power Unit H. Helmholtz Power Supplies J. Lens Supply K- Ionization Gauge Supply L- Microtherm Unit M. High Voltage Supply CALIBRANT & SAMPLE INLET To Vacuum Chamber To Forepump VACUUM PUMP SYSTEM SPECTROMETER CHASSIS l « ' — 52' B -1 i iz =?<=! Light Source . Forepump lo / Sample & .-" Roughing .•' Forepump FIGURE . 2 - 28 -the main chamber c y l i n d e r to f a c i l i t a t e m o d i f i c a t i o n or r e p a i r of the a n a l y s er. The O-ring grooves machined i n t o the end p l a t e s a l l o w s l i g h t c o m p r e s s i b i l i t y of the 1 / 4 " diameter V i t o n O-ring which permits a nominal —6 vacuum of 10 Torr. The e n t i r e vacuum chamber and adjacent d i f f u s i o n pump gate-valve were constructed of aluminum to minimize magnetic f i e l d g r a d i e n t s i n the analyser r e g i o n . A major shortcoming of the present design i s the i n a b i l i t y to u t i l i z e the top p l a t e and c y l i n d e r w a l l f o r e l e c t r i c a l or other a c c e s s o r i e s and so j u d i c i o u s use of the base p l a t e area i s r e q u i r e d . Three set s of 3 2 " square Helmholtz c o i l s and a c y l i n d e r of c o n e t i c Hu-metal i n s i d e the vacuum chamber were found necessary f o r best r e s o l u t i o n and beam i n t e n s i t y . The peak shape and absolute energy value a l s o depend markedly upon the magnetic environment. The m a j o r i t y of the e l e c t r i c a l components are t a b u l a t e d and i l l u s t r a t e d i n Figure 2 , t h e i r s p e c i f i c f u n c t i o n s being p r e v i o u s l y 28 29 documented i n the theses of Vroom and Sandhu. The data readout 59 system has a l s o been described most s p e c i f i c a l l y by Branton et a l . , and i n v o l v e s a F a b r i t e k 1024 s i g n a l averaging computer, Molesley chart r e c o r d e r , and Nuclear Chicago X-Y p l o t t i n g system (see Figures 3 and 4 ) . The remainder of the spectrometer s e r v i c e s , l i g h t source, gas i n l e t systems, f r e e r a d i c a l generators, Helmholtz c o i l s and a n a l y s e r u n i t s , are i n d i v i d u a l l y d escribed below. 3 . 3 L i g h t Source Unit o Two Hel 584 A source u n i t s were constructed and t e s t e d although 38 no thorough comparative a n a l y s i s such as that of Samson was conducted. C . Frost , C . A . McDowell , and D . A . V r o o m P H Y S I C A L R E V I E W L E T T E R S V O L U M E 1 5 , N U M B E R 15 11 O C T O B E R 1 9 6 5 K«lluffl to m i c r o therm unit reeii F I G . 1. Diagram of the microwave photon source and the spherical photoelectron energy analyzer. Recent Developments in Mass Spectroscopy RAMP TO ENERGY ANALYZER G . R . BRANTOX, D . C . FROST, T . MAKITA* C . A . MCDOWELL and I . A . STEXHOUSE MAGNETIC TAPE FABRITEK tOOO CHAN ANALYZER - C H S C R I M X - Y PLOTTER P U L S E AMP CHANNEL MULTIPLIER SAMFwE MOLECULES ISOLATION VALVES MICROWAVE DISCHARGE POWER SUPPLY L1 r FORE FORE PUMP PUMP >HL NEEDLE M VALVE CONTROLS FOR ENERGY ANALYZER CONTROLS ELECTRON LENS SYSTEM X SCANNING POTENTIAL FOR COLLISION CHAMBER X HIGH SPEED FORE PUMP Fig. 1 . Schematic diagram of high resolution photoelectron spectrometer. F I G U R E 3 PLATE O F T H E P H O T O E L E C T R O N S P E C T R O M E T E R F IGURE 4 - 31 -A water cooled condensed DC discharge between monel e l e c t r o d e s i n helium w i t h i n a 2 mm diameter 6.3 cm long c a p i l l a r y was operated under c o n d i t i o n s s i m i l a r to those reported by Turner et al.'"'' and Brundle 27 et a l . This source d i d not appear to be s u p e r i o r to the 2450 Mcs 153 microwave discharge apparatus of F r o s t f o r our purposes. The micro-wave apparatus designed f o r these s t u d i e s i s c a r e f u l l y a l i g n e d to o a t t a i n maximum tr a n s m i t t e d f l u x of 584.33 A (21.2186 e y ) resonance r a d i a t i o n as i l l u s t r a t e d i n F i g u r e 5. A brass sheathing i n t o which f i t s both the quartz discharge tube and c o l l i m a t i n g c a p i l l a r y i s constrained to a l i n e a r c o n f i g u r a t i o n by the s o l i d brass housing, and each i s sealed by number 009 V i t o n 0-rings. P r o v i s i o n i s a l s o i n c l u d e d f o r d i f f e r e n t i a l pumping of the t a r g e t chamber, the v e r t i c a l s e c t i o n of the pumpoff housing m a i n t a i n i n g a x i a l alignment of the t a r g e t chamber w i t h the photon source, but being e l e c t r i c a l l y i n s u l a t e d from i t . The design s p e c i f i c a t i o n s and o p e r a t i o n of the type of microwave 28 c a v i t y used, are f u l l y reported i n the Ph.D. t h e s i s of D.A. Vroom. Secondary i n l e t s , ( l a b e l l e d L ) , f o r water c o o l i n g l i n e s and sample gas i n l e t through the l i g h t source flange connected to the base p l a t e CF)» are used i n c o n j u n c t i o n w i t h the pseudo-molecular beam p y r o l y s i s u n i t f o r the p r o d u c t i o n of methyl r a d i c a l s , ( S e c t i o n 3.4.7). Fi g u r e 5 a l s o i n c l u d e s a 180° double f o c u s s i n g , h e m i s p h e r i c a l e l e c t r o -s t a t i c analyser of the type most commonly used throughout t h i s study. 3.4.1. Sample Gas Storage and I n l e t Systems A sample system f o r the a n a l y s i s of non-corrosive v o l a t i l e s o l i d s , l i q u i d s , and gases i s i l l u s t r a t e d i n Figure 2. The two G r a n v i l l e - 32 -I G H T S O U R C E A N D A N A L Y S E R U N I T A. 180' HEMISPHERICAL ANALYSER B. CHANNELTRON MULTIPLIER C- SAMPLE GAS INLET D. LENS E. COLLISION CHAMBER F. SLIT ASSEMBLY G. FAST FLOW PUMPOFF H. COLLIM ATING CAPILLARY J O-RING SEAL K. ALIGNMENT SHEATHING L- SECONDARY INLET M. BORON NITRIDE CONSTRICTION N- He PUMPOFF P- VACUUM CHAMBER BASE PLATE Q AIR COOLING R- MICROWAVE CAVITY S. TUNING STUB T. MICROTHERM JUNCTION U. QUARTZ DISCHARGE TUBE V. NEEDLE VALVE W- He INLET FIGURE 5 - 33 -P h i l l i p s v a r i a b l e leak valves permit easy c a l i b r a t i o n by simultaneous monitoring at s u i t a b l e pressures of both sample and s u i t a b l e r a r e gas c a l i b r a n t i n r e l a t i o n to t h e i r i n d i v i d u a l p h o t o e l e c t r o n c r o s s - s e c t i o n s . The freeze-out stem on the sample r e s e r v o i r f a c i l i t a t e s degassing of v o l a t i l e i m p u r i t i e s whereas the 1 l i t r e bulb volume permits maintainance of a constant pressure head at the l e a k v a l v e . The use of Veeco v a l v e s w i t h O-ring seats permits r e l a t i v e l y easy r e c o n d i t i o n i n g and r e s t o r a t i o n of the system. 3.4.2 S p e c i a l i z e d Sample and I n l e t Systems For a n a l y s i s of r o u t i n e non-corrosive v o l a t i l e samples the general system described i s adequate. However, s t u d i e s i n v o l v i n g very v o l a t i l e l i q u i d s o f t e n r e q u i r e use of c o o l i n g baths of temperatures a p p r o p r i a t e to reduce the vapour pressure of the substance to a c o n c e n t r a t i o n below that which leads to excessive Joule-Thompson f r e e z i n g and blockage at the l e a k v a l v e . A dismountable pyrex g l a s s system composed of twin storage b u l b s , 3-way stopcocks, and B 10/19 ground g l a s s b a l l and socket connections was assembled using Kel-F vacuum grease f o r the study of r e a c t i o n m ixtures, charge t r a n s f e r complexes, and discharge experiments r e q u i r i n g e x c i t a t i o n of one gas and subsequent mixture w i t h a second and/or an i n e r t c a r r i e r gas. The study of very c o r r o s i v e substances was most e a s i l y f a c i l i t a t e d by a f l u o r i n a t e d copper or s t a i n l e s s s t e e l i n l e t l i n e , 'swage-lock' or compression f i t t i n g s , and a s t a i n l e s s s t e e l sample r e s e r v o i r . P r o v i s i o n i s made f o r the study of substances ( s o l i d s ) having - 34 -very low v o l a t i l i t y by d r i l l i n g and s o l d e r i n g the i n s u l a t e d center tap of an e l e c t r i c a l o c t a l feedthrough to a metal i n l e t l i n e c o n t a i n i n g a sample r e s e r v o i r and roughing l i n e . The e n t i r e i n l e t l i n e both i n s i d e and outside the vacuum chamber i s wrapped w i t h an i n s u l a t e d e l e c t r i c a l h e a t i n g tape. E l i m i n a t i o n of a l l leak c o n s t r i c t i o n s o c c a s i o n a l l y i s necessary, and the sample vapour pressure c a r e f u l l y determined by c o n t r o l l i n g the a p p l i e d temperature. 3.4.3 Low Temperature P y r o l y s i s U n i t A d e s c r i p t i o n of the p y r o l y s i s apparatus used f o r the thermal cleavage of N_F, and S„0,F_ dimers to produce NF„ and SO_F r a d i c a l s 2 4 2 6 2 2 3 r e s p e c t i v e l y , has been described"'"'" and i s i l l u s t r a t e d i n F i g u r e 6. A non-inductive e l e c t r i c a l winding of approximately 4 ohms/foot nichrome wire around a 10 cm long s i l i c a tube i n s i d e the vacuum chamber was capable of producing and m a i n t a i n i n g temperatures up to 250°C. A thermocouple r i n g surrounding and touching the o u t s i d e of the s i l i c a tube d i r e c t l y above the t a r g e t chamber gave an i n d i c a t i o n of the temperature of the gaseous species being analysed. The nichrome w i r e was powered by a 150 v o l t , 15 amp DC V a r i a c to e l i m i n a t e AC noise pickup by the m u l t i p l i e r . During the p r o d u c t i o n of SO^F r a d i c a l s s e v e r a l hours were re q u i r e d f o r e q u i l i b r a t i o n , during which time the a n a l y s e r assumed a temperature c l o s e to that of the s i l i c a tube, and the vacuum chamber, a c t i n g as a heat s i n k , rose to about 80°C. D i f f e r e n t i a l pumping of the t a r g e t chamber appeared to reduce d e t e r i o r a -t i o n of a n a l y s e r r e s o l u t i o n , countrate, and m u l t i p l i e r s a t u r a t i o n . PLATE O F P Y R O L Y S I S UNITS F O R P R O D U C T I O N O F FREE R A D I C A L S A N D T H E 2^" R A D I U S H E M I S P H E R I C A L A N A L Y S E R F IGURE 6 - 36 -3.4.4 High Temperature P y r o l y s i s Unit A p y r o l y s i s furnace, Figure 6, capable of a t t a i n i n g temperatures of 1150°C was constructed to t h e r m a l l y d i s s o c i a t e ' s t a b l e ' s p e c i e s . This 'cracker' proved s u f f i c i e n t to d i s s o c i a t e chemical bonds of approximately 60 K c a l or l e s s energy. A 4-5' l e n g t h of 1.8 ohm/foot Kanthal w i r e was n o n - i n d u c t i v e l y wrapped over asbestos tape about a 3/8" brass rod, and subsequently coated w i t h l i q u i d p o r c e l a i n (Saureisen) f o r reduced o x i d a t i o n , support, and i n s u l a t i o n . The 6" long furnace and heavy copper leads to a 20 v o l t , 5 amp DC power supply were given s e v e r a l s u c c e s s i v e l a y e r s of asbestos and Saureisen. The brass rod was removed and the u n i t d r i e d and p r e c o n d i t i o n e d i n an oven at 450°C. I t was f i t t e d over a 5/16" diameter, 12" long s i l i c a tube which passed through an a i r cooled flange recessed i n t o the vacuum chamber l i d . The lower end of the heater i t s e l f was removed 2" from the top of the t a r g e t chamber. The temperature was measured by a c a l i b r a t e d chromel (nickel-chromium) thermocouple i n s e r t e d between the heater and s i l i c a tube touching the tube w a l l . To date the furnace has been s u c c e s s f u l i n p y r o l y s i n g dimethyl mercury, (CH^^Hg and t r i f l u o r o -methyl i o d i d e , CF^I (see Figure 7 ) , although s u i t a b l e c a r r i e r gas flow c o n d i t i o n s and sample pressures f o r the d e t e c t i o n of photoelectrons from the f r e e r a d i c a l s produced r e q u i r e f u r t h e r experimentation. 3.4.5 Very High Temperature P y r o l y s i s 3.4.5.1 Filament In order to reduce the recombination of r a d i c a l s and t r a n s i e n t s by means of c o l l i s i o n a l d e a c t i v a t i o n w i t h each other and w a l l s u r f a c e s , - 37 -PYROLYSIS U S I N G A H O T F ILAMENT A N D H I G H TEMPERATURE F U R N A C E 9.538 c m " 1 9 2000 4000 6000 8000 eV 9 . 5 0 10.0 10.5 10 11 12 13 K I O N I Z A T I O N P O T E N T I A L U V ) I O N I Z A T I O N P O T E N T I A L U V ) FIGURE 7 - 38 -a hot filament source for the production of unstable species was placed inside the target chamber approximately 2 mm above the photo-electron exit s l i t to the analyser. However, the inherent advantages of this method are reduced by the following shortcomings. The low energy thermal electrons produced obscure and saturate the region of the spectrum near the cutoff (21.22 eV). The filament winding also creates a magnetic inhomogeneity close to the electrostatic analyser that varies with applied current and/or deterioration of the filament wire. Furthermore, i t is very d i f f i c u l t to accurately monitor the filament temperature by means of a thermocouple, although in the present apparatus, the photolysis window of the microwave unit (shown in Figure 8) provides observation of the approximate light temperature. Consequently, i t i s d i f f i c u l t to reproduce threshold conditions or those giving maximum yield of a desired transient species. Also, reaction of the exposed hot filament with the sample gas causes recombination and reaction of short lifetime species with production of more stable secondary products, the most stable of which dominate the P E spectrum. Nevertheless, preliminary evidence to substantiate these arguments and to demonstrate the f e a s i b i l i t y of i exploring this mode of production of transients i s available from a brief study of methyl iodide. Results are shown in Figure 7 and are described in Section 5.2.4. The filament raises the temperature of the analyser unit considerably and the analyser must, therefore, be constructed in such a way and of appropriate materials to withstand temperatures of the order 500° to 1000°C without appreciably affecting i t s performance. PLATE O F M I C R O W A V E D I S C H A R G E A S S E M B L Y F O R P H O T O E L E C T R O N S T U D I E S FIGURE 8 - 40 -3.4.5.2 E l e c t r o d e Fusion A s i m i l a r proposed u n i t i n which the f i l a m e n t i s replaced by a f u s i o n c r u c i b l e cup supported by very high r e s i s t a n c e e l e c t r i c a l furance leads i n s i d e the t a r g e t chamber, w i l l f a c i l i t a t e v a p o r i z a t i o n of substances having very low v o l a t i l i t y (e.g. t r i a t o m i c oxides and f l u o r i d e s of the heavier elements) f o r gas phase p h o t o e l e c t r o n study. A p e r f o r a t e d l i d to c o n t a i n s p u t t e r i n g , a c o o l probe to recondense the sample to reduce contamination of the system, and a d d i t i o n a l c o o l i n g of the analyser u n i t may be r e q u i r e d . 3.4.6 Microwave Discharge U n i t s and P h o t o l y s i s U n i t A p r e l i m i n a r y study of the products of an e l e c t r o d e l e s s 2450 Mcs microwave discharge of some carbon f l u o r i d e s and other s m a l l molecules gave i n d i c a t i o n s that the technique has promise f o r p h o t o e l e c t r o n s p e c t r o s c o p i c study. The c o n f i g u r a t i o n of the discharge apparatus and a s s o c i a t e d p h o t o l y s i s f a c i l i t y i s i l l u s t r a t e d i n F i g u r e s 6 and 8. The compactness of the microwave c a v i t y and the recessed vacuum chamber l i d permit a minimum t r a v e r s a l path f o r the species produced. The 1/2" diameter flow tube and T e f l o n polymer coating on the exposed i n n e r s u r f a c e , decrease c o l l i s i o n a l d e a c t i v a t i o n w i t h i n the flow stream and at the w a l l s u r f a c e s . The microwave r e g i o n i n the s i d e arm of the a l l - s i l i c a u n i t prevents ions and e l e c t r o n s produced i n the discharge from d i r e c t l y e n t e r i n g the t a r g e t chamber, and e l i m i n a t e s the n e c e s s i t y of a guard r i n g . The p h o t o l y s i s window (mounted w i t h epoxy r e s i n to the v e r t i c a l s e c t i o n of the flow tube f o r p h o t o l y s i s experiments), using a mercury discharge lamp or s i m i l a r source, has not as yet been u t i l i z e d f o r - 41 -r a d i c a l p r oduction. A s i m i l a r u n i t (minus the p h o t o l y s i s c e l l ) was designed s p e c i f i c a l l y f o r mixture of a second gas w i t h the e x c i t e d discharge products of a f i r s t at a p o i n t immediately before e n t e r i n g the t a r g e t chamber. The l i f e t i m e of species detected without a c a r r i e r gas present i s r e f l e c t e d by v i b r a t i o n a l l y hot s p e c i e s , CO and HF i l l u s t r a t e d i n Fi g u r e A (91. The m a j o r i t y of the discharges i n carbon c o n t a i n i n g compounds i n i t i a l l y produced an abundance of carbon monoxide, (presumably from oxygen or oxides i n or on the gl a s s ) and l a r g e amounts of carbon t e t r a f l u o r i d e , whereas i n c r e a s i n g amounts of hydrogen f l u o r i d e appeared w i t h increased hydrogen s u b s t i t u t i o n i n the parent 3 species discharged. Evidence f o r the d e t e c t i o n of atomic oxygen ( P) i s presented and compared w i t h r e s u l t s of Jonathan et a l . ^ i n S e c t i o n 5.2.6.1. 3.4.7 M o l e c u l a r Beam and P y r o l y s i s U n i t ( f o r the production of methyl r a d i c a l s ) . To minimize c o l l i s i o n a l d e a c t i v a t i o n of methyl r a d i c a l s , a hig h temperature p y r o l y s i s u n i t was coupled w i t h a pseudo-molecular beam apparatus s i t u a t e d e n t i r e l y w i t h i n the vacuum chamber and cooled w i t h a water j a c k e t (see Figure 6). The heater was d i f f e r e n t i a l l y pumped and encased i n a g l a s s c y l i n d e r to minimize c o r r o s i o n and c a t a l y s i s at i t s s u r f a c e . The ta r g e t chamber, mounted from a boron n i t r i d e d i s c , enables i t to withstand the high temperatures a t t a i n e d d u r ing the experiment. The main design s p e c i f i c a t i o n s were by Dr. T. Makita. - 42 -3.5 Magnetic S h i e l d i n g , Helmholtz C o i l s and Mu-Metal The operating e f f i c i e n c y of an e l e c t r o s t a t i c energy anal y s e r i s dependent upon s e v e r a l f a c t o r s , p o s s i b l y one of the most important of which i s the magnetic environment i n the region of the e l e c t r o n t r a j e c t o r y . The magnetic f i e l d i n the v i c i n i t y of the spectrometer i s g e n e r a l l y inhomogeneous, and t h e r e f o r e , the use of Helmholtz c o i l s to completely c a n c e l the f i e l d and magnetic gradients i s i m p o s s i b l e . N e v e r t h e l e s s , the use of such c o i l s does serve to s u f f i c i e n t l y reduce these f i e l d s i n order to i n i t i a l l y detect photoelectrons and subsequently modify the beam t r a n s m i s s i o n c h a r a c t e r i s t i c s . 27 Brundle and Kuebler r e s t a t e the equation g i v i n g the f i e l d produced by two c o i l s of radius ' r ' and separated by a di s t a n c e ' r 1 at the c e n t r a l p o s i t i o n , the f i e l d being constant f o r the m a j o r i t y of the di s t a n c e between the c o i l s . S i m i l a r l y , a c a l c u l a t i o n f o r a set of square c o i l s , ' r ' to a s i d e , p o s i t i o n s them a di s t a n c e 0.545 r apart to e l i m i n a t e the f i r s t order magnetic f i e l d and the second order g r a d i e n t s at t h e i r center. Magnetic inhomogeneities could best be n u l l e d by using three p a i r s of c o i l s mounted at r i g h t angles, see Figure 2, at the ap p r o p r i a t e spacing. We independently vary the current through each p a i r v i a 20 v o l t , 5.7 amp. Lambda power s u p p l i e s ( l a b e l l e d H i n the i l l u s t r a t i o n ) . The analy s e r was i n i t i a l l y r e placed by a magnetic f l u x gate and the f i e l d n u l l e d although no absolute magnetometer value of r e s i d u a l magnetic f i e l d was measured. An enclosed c y l i n d e r of c o n e t i c Mu-metal was used to l i n e the i n s i d e of the vacuum chamber, w i t h the r e s u l t that only the v e r t i c a l c o i l p a i r remained e f f e c t i v e due to p e n e t r a t i o n by the earth's magnetic f i e l d . - 43 -Siegbahn et a l . ^ have gone to the extent of constructing a servo system i n conjunction with the c o i l s to counteract magnetic f i e l d changes and f l u c t u ations occurring during the course of a run. In the presence of the i n e v i t a b l e f i e l d s remaining at the present l e v e l of s h i e l d i n g technology, one must compromise between analyser i n t e n s i t y and r e s o l u t i o n . We have made the following observations. There are usually several combinations of c o i l currents that w i l l focus the e l e c t r o n beam at the m u l t i p l i e r s l i t opening, although there i s one d e f i n i t e set f o r which a -maximum i n t e n s i t y i s achieved. However, t h i s s e t t i n g does not preclude the best r e s o l u t i o n as w e l l . One may also magnetically s h i f t the absolute energy value of a given peak with respect to the retarding p o t e n t i a l voltage ( i . e . transmit a d i f f e r e n t energy electron beam through the analyser) and e f f e c t peak broadening, asymmetry, or doubling. Consequently, the magnetic f i e l d may, i n c e r t a i n cases, be manipulated to p a r t i a l l y o f f s e t minor focusing, alignment, and/or machining e r r o r s . Magnetic e f f e c t s are one of the major causes f o r lack of current increases i n r e s o l u t i o n of e l e c t r o s t a t i c analysers i n general, and d e f i n i t e l y an important c r i t e r i o n to the l i m i t e d success of both the two-stage double-hemispherical and ' b a r r e l ' s p h e r i c a l analysers tested and described subsequently. 3.6 E l e c t r o s t a t i c Energy Analysers During the i n i t i a l stages of t h i s study, there was s t i l l a large amount of conjecture about the performance and a p p l i c a b i l i t y of d i f f e r e n t energy analysers for use i n photoelectron spectroscopy. A b r i e f study of the e x i s t i n g analysers employed i n t h i s laboratory - 44 -(Figure 3 ) , and c o n s t r u c t i o n of v a r i o u s others of d i f f e r e n t design was prompted to achieve best t r a n s m i s s i o n and r e s o l u t i o n c h a r a c t e r i s t i c s w h i l e m a i n t a i n i n g as simple and durable a device as p o s s i b l e f o r s t u d i e s of very c o r r o s i v e substances. The f o l l o w i n g i s an account of t h i s study. 3.6.1 Double S p h e r i c a l G r i d Analyser 33 The s p h e r i c a l g r i d analyser of F r o s t et a l . was designed as a t o t a l c o l l e c t i o n improvement on the c y l i n d r i c a l g r i d apparatus of 64 Schoen (see Figure 3). I o n i z a t i o n occurs at the center of two concentric g r i d s and a p h o t o e l e c t r o n stopping curve i s obtained by applying a g r a d u a l l y i n c r e a s i n g r e t a r d i n g p o t e n t i a l to the i n n e r g r i d . A s m a l l constant p o s i t i v e p o t e n t i a l of a few v o l t s between the i n n e r and outer g r i d s turns back any e n e r g e t i c p o s i t i v e i o n s . A s l i g h t m o d i f i c a t i o n of t h i s apparatus was implemented i n c o n j u n c t i o n w i t h 65 W.R. Leeder by scanning both g r i d s together w i t h a r e s u l t a n t sharper d e f i n i t i o n i n the stopping curves. This a n a l y s e r was a l s o operated 66 w i t h a s i n g l e g r i d as p r e v i o u s l y described by F r o s t et a l . w i t h a r e s u l t a n t s i g n i f i c a n t i n c r e a s e i n r e s o l u t i o n . A best r e s o l u t i o n obtained here f o r these analysers was 40 mv. For p h o t o e l e c t r o n s t u d i e s , t h i s type of u n i t does have the advantage of almost t o t a l c o l l e c t i o n of e l e c t r o n s and hence has l i t t l e angular d i s c r i m i n a t i o n against e l e c t r o n s from o r b i t a l s g i v i n g d i f f e r e n t s p a c i a l o r i e n t a t i o n . However, s h o r t -comings i n c l u d e c o m p l i c a t i o n s from t o t a l g r i d transparency, the e f f e c t that w h i l e g r i d s generate a w e l l defined p o t e n t i a l b a r r i e r there may be an accumulation of space charge at the g r i d s u r f a c e , e f f e c t s of - 45 -f i e l d p e n e t r a t i o n and a l s o contact p o t e n t i a l at the c o l l e c t o r s u r f a c e . S i g n a l - t o - n o i s e r a t i o i s a l s o a problem and may be aided by such techniques as superimposing a s m a l l ac s i g n a l on the r e t a r d i n g v o l t a g e and using a synchronous d e t e c t o r as des c r i b e d by Leder and Simpson.^ 7 The m a j o r i t y of these detriments are avoided and in c r e a s e d r e s o l u t i o n obtained by use of e l e c t r o s t a t i c d e f l e c t i o n a n a l y s e r s . 3 . 6 . 2 127° E l e c t r o s t a t i c Analysers 37 Turner i n d i c a t e d that the 127° c y l i n d r i c a l e l e c t r o s t a t i c 6 8 f i e l d , c a l c u l a t e d by Hughes and Rojansky, was to be p r e f e r r e d to the 180° f o c u s i n g s p h e r i c a l e l e c t r o s t a t i c f i e l d , c a l c u l a t e d by 61 P u r c e l l , on account of i t s g r e a t e r s u i t a b i l i t y f o r u t i l i z i n g the ph o t o e l e c t r o n f l u x from a l i n e source. The source and e x i t s l i t s of 1 cm and p l a t e r a d i i of 9 and 11 cm, l e a d to a maximum s o l i d angle of 0 . 0 1 2 8 s r compared w i t h 0 . 6 3 s r f o r a f u l l y u t i l i z e d h e m i s p h e r i c a l condenser of the same r a d i i , although the l a t t e r f i g u r e would be much reduced by the use of lenses at the f o c i as i n the case of the 59 a n a l y s e r of Branton et a l . (see Figure 3 ) . The s m a l l e r s o l i d angle f o r the c y l i n d r i c a l case i s o f f s e t by the gr e a t e r l e n g t h of s l i t used. The advances i n design, such as cusp-form entrance and e x i t s l i t s which reduce l i m i t a t i o n s i n r e s o l u t i o n caused by d i s t o r t e d f r i n g e f i e l d s o c c u r r i n g near plane s l i t s (see f o r example 69 Y . B a l l u ) r e s u l t e d i n a best r e s o l u t i o n of 0 . 0 1 5 eV f o r Turner's apparatus. In the present study, two 127° p a r a l l e l p l a t e analysers were c o n s t r u c t e d , each of s l i g h t l y d i f f e r e n t design. - 46 -37 The f i r s t has comparable dimensions to t h a t of Turner; r a d i i 9 and 11 cm and p l a t e height 10 cm. The condenser e l e c t r o d e s were formed from r o l l e d brass s h e e t i n g , 1 mm t h i c k , and h e l d i n p o s i t i o n by a c c u r a t e l y machined s l o t s i n top and bottom T e f l o n formers which were t i e d together by s i x expansion rods b o l t e d through brass p l a t e s on the outsi d e s of the t e f l o n . The m a j o r i t y of the t e f l o n exposed to the analyser center was cut away to reduce space charge e f f e c t s at i t s s u r f a c e , although the m a j o r i t y of the brass covers was r e t a i n e d . No rods intermediate between the condensor p l a t e s at the top and bottom were i n s t i t u t e d to reduce f r i n g i n g f i e l d s as described by Siegbahn et a l . nor were v e r t i a l s t r i p e l e c t r o d e s used at the center of each 37 condenser p l a t e as reported by Turner. End p l a t e s w i t h recessed plane s l i t s defined the t r a n s m i t t e d beam, and the t a r g e t chamber e x i t was i n s e t i n t o a recess i n one end p l a t e . The second apparatus, Figure 9, was constructed to s m a l l e r dimensions, w i t h only a 1 cm p l a t e s e p a r a t i o n , and condenser p l a t e s h e l d i n p o s i t i o n about upper and lower a c c u r a t e l y machined s o l i d brass formers by brass s t r a p s from both of which they were i n s u l a t e d by mica l i n e r s . The brass formers were d r i l l e d i n s e v e r a l places to provide b e t t e r pumping of the analyser i n t e r i o r . In both cases the analysers were operated at the lowest p o s s i b l e energy without severe l o s s of i n t e n s i t y to minimize space charge 58 e f f e c t s . As c a l c u l a t e d by Simpson, the wider gap i n the f i r s t a n a l y s e r apparently reduced such e f f e c t s w i t h respect to the second, d e s p i t e the l a r g e t e f l o n formers. The increased d e f l e c t o r w i d t h , w h i l e r e q u i r i n g a l a r g e r p o t e n t i a l between the condenser p l a t e s to focus the - 47 -PLATE O F TYPES OF A N A L Y S E R UNITS C O N S T R U C T E D 360° Analyser 180 Analyser - 48 -same energy e l e c t r o n beam, a l s o reduces e f f e c t s of nonuniformity i n the p l a t e s u r f a c e s . This provides f o r a l a r g e r acceptance angle of i n c i d e n t e l e c t r o n s without producing t h e i r g r a z i n g i n c i d e n c e at the inner w a l l s u r f a c e s . The u n i t s were g o l d p l a t e d , coated w i t h c o l l o i d a l g r a p h i t e and then w i t h benzene soot w i t h a consequent r e d u c t i o n i n s c a t t e r e d e l e c t r o n background. In both cases a nominal r e s o l u t i o n of 35 mv was a t t a i n e d , however, i n g e n e r a l , the performance of the an a l y s e r w i t h the wider e l e c t r o d e s e p a r a t i o n was s u p e r i o r . 3.6.3. 180° Hemispherical E l e c t r o s t a t i c Energy Analysers 3.6.3.1 1" Mean Radius Uni t The f a c i l i t y f o r the h e m i s p h e r i c a l a n a l y s e r to accept a r e l a t i v e l y l a r g e s o l i d angle permits t r a n s m i s s i o n of a high e l e c t r o n f l u x . A l s o , the inherent double f o c u s s i n g c h a r a c t e r i s t i c s permit c o l l e c t i o n of the t r a n s m i t t e d beam at a p o i n t compatible w i t h a c o l l i m a t i n g lens system of c i r c u l a r symmetry and use of channeltron m u l t i p l i e r s . But t h i s f a c i l i t y a l s o creates the d i f f i c u l t y of more c r i t i c a l magnetic s h i e l d i n g i n two dimensions. The more s t r i c t alignment and launch angle l i m i t a t i o n s than f o r the p a r a l l e l p l a t e condenser, however, present minor a d d i t i o n a l d i f f i c u l t i e s that have s i n c e been overcome. This type of u n i t has a r e s o l v i n g power almost twice that of a c y l i n d r i c a l s e l e c t o r of the same r a d i u s . Therefore f o r the same energy w i d t h , a hemispherical s e l e c t o r can employ an op e r a t i n g v o l t a g e n e a r l y twice that of the c y l i n d r i c a l s e l e c t o r . Several other f a c t o r s are noted i n a 'Comparison of S p h e r i c a l and C y l i n d r i c a l M i r r o r A n a l y s e r s ' by Hafner et a l . ^ Kayatt and Simpson,^ and i n a t h e o r e t i c a l treatment of c y l i n d r i c a l , t o r o i d a l and s p h e r i c a l e l e c t r o s t a t i c prisms by H. W o l l n i k . 7 ^ - 49 -59 A s i m p l i f i e d m o d i f i c a t i o n of the u n i t of Branton e t a l . f u n c t i o n s more c o n s i s t e n t l y under c o n d i t i o n s of v a r i a b l e temperature and extreme r e a c t i v i t y without s a c r i f i c i n g r e s o l u t i o n or' i n t e n s i t y . The a n a l y s e r e l e c t r o d e s , a hemisphere and cup, machined from s o l i d brass have r a d i i 7/8" and 9/8" r e s p e c t i v e l y , w i t h a mean e l e c t r o n t r a j e c t o r y of 1", (metric dimensions being approximately 21, 29 and 25 cm r e s p e c t i v e l y ) (see Figures 5 and 9). Two such u n i t s were cons t r u c t e d ; the e l e c t r o d e surfaces exposed to the e l e c t r o n t r a j e c t o r y of only one u n i t was gold p l a t e d and then coated w i t h benzene soot. The other was r e t a i n e d w i t h p o l i s h e d brass s u r f a c e s . The t r e a t e d surfaces produced a much lower c o n c e n t r a t i o n of low energy s c a t t e r e d e l e c t r o n s near zero r e t a r d i n g energy, although the carbon s u r f a c i n g reacted w i t h s e v e r a l of the i n t e r h a l o g e n f l u o r i d e s making the u n i t t o t a l l y i n e f f e c t i v e . The uncoated brass was merely f l u o r i n a t e d , remaining i n e r t and extremely s t a b l e w i t h l i t t l e d e t r i m e n t a l e f f e c t to the analy s e r p r o p e r t i e s . I t was noted, however, that i n t r o d u c t i o n of r e a c t i v e f l u o r i n e substances to the system caused d r a s t i c s h i f t s of absolute energy values (as much as 1 eV), w i t h respect to the monitored r e t a r d i n g v o l t a g e , and r e q u i r e d refocussing of the t r a n s m i t t e d beam using both the lens and Helmholtz c o i l s . The lens element, a 2 mm c i r c u l a r knife-edge a p e r t u r e , served two main f u n c t i o n s . I t was i n s t r u m e n t a l i n focussing the photoelectrons emerging from the t a r g e t chamber, and d e f i n i n g t h e i r launch angle of i n c i d e n c e , designated a, upon the entrance s l i t to the an a l y s e r . Consequently, the p o t e n t i a l on the lens was compromised between two a l t e r n a t i v e s . A high v o l t a g e causes strong focussing and c o l l e c t i o n of - 50 -more e l e c t r o n s w h i l e i n c r e a s i n g the t o t a l e f f e c t i v e launch angle and degree of g r a z i n g incidence of e l e c t r o n s at the analy s e r s u r f a c e s . A lower v o l t a g e r e s u l t s i n reduced i n t e n s i t y and p o s s i b l e nonfocussing at the analyser s l i t w i t h a subsequent decrease i n r e s o l u t i o n . A low vol t a g e c u t o f f f o r the lens occurred a f t e r f l u o r i n a t i o n below which no s i g n a l was detected, perhaps due to nonfocussing or a t h r e s h o l d requirement to overcome f r i n g i n g f i e l d s or space charge at the s l i t . However, above t h i s t h r e s h o l d value a t a s l i g h t l y higher lens p o t e n t i a l the beam could be r e a d i l y maximized. I f good alignment of the t a r g e t chamber aperture, l e n s ( and analy s e r apertures p r e v a i l e d , o p e r a t i o n of the lens was unnecessary f o r d e t e c t i o n of the beam but d i d serve to l i m i t the acceptance angle to the condenser. This angle i s the major c r i t e r i o n i n e s t a b l i s h i n g the "gap wid t h " between the e l e c t r o d e s as 5 8 described by Simpson. The t a r g e t chamber was constructed from a 1 3/4" l e n g t h of 1/4" . brass tubing w i t h a 0.020-0.030" counter-sunk k n i f e edge aperture d r i l l e d i n the center f o r an e x i t s l i t . This was mounted through a 1/4" hole d r i l l e d i n a t e f l o n d i s c which f i t snugly i n t o the lens mounting. The whole u n i t was mounted on the s l i t bar v i a four nylon screws and boron n i t r i d e washers f o r e l e c t r i c a l i n s u l a t i o n . I n i t i a l l y , s a t i s f a c t o r y aperture alignment was made through the lens "by eye", although a more c o n s i s t e n t arrangement using a l o c a t i n g c o l l a r on the chamber was l a t e r adopted. The t e f l o n was a l s o replaced w i t h boron n i t r i d e , and the nylon screws w i t h b r a s s , f o r high temperature s t u d i e s . The s i z e of the e x i t aperture of the target chamber determines the launch angle, the transmission focal point and s p h e r i c a l a b e r r a t i o n , but only s l i g h t l y - 51 -the resultant intensity of the signal. It did more spe c i f i c a l l y limit the base pressure in the vacuum chamber and set a limit on the sample pressure in the c o l l i s i o n region. The operating conditions were - 4 generally established using 1 x 10 Torr as an upper vacuum chamber base pressure limit for best multiplier operation, and the signal maximized by monitoring the sample pressure, generally of the order of a few microns. The maintainance of a constant pressure within the target chamber was observed to be c r i t i c a l because any shift in pressure resulted in a d r i f t in the energy value of a peak with respect to the retarding energy scale. This is attributed to contact potential changes at the inner target chamber surface. This effect i s also manifested when a second gas is mixed with the sample under study, for example, during calibration. The inner target chamber surface was coated with colloidal graphite and the chamber i t s e l f constructed of stainless steel although both did not appear to enhance the signal characteristics or resolution. The graphite did however reduce production of low energy scattered electrons. Poisoning of the inner surface occurred only for studies of reactive species, and a monolayer of the substance i t s e l f or the products of reaction with the wall caused marked reductions in intensity. The chamber could be almost completely restored to i t s original condition by dipping i t in a concentrated chromic acid solution. For ease of machining and alignment, the entrance and exit circular apertures to the monochromator were inset and d r i l l e d (0.020" diameter knife-edges) in opposite ends of a single bar used for mounting the electrodes, lens assembly, and multiplier housing. The - 52 -target chamber o J i S C A N ) t—,J UNIT V=Vl(R,/Rl)-(R1/R2)2. The potential of the inner sphere is r o [ 3 - 2 ( / ? 0 / J ? i ) l the potential of the outer sphere is l /o[3-2(i? 0/i?2)],' a2=w/4Ro, W MAX A £ j / 7 i 0 = w / 2 J R 0 . X z * ^ * » = - ( V * o ) + 2 ( A E / £ , ) _ 2 a * , 180° HEMISPHERICAL ANALYSER UNIT FIGURE 10 - 53 -bar was e l e c t r i c a l l y i n s u l a t e d from the e l e c t r o d e s by a t h i n mica sheet cut w e l l away from the s l i t apertures to avoid space charge bu i l d u p . S e v e r a l d i f f e r e n t s l i t arrangements were t e s t e d and those recessed as c l o s e as p o s s i b l e to the t h e o r e t i c a l 180° focus p o i n t s adopted. No doubt f r i n g e f i e l d s e x i s t e d at the plane c i r c u l a r s l i t a p e rtures, although m a i n t a i n i n g them at ground p o t e n t i a l reduced space charge accumulation and minimized the t r a n s m i s s i o n of e l e c t r o n s that h i t the aperture edges. The m u l t i p l i e r housing i s e l e c t r i c a l l y connected to the grounded s l i t bar and i s provided w i t h a sidearm f o r d i f f e r e n t i a l pumping. This was only necessary i n experiments using c o r r o s i v e gases and r a d i c a l s . A lead g l a s s channeltron m u l t i p l i e r was used, i t s o p e r a t i o n c h a r a c t e r i s t i c s being p r e v i o u s l y w e l l documented i n the l i t e r a t u r e . 4 5 I t s r a t h e r low s a t u r a t i o n l e v e l i n the r e g i o n of 10 -10 cps i s balanced by i t s s i z e , r e p r o d u c i b i l i t y , s t a b i l i t y , and the f a c t t h a t i t i s not r e a d i l y poisoned. 3.6.3.2 1" Radius, Two-Stage Double Hemispherical Analyser The m e r i t s and o p t i m i z a t i o n of t h i s type of design have been d e a l t w i t h i n d e t a i l by both Kuyatt and Simpson,"' 7 and Maeda, 72 Semeluk, and Lossing. Applying optimum design c o n s i d e r a t i o n s , the 5 8 double h e m i s p h e r i c a l s e l e c t o r and a n a l y s e r system of Simpson was -14 capable of a h a l f width of 0.005 eV and i n t e n s i t y of 3 x 10 amps at an a n a l y s e r energy of 0.470 eV. The two h e m i s p h e r i c a l u n i t s described i n s e c t i o n 3.6.3.1 were assembled together to form a two stage double-hemispherical s e l e c t o r - 54 -J . A R O L S I M P S O . V r'ic. 2. Detail of s.herical analyzer show-use of precision sap-:'»re balls to insulate position elements to :"l<rancc 0.02S mm. K. MA E D A, G . P. S E M E L U K , F. P. L O S S I N G SECTMH »-* • F i g . 1. Schematic d iagram o f double-hemispherical electron energy selector. FIGURE 11 - 55 i n a l i n e a r c o n f i g u r a t i o n using a longer s l i t bar w i t h a common aperture at i t s center. However, papers by Redington, Saum, and 62 73 72 Van Heerden, Armstrong, and Maeda, Semeluk and L o s s i n g p r e f e r p o s i t i o n i n g of the two hemispheres so that the e l e c t r o n t r a j e c t o r i e s 74 i n the two stages are mutually p e r p e n d i c u l a r (see F i g u r e 11). Scherer has pointed out that a r o t a t i o n a l l y symmetric lens cannot be f r e e of s p h e r i c a l a b e r r a t i o n . Consequently, i n the f i r s t s t a g e, the energy d i s p e r s i o n of the beam causes the image of the entrance aperture to be spread along a l i n e and that p a r t of the image f a l l i n g on the i n t e r s t a g e aperture i s then focussed i n the second stage to an image roughly 62 t r i a n g u l a r i n shape. Therefore i n the n o n - r o t a t i o n a l l y - s y m m e t r i c 6 2 f o u r - l e n s scheme of Redington et a l , the s p h e r i c a l a b e r r a t i o n i s e l i m i n a t e d except f o r f r i n g e f i e l d s . In the apparatus c o n s t r u c t e d i n t h i s study, the second stage would only f u r t h e r d i s t o r t the image of the f i r s t , i n the same plane. However the device was never f u l l y o p e r a t i v e due to the p r e v a i l i n g magnetic environment. Once a s i g n a l was detected i n the f i r s t stage and maximized using the Helmholtz c o i l s , the c o n d i t i o n s were unfavorable f o r simultaneous focussing of the beam at the e x i t aperture of the second stage. Only at much higher e l e c t r o d e p o t e n t i a l s , f o r extreme r e d u c t i o n i n r e s o l u t i o n , could the s i g n a l be detected i n the present c o n f i g u r a t i o n . A f u r t h e r shortcoming of t h i s type of device f o r use i n photo e l e c t r o n spectroscopy i s the l o s s of t r a n s m i t t e d beam f l u x i n successive stages. 3.6.3.3 2 1/2" Mean Radius Unit S i n c e , to a f i r s t approximation, the t h e o r e t i c a l r e s o l u t i o n is p r o p o r t i o n a l to the mean rad i u s of the a n a l y s e r , equat ion 2 ( 2 5 ) , - 56 -an experimental advantage i s to be gained by constructing the electrodes of an e l e c t r o s t a t i c condenser to the larges t f e a s i b l e dimensions. Hemispherical electrodes were machined from a 6" OD s o l i d brass c y l i n d e r , with f i n a l r a d i i 2 3/4" and 2 1/4" and a mean radius of 2 1/2". To avoid grazing incidence of the beam with the inner electrode surfaces ( t h e o r e t i c a l l y allowing a beam d i s p e r s i o n W J G c max fl of i t s width i n c i d e n t i n t o the condenser) the i n l e t e l e c t r o n optics assembly i s l i m i t e d to a launch angle a, determined by 2 w a = 4 R ~ o For very moderate s p e c i f i c a t i o n s of s l i t (aperture) s i z e , i . e . 0.020", a t h e o r e t i c a l r e s o l u t i o n of 2 mV i s attai n a b l e assuming the system i s adjusted to focus 1 eV electrons and there are no magnetic or focussing aberrations. The spheres were simultaneously aligned and i n s u l a t e d from a 1/4" t h i c k , 6" diameter, face-plate d i s c by 2 mm sapphire b a l l s . Holes were machined for removable apertures and pumping of the analyser i n t e r i o r . The use of a d i s c availed easier machining and alignment of the apertures to within 0.001". A s i m i l a r m u l t i p l i e r housing and target chamber-lens assembly as described for the 1" unit were constructed with the t e f l o n replaced with boron n i t r i d e and a s e l f -a l i g n i n g target chamber. With the use of Mu metal s h i e l d i n g and Helmholtz c o i l s , the analyser was capable of c o n s i s t e n t l y reproducing 2 between 5,000 and 10,000 cps on the P.,„ peak of argon with a nominal - 57 -working r e s o l u t i o n of 22 mV. A best r e s o l u t i o n of 15 mv has been achieved but i s l i m i t e d by the magnetic environment, space charge, focussing, and l i k e e f f e c t s . This design i s acceptable f o r h i g h temperature s t u d i e s (see Figure 6 ) . 3.6.4 360° S p h e r i c a l E l e c t r o s t a t i c Analyser The p o s s i b i l i t y of d e f l e c t i n g and focussing a s l i g h t l y d i v e r g i n g beam of charged p a r t i c l e s by means of a c y l i n d r i c a l condenser was 68 f i r s t demonstrated by Hughes and Rojansky, and t h a t of u s i n g a p o r t i o n of a s p h e r i c a l condenser as an analyser was suggested by A s t o n . 7 ^ The f a c t that conjugate focus p o i n t s l i e on a l i n e through the center of r o t a t i o n , was f i r s t p o i n ted out by B a r b e r 7 ^ f o r a magnetic spectrograph, and a s i m i l a r r e l a t i o n was shown by Purcell,^"'" p e r m i t t i n g c o n s t r u c t i o n of a three dimensional e l e c t r o s t a t i c a n a l y s e r of very l a r g e u s e f u l aperture and any d e s i r e d r e s o l v i n g power. I n the instrument of P u r c e l l ^ (see Figure 12), the u s e f u l a p e r t u r e , measured i n accepting s o l i d angle at the source, i s 0.210 or 1/60 of the whole sphere and could be i n c r e a s e d without s e r i o u s d e f e c t s . As a more recent improvement, Weichert and H e l m e r 7 7 use annular s l i t s at both source and d e t e c t o r to provide a l a r g e r e f f e c t i v e source area. The source can have a f i n i t e width comparable to the s i z e of the d i s c source i n the P u r c e l l type, but s i n c e i t has a length (2TTr) many times l a r g e r than i t s w i d t h , a tremendous i n c r e a s e i n source area i s achieved. However, the width of the aperture has to be balanced against the source area i n order to o b t a i n the highest p o s s i b l e l u m i n o s i t y at a given r e s o l u t i o n . A disadvantage i s the inherent d i f f i c u l t y of c o n s t r u c t i o n and magnetic f i e l d e f f e c t s . - 58 r N . H . Weichert and J. C . Helmer a -I 1 I I I I I I I I • \ \ E- M. P U R C E L L c focusing in the case of an annular source. FIGURE 12 - 59 -An i d e n t i c a l procedure to that of Purcell,^"'" and Weichert and H e l m e r 7 7 was adopted here i n e s t a b l i s h i n g the t o t a l d e f l e c t o r angle at <j> = 90°, and 6^  = ~ ^5°, both f o r convenience of machining and s i m p l i c i t y i n the c a l c u l a t i o n s . S e v e r a l of the present a n a l y s e r dimensions and c o n f i g u r a t i o n s are i l l u s t r a t e d i n Figur e s 9 and 13. A l s o a summary of the c a l c u l a t i o n parameters reported by Weichert and H e l m e r 7 7 a p p l i c a b l e f o r focussing of an annular source of f i n i t e w i d t h , are c o l l e c t e d i n Figure 13 . The appropriate symbols are de f i n e d as f o l l o w s : B the t o t a l width of the image produced by a r e p r e s e n t a t i v e part of the source (see Figure 13) = t r a c e width, r mean rad i u s of the annular source, w mean width of the annular source. +$> angle d e f i n i n g the source area, r ^ r a d i u s of c i r c u l a r detector s l i t , w^ c o n t r i b u t i o n from f i n i t e source width. 2 2r°a a b e r r a t i o n due to the angular spread. — - — increased width of the ring-shaped area scanned i n cosa n o n r a d i a l d i r e c t i o n . <|> angle of d e f l e c t i n g f i e l d . Rg=D/B r e s o l u t i o n . D energy d i s p e r s i o n ( f o r 8^ = = 45°, D = ^ r 0 J ' A source area = 2rTTW. L l u m i n o s i t y . DESIGN PARAMETERS FOR SPHERICAL ANALYSER WITH ANNULAR SOURCE FIGURE 13 - 61 -The r a d i u s of the inner and outer s p h e r i c a l e l e c t r o d e s were chosen to be 2.6 and 3.4 cm r e s p e c t i v e l y , a l l o w i n g a maximum s u i t a b l e launch angle approximating 6 1/2°. The gap i s s m a l l compared to the t o t a l path t r a v e l l e d i n the condenser and, t h e r e f o r e , f i r s t order focus c a l c u l a t i o n s should s t i l l be v a l i d . As i n the u n i t of P u r c e l l , ^ a grounded guard diaphram of approximate p r o p o r t i o n s as suggested by 39 Herzog was implemented t o compensate f o r edge e f f e c t s and f r i n g e 39 f i e l d s as f a r as p o s s i b l e . However, Herzog's c a l c u l a t i o n s were made only f o r a plane condenser and t h e r e f o r e cannot be r e l i e d upon to 2 the order of a because of the v a r i a t i o n of the f i e l d s t r e n g t h across the s p h e r i c a l condenser. Replaceable s l i t d i s c s w i t h c i r c u l a r k n i f e edges a l l o w v a r i a t i o n of annular width of acceptance to the condenser. These are mounted on the diaphram which i s a l s o used to l o c a t e and support the inner sphere e l e c t r o d e . Three narrow segments of the condenser are c l o s e d to the beam due to the diaphram support assembly, and the leads to the inner sphere and i n n e r segments of the o c t a l lens 61 cross at one of these segments. P u r c e l l ' s study at higher energy i n d i c a t e d t h a t these support t i e s d i d not n o t i c e a b l y d i s t u r b the adjacent p a r t s of the beam i n h i s apparatus. An accurate treatment of the i n f l u e n c e of the de t e c t o r s l i t i s very complicated because the a c t u a l l i n e shape has to be c a l c u l a t e d to get the most e f f e c t i v e s l i t w idth. An estimate of Weichert and H e l m e r 7 7 showed that a s l i t of h a l f the t o t a l width B was a proper choice g i v i n g a r e s u l t a n t FWHM of 3/4 B and f i n a l r e s o l u t i o n (FWHM) of R = 4/3Rg. The e f f e c t s of image m a g n i f i c a t i o n are accounted f o r i n the equations of r ^ and w^ i n Figure 13. - 62 -The a p p l i c a t i o n of t h i s type of analyser to p h o t o e l e c t r o n s p e c t r o -scopy r e q u i r e s i t s o p e r a t i o n at lowest p o s s i b l e d e f l e c t o r p o t e n t i a l . Under these c o n d i t i o n s magnetic e f f e c t s become much more important than f o r previous o p e r a t i o n c o n d i t i o n s at very high p o t e n t i a l s . 61 P u r c e l l minimized t h i s e f f e c t by mounting the analyser w i t h i t s a x i s along the earth's magnetic f i e l d a x i s , the e f f e c t of which i s sm a l l but symmetrical on the focussing. However, t h i s alignment was not p o s s i b l e i n the present spectrometer system and r e s i d u a l magnetic e f f e c t s , d e s p i t e the use of Helmholtz c o i l s , were keenly f e l t . During the p r e l i m i n a r y o p e r a t i o n of the u n i t , the o c t a l lens elements were e l e c t r i c a l l y connected together and a d e f l e c t o r p o t e n t i a l of 1 eV maintained, while a v a r i a b l e r e t a r d i n g v o l t a g e was a p p l i e d to the t a r g e t chamber. I n i t i a l l y no c o a x i a l refocussing c y l i n d r i c a l e l e c t r o d e has been i n s t a l l e d a f t e r the d e t e c t o r s l i t , but a l a r g e channeltron m u l t i p l i e r has t e m p o r a r i l y been s i t u a t e d at the (nonfocus) crossover p o i n t to i n t e r c e p t the beam. The three hundred v o l t p o s i t i v e p o t e n t i a l at the entrance to the m u l t i p l i e r should prove s u f f i c i e n t to i n t e r c e p t the m a j o r i t y of the t r a n s m i t t e d s i g n a l s i n c e 20,000 cps 2 were obtained on the -?2/2 P e a ^ °^ a r g ° n without s p h e r i c a l focussing. Only p r e l i m i n a r y s t u d i e s have been begun to date, but the i n i t i a l successes and r e s o l u t i o n at h i g h count r a t e s so f a r achieved appear to h o l d promise f o r t h i s analyser i n both of these areas. A c o n t i n u i n g s l u d y of o p e r a t i o n c h a r a c t e r i s t i c s i s i n progress and w i l l be f u l l y reported at a l a t e r date. CHAPTER 4 PHOTOELECTRON SPECTROSCOPY OF INTERHALOGEN MOLECULES 4.1 The Diatomic Interhalogens P h o t o e l e c t r o n s p e c t r a of the halogens have been reporte d by 78 50 85 Fro s t et a l . , more r e c e n t l y by Cornford et a l . , Evans and Orchard, 124 and P o t t s and P r i c e . The more recent complementary s t u d i e s i n c l u d e 79 81-83 mass spectrometry, p h o t o i o n i z a t i o n r e v e a l i n g hot bands, and e l e c t r o n impact r e s u l t s . ^ Recent reviews by Wiebenga^ concerning 87 the i n t e r h a l o g e n s , and by S t e i n , on the p r o p e r t i e s of the i n t e r h a l o g e n f l u o r i d e s , reference the m a j o r i t y of the s p e c t r a l evidence a v a i l a b l e before 1967. A reasonably complete, high r e s o l u t i o n p h o t o e l e c t r o n s p e c t r o s c o p i c study of the diatomic i n t e r h a l o g e n s , i s now presented, i n c l u d i n g comparisons w i t h the diatomic halogens'^ and molecular v . . i i i 8 8 " 9 0 o r b i t a l c a l c u l a t i o n s . 4.1.1 Experimental 87 S t e i n remarks that when bromine i s mixed w i t h bromine t r i f l u o r i d e at 25°C or higher temperatures, the f o l l o w i n g e q u i l i b r i u m i s e s t a b l i s h e d : B r 2 + B r F 3 3BrF (1) - 64 -The bromine monofluoride formed cannot be i s o l a t e d as a pure substance. The f i r s t I.P. of BrF assigned to fea t u r e s observed i n the PE spectrum of d i s s o c i a t i o n products from BrF^ (Sec t i o n 4.4.3) c o r r e l a t e w e l l , w i t h the r e s t of the s e r i e s and are described i n S e c t i o n 4.2.4. Iodine monofluoride, l i k e bromine monofluoride, cannot be i s o l a t e d as a pure substance at room temperature s i n c e i t a l s o dispropor-t i o n a t e s r a p i d l y : 5IF ^ ± 2 I 2 + I F 5 (2) Only above 800°C does the reverse r e a c t i o n dominate and i s IF a s t a b l e 91 92 and abundant s p e c i e s . ' B r C l i s c h e m i c a l l y u n s t a b l e , being the l e a s t s t a b l e of a l l the ten diatomic halogens: 2BrCl T±- B r 2 + C l 2 (3) The expected s p i n - o r b i t components of the f i r s t I.P. might p o s s i b l y be obscured by the fj components of the bromine and c h l o r i n e formed as d i s s o c i a t i o n products. A p r e d i c t e d spectrum ( S e c t i o n 4.2.6) i s presented i n Figure 16 and I.P.'s are compared w i t h l i t e r a t u r e data i n Table 3(b). Samples of IC1 and IBr were s u p p l i e d by A l f a I n o r g a n i c s , and both were r e c r y s t a l l i z e d by slow c o o l i n g to approximately 15°C. The P E spectrum of IC1 shows minute t r a c e s of i o d i n e near the a d i a b a t i c r e g i o n of the f i r s t s p i n - o r b i t component of the f i r s t I.P., obscuring - 65 -p o s s i b l e hot bands. The spectrum of IBr shows t r a c e s of Br^, (Figure 14), although the I^, which i s a l s o i n e v i t a b l y p r esent, u n d e r l i e s the IBr v i b r a t i o n a l peaks. These l a t t e r i m p u r i t i e s r e s u l t as a d i r e c t consequence of the 8% d i s s o c i a t i o n of IBr at room temperature reported 94 by Sneed et a l . U n l i k e the monofluorides of bromine and i o d i n e , c h l o r i n e mono-f l u o r i d e i s s t a b l e at room temperature and may be i s o l a t e d i n the 86 pure s t a t e . The sample was used d i r e c t l y as s u p p l i e d by Matheson Chemical Co. and was k i n d l y provided by Dr. F. Aubke f o r t h i s study. There was no evidence of i m p u r i t i e s i n the s p e c t r a . 4.1.2. P r e l i m i n a r y D i s c u s s i o n and Summary of General Trends The ground e l e c t r o n i c c o n f i g u r a t i o n of I B r , IC1 and FBr, FC1 may be w r i t t e n : 4 1 + ...(n.pa + n.pa ,0") (n.pir ) (n.pir ) , E i x . 3 y. i x . l y . i 3 i J where, Xj = smaller atom yj = larger atom != 4 or 3 f o r Br and C l r e s p e c t i v e l y = 5 or 2 f o r I and F r e s p e c t i v e l y (4) The f i r s t and second I.P.'s correspond to removal of a (n.piT ) and a Cn.pir ) e l e c t r o n r e s p e c t i v e l y l e a v i n g the i o n i c s t r u c t u r e s : i I O N I Z A T I O N P O T E N T I A L ( e V ) S P I N O R B I T C O M P O N E N T S O F 1st I P ' S FIGURE 14 - 67 -2 4 3 2 (5) x^ x. y.' i r x. j - y. 1 A3/2,l/2g x j x J j (n pa + n pa a ) 2 ( n p,r ) 3 ( n prr ) 4 , ^ 1 / 2 u (6) x j 1 J In each of the f i r s t 2 I.P.'s, the ^2/2 s p i n - o r b i t component l i e s lowest i n b i n d i n g energy by Hund's r u l e . 88 S e v e r a l of the most recent MO c a l c u l a t i o n s by Boyd, S i c h e l 102 90 and Whitehead, Deb and Coulson and references c i t e d t h e r e i n are l i s t e d i n Tables 3(a)-(c) and compared w i t h a v a i l a b l e experimental data. In general the p o s i t i v e i o n s t a t e s of a l l the i n t e r h a l o g e n s c a l c u l a t e d by the above authors are ordered ...a , i r , 0 , i r . Although u' u' g' g t h i s assignment i s maintained throughout the e n t i r e s e r i e s of the ten diatomic halogens, the 2^ and riu s t a t e s i n a l l cases show a very c l o s e correspondence i n o r b i t a l energy. However, from s p i n - o r b i t i n t e r a c t i o n c o n s i d e r a t i o n s , and i n view of the ph o t o e l e c t r o n assignments of the 50,85,124 . + „r. A • prevxous papers, xt xs apparent that thxs orderxng xs an a r t e f a c t of the c a l c u l a t i o n s , and the LT s t a t e d e f i n i t e l y has a lower u J 125 i o n i z a t i o n energy i n a l l but p o s s i b l y F^. The Hartree-Fock and CNDO/2 I.P.'s c a l c u l a t e d u s i n g Koopmans' 44 theorem overestimate the observed I.P.'s and, w i t h few e x c e p t i o n s , 88 the SCF-M0-CND0 method of Boyd i s c o n s i s t e n t l y b e t t e r , e s p e c i a l l y 95 f o r the monofluorides. The p r e d i c t e d I.P.'s reported by M u l l i k e n , (Table 3), are based s o l e l y on correspondence w i t h atomic halogen I.P.'s. Although both approaches overestimate the f i r s t I.P. by approximately 1 eV and g e n e r a l l y underestimate the v e r t i c a l energy d i f f e r e n c e s , they do provide a s a t i s f a c t o r y order-of-magnitude TABLE 2. I . P . ' s o f t h e D i a t o m i c H a l o g e n s and I n t e r h a l o g e n s (eV + 0 . 0 2 ) . D e t e r m i n e d by P h o t o e l e c t r o n S p e c t r o s c o p y H a l o g e n I 2 I B r I C 1 B r 2 B r C l C l 2 I F B r F C1F F 2 A ( c m - 1 ) 5125 4640 4650 2820 ( 2 2 7 5 ) * 645 ( 4 7 0 0 ) * 2620 870 1050 v ' ( c m - 1 ) 220 290 420 360 ( 4 7 5 ) * 645 ( 2 0 0 ) * 750 670 337 Hot Band A d i a b a t i c V e r t i c a l S / 2 g Hot Band A d i a b a t i c V e r t i c a l S / 2 g A d i a b a t i c V e r t i c a l 7 T 3 / 2 u A d i a b a t i c V e r t i c a l 2 W 9 . 2 9 9 . 3 2 9 . 3 4 9 . 3 7 9 . 9 4 5 9 . 9 7 1 1 0 . 0 0 1 1 0 . 0 2 8 1 0 . 8 0 1 1 . 0 3 1 1 . 8 2 1 0 . 0 2 9 1 0 . 4 3 9 . 7 1 0 1 0 . 0 7 8 1 0 . 4 7 9 . 7 4 4 9 . 7 8 0 1 0 . 1 3 1 0 . 5 1 9 . 8 1 7 1 0 . 1 8 2 1 0 . 5 6 9 . 8 5 0 1 0 . 2 4 0 1 0 . 6 0 9 . 9 8 8 1 0 . 2 9 1 1 0 . 6 5 ( 1 0 . 6 9 : 1 0 . 2 8 5 1 0 . 6 1 0 1 0 . 7 8 1 0 . 3 1 7 1 0 . 6 5 6 1 0 . 8 2 1 0 . 3 5 6 1 0 . 7 1 0 1 0 . 8 6 1 0 . 3 9 2 1 0 . 7 6 1 1 0 . 9 1 1 0 . 4 2 6 1 0 . 8 1 3 1 0 . 9 5 1 0 . 4 6 6 1 0 . 8 6 5 1 1 . 0 0 1 0 . 9 1 2 1 1 . 0 4 1 1 . 5 6 1 2 . 4 3 1 2 . 5 2 1 2 . 0 4 1 2 . 6 2 1 2 . 8 5 1 2 . 1 8 1 2 . 3 4 1 2 . 8 2 1 3 . 0 8 ( 1 0 . 9 ) ' mean ( 1 1 . 1 ) ' + 0 . 2 ( H . 3 ) mean ( 1 3 . 7 5 ) ' + 0 . 4 1 1 . 4 2 1 1 . 4 9 1 1 . 5 7 1 1 . 6 5 1 1 . 7 3 1 1 . 8 2 1 1 . 8 9 1 1 . 9 7 1 1 . 4 9 1 1 . 6 5 1 4 . 0 3 1 4 . 4 3 ( 1 0 - 1 ) + 0 . 4 mean ( 1 0 . 5 ) ' + 0 . 4 ( 1 0 . 9 ) ' + 0 . 4 mean ( 1 6 . 5 5 ) 1 1 . 7 2 1 1 . 8 1 1 1 . 9 0 1 2 . 0 5 1 2 . 1 3 1 2 . 2 1 1 4 . 4 3 + 0 . 4 mean ( 1 6 . 7 5 ) + 0 . 4 1 2 . 6 4 9 1 2 . 7 6 0 1 2 . 8 7 1 1 2 . 9 7 5 1 3 . 0 8 2 1 2 . 3 7 5 1 2 . 8 4 4 1 2 . 9 5 1 1 3 . 0 5 6 1 6 . 5 0 1 7 . 1 8 1 7 . 1 8 1 5 . 7 0 1 5 . 8 3 1 5 . 9 6 1 6 . 0 9 1 5 . 7 3 1 5 . 8 7 1 6 . 0 0 1 6 . 1 3 1 8 . 4 3 1 8 . 9 8 1 8 . 9 8 oo TABLE 2 ( C o n t i n u e d ) H a l o g e n h I B r I C 1 B r 2 B r C l c i 2 I F B r F C1F F 2 A d i a b a t i c V e r t i c a l 1 2 . 7 0 1 2 . 9 2 1 3 . 4 1 1 3 . 7 2 1 3 . 9 9 1 4 . 2 6 1 4 . 3 3 1 4 . 6 0 * ( 1 5 . 5 ) +0 .4 1 5 . 8 1 1 6 . 1 0 ( 1 8 . 0 ) * + 0 . 4 ( 1 8 . 1 5 ) * + 0 . 4 1 8 . 0 1 1 8 . 5 1 ( 1 9 . 0 ) : ( 2 0 . 8 -2 1 . 0 ) ? TT (mean) g 9 . 6 6 1 0 . 0 7 1 0 . 4 2 1 0 . 6 9 A ( l l.D 1 1 . 6 5 ( 1 0 . 5 ) * 1 1 . 9 7 1 2 . 8 0 1 5 . 8 5 TT u (mean) 1 1 . 4 0 1 2 . 1 9 1 2 . 7 2 1 2 . 9 7 ( 1 3 . 7 5 ) * 1 4 . 4 3 ( 1 6 . 5 5 ) * ( 1 6 . 7 5 ) * 1 7 . 1 8 1 8 . 9 8 (TT -TT )mean g u 1 0 . 5 3 1 1 . 1 3 1 1 . 5 7 1 1 . 8 3 ( 1 2 . 4 ) * 1 3 . 0 4 ( 1 3 . 5 5 ) * ( 1 4 . 3 6 ) * 1 4 . 9 9 1 7 . 4 2 TT -TT s e p a r a -g u . B t i o n 1 . 7 4 2 . 1 2 2 . 3 0 2 . 2 8 ( 2 . 6 5 ) * 2 . 7 8 ( 6 . 0 5 ) * ( 4 . 8 0 ) * 4 . 3 8 3 . 1 3 P r e d i c t e d - 70 -TABLE 3(a). Ioniza t i o n P o t e n t i a l s (eV) IBr Ref. 2 1 T 3/2,g (mean) 2 7 T l / 2 , g 2 "3/2,11 (mean) 2 1 T l / 2 , u V ™ s * work 9 . 7 8 ( 0 ) ( 1 0 . 0 7 ) 1 0 . 3 5 ( 6 ) 1 2 . 0 4 ( 1 2 . 1 9 ) 1 2 . 3 4 1 3 . 7 2 * 85 9 . 8 3 ( 1 0 . 1 2 ) 1 0 . 4 1 1 2 . 0 0 ( 1 2 . 1 8 ) 1 2 . 3 6 1 3 . 7 0 * 84 9 . 9 8 ( 1 0 . 2 3 ) 1 0 . 4 9 1 1 . 5 9 ( 1 1 . 6 4 ) ^ 1 1 . 7 — * 79 < 1 0 . 3 + . 2 * 124 9 . 8 5 1 1 . 9 9 1 3 . 7 0 95 1 1 . 1 6 1 2 . 4 3 > 1 3 . 6 8 88(a) (TT) 1 1 . 0 6 ( o ) 1 3 . 6 7 0 0 1 3 . 8 5 8 8 ( b ) 0 0 1 1 . 3 7 ( a ) 1 3 . 3 2 0 0 1 3 . 5 4 8 8 ( c ) ( T T ) 1 1 . 8 9 102 1 1 . 8 5 90(a) (TT) 7 . 4 3 ( a ) 1 5 . 3 0 0 0 1 6 . 5 1 9 0 ( c ) I C 1 0 0 1 1 - 7 5 0 0 1 1 . 7 7 0 0 1 3 . 1 4 T h i s * work 1 0 . 1 3 ( 1 0 . 4 2 ) 1 0 . 71 1 2 . 6 2 ( 1 2 . 7 2 ) 1 2 . 8 2 1 4 , 2 6 A 85 1 0 . 1 3 ( 1 0 . 4 2 ) 1 0 . 7 2 1 2 . 8 3 1 4 . 2 9 * 84 1 0 . 3 1 ( 1 0 . 5 5 ) 1 0 . 7 9 1 2 . 1 3 ( 1 2 . 1 6 ) ^ 1 2 . 2 — * 79 < 1 0 . 4 + . 2 * 124 1 0 . 1 0 1 2 . 8 8 1 4 . 2 6 95 1 1 . 1 6 1 3 . 6 6 > 1 4 . 3 8 88(a) 0 0 1 1 . 7 6 ( c ) 1 4 . 0 9 Or) 1 4 . 49 8 8 ( b ) (TT) 11.92 (a) 1.3.9 4 0 0 1 4 . 3 2 8 8 ( c ) 1 2 . 5 1 102 1 2 . 4 6 90(a) (TT) 8 . 1 9 ( a ) 1 5 . 3 3 0 r ) 1 6 . 5 1 9 0 ( b ) (TT) 8 . 4 9 ( a ) 1 5 . 7 8 0 0 1 6 . 4 7 9 0 ( c ) (a ) 1 2 . 35 0 0 1 2 . 4 0 0 0 1 3 . 8 4 Experimental r e s u l t s . - 71 -TABLE 3(b). V e r t i c a l I o n i z a t i o n P o t e n t i a l s (eV). IF Reference (mean) (mean) ** This work ^10.5+.4 vL6.55+.4 ^18.0+.4 * 79 <10.5+.2 88(a) (TT)12.65 (a)15.46 (TT)16.52 88(b) (IT) 12. 71 (a)15.34 (TO 16.63 88(c) 13.47 102 13.45 90(a) 0010.25 (a)16.46 (TT)19.25 90(b) Or) 11.17 (a)17.06 (TT)18.35 90(c) (a)13.25 0013.65 0015.72 B r C l ** This work ^11.1+.2 ^13.75+.4 ^15.5+.4 * 79 < l l . l + . 2 95 12.43 13.66 >15.14 88(a) (TT) 11.55 (a)14.61 (TT)14.77 88(b) 0011.67 (a)14.72 0014.64 88(c) 12.41 102 12.35 90(a) (a)13.40 (TT)13.73 0015.02 90(b) 0012.84 (a)13.88 0014.27 90(c) 0012.29 (a)12.70 0014.00 * ** Experimental r e s u l t s . P r e d i c t e d . - 72 -TABLE 3(c). V e r t i c a l I o n i z a t i o n P o t e n t i a l s (eV). BrF 2 2 2 + Reference ^r>/0 (mean) n .„ (mean) £ 3/2 ,g 1 /2 ,g This work* 11.81 (11.97) 12.13 ^16.75+.4** ^18.15+.4*' 79* 11.8+.2, L11.9+.3' 88(a) (TT)12.49 (a) 15.81 (TT)16.79 88(b) (TT)12.58 (a) 15.70 0016.81 88(c) 12.96 102 12.94 90(a) 0013.80 (a)18.25 0020.19 90(b) 0012.77 (a) 18.00 0019.14 90(c) (rr)13.20 (a) 14.14 Or) 16.29 C1F This work * 129 * 79 * 81 88(a) 88( 88(b) 99( 88(c) 102 90(a) 90(b) 90(c) 128 12.76 12.78 (12. 12. <12. 12. Cir) 13. 0 0 1 1 . 0013 . Or) 12. 14. 14. Or) 14. 0013 . 0014 . 0013 . 80) 82 7+.3 65 71 98 88 30 37 34 43 67 44 36 12.84 12.85 17.18 17.01 (<j)17.04 (a)15.83 (a)16. 85 (a)15.45 (a)19.31 (a)19.10 (a)15.14 (a)19.08 18.51 18.36 Or) 17. 82 0016 .49 0017 .75 Or) 16. 34 OO20.91 0019.93 (TT)17.24 Or) 19.69 * ** Experimental R e s u l t s . P r e d i c t e d . - 73 -correspondence between theory and experiment. The f o l l o w i n g observations f o r the diatomic halogens and interhal o g e n s are i l l u s t r a t e d i n the f i g u r e s 14, 15, and 16. 2 The t r a n s i t i o n p r o b a b i l i t y to the 17.^ /2 c o m P o n e n t 1 S g r e a t e r than to 2 2 the I I3/2 component of the rj ^ s t a t e s , but i s i n general compensated 2 f o r by an in c r e a s e i n band width of the II3/2 s t a t e * W i t h i n the same s p i n - o r b i t doublet, t h i s compensation of a r e d u c t i o n i n peak i n t e n s i t y by g r e a t e r peak w i d t h , i s presumably a s s o c i a t e d w i t h a d i f f e r e n c e i n 2 2 the shape of the ^2/2g a n C j ^ l / 2 g P o t e n t i a - 1 - s u r f a c e s , the former showing g r e a t e r i n c r e a s e i n bonding r e l a t i v e to the n e u t r a l molecule than the l a t t e r . L ^ V i b r a t i o n a l hot band c o n t r i b u t i o n s to the XZ + g 2 (n e u t r a l ) -> r j ^ (ion) t r a n s i t i o n s are discussed f o r the i n d i v i d u a l molecules w i t h the a v a i l a b l e p h o t o i o n i z a t i o n evidence. The g r e a t e r PE band widths observed f o r the second and t h i r d I.P.'s r e f l e c t the d i f f e r e n c e between t h e i r bonding c h a r a c t e r w i t h respect to the narrower and weakly antibonding components of the f i r s t I.P. The apparent 2 l a c k of f i n e s t r u c t u r e i n the r j ^ s t a t e s may r e s u l t from one of unresolved v i b r a t i o n a l p r o g r e s s i o n s , p r e d i s s o c i a t i o n , or a r e p u l s i v e 38 i o n i c s t a t e . The d i s c u s s i o n i n S e c t i o n 2.1.2 i n d i c a t e s that PE band i n t e n s i t i e s r e f l e c t the r e l a t i v e p r o b a b i l i t i e s of i o n i z a t i o n from d i f f e r e n t molecular s u b s h e l l s . The i n t e n s i t y of the t h i r d band i n the s p e c t r a i s roughly one-half that of the t o t a l band i n t e n s i t y of the 1 + 2 E -* fjg u processes which i n v o l v e doubly degenerate i o n i c s t a t e s , and i s t h e r e f o r e a s s o c i a t e d w i t h i o n i z a t i o n of a a - e l e c t r o n and 0 35 a t t r i b u t e d to a Z i o n i c s t a t e . He 304 A and ESCA s t u d i e s of the r e l a t i v e i n t e n s i t y of t h i s band w i t h respect to those of the I I s t a t e s c o n f i r m i t s . assignment. - 74 ^ C O R N F O R D , F R O S T , M c D O W F . L L , R A G L E , A N D S T E N H O U S E P H O T O E L E C T R O N S P E C T R A O F T H E H A L O G E N S (a) —i 1 : 1 1 r 16 17 18 19 2 0 21 IONIZATION POTENTIAL (eV) n* J —r— 13 - 1 — 16 — I — 17 IONIZATION POTENTIAL (eV) (c) Id) 2 2 —I— 14 II 12 13 IONIZATION POTENTIAL (eV) Xe -1— 13 — I — 14 t O II 12 IONIZATION POTENTIAL (eV) —r— 15 I'ic. 1. The 584-A. photoelectron spectra of ia) K., ib) C l : , (c) Ltr., and (d) Ij. v-'l.v'oO 2 O 2 • = 0 1 2 3 4 i I I t I I — I I — i v ' = 0 I 2 3 4 0 I 2 3 4 _ , „ _ 1 i i i I . 2 v ' = 0 , v ' = 0 2 I 1 I I I 1 2 2 ^ > 2 o i 2 3 4 n 3 ' C ., 5125 c m FIG. 2. Spin-orbit splittings and vibrational levels in the ground states of the halogen molecule ions. FIGURE 15 - 75 -S C H E M A T I C P H O T O E L E C T R O N S P E C T R A O F THE 10 D I A T O M I C H A L O G E N S A 2 IBr IC1 B R 2 BrCl C l a IF BrF C1F F 0 1 .1 A A A ~ i — 1 0 D I A T O M I C H A L O G E N S ,l lul /\ ! Hi: I .--<-'^\l L_ L _ - 0 r • 1 2 i— 1 4 l 1 6 i— 1 8 — I — 2 0 I O N I Z A T I O N P O T E N T I A L ( e V ) FIGURE 16 - 76 -A few apparent anomalies e x i s t i n t h i s diatomic halogen s e r i e s (Figure 16). The monofluorides form a s e r i e s apart from the c h l o r i d e s , bromides and i o d i d e s (see S e c t i o n 4.2.3) and the f i r s t I.P. of f l u o r i n e , T^, r e f l e c t s the extremely high e l e c t r o n e g a t i v i t y of the f l u o r i n e atom w i t h respect to the other halogen atoms. P o t t s and P r i c e ^ ^ and Cornford et a l . " ^ give evidence f o r the o r b i t a l o r d e r i n g 125 ...a ,TT , T T f o r F„ although Wahl p o s t u l a t e s a reverse b i n d i n g g u' g 2 energy o r d e r i n g of the TT and a o r b i t a l s . Measurement of the i n t e g r a t e d peak area of the second PE band has not confirmed the p r o x i m i t y or overlap of these two I.P.'s and p l o t s of e l e c t r o n e g a t i v i t y a gainst I.P. values might suggest e i t h e r assignment (see F i g u r e 17). An assessment of bond s t r e n g t h data (Table 4) supports the lower I.P. value f o r the I s t a t e of F_. g 2 2 The s p i n - o r b i t s p l i t t i n g i n the [ T ^ band i s l a r g e r than that 2 observed f o r the 17.^  band of molecular i o d i n e , I ^ , but i s not p a r a l l e l e d i n the r e s t of the s e r i e s . This suggests that the T T ^ bonding e l e c t r o n s are more ' c o r e - l i k e ' and are t h e r e f o r e c l o s e r to the n u c l e i (see S e c t i o n 2.1.4.2) than are the antibonding TT e l e c t r o n s . 4.2 I n d i v i d u a l Molecules 4.2.1 Iodine Monobromide (IBr) The PE spectrum of I B r i s i l l u s t r a t e d i n F i g u r e 14 and the observed I.P. values are reported i n Tables 2 and 3(a). The appearance 79 p o t e n t i a l value of I r s a and Friedman gives the best experimental 2 agreement w i t h the PE data f o r the f i r s t I.P. The l/2g s t a t e s (A = 4640 + 40 cm "*") both d i s p l a y v i b r a t i o n a l progressions w i t h IONIZATION ENERGY TRENDS IN THE DIATOMIC HALOGENS ANO INTERHALOGENS FIGURE 17 TABLE 4 . Summary o f S p e c t r o s c o p i c and P h o t o e l e c t r o n D a t a f o r t h e D i a t o m i c H a l o g e n s and I n t e r h a l o g e n s M o l e c u l e O b s e r v e d S p i n O r b i t O b s e r v e d S p e c t r o s . „ , , 1 0 1 O b s e r v e d I o n i c ,"A" S p i n ( I on ) C o u p l i n g C o n s t a n t I o n i c F r e q u e n c y ^ v - v C B (cm 1 ) O r b i t C.alc • (A c m - 1 ) F r e q u e n c y (v cm~l ) A n h a r m o n i c i t y (oi ,a) X cm~l ) (cm 1 ) 2 IT g 2 TT u 2 TT g e e e 2 TT g 2 TT g 2 Tt U : f . 5125+40 ^ 6 4 0 0 ^220+40 2 1 4 . 6 , ( 0 . 6 1 ) * I B r 4640+40 %2420 290+40 5400 2745 I B r 2 6 8 . 4 , ( 0 . 7 8 ) 4668 I C 1 + 4650+40 ^ 1 6 1 5 420+40 it 5080 650 I C 1 3 8 4 . 2 , ( 1 . 4 7 ) 4713 c 2 4. 2820+40 ^ 2 1 0 0 360+40 3 2 3 . 2 , ( 1 . 0 7 ) * — — B r C l — — — 2600 650 B r C l (430) 2238 c i 2 + c i 2 645+40 n o t o b s . 645+40 6 4 5 . 3 , ( 2 . 9 0 ) * 5 6 4 . 9 , ( 4 . 0 ) * — — IF+ — — — — — I F — B r F + B r F 2620+40 — 750+40 6 7 1 , ( 3 ) * 9 2 2745 — C 1 F + (628+30) 670+40 129 n o t o b s . 129 (912+30) y 870+40 ( 7 8 6 . 3 4 )1 2 9A 765 C1F 7 9 3 . 2 . ( 9 . 9 ) ( 7 7 3 ) 8 1 F , + 337+40 n o t o b s . 1050+40 — — z (802) TABLE 4 (Continued) Atomic Data I Br C l F Spin-Orbit Coupling Constants (A) (eV) 96-99 I o n i z a t i o n P o t e n t i a l s (eV) 90 0.628 0.305 0.073 0.033 10.347 1 0 . 4 5 4 1 0 1 11.574 12.686 17.141 Diatomic Halogen Bond Strengths  Molecule Bond Strength (Kcal/mole) VO F-Cl 61.4 C l - C l 57.9 F-Br 56 Cl-Br 53 C l - I 50.5 Br-Br 46.1 I-Br 42.5 F-F 27_ I - I 36.1 - 80 -v! = 290 + 40 cm \ The f i r s t r e s o l v e d v i b r a t i o n s i n both s t a t e s i o n — 2 - 1 2 are assigned to hot bands, v'( ft-j^g) = 2 ^ — ^ c m a n c* ^ l / 2 g ^ = 242 + 40 cm \ Both c o r r e l a t e w e l l w i t h the n e u t r a l molecule v i b r a t i o n a l frequency, v" = 268.4 cm \ and suggest a frequency r e d u c t i o n i n the J = 1/2 s t a t e w i t h respect to the J = 3/2 s t a t e . The frequency d i f f e r e n c e v! - v" , r e f l e c t s the degree of antibonding of the i o n n e u t r a l ° ° Il s t a t e (Figure 17(e)) w i t h respect to that observed i n the other molecules of the s e r i e s . 2 The ^2/2 l/2u s t a t e s o v e r l a p , the J = 1/2 s t a t e being more inte n s e and narrower (see S e c t i o n 4.1.2) w i t h a semblance of f i n e s t r u c t u r e ( v 1 ^ 560 cm \ approximately double that of the IT s t a t e ) g which must be viewed w i t h s u s p i c i o n i n view of the bonding and p o s s i b l e 121 r e p u l s i v e nature of these s t a t e s . The s p i n - o r b i t s p l i t t i n g , Aj.gr+ = 2420 + 40 cm \ c l o s e l y approximates that of the EI s t a t e of B^"1", A g r + (J[ ) = 2820 + 40 cm \ and shows the p r e f e r e n t i a l l o c a l i z a t i o n 2 S of the IT o r b i t a l on the bromine atom. The II s t a t e i s s i m i l a r l y u g dominated by i o d i n e c o n t r i b u t i o n s (A T T 1 + = 4640 + 40 cm \ A T + = J IBr — ' 1^ 5125 + 40 c m - 1 ) . The t h i r d PE band at 13.72 eV i s s t r u c t u r e l e s s and most l i k e l y 95 i s p r e d i s s o c i a t e d or i s a r e p u l s i v e s t a t e . 4.2.2 Iodine Monochloride (IC1) Figure 14 and Tables 2 and 3(a) c o n t a i n the PE data f o r IC1. 2 Any hot band c o n t r i b u t i o n to the ^3/2g s t a t e ^ s obscured by 2 i m p u r i t y although a hot band at 10.61 eV i n the -^2/2g s t a t e > separated from the a d i a b a t i c t r a n s i t i o n by v' = 370 + 40 cm \ shows e x c e l l e n t agreement w i t h the n e u t r a l molecule ground s t a t e frequency \>" = 384.2 cm The i o n i c frequency f o r the L7. s t a t e , v' = 420 + 40 cm \ i n d i c a t e s i o n i z a t i o n of an antibonding e l e c t r o n (Figure 17(e)) and stronger bonding i n the i o n by the net a c t i o n of the remaining 95 seven valence e l e c t r o n s . The molecular ground s t a t e may then be represented as: 2 4 4 . .. (a + a ,a) (TT + TT ,TT) (TT - TT ,TT) (7) y x ' y x ' x y This a l s o i m p l i e s that there i s a consi d e r a b l e d i f f e r e n c e between the I.P.'s of the FI and n s t a t e s as observed (Table 2 ) . To the l i m i t s 95 of t h i s approximation these two s t a t e s should have about the same i n t e r n u c l e a r d i s t a n c e , r , as the n e u t r a l IC1 molecule s i n c e the e' 2 + bonding c o n f i g u r a t i o n (o" c^ + o^) i n IC1 i s not r a d i c a l l y d i f f e r e n t from that i n IC1. The d i s t r i b u t i o n of i n t e n s i t i e s i n the v ' •*- 0 bands may be used to c a l c u l a t e Av^, (the change i n r e i n going from the n e u t r a l molecule to the i o n ) . This procedure was a p p l i e d to the ^2^"' C^"1", and B r ^ + ^ 3 / 2 g S r o u n ( 3 s t a t e s , a n d i n d i c a t e d a decreasing p o s i t i v e increment i n r g from to C l ^ + to B^"*", w i t h B r ^ + a c t u a l l y showing a very s m a l l decrease i n r^. As f u r t h e r j u s t i f i c a t i o n of the use of equation (7) we compare the 2 2 + observed I I - I I U v e r t i c a l energy s e p a r a t i o n of 2.30 eV i n IC1 to 95 the energy d i f f e r e n c e p r e d i c t e d by M u l l i k e n , i . e . , 2.50 eV. I f the * lowest e x c i t e d s t a t e of IC1 be obtained by adding an e l e c t r o n to the a o r b i t a l of the normal s t a t e of the ( I C 1 ) + i o n , (which may a l s o be represented to a f i r s t approximation as I + C 1 ) : - 82 -2 A 3 2 (3pa c l + Spoj.a) ( 3P 7 T C 1) (5PTTJ.) , 11^2 (8) the r e s u l t i n g e l e c t r o n i c s t a t e of IC1 may then be represented as: 2 U 3 * 1 1 3_ ...(o) Gr^rcTTj-rco r , 1 , J n (9) In 3 T r a n s i t i o n s from the normal s t a t e of IC1 to the IJ.Q+ and TJ^ l e v e l s of (9) give the w e l l known v i s i b l e and i n f r a r e d a b s o r p t i o n bands of IC1. 1 3 1 3 Above these ' II s t a t e s of IC1, there should be another ' II doublet: . . . C a ) 2 ^ ) 3 ^ ) ^ * , 1 , 3 n (10) 1 3 The energy i n t e r v a l , ALT, between ' LT of (9) and (10) would be 95 + expected to be approximately equal to the i n t e r v a l (AID between the 2 two low energy IT s t a t e s of the molecule-ion (8) and (11) below: . .. (3pa + 5po,o) (3p7rcl) (SpiTj.) , ^3/2 1/2 For IC1, (AlD+ should then be approximately equal to the d i f f e r e n c e between the ir ^  and atomic I.P.'s ( a p p r o p r i a t e f o r both bonding and antibonding o r b i t a l s ) . This c a l c u l a t e d energy d i f f e r e n c e i s i n extremely good agreement f o r I C 1 + but i s a l i t t l e low f o r I B r + . As an extension to t h i s approximation, i n most of the XY molecules the bonding i s more n e a r l y homopolar than h e t e r o p o l a r , and a good atomic comparison p o t e n t i a l f o r the o-bonding o r b i t a l corresponding to the t h i r d P E band, appears to be l/2(atomic I.P. of X + atomic I.P. of Y) - 83 -Comparison of these p r e d i c t e d values w i t h the PE r e s u l t s f o r IC1 and IBr d i f f e r by no more than 0.12 eV (Tables 3 ( a ) , ( b ) , ( c ) ) . In the i n t e r p r e t a t i o n of the uv spectrum of IC1 r e p o r t e d by 112 113 Cordes and Sponer, ' the upper l e v e l s of the s t a t e s used were * assumed to be a t o m i c - l i k e o o r b i t a l s c o n t a i n i n g a l o o s e l y bound e l e c t r o n that may be thought of as added to the I + C 1 core. The observed Av values are 4713, 4668, and 2238 cm 1 as compared w i t h p r e d i c t e d A ( s p i n - o r b i t s p l i t t i n g ) values of 5080, 5400, and 2600 cm 1 f o r the FJ s t a t e s of I C 1 + , I B r + , and B r C l + r e s p e c t i v e l y , determined from the s p i n _ o r b i t : coupling of tl-.e l a r g e r atomic i o n . For example, the 2 + magnitude of the s p l i t t i n g of the II s t a t e of IC1 may be approximated by: A = C 2 / 3 ) ( 2 P 1 ^ 2 - 2 P 3 / 2 ) = (2/3)(0.94) = 0.63 eV = 5080 cm"1 One does not expect to f i n d t h i s exact value (see Table 4) s i n c e the 5piT o r b i t a l of I + i s a p p r e c i a b l y a f f e c t e d by the presence of the I - C I bond. A c o l l e c t i o n of t h i s s p i n - o r b i t data appears i n Table 4 . 2 -1 The I I 3 / 2 i/2u s t a t e s a r e b a r e l y r e s o l v e d (A ^ 615 cm ) and are most l i k e l y r e p u l s i v e . Apparent s t r u c t u r e may r e s u l t from the s p i n -95 o r b i t doublet of HCl i m p u r i t y at 12.79 eV. M u l l i k e n provides f u r t h e r evidence to support the observed s t r u c t u r e l e s s nature of t h i s 3 + 2 band. I f t h i s s t a t e on d i s s o c i a t i o n gives P 2 (I ) p l u s F3/2 (^1) as i s t h e o r e t i c a l l y p o s s i b l e , then i t should probably be unstable s i n c e the energy of 2.8 eV r e q u i r e d f o r i t s v e r t i c a l e x c i t a t i o n (observed to be 2.59 eV), c o n s i d e r a b l y exceeds the estimated bond d i s s o c i a t i o n - 84 -2 energy D = 1.87 eV ( f o r the ^2/2 ^ewe^S/ °^ unexcited IC1. Also the known d i s s o c i a t i o n energy of IC1 i s 2.14 eV f o r the p r o d u c t i o n of K 2 P 3 / 2 ) + C K 2 P 3 / 2 ) . 18 The t h i r d s t a t e (3rd PE band), i s p r e d i c t e d to be, i n a l l 4 4 2 p r o b a b i l i t y , r e p u l s i v e and may be denoted . . . a r r TT , E. 4.2.3 C h l o r i n e Monofluoride (C1F) 2 The l " L / 0 i /<•> s t a t e s of the f i r s t I . P . of C l F are b a r e l y r e s o l v e d 3/2,l/2g (see F i g u r e 14 and Table 2) w i t h A = 670 + 40 cm"1 and v' = 870 + -1 81 40 cm (Table 4 ) . D i b e l e r e t a l . observe two weak p h o t o i o n i z a t i o n o onsets at 12.55 eV and 12.65 eV (980 A) assigned to a hot band t r a n s i t i o n ( r e l a t i v e p o p u l a t i o n of 0.025 i n the f i r s t e x c i t e d s t a t e 81 of the molecule ) and the a d i a b a t i c t r a n s i t i o n r e s p e c t i v e l y to the 2 rL / 0 s t a t e . The rr MO i s almost t o t a l l y l o c a l i z e d on the c h l o r i n e 3/2g g 3 96 atom (atomic I . P . of 12.686 eV ) see equations (4) and (5) S e c t i o n 4.1.2., as evidenced by the observed I . P . at 12.80 eV. The p o l a r i z a t i o n of the XF molecule makes X p o s i t i v e , and t h i s n e c e s s a r i l y r a i s e s the I . P . of X e l e c t r o n s i n XF above th a t of X e l e c t r o n s i n X,, (Figure 16). 79 D e s c r i b i n g t h i s i n an a l t e r n a t e way I r s a and Friedman note that i n C l 2 i o n i z a t i o n i n v o l v e s l o s s of s t r o n g l y antibonding e l e c t r o n s , w h i l e i n C l F , the s t r u c t u r e i s such that e s s e n t i a l l y nonbonding e l e c t r o n s are removed. The observed i o n i c frequency v' compared w i t h v" of the n e u t r a l molecule and P E band shapes of the LT s t a t e s both i n d i c a t e g i o n i z a t i o n of an e l e c t r o n from an antibonding o r b i t a l i n C l F . The C l F bond energy i s c l o s e r to C l 2 than F 2 (Table 4 ) and may suggest weaker antibonding of IT e l e c t r o n s i n C l F than Cl,,. C a l c u l a t i o n 79 - 85 -of the 2pTr- 3prr and 3pTr - 3p7i overlap i n t e g r a l s 1 " ^ f o r C1F and Cl^ 118 r e s p e c t i v e l y , support t h i s c o n c l u s i o n . M u l l i k e n suggests t h a t a s m a l l amount of 3d h y d r i d i z a t i o n w i t h second-row and higher elements which strengthens second-row and higher bonds, a l s o helps lower the antibonding i n C1F. This decrease i n antibonding nature of the u e l e c t r o n s i s r e f l e c t e d by the change of frequency observed i n going to s i m i l a r s t a t e s of the ions i n the s e r i e s C l ^ , C l F , and Br^, BrF, F 2 (Figure 1 7 ( e ) ) : T A B L E 5 T r a n s i t i o n Change i n Frequency - 2n 8 ( n e u t r a l ) 8 ( i o n ) 1Z + »• 2LT Av(cm 1 ) 1. C l 2 (645) C l 2 + (565) 80 • • ,(870) +,(793) , 77 U A * l(912) U 1 J ! l(786) l126 (Ref. 129) F 2 (1050) F 2 + (802) 248 2. B r 2 (360) B r 2 + (323) 37 BrF (750) B r F + (671) 79 F 2 (1050) F 2 + (802) 248 The c l o s e correspondence of the s p i n - o r b i t s p l i t t i n g s i n the C l 2 > ClF and B r 2 > BrF p a i r s (Table 4) may f u r t h e r support the decrease i n 79 antibonding e f f e c t proposed by I r s a and Friedman. Figures 14 and 16 i l l u s t r a t e the l a r g e 11—11 energy s e p a r a t i o n and g u p r o x i m i t y of the TI and £ o r b i t a l s unique to the observed PE spectrum of ClF and p r e d i c t e d f o r the IF and BrF members of the monofluoride s e r i e s . F„, however has a 11-11 s e p a r a t i o n c o n s i s t e n t w i t h the 2' g u c h l o r i d e s , bromides and i o d i d e s (Table 2), although the FI and £ ^ 8 - 86 -88 states may overlap (see Section 4.1.2). Boyd's calculations give reasonable agreement with the experimental I.P.'s of the monofluoride series as a result of this large ^-17^ energy separation (Table 3(c)). The LIu I.P. of ClF at 17.18 eV closely approximates the atomic fluorine I.P., 17.141 eV, and the m - I I ) mean I.P. of F„ at 17.42 eV g u 2 (Table 2). Although this orbital is -n—bonding and may be designated (2pTr„ + 3p-rr,7r) , the overlap contribution to the bonding provided by r LI chlorine, possibly through some 3d hybridization appears to be negligible. The rising background enhances the apparent observed intensity for the third P E band centered at 18.51 eV, but this i s undoubtedly 2 + + the Z state of the ClF molecule-ion (Figure 14). The IT —E energy separation, 1.3 eV, i s an approximate measure of the energy difference between a a-bonding and a Tr-bonding fluorine electron, reduced only marginally by the amount of the chlorine-fluorine a-overlap (a_+a,,,). If this value is 0.3 eV or greater, then the n band w i l l obscure the 125 presence of the E band of i n support of the calculations of Wahl. The foregoing analysis i s now used to predict the PE spectro-scopically unobserved I.P.'s of BrF, IF and BrCl. 4.2.4 Bromine Monofluoride (BrF) Observed and Predicted I.P.'s Only the f i r s t I.P. in the P E spectrum of BrF was unobscured and easily identified (see Figure 14 and Table 2). An observed frequency 2 -1 in both ^2/2 l/2g i o n i c states ( vg r F+ = 750 + 40 cm compared with v " = 671 cm 1 • ^ -03) reflects t n e antibonding nature exhibited in the rest of the series (Figure 17(e)). The spin-orbit s p l i t t i n g , A = 2620 + 40 cm \ compares favourably with A_^  + = 2745 cm x (approximately - 87 -95 11 1/2% g r e a t e r than f o r ) (see Table 4) showing reduced (4p7TT3 - 2pTr_) overlap compared w i t h (3p,r ..-2pTr ) of ClF above. The Br F 01 r strong e l e c t r o n withdrawal of the h i g h l y e l e c t r o n e g a t i v e f l u o r i n e atom combined w i t h the more r e a d i l y p o l a r i z a b l e charge of bromine w i t h respect to that of c h l o r i n e , may account f o r the s l i g h t l y l a r g e r energy d i f f e r e n c e of the f i r s t I.P. of BrF from t h a t of the atomic 90 I.P. v a l u e , i . e . , 11.97 eV (mean) (Br atomic I.P. 11.574 eV \ compared w i t h the f i r s t I.P. of C l F 12.80 eV ( C l atomic I.P., 12.686 eV). Thi s II mean I.P. i s i n e x c e l l e n t agreement w i t h the experimental I.P. g 79 observed by I r s a and Friedman , Table 3(c). The mean value of the LTu I.P. of BrF C A g r F + as A^+ « 300 cm"1) i s expected t o c l o s e l y approximate the atomic f l u o r i n e I.P., 17.141 eV f o l l o w i n g arguments s i m i l a r to those f o r C l F (S e c t i o n 4.2.3). This p r e d i c t s a ^ g - ^ energy s e p a r a t i o n i n BrF g r e a t e r than t h a t f o r ClF and an e s s e n t i a l l y ir-bonding f l u o r i n e o r b i t a l (TTu i n BrF) at higher b i n d i n g energy than the C l a-bonding o r b i t a l i n C ^ (Figure 16). An upper l i m i t f o r the LT I.P. of BrF i s e s t a b l i s h e d by the second I.P. of C l F , 17.18 eV from general s e r i e s trends. A p l a u s i b l e c r i t e r i o n to employ i s to reduce the value of the second I.P. from the atomic f l u o r i n e value by approximately the same amount as the f i r s t I.P. was inc r e a s e d from the atomic bromine v a l u e (Table 4). This i s c o n s i s t e n t w i t h the C l F data and assumes r e c i p r o c a l but reverse magnitudes of the e f f e c t s o c c u r r i n g i n the antibonding and bonding r r - o r b i t a l s w i t h i n the same molecule. The p r e d i c t e d II mean I.P. i s then 16.75 + 0.4 eV u — f o r BrF. - 88 -The magnitude of the LT^- energy s e p a r a t i o n i n ClF should give a reasonable and approximate order of magnitude f o r the E I.P. of BrF. This s e p a r a t i o n i s reasonably constant i n the e n t i r e s e r i e s (Figure 16) w i t h a s l i g h t r e d u c t i o n towards the f l u o r i d e s . The £ I.P. of & BrF i s p r e d i c t e d at 8.15 +0.4 eV. 4.2.5 P r e d i c t e d Iodine Monofluoride (IF) 90 The reported atomic I.P. values f o r i o d i n e , 10.347 eV and 103 10.454 eV are almost c o i n c i d e n t w i t h the experimental f i r s t I.P. 79 of I F , 10.5 +0.2 eV observed by I r s a and Friedman. A reasonable estimate of the LI mean I.P. of IF i s then 10.5 + 0.4 eV but may be g -a l i t t l e low by comparison w i t h the observed values f o r BrF ( i . e . I . P . B r p ( l l . 9 7 eV)- I . P . B r ( l l . 5 7 4 eV) = 0.4 eV) . A p p lying s i m i l a r arguments to IF as f o r BrF above and n o t i n g general trends i n the e n t i r e s e r i e s and monofluoride s u b s e r i e s (Figure 16), p r e d i c t e d second and t h i r d I.P.s f o r I F are 16.55 + 0.4 eV and 18.00 + 0.04 eV r e s p e c t i v e l y . These values are l i s t e d i n Table 2. 4.2.6 P r e d i c t e d Bromine Monochloride (BrCl) The P E spectrum of B r C l i s not expected to d i f f e r g r e a t l y from those of the i o d i d e s , bromides and c h l o r i d e s i n Figure 16. I r s a and 79 Friedman re p o r t a f i r s t I.P. value of 11.1 +0.2 eV which compares 95 88 favourably w i t h M u l l i k e n ' s p r e d i c t i o n s and Boyd's c a l c u l a t i o n s . both a p p r o p r i a t e l y reduced (see S e c t i o n 4.1.2). A s m a l l amount of B r - C l overlap i s favourable and the LI mean I.P. i s t h e r e f o r e s l i g h t l y fi l e s s than the Br atomic I.P., 11.574 eV. The s p i n - o r b i t s p l i t t i n g w i l l approximate the s p e c t r o s c o p i c Av value of Cordes and Sponer,^"^ - 89 --1 2 2238 cm (Table 4 ) and both I L ^ l /2g s t a t e s s h o u l d display vibrational progressions with vT fa 470-480 cm 1 (see Figure 17(e)). Extrapolation of the energy separations (Table 2) for the iodide, bromide, chloride series gives a value of 2.65 eV for BrCl and 2 a predicted II I .P . of 13.7(5) + 0.4 eV. The L T 0 / 0 , , 0 spin-orbit ^ u — 3/2, l /2u components are expected to be just at the l imi t of resolution for these broad P E bands (A fa 650 cm "*") . A similar procedure using H^-E separations (^1.8 eV) predicts the E I .P. of BrCl to be at 15.5 + 0.4 eV. These values are in excellent agreement with those predicted by 95 Mulliken. A schematic P E spectrum of BrCl i s i l l u s t r a t e d i n Figure 16. 4.3 Some Trends i n the Diatomic Halogen Series In general for the molecules discussed i n this series the atomic predictions of the molecular I . P . ' s and spin-orbit s p l i t t i n g s , having due regard for electronegativity and overlap considerations, are very adequate and possibly superior to the theoretical MO calculations in Table 3 . It appears that one may represent MO delocalization as an 85 effective covalent contribution to an ionic bond. Evans and Orchard state that the decrease i n II -IT energy separation from C l _ to I„ demon-g u a j r 2 2 strates a weakening in covalent interactions i n agreement with the similar decreasing dissociation energy. 4.3.1 Energy Level Diagram of the I . P . ' s of the Diatomic Halogens The observed and predicted I . P . ' s are graphically correlated in Figure 16 and show the subseries groupings mentioned in the text, i . e . , I 2 , , IBr, I C l , B r 2 , BrCl and C l 2 ; IF, BrF and C l F ; and F 2 -- 90 -4 . 3 . 2 The Relationship Between Diatomic Halogen I.P.'s and Electronegativity Assuming the validity of equation ( 4 ) , that i s , almost pure ionicity or complete localization of the TT and TT orbitals on the g u larger and smaller atoms of the diatomic respectively, the II and II v g u I.P.'s are plotted against Pauling electronegativity (Figure 17). The E g I.P.'s and (ng-LIu) mean I.P.'s (Table 2) are plotted against the average of three sets of electronegativity values for the appropriate halogen atoms. Figures 17(a) and (c) show the definite grouping of the mono-fluorides for each I.P. and possibly apart from the rest of the series depending, of course, on the v a l i d i t y of the predictions and the concept of electronegativity. Two different linear plots for the E I.P.'s in Figures 17(a) and (c) do not resolve the E I.P. value 8 of but suggest a more l i k e l y value near 19 eV (i.e., superimposed on the II u I.P.). From Figures 17(a) and (b) the Huggins electro-negativity values show best linearity for the observed I.P.'s. 4 . 3 . 3 Ionic Frequencies and Bonding Trends A plot of AG.JY2 f° r the neutral molecule against AG^^ r o r t n e ( l 7 T g ) ^ and (1T u ) L states of the halogens is illustrated i n Figure 17(e) showing the approximate linear increase in the extent of anti-bonding and bonding nature respectively from I^ to F 2 . 4 . 3 . 4 The Relationship Between Halogen Molecular and Atomic I.P.'s An approximately linear relationship exists for a plot of the average atomic I.P. (Table 4) of the appropriate halogen atoms against - 91 -the (Ii-II) mean I.P. and the 1st I.P. of the homonuclear diatomics g " (Figure 1 7 ( f ) ) . LI values of int e r h a l o g e n s r e f l e c t i n g a g >u dominant e l e c t r o n l o c a l i z a t i o n are a l s o noted opposite the atomic I.P. f o r t h a t atom. 4.3.5 (1^-11) Energy Separation C o r r e l a t i o n w i t h Dipole Moment Figure 17(d) i l l u s t r a t e s a p l o t of the d i p o l e moment data of 90 Deb and Coulson. The PE data may provide a f u r t h e r assessment of t h i s parameter f o r f u t u r e s t u d i e s . 4.4 Polyatomic Interhalogen F l u o r i d e s and Xenon F l u o r i d e s 4.4.1 I n t r o d u c t i o n P h o t o e l e c t r o n s p e c t r o s c o p i c s t u d i e s of the homonuclear^ and heteronuclear diatomic halogens have been presented. In view of t h i s data, and known d i s s o c i a t i o n product s t u d i e s , a p r e l i m i n a r y , i n t e r -mediate r e s o l u t i o n study of the f o l l o w i n g f i v e polyatomic i n t e r h a l o g e n s p e c i e s : BrF^, CIF^, BrF<-, IF,., and I F ^ i s now given. C o r r e l a t i o n i s a l s o made w i t h the xenon f l u o r i d e s i n Figure 22. 4.4.2 Experimental D i s s o c i a t i o n Considerations A l l of the samples were used as s u p p l i e d by the Matheson Chemical Co. I m p u r i t i e s (e.g. HF) other than those from d i s s o c i a t i o n were undetected i n the f i n a l runs. The r e s o l u t i o n of the spectrometer d e t e r i o r a t e d s l i g h t l y but d i d r e s o l v e e a s i l y the v i b r a t i o n a l progressions (v* = 360 + 40 cm ^) i n the f i r s t I.P. of the bromine i m p u r i t y i n BrF,.. - 92 -4.4.3 Bromine and Chlorine Trifluorlde (BrF,, and CIF^) A schematic study of the dissociation species that could possibly obscure or enhance the spectral peaks or their intensities is presented in Figure 19. The f i r s t I.P. of ClF almost coincides with that assigned to ClF-j (Table 6) and has approximately the same vibrational envelope but slightly reduced frequency, vQitp+ = ^70 + 40 cm 1 whereas v' + = 810 + 80 cm \ The relative intensities of C1F3 similar bands in both BrF^ and CIF^, similar vibrational progressions in the f i r s t I.P., and the apparent absence of BrF from the PE spectrum of BrF^ indicate that any contribution from ClF to the band intensities of CIF^ is minimal. The PE spectra reproduced in Figure 19 show no evidence for the presence of other impurities or a CIF^ dimer. 4.4.4 Bromine and Iodine Pentafluoride (BrF,. and IF,.) The PE spectrum of BrF,. (Figure 19) shows a Br^ impurity and no doubt includes the complementary dissociation species BrF-j which is not as intense because of a lower photoionization cross-section compared to Br^ (see Section 2.1.2). Comparison with the PE spectrum of IF,. (Figure 19) , XeF^ (Figure 21) and the correlation diagram of the I.P.'s of the polyatomic fluorides (Figure 22) substantiates the observed spectrum as that of BrF,.. The slight shift of the f i r s t I.P. of BrF,. to higher energy than that of IF,, is not parallelled i n the o remainder of the I.P.'s (or the other isoelectronic fluorides) but is slightly reversed (Figure 22). PHOTOELECTRON SPECTRA OF S O M E POLYATOMIC INTERHALOGEN FLUORIDES IP. (Vert.) SCHEMATIC DIAGRAM OF POSSIBLE DISSOCIATION PRODUCTS O 2 <°v) T U - T - 1 1 - 6 ' 2 ' i~ IONIZATION POTENTIAL («V) FIGURE 19 - 94 -TABLE 6 C1F 3 B r F 3 B r F 5 I F 5 I F 7 XeF 2 XeF, 1st Adiabatic 12.55 12.11 13.28 12.98 13.05 Adia. 12.37 12.90 12.66 12.20 Vert. 12.43 13.08 12.75 12.29 (505) ( — ) V e r t i c a l 12.85 12.37 13.48 13.37 13.39 12.97 12.46 Adia. 12.90 13.38 (+0.02eV) 13.08 12.55 Vert. 12.90 13.44 (810) (725) (505) (520) (245)? ( ? ) 240 3rd Adia. Vert. 14.47 14.81 14.59 14. 15. 96 71 15. 15. 57 97 15.08 15.49 4th Adia. Vert. 15.11 15.40 15.29 16. 17. 85 47 17. 17. 09 56 15.77 16.33 5 th Adia. Vert. 15.86 16.07 15.66 18. 14 16.85 17.51 6 th Adia. Vert. 16.69 17.26 (810) 16.17 16.65 (810) 19. 19. 02 83 18.35 18.66 7th Adia. Vert. 18.01 18.68 17.79 20.36 20.88 8 th Adia. Vert. 19.27 19.70 18.12 19.02 C1F 3 B r F 3 V l ( a l } 752 675 v 2 ( a l } 529 552 V 3 ( a l } 337 233 XeF? 1. 11.56(1) 2. 11.65(6) 3. 11.73(4) (+0.05eV) 2nd Adia. 13.69 — 14.31 14.83 14.14 — 14.23 Vert. 14.06 13.86 14.84 15.37 14.42 13.64 14.53 14.0 14.8 14.33 15.31 Adia. 15.3 Vert. 15.62 Adia. — 15.8 Vert. 16.0 17.6 17.0 17.93 17.48 — 18. 53 19. 2 19. 78 (702) (629) SCHEMATIC R E . SPECTRA OF INTERHALOGENS & X E N O N FLUORIDES BrF, A /WVil l l l . IONIZATION POTENTIAL ( « V ) I O N I Z A T I O N P O T E N T I A L (ev) FIGURE 20 - 96 -R E . S P E C T R A OF INTERHALOGENS & X E N O N FLUORIDES xeF 2 J I I I I ' ! ' I 12 14 16 18 21 IONIZATION POTENTIAL (eV) F i c . 1. H e i photoelectron spectra of X e l " ; , X e F i , and X c F « . " P u r e F R e g i o n " refers to the energy range in which the orbitals concerned have negligible xenon atomic orbital contribution, and which fvrinMy correspond to iluorine lone pair orbitals. Brundle et al . Ref. 131 FIGURE 21 IONIZATION ENERGY CORRELATION DIAGRAM FOR INTERHALOGEN & XENON FLUORIDES XeF2 BrR> (c2v) (Cjv) XeF^  (D4h) Brf| l l | (XeOFA) XeF6 (C4V> (C4v) (Oh) 1F7 13.43 1 3 - 3 7 o, 17,90 /... / 1144 /ff f l ,''Jil4'', b, I 16.0 ; ! i 2 » : b , - . ; -1 1 ' 1i£66. j o , V 16.65 b, S Ml ° i • • ]il°ju~~" . 13.48 °l . 13.37 14.81 !<J, I 1&2Z. ia,-... j ^ J , L . . . |b, !°l « L lb, . !£24 b, \ \ •> 18.68 . \ I 18.3 ft. 13JZ. |o, " b, > — . ! ° i * 1 e jb, 17.56 H5J.O, \ (as.) (IF,?) \ 1 I 16JJ w '7U~ -v 1 t f l i » 1 • •aaa a a v " ' " m • .«. 1M4 Ej / 1 / ffliBS A, FIGURE 22 - 98 -4.4.5 Iodine H e p t a f l u o r i d e (IF.,) In the schematic diagram of p o s s i b l e d i s s o c a t i o n products (Figure 19) there i s a s t r i k i n g correspondence between s e v e r a l I.P.'s of I F ^ and those of IF,.. However no t r a c e s of other l i k e l y d i s s o c i a t i o n products i s observed and the r e l a t i v e r a t i o s of band i n t e n s i t i e s i n the P E s p e c t r a of these two species i s not the same e s p e c i a l l y w i t h respect to the lowest and highest energy peaks. The weak I.P. i n I F ^ at 13.39 eV (see Table 6 and Figure 22) i s most l i k e l y r e p r e s e n t a t i v e of IFj. i m p u r i t y but may a l s o be assigned to I F ^ s i n c e the MO c o n f i g u r a t i o n s may be approximately s i m i l a r f o r t e t r a g o n a l pyramid s t r u c t u r e of IF,, and the proposed pentagonal bipyramid s t r u c t u r e of IF^. 4.4.6 Xenon F l u o r i d e s (XeF^,, XeF^) Approximately one-half hour c f p r e c o n d i t i o n i n g was r e q u i r e d before the s p e c t r a of both XeY^ and XeF^ were d i s t i n g u i s h a b l e and the spectrum of Xe reduced t o a low l e v e l (Figure 20). Figure 21 shows the e f f e c t of the s c a t t e r e d e l e c t r o n background i n the spectrometer, and a l s o d i r e c t c o r r e l a t i o n of the s p e c t r a w i t h those reported by Brundle et a l . ^ ' 1 ^ 1 No d e t a i l e d d i s c u s s i o n of the s p e c t r a i s given here i n view of these p u b l i c a t i o n s but I.P. energy l e v e l c o r r e l a t i o n s are presented i n Figure 22 and Table 6 . 4.5 D i s c u s s i o n and P r e l i m i n a r y I n t e r p r e t a t i o n of the Spectra 4.5.1. BrF- and C1F-— J J The lowest energy I.P. of both BrF^ and C l F ^ most l i k e l y i n v o l v e s removal of an e l e c t r o n from e i t h e r of the two e q u a t o r i a l lone p a i r s i n - 99 -132 the t r i g o n a l bipyramidal structure (see papers by G i l l e s p i e and Nyholm, 133 87 86 G i l l e s p i e and reviews by Stein and Wiebenga et a l . ). The f i r s t I.P. i n both molecules compares favourably with the atomic Br and C l I.P.'s (Table 4) r e s p e c t i v e l y and the appearance p o t e n t i a l data of 79 I r s a and Friedman (Table 7(a)). Figure 19 and 22 i l l u s t r a t e the trend of increasing binding energy of the f i r s t I.P. with i n c r e a s i n g f l u o r i n e content i n halogen f l u o r i d e systems (e.g. BrF^ > BrF^ > BrF). This trend shows the f i r s t observed PE band i n the IF^ spectrum to be most l i k e l y associated with IF,, impurity (Section 4.4.5). Io n i z a t i o n of an e q u a t o r i a l lone p a i r e l e c t r o n (a^ symmetry) relaxes the lone pair-lone p a i r repulsion and may p o s s i b l y e x c i t e both symmetric s t r e t c h i n g and bending v i b r a t i o n s i n the i o n . Molecule Observed Freq. v' Assignment Type of Molecular (+ 80 cm - 1) (see Table 8) O r b i t a l Indicated C1F„ 810 v''(a..) 752 nonbonding or (245) s l i g h t l y v' 3(a^) 337 antibonding BrF 725 v"(a ) 675 J (240) 1 1 v j ( a ) 233 The s i x t h I.P.'s of BrF^ and C1F 3 (Table 6) may also show resolved v i b r a t i o n a l progressions of v' ^ 810 cm ^suggesting e x c i t a t i o n of the symmetric s t r e t c h i n g frequency v | ( a ^ ) . From c o r r e l a t i o n s i n Figure 22 and an I.P. value s i m i l a r to that of atomic f l u o r i n e , 17.141 eV, t e n t a t i v e assignment to a pure F 2pa i o n i z a t i o n (F lone p a i r ) i s proposed. I.P. Calculations and o r b i t a l symmetries are c o l l e c t e d i n 131 Table 7(a). Brundle et a l . c a l c u l a t e I.P.'s wit h i n a defined energy - 100 -TABLE 7(a). C1F 3 Reference 1st I.P. 2nd I.P. 3rd I.P. 4th I.P. 5th I.P. 6th I.P. This work 12.85 14.06 14.81 15.40 16.07 17.26 79* <13.0+0.2 123(a) (CNDO/2) 10.06 137 ( a -only, no. 3d) 11.63 (3a L) 13.10 db 2) 22.84 (2a x) 23.67 (lb x) 35.87 (la L ) 137(b) ( a -only inc. 3d) 12.46 12.67 (lb 2) 14.73 14.90 (3a x) 22.98 23.15 (2 a ; L) 24.37 24.46 (lb x) 36.57 36.68 (la x) 136(a) 87° (approx.) 12.2 ( a x ) a 14.2 ( b 2 ) r r 16.3 ( a 1 ) a 16.6 ( b L ) a 17.4 ( a 2 ) r r 18.5 (b2)7r 21.8 ( b L ) a 136(b) 120°,D 3 h) ( a 1 ' ) a antib. BrF 3 (a2*')7r antib. ( e ' ) a Fnb. ( a 2 ' ) a Fnb. ( e " ) 7 r Fnb. * This work 12.37 13.86 14.59 15.29 15.66 16.65 7 9 * < 1 2 . 9 + 0 . 3 123(a) 10.07 (CNDO/2) 123(b) 10.75 (INDO) * Experimental - 101 -region i n the s p e c t r a of the xenon f l u o r i d e s to be produced by i o n i z a t i o n s from MO's composed s o l e l y of f l u o r i n e c o n t r i b u t i o n s . These r e g i o n s , designated the "pure f l u o r i n e r e g i o n " , are confined i n braces i n Figure 22. 4.5.2 B r F 5 and I F 5 134 The c a l c u l a t i o n s of Berry et a l . g i v e a f i r s t I.P. w i t h a^ symmetry f o r BrF,. (Table 7(b)) which i s both F-Br bonding above the e q u a t o r i a l f l u o r i n e plane and antibonding below i t . The Br-F r e p u l s i o n s below and a t t r a c t i o n s above t h i s plane produce the observed d i s t o r t i o n of the square pyramidal s t r u c t u r e , i . e . , the energy of the system i s lowered i f the lone p a i r spreads out, by having the f o u r f l u o r i n e atoms i n the e q u a t o r i a l plane move s l i g h t l y up toward the 134 a x i a l f l u o r i n e . Berry et a l . suggest c o n s i d e r a b l e Br-F mixing and l a r g e f l u o r i n e 2po but l i t t l e or no bromine d - o r b i t a l c o n t r i b u t i o n to t h i s MO. This i s c o n s i s t e n t w i t h the f i r s t I.P. being removed approximately 2 eV to higher energy than the atomic I.P.'s of Br and I f o r BrF^ and IF^ r e s p e c t i v e l y . An energy gap of approximately 1.4 eV, c o n s i s t e n t w i t h the PE 134 spectrum (Figure 22), i s c a l c u l a t e d between the 1st and 2nd I.P.'s, the l a t t e r designated as being d e r i v e d from a pure f l u o r i n e 2p e q u a t o r i a l o r b i t a l . The second low energy I.P. of XeF^ at 13.44 eV, not observed i n BrF,. or !F^, i s d e r i v e d from a second o r b i t a l 131 predominantly l o c a l i z e d on xenon. 134 C a l c u l a t i o n s show the second to n i n t h I.P.'s to be d e r i v e d from MO's c o n t a i n i n g e s s e n t i a l l y F 2pa c o n t r i b u t i o n s w i t h c o n s i d e r a b l e e q u a t o r i a l 2pa ( p e r p e n d i c u l a r to the f l u o r i n e plane) and a x i a l 2po - 102 -TABLE 7(b). B r F 5 Reference 1st I.P. 2nd I.P. 3rd I.P. 4th I.P. 5th I.P. 6th I.P. This work* 13.48 14.85 15.71 17.47 123(a) 12.24 (CNDO/2) 123(b) 13.28 (INDO) 134(a) (sq. pyr.) 134(b) 137(b) (o-only) ( i n c . 3d) 9.14 17.46 17.58 17.69 17.88 18.13 18.30 (6 a ; L) ( l a 2 ) (3b j ) (5e) (5a x) (4e) (2b 1) rr y eq. a eq. TTx eq. 5.60 16.96 17.49 17.60 17.95 18.44 18.91 (5e') (4e') ( 4 a i ' ) (3e") (3a 2") d a 2 ' ) 12.84 16.29 25.59 26.87 26.87 41.21 (3a x) (lb;,) (2a x) ( l 8 ( b ) ) ( l a x ) Hi This work* 13.37 15.37 15.97 17.56 18.14 19.83 79* <13.5+0.2 123(a) 13.51 (CNDO/2) 123(b) 13.65 (INDO) 141 (approx. e' a,"' a^' b1 &u a± s t a t e o r d e r -i n g only) * Experimental - 103 -mixing, and some 2 p r r e q u a t o r i a l in-plane and a x i a l 2pTT mixing. These are grouped i n braces i n Figure 2 2 and designated the "pure f l u o r i n e region" as above i n Section 4.5.1. 134 The 10th and 11th calculated o r b i t a l s are 1 . 2 eV to higher binding energy and contain F-Br pa-pa contributions i n the e q u a t o r i a l plane and a Br c o n t r i b u t i o n to the a x i a l bond r e s p e c t i v e l y . These calculated trends i n the I.P.'s of BrF,. are very c l o s e l y p a r a l l e l l e d i n the rest of the polyatomic interhalogen and xenon f l u o r i d e s studied, as shown i n Figure 2 2 . The higher binding energy of the inner I.P.'s of IF,, with respect to s i m i l a r I.P.'s of BrF,. may i n d i c a t e stronger f l u o r i n e bonding and greater s t a b i l i t y towards d i s s o c i a t i o n . A s i m i l a r P E spectrum i s expected f o r XeOF^, i s o e l e c t r o n i c with and of s i m i l a r structure to BrF,. and IF,., with a d d i t i o n a l I.P.'s from oxygen. 4.5.3 I F ? Although the structure of IF^ i s reasonably w e l l established as a pentagonal bipyramid,"'"'^ ,^^ Oakland and Duffey"*""^ have c a l c u l a t e d energy l e v e l s f o r several p o s s i b l e configurations, Table 7(c). Neglect of u-bonding d e f i n i t e l y reduces the usefulness of t h e i r c a l c u l a t i o n s f o r comparison with P E spectra. However, i f a s i m i l a r trend e x i s t s with i n c l u s i o n of TT contributions to the bonding, i . e . , i f no large energy differences occur between successive energy l e v e l s , then good correspondence e x i s t s with the P E I.P.'s, (Figures 19 and 2 2 ) . The "pure f l u o r i n e region" i s comparable to that observed 131 by Brundle et a l . i n XeF^ (see Figure 2 2 ) , and no doubt contains - 104 -TABLE 7 ( c ) . IF ^ (Bipyramidal s t r u c t u r e c a l c u l a t e d to be most s t a b l e Ref. 138) • Reference 1st I.P. 2nd I.P. 3rd I.P. 4th I.P. 5th I.P. 6th I.P. This work (13.39) 14.42 15.49 16.33 17.51 18.66 20.88 123(a) (CNDO/2) 14.78 123(b) (INDO) 14.71 1 3 8 ( a ) ( C 3 v ) (ext. Huckel a-on 7.50 ( A . ) iy) 7.51 (E) 8.28 (A x) 10.00 (A1) 10.37 (E) 19.58 (A1) 19.67 (E) 138(b) ( D5h> 6.41 CA1') 7.50 (E 2') 7.99 ( A 1 * ) 9.59 (E 1') 10.64 ( A 2 " ) 19.54 (E 2*) 20.53 (A1') 138(c) ( V C 2 v > 7.37 ( A X ) 7.43 (A 1) 7.53 (B 2) 8.62 (Aj_) 9.31 (B 2) 9.98 ( A X ) 11.17 ( V 138(d) (V 5:C 2) 7.37 ( A ) 7.43 ( A ) 7.53 (B) 7.59 ( A ) 7.61 (B) 8.40 ( A ) 9.06 (B) * Experimental TABLE 7 ( d ) X e F 2 R e f e r e n c e 1 s t I . P . 2nd I . P . 3 r d I . P . 4 t h I . P . 5 t h I . P . * T h i s w o r k 1 2 . 43 (12 . 6 7 ) 1 2 . 9 0 1 3 . 6 4 1 4 . 3 3 15 , . 6 2 15 . 8 1 6 . 0 1 7 . 48 * 47 1 2 . 42 (12 . 6 6 ) 1 2 . 8 9 1 3 . 6 5 1 4 . 3 5 15 , . 6 0 15 . 8 0 1 6 . 0 0 1 7 . 35 * 146 1 2 . 44 (12 . 6 9 ) 1 2 . 9 4 1 3 . 6 8 1 4 . 3 6 15 . 9 1 7 . 5 1 4 2 * ( A d i a . ) 1 2 . 28 1 4 5 * ( A d i a . ) 1 2 . 35 47 12 . 5 1 - 1 1 . 7 9 1 4 . 7 1 15 . 9 2 1 6 . 93 (92% KT c a l c . ) (5a i s » (10og) (3rr ) g (4TT. u> <6ou) X e F . h R e f e r e n c e 1 s t I . P . 2nd I . P . 3 r d I . P . 4 t h I . P . 5 t h I . P . 6 t h I . P . 7 t h I . P . 8 t h I . P . 9 t h I . P . 1 0 t h I * T h i s work 1 3 . 08 13 . 4 4 1 4 . 5 3 1 5 . 3 1 15 . 8 1 7 . 93 1 8 . 5 2 ft 131 1 3 . 06 13 . 3 8 1 4 . 4 6 1 5 . 4 1 15 . 7 8 (16 . 3 ) 1 7 . 94 1 8 . 4 0 1 4 5 * ( A d i a . ) 1 2 . 65 ft 131 1 1 . 7 13 . 9 1 5 . 2 1 5 . 8 1 6 . 2 16 . 5 16 . 6 1 6 . 8 1 9 . 0 1 9 . 6 (92% KT c a l c . ) ( 1 0 a l g ) (5a < l a 2g> ( 7 e u ) ( l b 2»>. (3e " V < 4 a 2u> ( 6 e u ) 147 1 4 . 79 16 . 9 2 1 7 . 3 5 1 7 . 4 0 1 7 . 4 2 17 . 4 2 17 . 4 2 1 7 . 4 3 (LCAO MO c a l c . ) <a2u> ( V <V < a 2 g » ( e u > <b2u> ( e g ) < » 2 g > 147 1 5 . 10 17 . 4 0 1 7 . 4 2 1 7 . 4 2 1 7 . 4 2 17 . 4 2 17 . 4 2 1 7 . 4 2 1 7 . 43 1 8 . 8 8 (p o - o n l y <a2u> ( V ( a 2 g > <a2u>" ( t 2 g > ( b 2u> ) ( e u > 148 1 0 . 6 12 . 0 1 7 . 6 1 7 . 6 1 7 . 8 18 . 2 18 . 5 . 1 9 . 7 ( s e m i - e m p i r . MO) ( 2 a 2 u > (3a ( a 2 g > ( V (2eu) ( l a 2u> (2e g> ( 2 b 2 g > 141 ( s t a t e o r d e r - (e i n g o n l y ) ( b l g ' <a2u> TABLE 7(e) Xe F, 6 Reference 1st I.P. 2nd I.P. 3rd I.P. 4th I . P . 5 th I.P. 6 th I.P. 7th I.P. 8th I.P. * 131 12.51 15. 2 16 .0 (16 .5) (17 .1) 17 .65 20.0 21 .0 145*(Adia.) 12.19 * 131 10.8 17. 2 17 .6 18 .0 18 .1 18 .8 22.3 29 .2 (92% KT ca l c . ) < 8 V ( 5 e s ) ( I t is' (7t 2u> ( I t 2u> (3t 2S' <6t2a> (7a is' 143 9.7 11. 3 11 .3 11 .4 11 .5 12 .3 14.0 (approx.) ( 2 V <1C2u> ( I t is' (It. 2s' (2t lu> ( l e s' - 107 -s e v e r a l I.P.'s many of which are not i n d i c a t e d i n the f i g u r e . I f the tren d of i n c r e a s i n g b i n d i n g energy of the 1st I.P. w i t h i n c r e a s i n g f l u o r i n e content i n the same halogen f l u o r i d e system ( S e c t i o n 4.5.1) i s v a l i d f o r f l u o r i n e systems, the 1st I.P. of I F ^ i s at 14.42 eV (as noted i n S e c t i o n 4.4.5), and the I.P. at 13.39 eV i s from IFf. i m p u r i t y . The l o c a l i z a t i o n of the PE I.P.'s of I F ^ i n bands s i m i l a r to those observed f o r IF,, and XeF^ p o s s i b l y s u b s t a n t i a t e s a r e g u l a r s t r u c t u r e (e.g. pentagonal bipyra m i d ) . CHAPTER 5 THE STUDY OF FREE RADICALS AND TRANSIENT SPECIES BY PES 5.1 I n t r o d u c t i o n 149 Herzberg s t a t e s that most simple organic and i n o r g a n i c r a d i c a l s are extremely short l i v e d s p e c i e s , being c h e m i c a l l y unstable but i n general p h y s i c a l l y s t a b l e and d i f f i c u l t to produce and study i n the f r e e s t a t e . T h i s chapter i s devoted to p r e l i m i n a r y s t u d i e s of molecules, atoms and ions w i t h an unpaired e l e c t r o n ( s ) or being unstable ( t r a n s i e n t s ) under normal l a b o r a t o r y c o n d i t i o n s . The techniques and apparatus used to produce these species i n s u f f i c i e n t c o n c e n t r a t i o n f o r PE d e t e c t i o n ( i n the order of a few % to approximately 100% f o r NF^) are d e s c r i b e d f u l l y i n the Experimental S e c t i o n (Chapter 3). 5.2 I n d i v i d u a l Species 5.2.1 Oxygen 0^, N i t r i c Oxide NO, and Nitrogen Dioxide An account of the s p e c t r a l d i f f e r e n c e s observed f o r the f i r s t o o I.P. of 0^ using Ne (736-744 A) and He (584 A) i o n i z i n g r a d i a t i o n s i s reported i n reference 8 of the Appendix. In the most recent high r e s o l u t i o n PE study of O2, Lindholm et a l . " * " " ^ best r e s o l v e the doublet and quartet s t a t e s and give a very complete account of the s p e c t r a l f e a t u r e s . - 109 -R E . S P E C T R A O F FREE R A D I C A L S IONIZATION POTENTIAL (eV) IONIZATION POTENTIAL UV) FIGURE 23 - 110 -The PE spectrum of NO (Figure 23) was obtained without improvement upon that of Turner et a l . ^ Only f i v e of the s i x p o s s i b l e I.P.'s r e s u l t i n g from exchange i n t e r a c t i o n s ( S e c t i o n 2.1.4.3) are observed f o r i o n i z a t i o n of a s i n g l e e l e c t r o n from the second lowest b i n d i n g energy MO. The spectrum of N0 2 (Figure 23), obtained i n c o n j u n c t i o n w i t h a study of N0C1 (Appendix S e c t i o n 13) and NOF, adds l i t t l e t o the analyses of Brundle et a l . " ^ and Lindholm et a l . ^ " ^ An I.P. of NOF i n the region of the f i r s t I.P. of N0 2 showed p o s s i b i l i t i e s of v i b r a t i o n a l f i n e s t r u c t u r e , however, the spectrum was overlapped w i t h t h a t from N0 2 present by decomposition and/or i m p u r i t y . Attempts to observe the f i n e 53 s t r u c t u r e reported by Brundle et a l . i n the f i r s t I.P. of r e l a t i v e l y pure NO^ were u n s u c c e s s f u l . 5.2.2 Difluoramino R a d i c a l NF 2, and C h l o r i n e Dioxide C10 2 D i s c u s s i o n of the PE s p e c t r a of the NF 2 and C10 2 i s o e l e c t r o n i c f r e e r a d i c a l s (Figures 23, 24 and 25 ) have been reported i n the 51 52 l i t e r a t u r e . ' A d d i t i o n a l i n t e r p r e t a t i o n s and c o r r e l a t i o n s w i t h N0 2 and s i m i l a r t r i a t o m i c s i s presented here. A Rydberg s e r i e s converging to the f i r s t I.P. of C10 2 c a l c u l a t e d 170 at 10.36 eV by Basco and Morse i s i n e x c e l l e n t agreement w i t h the 52 a d i a b a t i c value reported from the PES measurements. The f i r s t I.P. of both C10 2 and NF 2 i n v o l v e s removal of an e l e c t r o n from a b^ o r b i t a l and e x c i t a t i o n of the symmetric s t r e t c h i n g frequency v|(a^) i n the ions w i t h no evidence f o r the bending v i b r a t i o n . S i m i l a r i n c r e a s e s i n the symmetric s t r e t c h i n g frequencies are observed f o r the ions of both 20 valence e l e c t r o n molecules, F 20 and C1 20 (Appendix S e c t i o n 6 ) , R E . S P E C T R A O F FREE R A D I C A L S P R O D U C E D BY PYROLYSIS O F DIMERS 9. (21.0) 9. 19.41 IONIZATION POTENTIAL Lv) IONIZATION POTENTIAL (eV) FIGURE 24 RECORDER TRACE OF PE SPECTRUM OF Nf| FIGURE 2 5 - 113 -the much l a r g e r overlap of the b^ pr o r b i t a l s i n Cl^O probably accounting f o r simultaneous e x c i t a t i o n of i n the C 1 2 0 + i o n . I o n i z a t i o n of an e l e c t r o n from the s i n g l y occupied a^ o r b i t a l of NO^ (17 valence e l e c t r o n s ) , however, produces a l a r g e geometry change f o r NO^ "*", expected to be s i m i l a r to the i s o e l e c t r o n i c l i n e a r molecule CO^, and e x c i t a t i o n of the bending mode l n the i o n . The dependence of the bond angle on the occupation of the a^ o r b i t a l i s i l l u s t r a t e d i n the d i o x i d e s of the f i r s t row elements of the p e r i o d i c t a b l e : Molecule No. of a^ e l e c t r o n s Bond angle C0 2 0 180° N0 2 1 134° 0 3 2 117° Assuming s i m i l a r o r d e r i n g f o r the f i r s t few occupied MO's i n the ground s t a t e s of both C10"2 and N0 2 (see Figure 27) , and n o t i n g the p r o x i m i t y of the a^, a 2 and b 2 o r b i t a l s and apparent r e - o r d e r i n g of a^ and b 2 i n the i s o e l e c t r o n i c d i o x i d e s S n 2 and 0^, the second and t h i r d I.P.'s (exchange components) of C10 2 are expected to i n v o l v e e x c i t a t i o n of the bending mode a s s o c i a t e d w i t h i o n i z a t i o n from an a^ o r b i t a l . The e f f e c t of the unpaired b^ e l e c t r o n , and hence the magnitude of the exchange 3 1 s p l i t t i n g between the corresponding B^ and B^ s t a t e s produced, should be minimal from symmetry c o n s i d e r a t i o n s . A s i m i l a r MO o r d e r i n g scheme appears to be app r o p r i a t e f o r the d i f l u o r i d e s NF 2 and 0 F 2 (Figure 26,27) however the stronger e l e c t r o n e t a t i v i t y of f l u o r i n e s h i f t s the I.P.'s to higher b i n d i n g energy. - 114 -CORRELATION DIAGRAM OF SMALL POLY/ NUMBER OF MOLECULE  ELECTRONS N 2 0 16 i i CO, 17 N 0 2 18 i i I i i i i i i SO2 NOCI HNSO CB, 19 NR, CIO, 20 C l 2 0 F0O 22 XeF2 3 * / r / * h M I. • I life t ll!" 1 Ilk .7°i \b? -P, /// \ i .„h. 1), I bj? bi \a, °2f fill„ 4 ^ ^ ^ _ ./£ . 1O1 'bV /bV \ •••Xb, X^b, —. v / -'— K> M 18 22 IONIZATION ENERGY (eV) 26 FIGURE 2 6 IONIZATION POTENTIAL (eV) O C 70 IO IONIZATION ENERGY eV ? 9 f £_ o , s 1 z c CD o o S J-.J L (I / /. Ll '. 111 \ I \ m z o m m O _ 1 _ -2=1"'ro n r i i / / i / ro.' «•/-I 'I - 1 r i i -T-+-/ 1 • ' ! I s , I > •o m x > z o /i J i ' i i J \ 'M I I I '.III* > / !'• I* |r II? f \ /.' 11 i i frffi y y w v Is ' ii b 5 o= g w o x o m co o ro 1 o ' i r w Q SE i; .8« S>- s » § P s m O w o P' 2' roTI g T l N 5 o z m z m s r o 5 2 n ° O g m IOTI ° co - 116 -The second P E band i n NF 2 (Figure 25) d i s p l a y s a long v i b r a t i o n a l progression (16 members) suggesting a l a r g e geometry change i n the i o n from that i n the parent molecule and predominant e x c i t a t i o n of v,,(in opposition to the published a s s i g n m e n t 5 1 ) . The frequency r e d u c t i o n v" =750 cm - 1 to -1 N F 2 =520 cm s u b s t a n t i a t e s the bonding nature of the a, o r b i t a l . 1 , F2 + The extent of the bend f o r NF 2 -s- NF 2 , (104° to <134°) , i s not expected to be as great as that f o r NC>2 -»• N 0 2 + , (134° to 180°), p o s s i b l y being represented by the more favourable Franck-Condon overlap c o n d i t i o n s i n NF 2 upon i o n i z a t i o n of a s i m i l a r a^ e l e c t r o n . An apparent d i s c o n t i n u i t y observed i n the 2nd I.P. of NF,^ 1 (see Figure 25) may be produced by overlap of a weak pro g r e s s i o n i n v| (almost double v^) or the coincidence of the s i n g l e t s t a t e v i b r a t i o n a l p r o g r e s s i o n i n v 2 s h i f t e d by a s m a l l exchange i n t e r a c t i o n . The i n t e g r a t e d peak area of t h i s I.P. r e l a t i v e to others i n the spectrum of NF 2 i s not c o n s i s t e n t w i t h overlap of two t r i p l e t s t a t e s . Evidence f o r the o r d e r i n g of the next two innner valence o r b i t a l s ( i . e . the 4th to 7th I.P.'s i n c l u s i v e ) of C10 2 and NF 2 i s l e s s c o n c l u s i v e , but there i s l i t t l e doubt that the a 2 and b 2 o r b i t a l s are i n v o l v e d (see Figure 27). The a n a l y s i s of the P E spectrum of N0 2 by Brundle 5 3 3 - 1 et a l . e s t a b l i s h e s the f o l l o w i n g MO scheme f o r N0 2: ... B 2 ( 2 b 2 ) , 3k1(3a1)~1 + 3 B 1 ( l b 1 ) " 1 , 1 B 2 ( 3 b 2 ) " 1 , 1 A 2 ( l a 2 ) " 1 , ^ ( l a ^ - 1 , 3 B 2 ( 3 b 2 ) " \ "^A^(4a^) L. The second to f i f t h I.P.'s f a l l i n a group between 12.5 and 15 eV and the bending v i b r a t i o n i s e x c i t e d i n each i o n i c s t a t e . The 5th I.P. (^B2) i s an exception i n that the symmetric s t r e t c h i n g mode seems to be dominant. The o r b i t a l s a s s o c i a t e d w i t h these B 2 s t a t e s have weak bonding or antibonding c h a r a c t e r whereas those r e l a t i n g - 117 -to the s t a t e s are n e a r l y nonbonding i n c h a r a c t e r . Noting the r e l a t i v e o r b i t a l b i n d i n g energies i n the d i o x i d e s e r i e s N C ^ , O^CSO^), ClO^ (Figure 27), a second e l e c t r o n i n the a^ o r b i t a l i n going from NC^ to 0^ causes f u r t h e r bending of the molecule. This pushes up the energy of the o r b i t a l without g r e a t l y a f f e c t i n g the o r b i t a l and causes an i n v e r s i o n of b^ and a^. The subsequent a d d i t i o n of a b^ e l e c t r o n does not a p p r e c i a b l y a f f e c t the bond angle (0^: 117°, SOy. 119°, C I C y 118°) and t h e r e f o r e probably has l i t t l e a f f e c t on a x as expected from symmetry c o n s i d e r a t i o n s . The b^ o r b i t a l , however, i n t e r a c t s more s t r o n g l y w i t h an o r b i t a l of a^ symmetry, the e f f e c t being r e f l e c t e d upon i o n i z a t i o n of t h i s e l e c t r o n by a l a r g e exchange s p l i t t i n g between the I.P.s. The I.P.'s r e s u l t i n g from i o n i z a t i o n of a b^ e l e c t r o n would be expected to l i e i n c l o s e p r o x i m i t y to one another. The very l a r g e i n t e g r a t e d peak area of the second P E band of ClO^ and the apparent shoulder at 13.4 eV mark the presence of more than one t r i p l e t and/or s i n g l e t s t a t e , (Figure 23^8). The t h i r d band of CIO,, at 15.45 eV i s very weak, suggesting, i t s assignment to a s i n g l e t s t a t e . Resolved v i b r a t i o n s w i t h an i n t e r v a l of 725 + 40 cm ^ compared w i t h vj' = 943 cm ^ and v^ ' = 445 cm ^ suggest e i t h e r i o n i z a t i o n from a bonding o r b i t a l and e x c i t a t i o n of o r , l e s s l i k e l y , i o n i z a t i o n from a s t r o n g l y antibonding o r b i t a l and e x c i t a t i o n of v^. Since the a^ and b^ o r b i t a l s are d e r i v e d from the TT^ o r b i t a l i n the l i n e a r c o n f i g u r a t i o n , the former assignment i s adopted. Assignment as a s i n g l e t s t a t e i s c o n s i s t e n t w i t h the l a r g e exchange i n t e r a c t i o n expected between the b^ and a^ o r b i t a l s . A comparison of the P E spectrum of C I O 2 w i t h those of C O 2 , N O 2 , S O 2 , and 0^ (Figure 27) i n d i c a t e s the presence of the b 2 - 118 -o r b i t a l of CIO,, at a s i m i l a r b i n d i n g energy although the exact MO o r d e r i n g i s unknown. I t i s suggested t h a t the s t a t e o r d e r i n g i n the C10„ + i o n e x i s t s as given below. E Band Approx . V e r t . I.P.(eV) State Ordering • 1 10.48 ( 12.94 to 1 V(a.) - 1 2 j 13.40 i - i j W and 3 A 2 ( b 2 ) " 1 \ i 1A 2(b 2)~ 1 and 3 B 2 ( a 2 ) _ : L 3 15.45 4 17.50 and \ C a - L ) - 1 and 3 w _ 1 5 17.95 6 19.36 3 A 2 ( b 2 ) _ 1 S i m i l a r s t a t e s and approximate MO o r d e r i n g are expected f o r NF,,.. The higher b i n d i n g energy of the a 2 and b 2 o r b i t a l s of NF,, w i t h respect to those of CIO,, r e f l e c t s the g r e a t e r e l e c t r o n e g a t i v i t y of f l u o r i n e . C o r r e l a t i o n w i t h the d i f l u o r i d e s e r i e s (Figure 27) places the b 2 o r b i t a l at lower energy than a 2 i n NF,, dependent upon the accuracy of the p r e d i c t i o n s f o r CF,, and the assignment of 0 F 2 . S i n g l e t s t a t e s above 17 eV are apparently not detected i n the 53 s p e c t r a of NO,,, C10 2 a n c* N F 2 , a n d n lS n e r energy s t a t e s are assigned to t r i p l e t s . In the d i o x i d e s e r i e s a d i s t i n c t energy i n t e r v a l of approximately three v o l t s i s observed between the lowest group of o r b i t a l s b 2 , a,,, a^, b^,(assuming a center of g r a v i t y near 14.5 eV f o r - 119 -the a,, s t a t e i n CIO,,) and the remainder of the valence e l e c t r o n s . In a recent paper on the use of the p h o t o e l e c t r o n spectrum of a molecule i n the i n t e r p r e t a t i o n of the e l e c t r o n i c spectrum of the corresponding c a t i o n by H e r r i n g and M c L e a n , c o m p a r i s o n i s made between the P E + 172 spectrum of CIO,, and the e l e c t r o n i c spectrum of ClO^ of Ca r t e r et a l . T h e i r i n t e r p r e t a t i o n i s c o n s i s t e n t w i t h an o r b i t a l c o n f i g u r a t i o n f o r + 2 2 2 C10 2 of ...b 2 a 2 a^ , the lowest unoccupied o r b i t a l being the b^. The or d e r i n g of the b 2 o r b i t a l i s d i f f e r e n t from that given above and suggested i n Figure 27. The lower p o r t i o n of the f i g u r e i l l u s t r a t e s a c o r r e l a t i o n of I.P.'s of C l 2 n and d i o x i d e s i n r e l a t i o n to both apex angle and the number of valence e l e c t r o n s s i m i l a r to that p r e d i c t e d by 173 M u l l i k e n f o r t r i a t o m i c oxides. The diagram i s c o n s i s t e n t w i t h the a n a l y s i s given and shows the energy grouping of the molecular o r b i t a l s throughout t h i s s e r i e s . The diagram may a l s o i n d i c a t e the stronger apex angle dependence on i o n i z a t i o n of an a^ e l e c t r o n w i t h respect to that of o r b i t a l s of symmetry b 2 , a 2 and b^. 5.2.3 The Fluorosulphate R a d i c a l (SC^F) 5.2.3.1 I n t r o d u c t i o n and Experimental The molecular p o i n t group of the SO^F r a d i c a l i s determined by e l e c t r o n d i f f r a c t i o n to be C_ . The c a l c u l a t e d ground s t a t e of the 3v & , . 156 2. . r a d i c a l xs A2• . . ( 4 e ) 4 ( 5 a 1 ) 2 ( 5 e ) 4 ( l a 2 ) 1 2 A £ 7 2 with a A] a r | d two E exci ted electronic states close above i t . From t h e s e f o u r - 120 -lowest energy s t a t e s (the next lowest i s c a l c u l a t e d to be 4-6 eV higher i n energy) one expects a t o t a l of seven I.P.'s i f a l l s i n g l e t -t r i p l e t s p l i t t i n g s are r e s o l v a b l e . The f l u o r o s u l p h a t e r a d i c a l e x i s t s i n e q u i l i b r i u m w i t h i t s dimer, p e r o x y d i s u l p h u r y l d i f l u o r i d e , the c o n c e n t r a t i o n of the r a d i c a l ... fc t 157-159,168,169 i n c r e a s i n g w i t h temperature: 2S0 3F ^ ± S 2 ° 6 F 2 The sample of p e r o x y d i s u l p h u r y l d i f l u o r i d e , prepared by the combination of r e d i s t i l l e d sulphur t r i o x i d e and f l u o r i n e over AgF^ i n a c a t a l y t i c 160 r e a c t o r , was k i n d l y s u p p l i e d by Dr. F. Aubke. The p y r o l y s i s of the dimer was achieved using a n o n - i n d u c t i v e l y wound e l e c t r i c c o i l d escribed i n S e c t i o n 3.4.3 and i l l u s t r a t e d i n Figure 6. The P E spectrum of ^2^(>^2 ( F^§ u r e ^4) completely disappeared above 150°C but best y i e l d of monomer was obtained at 185°C. Peri o d s of 3-4 hours were r e q u i r e d f o r approximate thermal e q u i l i b r a t i o n of the system during p y r o l y s i s and reduced the e f f e c t i v e n e s s of m u l t i p l e scanning techniques. Traces of a i r provided n i t r o g e n peaks used f o r c a l i b r a t i o n without obscuring any strong bands i n the r a d i c a l spectrum (see Figure s 24, 28 and Table 8 ) . 5.2.3.2 D i s c u s s i o n of I n d i v i d u a l I.P.'s The f i r s t band (Figure 28) i s s h a r p , e x h i b i t i n g a st r o n g a d i a b a t i c peak at 12.85 eV and p o s s i b l e weaker peaks at 12.99 eV (1220 cm"1 i n t e r v a l ) and 13.15 eV (2400 cm 1 i n t e r v a l ) . A c a l c u l a t i o n by King et al.''""'*' shows the fou r lowest l y i n g MO's to be d e r i v e d from oxygen - 121 -TABLE 8. S03F P.E. Band Designation I .P.(+ 0.02 eV) V i b . Freq. ( c m ) 1st A d i a b a t i c 1. 12.85 V e r t i c a l 2. 12.99 2nd A d i a b a t i c 1. 13.83 V e r t i c a l 2. 14.06 9 A d i a b a t i c 1. (14.41)* V e r t i c a l (14.54) 3rd A d i a b a t i c 1. 14.87 V e r t i c a l 2. 14.96(5) 3. 15.08(0) 4. 15.18(7) 5. 15.29(8) 6. 15.41(1) 4th A d i a b a t i c 000 17.91(0) 010 17.97(8) 100 18.02(4) 110 18.09(7) 200 18.14(5) 210 18.20(7) 300 18.25(2) 310 18.32(8) 5 t h A d i a b a t i c 19.27 V e r t i c a l 19.51 Observed V i b r a t i o n a l Frequencies (SO^F ) 1220 + 40 (605)? 850 + 40 920 + 40 945 + 40 550 + 40 I.P. (molecule ion) v 1(a 1) V i b r a t i o n a l Frequency (cm X ) v 2 ( a l ) V 3 ( a l } v 4 ( e ) v 5 ( e ) v 6 ( e ) 1st 2nd * ? 3rd 4th 5th (molecule) 2 Ground S t a t e ( A_) 2 Upper State ( E) 1220 (1220) 945,920 850 945,920 (605)? 550 (605)? (550) 1055.5 839.3 533.5 1177.5 604.1 369.4 952.9 800.5 515.0 1111.5 505.7 346.9 - 122 -R E . SPECTRA OF THE FLUOROSULPHATE FREE RADICAL AND RELATED COMPOUNDS :5S0 cm"* 1220 cm"1 850 cm" S O , F " C I O A F S O , F , F O S O , F S 2 0 6 F 2 ill..'. 11. A / A A 18 20 I O N I Z A T I O N P O T E N T I A L (ev) S O 3 F K clo>F J SOsFj, 13 14 IS 16 17 IS 19 20 13 14 15 16 17 11 t9 30 IONIZATION POTENTIAL UV) FIGURE 28 - 123 -TABLE 9. I.P.'s of the Fluorosulphate R a d i c a l and Related Compounds. I.P. Molecule S0 3F' C10 3F S 0 2 F 2 SOjFCl FOS0 2F S 2 ° 6 F 2 1. 12.85 13.10 13.51 12.66 13.62 13.37 (1220) (630,460) 2. 14.06 14.09 13.74 13.31 14.40 14.01 (495) (513) 3. 14.96 14.40 15.12 14.1 14.82 14.56 (850) (495) (326) 4. 17.91 15.76 15.46 14.7 15.72 15.16 (920,945,550) (1046) 5. 19.51 — ? 16.68 15.07 15.82 15.82 (1132,524) 6. — 17.34 18.34 18.34 17.78 16.68 (480) 7. — 19.96 19.89 16.74 18.10 17.00 (553) 8. — — (20.8) 18.4 19.43 18.14 ( I o n i c frequencies i n b r a c k e t s ) . Comparison of Molecular O r b i t a l C a l c u l a t i o n s of I.P.'s w i t h P h o t o e l e c t r o n R e s u l t s I.P. Molecule, o r b i t a l , and energy (eV) 156 S0 3F CA 2) S0 3F CA X) (mean) P.E.S. S0 3F" 1. 2. 3. 4. e 13.04 a 2 14.95 e 15.50 a x 16.56 a 2 12.78 e 12.97 e 14.65 a 1 20.47 1. 12.91 2. 13.96 3. 14.93 4. 17.52 12.85 14.06 14.97 17.91 - 124 -lone p a i r A.O.'s, the (4e), (5a^) and (5e) being s l i g h t l y d e l o c a l i z e d over the other n u c l e i . The l a ^ o r b i t a l i s nonbonding and e n t i r e l y comprised of oxygen p - o r b i t a l s p e r p e n d i c u l a r to the t h r e e - f o l d a x i s , supported by the nonbonding P E band shape observed (see S e c t i o n 54 2.1.3). A c a l c u l a t i o n by H i l l i e r and Saunders shows the hi g h e s t occupied o r b i t a l of the SO^F i o n a l s o to be a^ and oxygen nonbonding Smaller sulphur 3d and oxygen 2p popu l a t i o n s i n SO^F compared w i t h i s o e l e c t r o n i c SO^ i n d i c a t e that the major change i n e l e c t r o n d i s t r i b t i o n upon removing two e l e c t r o n s to form SO^F"*" i s a l a r g e r e d u c t i o n i n oxygen 2p and a s m a l l e r r e d u c t i o n i n sulphur 3d p o p u l a t i o n s . o 161 V i b r a t i o n a l a n a l y s i s of the 5160 A abso r p t i o n spectrum of SO^F 16 2 and E S R data both suggest a nondegenerate ground s t a t e . Assignment of the observed i o n i c v i b r a t i o n a l frequency v' = 1220 + 40 cm 1 to an i n c r e a s e i n the n e u t r a l molecule symmetric s t r e t c h i n g —1 161 frequency v'^(a^) = 1055.5 cm (Table 8 ) , i s c o n s i s t e n t w i t h an antibonding a^ o r b i t a l . The second P E band (having onset 13.8(3) eV, maximum 14.06 eV and band width <vl eV) i s p a r t i a l l y obscurred by e i t h e r n o i s e or a second band w i t h apparent maximum at 14.5(4) eV. An i n t e r v a l of approximately 605 cm X f o r the former band would not be i n c o n s i s t e n t w i t h one of the observed s p e c t r a and may i n d i c a t e e x c i t a t i o n of the bending mode v-j(a^) or even v^(e) (Table 8 ) i f i o n i z a t i o n occurs from a degenerate o r b i t a l . Ab_ i n i t i o 5 4 and CNDO 1 5^ c a l c u l a t i o n s p r e d i c t the hi g h e s t f u l l y occupied o r b i t a l to have 3 e symmetry, i o n i z a t i o n of an e l e c t r o n from which w i l l produce E and ^E (the p o s s i b l e band at 14.5 eV) s t a t e s of the S0,jF + i o n . - 125 -I n t e n s i t y c o n s i d e r a t i o n s are c o n s i s t e n t w i t h t h i s assignment. The apparent magnitude of a J a h n - T e l l e r / s p i n - o r b i t i n t e r a c t i o n f o r an 154 e x c i t e d e l e c t r o n i c s t a t e of the r a d i c a l discussed by King et a l . i s i n s u f f i c i e n t to cause s t a t e r e o r d e r i n g but no extension of the d i s c u s s i o n i s o f f e r e d f o r other s t a t e s or f o r the p o s i t i v e i o n . The t h i r d I.P. ( v e r t i c a l I.P. 14.96 eV) has a v i b r a t i o n a l p r o g r e s s i o n 850 + 40 cm ^ and band shape i n d i c a t i n g removal of a bonding e l e c t r o n . Comparison w i t h ground s t a t e frequency data (Table 8) gives the f o l l o w i n g most l i k e l y p o s s i b i l i t i e s . E x c i t a t i o n of v^(a^) (v*2 = 839 cm \ the S-F symmetric s t r e t c h ) i n d i c a t e s i o n i z a t i o n from a non-degenerate o r b i t a l of a^ symmetry and l i t t l e change i n geometry i n the i o n from that i n the molecule. However, e x c i t a t i o n of v ^ ( a ^ ) , s i m i l a r to that assigned the f i r s t P E band, i n d i c a t e s removal of a bonding e l e c t r o n and may i n v o l v e a degenerate o r b i t a l , a l s o c o n s i s t e n t w i t h the c a l c u l a t i o n s . T h i s band i s most l i k e l y r e p r e s e n t a t i v e of a t r i p l e t s t a t e ; the s i n g l e t exchange component p o s s i b l y i s unresolved from the n o i s e or u n d e r l y i n g the observed band. The 4th P E band has an i n t e n s e onset peak at 17.91 eV and v i b r a t i o n a l progressions 920 + 40, 945 + 40 and 550 + 40 cm (Figure 28 and Table 8 ). The corresponding molecular v i b r a t i o n s are most l i k e l y v^(a^) = 1056 cm L and ^C 3^) = 534 cm ^ (the SO^ symmetric bend). The P E band shape and magnitude of the frequency change i n producing the i o n both represent i o n i z a t i o n of a predominantly non-bonding or s l i g h t l y bonding e l e c t r o n . L i t t l e change i n bond angle i s expected s i n c e the maximum band i n t e n s i t y i s found i n the a d i a b a t i c t r a n s i t i o n . As noted f o r the t h i r d I.P., the p o s s i b i l i t y e x i s t s f o r e x c i t a t i o n of - 126 -v^Ca^) ( i o n i z a t i o n from a s l i g h t l y antibonding o r b i t a l ) and simultaneous e x c i t a t i o n of \)^(a^). The I.P. shows comparable b i n d i n g energy to the atomic f l u o r i n e I.P. at 17.14 eV and may p o s s i b l y s u b s t a n t i a t e a high 162 s p i n d e n s i t y on f l u o r i n e i n t e r p r e t e d from ESR data. Unequivocal assignment of t h i s P E band to i o n i z a t i o n of an e l e c t r o n from a d e l o c a l i z e d oxygen lone p a i r MO or a predominantly f l u o r i n e MO i s not p o s s i b l e , however, the former i s more l i k e l y and the r e s u l t i n g i o n i c s t a t e i s most probably a t r i p l e t . There i s no f i n e s t r u c t u r e observed on the weak f i f t h I.P. at 19.5 eV. Despite the l a c k of i n t e n s i t y (see F i gure 24) the p r o b a b i l i t y of i t s assignment to a t r i p l e t s t a t e i s supported by the l a c k of evidence f o r o b s e r v a t i o n of s i n g l e t s t a t e s i n NO^, ClO^ and NF^ above 17 eV. An in t e n s e peak i n the dimer P E spectrum at 19.41 eV and r i s i n g background make i t s d e f i n i t e assignment to SO^F suspect. 5.2.3.3 Comparisons w i t h Related Molecules The P E spectrum of ClO^F (Figure 28) i s expected to c l o s e l y approximate that of SO^F except f o r exchange i n t e r a c t i o n s and those s t a t e s i n v o l v i n g i o n i z a t i o n of an e l e c t r o n from an o r b i t a l c o n t a i n i n g a l a r g e c e n t r a l atom c o n t r i b u t i o n . I.P. c o r r e l a t i o n s are made w i t h the e n t i r e i s o e l e c t r o n i c s e r i e s ClO-jF, S O ^ , F 3PO and F 3NO, S0^~ and - 164 C10 4 (Figure 29). X-ray (ESCA) P E s p e c t r a of the valence I.P.'s of i s o e l e c t r o n i c c a t i o n s ClO^ and SO^ have been reporte d and assigned on the b a s i s of e x i s t i n g MO c a l c u l a t i o n s ' * " ^ and X-ray fluor e s c e n c e data (Table 10). ESCA s p e c t r a of s e v e r a l f l u o r o s u l p h a t e s a l t s are p r e s e n t l y being i n v e s t i g a t e d to a i d i n the i n t e r p r e t a t i o n of the SO^F r a d i c a l P E spectrum,(Figure 18). - 127 -LP CORRELATION DIAGRAM OF THE FLUOROSULPHATE FREE RADICAL AND RELATED COMPOUNDS 8H 10 < r — z LL1 5 a. ^2A 14 Z o M 16 Z o 18 20 dominant atom contribution to M O _bi_ b2 °2 nb. e _ • t o N & P Ol TT Oj s-o ,e F -°2_ a 2  r , J 2 - S - 0 Q] .a e / j i e character " ' / / \ / - \ » \ / some \ .• \ •central . \\ • / atom \ »\ \ Q2 w / — 2 L e / s / character °1 \ \ \ w \ e a i or ° e M O L E C U L E : S02F2 ^NO V A L E N C E e ' s : 32 SYMMETRY = Coy 32 C 3V F3PO 32 C 3 V C103F a o ; 32 C 3V 32 Td SO/ SO3F 32 Td 31 •^3V FIGURE 29 TABLE 10 S p e c i e s N ° 3 S ° 3 F S ° 3 F ( L i N 0 3 ) ( S 2 ° 6 F 2 ) ( K S 0 3 F ) T e c h n i q u e ESCA C a l c . PES ESCA R e f e r e n c e 187 187 T h i s T h i s work w o r k I.P. eV. 1 * 6 5 . 8 l a ! 1 2 . 8 5 * 7 . 5 6 . 0 l e " a . 2 2 4 . 6 6 . 4 j 1 4 . 0 6 ( 3 e ) 1 ' 8 , 2 7 1(14.54) ( 1 e ) * 7 . 5 7 . 3 4e« U ' 9 6 6 ° r 1 1 . 0 a . * 1 3 1 3 . 6 3 e ' j 1 ™ 1 a l ° r [ 1 5 . 6 ^ 1 6 1 4 . 8 l a 2 " j 1 ^ 1 ? j 1 7 . 2 6 ^ 2 8 1 6 . 4 4 a 1 ' 2 5 . 6 7 ^ 3 4 3 1 . 6 2 e ; L ' 3 1 . 2 SO. C IO . 4 4 ( L i 2 S 0 4 ) ( L i C 1 0 4 ) ESCA C a l c . C a l c . ESCA C a l c . 164 55 166 164 166 5 . 8 - ( 2 . 6 3 ) - ( 4 . 1 2 ) 6 . 3 6 . 4 8 7 . 7 - ( 4 . 8 2 ) - ( 3 . 3 1 ) 3 t 0 9 . 0 6 . 9 4 t 0 z n zn 7 . 7 0 . 1 2 - ( 2 . 5 3 ) e 9 . 0 7 . 7 8 e n n 1 1 . 4 3 . 7 2 5 . 8 8 2 t „ 1 3 . 4 1 6 . 8 9 t_ z zn 1 4 . 3 6 . 2 6 7 . 5 0 2 g 1 1 6 . 5 1 9 . 8 7 a± 2 5 . 3 1 7 . 2 4 t 2 2 7 . 0 2 8 . 0 8 l t 2 ( 2 9 . 0 ) 2 3 . 0 9 a x 3 4 . 4 3 7 . 9 5 l a x - 129 -COMPARISON OF PES AND ESCA VALENCE LP' FIGURE 18 - 130 -5 . 2 . 4 The Methyl R a d i c a l CH 3 P r e l i m i n a r y evidence f o r the pro d u c t i o n of the methyl r a d i c a l by p y r o l y s i s over a hot f i l a m e n t , ( s e e S e c t i o n 3 . 4 . 5 . 1 ) i s summarized below. Experimental and s p e c t r o s c o p i c values reported f o r the f i r s t I.P. of CH^ are l i s t e d i n Table 11 along w i t h values of the f i r s t I.P. s p i n -2 2 o r b i t components ( £3/2 ' ^ 1 / 2 ^ °^ u s e c * ^ o r c a l i b r a t i o n . The peak 2 observed to the high energy s i d e of the CH^I Ej_/2 peak i n Fi g u r e 7 i s 103 a t t r i b u t e d to one of I ( s p e c t r o s c o p i c I.P. 1 0 . 4 5 4 eV) or HI ( s p e c t r o -s c o p i c I.P. 1 0 . 3 9 eV) produced at the f i l a m e n t during p y r o l y s i s . Non-e q u i l i b r i u m thermal c o n d i t i o n s and magnetic inhomogeneity from the f i l a m e n t permitted only s i n g l e scans, and r e s u l t e d i n a h i g h n o i s e l e v e l . Two apparent peaks at 9 . 9 6 and 1 0 . 0 7 eV are observed. Using t h e i r energy s e p a r a t i o n (1160 + 40 cm "*"), the asymmetry to the low energy s i d e of the peak at 9 . 9 6 eV, and the i n d i c a t i o n of an antibonding v i b r a t i o n a l envelope (from the r e l a t i v e i n t e n s i t i e s of the two r e s o l v e d peaks), the evidence suggests e x i s t e n c e of an a d i a b a t i c v i b r a t i o n at 9 . 8 4 eV unresolved from the n o i s e . This i s i n e x c e l l e n t agreement w i t h 180 the spectroscop i c a d i a b a t i c I.P. of 9 . 8 4 3 eV. O J The l a ^ o r b i t a l i s 30 p r e d i c t e d to be non-bonding i n a planar CH^ molecule. P h o t o i o n i z a t i o n 174 179 s t u d i e s of Chupka and L i f s h i t z and Lossing e t a l . support Rydberg i n t e n s i t y d i s t r i b u t i o n s suggesting that the CH^ molecule and C H 3 + i o n are both planar or n e a r l y so. A p h o t o i o n i z a t i o n step at 1 0 . 1 8 5 eV observed by both Chupka and L i f s h i t z " * " ^ and E l d e r et a l . " * " ^ compares favourably w i t h the Rydberg band at 2740 cm ^ from the main band and p o s s i b l y represents 2v^ or more l i k e l y v | i n the i o n (Table 11 ) TABLE 11 R e f e r e n c e C H 3 I I . P . (eV) " i l l J l / 2 A d i a . I . P . ( e V ) M e t h y l R a d i c a l CH, V i b . F r e q . (cm ) 180 (Rydberg ) 11 (PES) 56 (PES) 30 (Rydberg ) 30 ( C H 3 + ) T h i s w o r k 178 ( C H 3 + ) 174 175 179 188 188 (CD 3 ) 9 . 5 3 8 9 . 5 5 9 . 5 0 1 0 . 1 6 5 1 0 . 1 6 1 0 . 1 3 9 . 8 4 3 9 . 8 3 ( 8 ) 9 . 8 2 5 9 . 8 2 9 . 8 4 2740 3256 3044 2153 (580) 390 o r 1360 1160 1526 617 463 3162 2381 1396 1026 i t—* CO r-> I - 132 -174 _ L An observed hot band, 565 + 80 cm below t h r e s h o l d and 710 cm below the s p e c t r o s c o p i c I.P. at 9.843 eV, was assigned to v^, (out-of-plane 30 bending). P r i c e e s t a b l i s h e s the e x i s t e n c e of superimposed on a p r o g r e s s i o n i n i n the f i r s t P E band of CH^ "*" as a r e s u l t of strong hydrogen r e p u l s i o n s i n the planar CH^ "*" c o n f i g u r a t i o n d u r i n g i n v e r s i o n . T h i s e f f e c t i s not expected to occur i n pl a n a r CH^ "*" and i s u , . , m + 186 not observed i n NH^ . T r a n s i t i o n s from the ground s t a t e of planar CF^ to non-planar CF^ + i n 820 cm "*" and 830 cm "*" have been observed i n photoionization"*"''^ and and e l e c t r o n i c absorption"*"^ s p e c t r a r e s p e c t i v e l y . These are assigned to which i s s i m i l a r to v^ ' i n i s o e l e c t r o n i c BF^ and t h a t observed f o r — 1 186 NH^ by PES (v^ = 950 cm ). The observed v i b r a t i o n a l i n t e r v a l f o r CH^, 1160 + 40 cm "*" (Table 11 ) suggests an assignment adopted by Herzberg:"^ v^ ' = 580 cm "*", = 1360 cm "*" ( l e s s favoured i s v^ ' = 580 cm ~ 390 cm "*" and v| = 2740 cm "*"). I t a l s o compares w e l l w i t h 178 the c a l c u l a t i o n of Von Burnau et a l . , i n d i c a t e s i t s assignment to v'2 i - n C^3+> and suggests i o n i z a t i o n from an antibonding o r b i t a l i n CH 3. 5.2.5 The B i s t r i f l u o r o n i t r o x i d e Free R a d i c a l (OF^^NO This r a d i c a l i s one of the s m a l l e r n i t r o x i d e compounds composed of two i d e n t i c a l l i g a n d s attached to the N-0 group t h a t has a s i n g l e unpaired e l e c t r o n . A c a l i b r a t e d spectrum of (CF^^NO appears i n Figure 23 and i n Appendix Figure A(6) along w i t h a s e r i e s of r e l a t e d R compounds. A systematic study of a s e r i e s of n i t r o x i d e s N-6 may R ' provide a meaningful c o r r e l a t i o n between o r b i t a l energies of s t a b l e - 133 -o p e n - s h e l l molecules. A study of t h i s system i s i n progress but i s not a stage to be f u l l y reported here. 5.2.6 T r a n s i e n t Species 3 5.2.6.1 Atomic Iodine I and Atomic Oxygen 0 ( P) A peak at 10.45 eV assigned to atomic i o d i n e ( s p e c t r o s c o p i c I.P. 10.454 eV) was produced during p y r o l y s i s of CF^I at 1140 °C and use of a helium c a r r i e r gas ( S e c t i o n 3.4.5.1 and Figure 7). Traces of atomic oxygen were detected during p r e l i m i n a r y 2450 Mcs microwave discharge experiments i n oxygen. A spectrum i s compared w i t h that 152 reported by Jonathan et a l . i n F i g u r e 30. 5.2.6.2 V i b r a t i o n a l l y "hot" CO and HF Discharge experiments of s e v e r a l f l u o r o c a r b o n compounds produced v i b r a t i o n a l l y e x c i t e d species and d e t e c t i o n of hot bands, see Appendix Figure A(9). 5.2.6.3 Bromine Monofluoride BrF The evidence f o r and the i n f o r m a t i o n d e r v i e d from the o b s e r v a t i o n of the f i r s t I.P. s p i n - o r b i t doublet of BrF i s reported f u l l y i n the d i s c u s s i o n of the diatomic i n t e r h a l o g e n s , S e c t i o n 4.2.4. I t i s mentioned here s o l e l y because of i t s p o s s i b l e d e s i g n a t i o n as a t r a n s i e n t s p e c i e s . 2450 Mcs DISCHARGE IN OXYGEN 9 7 6 3 1 Electron energy <eV> FIGURE 30 - 135 -5.2.6.4 Evidence f o r the P E Detection of Xenon Monofluoride XeF One views the e x i s t e n c e of such a species w i t h grave doubt mainly because of the i n s t a b i l i t y of xenon compounds i n g e n e r a l . I f t h i s r a d i c a l were to e x i s t i n the f r e e s t a t e one would a l s o p o s s i b l y 92 expect to observe I F , s p e c t r o s c o p i c a l l y observed by Durie only i n flames. Therefore the f o l l o w i n g i n f o r m a t i o n i s presented but no d e f i n i t e assignment i s made i n view of the nature of t h i s s p e c i e s . The v i b r a t i o n a l p r o g r e s s i o n i n question (Figure 20, Table 6 ) v = 664 + 40 cm 1 was observed i n the s p e c t r a of both XeF„ and XeF. — 2 4 181 182 and i n each case was extremely weak. Both XeF and KrF have been observed by ESR i n f r o z e n m a t r i c e s . V i b r a t i o n a l ground s t a t e 183 -1 -1 frequencies are c a l c u l a t e d to be 810 cm and 621 cm f o r KrF and K r F + r e s p e c t i v e l y and may be p a r t i a l l y s u b s t a n t i a t e d by f o r t u i t o u s agreement w i t h an observed value of 626 cm 1 a t t r i b u t e d to K r F + from 184 a study of K r S b F ^ * Although the v i b r a t i o n a l frequency u s u a l l y decreases toward the species of higher atomic number i n a s e r i e s (see the diatomic i n t e r h a l o g e n s , F i g u r e 17), the observed i n t e r v a l i s of the c o r r e c t order of magnitude. 181 From the ESR study, i t i s apparent t h a t XeF i s a o - e l e c t r o n r a d i c a l , i . e . , the unpaired e l e c t r o n occupies an antibonding a - o r b i t a l 181 composed of approximately 47% F^^ and 36% Xe 5p c h a r a c t e r : ( ° " 9 N + 0 S T 1 » A 1 ) 2 ( 7 R , T 1 , " -I , ) V , " T T . , T T „ . ) 4 2 p F 5 p X e 1 2 p F 5 p X e 1,2 2p F 5 p X e 3,4 2 p F 5 p X e 2 The P E band shape i s c o n s i s t e n t w i t h removal of an e l e c t r o n from a - 136 -weakly antibonding o r b i t a l . 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Robin, N.A. Keubler and C R . Brundle, I n t . Conference on E l e c t r o n Spectroscopy, Asilomar C a l i f o r n i a , U.S.A., Sept. 7-10, 1971, Se c t i o n I I I - 9 . 186. G.R. Branton, D . C F r o s t , F.G. H e r r i n g , C A . McDowell and I.A. Stenhouse, _3> 5 8 1 (1969). 187. M. Barber, J.A. Connor, I.H. H i l l i e r , and V.R. Saunders. I n t . Conf. on E l e c t r o n Spectros. Asilomar C a l i f . Sept. 7-11,1971, S e c t i o n V-13. 188. A. Snelson. J . Phys. Chem., _74, 537 (1970). - 148 -APPENDIX (A) References fo r papers p u b l i s h e d to date are presented i n t h i s s e c t i o n and are not el a b o r a t e d i n the body of the t h e s i s except f o r those concerning the NF^ and ClC^ r a d i c a l s . I l l u s t r a t i o n s of s p e c t r a not provided i n the pub l i s h e d t e x t s are i n c l u d e d here. 1. The V i b r a t i o n a l Constants of the Ground States of H 2 + , HD +, and D 2 +, from P h o t o e l e c t r o n S p e c t r o s c o p i c Measurements, Chem. Phys. L e t t e r s , 5_, 486 (1970). 2. The E l e c t r o n i c Levels of the Methyl Amines by P h o t o e l e c t r o n Spectroscopy and an INDO C a l c u l a t i o n , Can. J . Chem. 4^ _, 1135 (1971). Figure A ( l ) , ( H y CH 3NH 2 > ( C H ^ N H , ( C H ^ N ) . 3. The I o n i z a t i o n P o t e n t i a l s of the Difluoroamino R a d i c a l by P h o t o e l e c t r o n Spectroscopy and INDO C a l c u l a t i o n s , J . Chem. Phys. 54_, 1872 (1971). Table A ( l ) , ( N F 2 ) . 4. P h o t o e l e c t r o n Spectra of the Halogens, J . Chem. Phys. 54, 2651 (1971). 5. The Ph o t o e l e c t r o n Spectrum of the Free R a d i c a l C h l o r i n e  D i o x i d e , Chem. Phys. L e t t e r s , 10, 345 (1971). See Figure A ( 8 ) , ( c i o 2 ) . 6. The Ph o t o e l e c t r o n Spectra of F 20 and C1 20, J . Chem. Phys. _5_5, 2820 (1971). Figure A ( 2 ) , ( F 2 0 , C l 2 0 , H 20). Table A ( 3 ) , ( F 2 0 , C1 20, H 20). (B) Papers presented at conferences, but not in c l u d e d i n the t h e s i s because of d u p l i c a t i o n of m a t e r i a l are l i s t e d , although a few of the appro p r i a t e diagrams are i n c l u d e d . - 149 -7. Recent Experiments i n P h o t o e l e c t r o n Spectrometry, S i x t h I n t e r n a t i o n a l Conference on the Phy s i c s on E l e c t r o n i c and Atomic C o l l i s i o n s , M.I.T. Cambridge, Mass., U.S.A., J u l y 28-August 2, 1969, p. 194. 8 . P h o t o e l e c t r o n Spectra and I o n i z a t i o n P o t e n t i a l s of H,,, HD, T>2t and the Halogens, Eighteenth Annual Conference on Mass Spectrometry and A l l i e d T opics, San F r a n c i s c o , C a l i f o r n i a , June 14-19, 1970, S e c t i o n Q 4 , p. B402. 9. P h o t o e l e c t r o n Spectra of Some Dihalocompounds, I n t e r n a t i o n a l Conference on E l e c t r o n Spectroscopy, Asilomar, P a c i f i c Grove, C a l i f o r n i a , U.S.A., Sept. 6-10, 1971. Figure A ( 3 ) , ( I . P . C o r r e l a t i o n Diagram of S0F 2, S O ^ , S i H ^ , C H ^ , H 2CCF 2, C1 2CCF 2, F 2 0 , and F 2C0). Figure A ( 4 ) , ( I . P . C o r r e l a t i o n Diagram of S0C1 2, S 0 2 C 1 2 , S i H 2 C l 2 , CH 2C1 2, C1 20, C1 2C0, C1 2CS, C1 2CCH 2, C1 2CCF 2, ci s - C l C C H C l , and trans-ClCCHCl). (C) T a b u l a t i o n of i o n i z a t i o n p o t e n t i a l s and i l l u s t r a t i o n s of photo-e l e c t r o n s p e c t r a of s e v e r a l s e r i e s of compounds s t u d i e d to date, but unpublished, are given f o r general r e f e r e n c e . No a n a l y s i s of the s p e c t r a i s presented but w i l l be a v a i l a b l e i n f u t u r e p u b l i c a t i o n s . 10. Hydrogen and Deuterium H a l i d e s , Figure A(5) (HF, HCl and DC1, HBr and DBr, HI and DI) . 11. B i s - t r i f l u o r o n i t r o x i d e Free R a d i c a l and Related Compounds, Figure A(6) ( ( C F ^ N O , (CF^NOH, ( C F ^ C O , ( C H ^ C O , ( C H ^ N H , CF 3N0, CF 3CN, CF 3CCH, C^CN, CH^CH, CH^HO) . Table A(4) ( C F ^ N O , (CF 3) 2N0H, (CF 3) 2CO, CF 3N0). Table A ( 5 ) , (CH 3CCH, CF 3CCH, CH3CN, CF CN, CF CCCF ). - 150 -12. HNSO, DNSO, S0 2 and Related Compounds, Figure A(7) (SC>2, HNSO, DNSO, HNCO, HNCS, HN^) . Table A(6) , (HNSO, DNSO, SO^. Table A ( 7 ) , (HNSO and S 0 2 , HNCO and C0 2, HNCS and COS, HN^ and N 20, (see Figure A(7) above)). 13. Small Polyatomic Molecules, Figure A ( 8 ) , (N0 2, HNSO, N0C1, ( C F 2 ) , NF 2, C10 2, 0C1 2, 0 F 2 ) . 14. V i b r a t i o n a l l y Hot Discharge Products. Figure A ( 9 ) , (CF^NO y i e l d i n g "hot" CO, plus some NO, CF^ and unknown; CH 2F 2 y i e l d i n g "hot" HF). 15. T r i f l u o r o m e t h y l Compounds. Figure A(10) ( C F 3 C E C C F 3 , and CF 3CN), (see Table A(5) above)). 16. Some S i l i c o n , Germanium, and T i n Complexes. Figure A(11), ( ( M e 3 S i ) 2 0 , and (Me 3Ge) 20; ( M e 3 S i ) 2 S and ( M e ^ e ^ S ; Me 3Si-SMe and Me0Sn-SMe; (Me„Si0) o, ( M e o S i 0 ) . , ( M e o S i 0 ) c and (Me^SiO)-). 17. Some S u b s t i t u t e d Benzenes, Methyl S i l i c o n and T i n Compounds. Figure A(12), ( C H c S i H 0 and C,.HrCH • CH CNH 0; CH CSH and C.Hc0H; D O 0 D J J O J £ D O O J M e 0 S i , , Me.Si and Me.Sn). R E . SPECTRA or THI M E T H Y L A M I N E S , H Y D R O G E N , AND A R G O N IONIZATION POTINTIAl UV> FIGURE Al - 152 -TABLE A ( l ) NF, P.E. Designation Band I.P. (+ 0.02 eV) INDO c a l c . Obs. V i b . Freq. (cm" 1) 3rd 4 t h A d i a b a t i c 1. 11.62 2. 11.79 V e r t i c a l 3. 12.10 4. 12.25 5. 12.40 6. 12.56 A d i a b a t i c 1. 14.05 2. 14.12 3. 14.20 4. 14.27 5. 14.34 6. 14.40 7. 14.47 8. 14.54 V e r t i c a l 9. 14.60 10. 14.66 D i s c o n t i n u i t y 11. 14.73 12. 14.79 13. 14.85 14. 14.92 15. 14.99 D i s s o c i a t i o n ? 16. 15.03 A d i a b a t i c (15.4) V e r t i c a l 16.38 V e r t i c a l 17.6 B x 11.95 13.9 B± 15.9 A 2 17.6 1250 + 40 (1013 + 40)? 520 + 40 - 153 -TABLE A(2) CIO 2 P.E. Designation I.P. V i b . Freq. (cm 1 ) Band (+ 0.02 eV) 1st A d i a b a t i c 1. 10.36 V e r t i c a l 2. 10.48 980 + 40 3. 10.60 4. 10.72 2nd A d i a b a t i c 12.32 V e r t i c a l 12.94 (Shoulder)? (13.40) 3rd A d i a b a t i c 1. 15.27 2. 15.36 V e r t i c a l 3. 15.45 725 + 40 4. 15.54 5. 15.64 6. 15.73 4th A d i a b a t i c 16.25 V e r t i c a l 17.50 5th A d i a b a t i c 17.7 V e r t i c a l 17.95 6th V e r t i c a l 19.36 R E . SPECTRA OF DIHYDROGEN AND DIHALOGEN MONOXIDES - 155 -TABLE A(3) F 20 c i 2 o H 20 CNDO/2 INDO CNDO/2 10.84 10.94 10.94 10.98 14.4 11.02 8.9 11.06 11.10 11.15 11.19 11.23 11.27 11.31 11.34 (670,300) 12.02 (12.13) (12.16) (12.19) (12.27) (12.31) (12.34) 15.8 12.37 10.4 (12.41) (12.45) (12.49) (12.53) (300) 12.58 16.7 12.65 10.7 (12^74) 18.1 12.79 12.5 12.84 12.90 ? (500)? (15.48) 19.9 (15.90) 16.9 (16.16) 20.2 (16.65) 18.1 (17.16) 17.68 20.3 20.64 > 21 1st A d i a b a t i c V e r t i c a l 2nd A d i a b a t i c V e r t i c a l 3rd 4th A d i a b a t i c V e r t i c a l A d i a b a t i c V e r t i c a l 5th A d i a b a t i c 6th 6 th 7 th A d i a b a t i c V e r t i c a l A d i a b a t i c V e r t i c a l A d i a b a t i c V e r t i c a l 13.12 13.25 13.37 13.50 13.62 13.75 (1005) 15.86 16.22 16.45 17.91 18.74 19.17 19.42-19.66 (20.9)? 15.4 16.6 17.3 19.1 20.5 20.9 12.62 (3200) (1380) 14.7 (975) 18.6 (2990) (1610) - 156 -ION IZAT ION E N E R G Y C O R R E L A T I O N D I A G R A M S (-) 20-CI I -S0 2 C I 2 C H 2 C ! 2 CI 2C0 C l j C C H j CIHCCHCI trans-SOCI2 S1H2CI2 C l 20 C I 2 C S CI2CCFJ CIHCCHCI -1 1 1 1 1 1 - -f i — -I 1- 1 h -1-- —I 1 1 H 1 8 -16-14 -12-10-8-1 °-a -b, _ . '. /_2_ "~bT ——N.> C l - b 2 — - - . ^ t \ b, \ b, ... O / 2 b. _ b, FIGURE A 4 w 20 18 16 14 12 10 S0F 2 S 0 2 F 2 S i H 2 F 2 C H J F A r+zCCE C ^ C C R E.0 F - C O —I I 1 1 i 1 1 1 i i 1 1 I I (= r-—— a" 0' \ „ . cij b? _2_—-a" ' 1 ' b, o' - . 0 2 b, b. / b. b, / FIGURE A3 - 157 -R E . SPECTRA OF THE HYDROGEN A N D DEUTERIUM HALIDES i-* is is rr is~ io M i i ~n K is It IONIZATION P O T E N T I A L (eV) IONIZATION P O T E N T I A L (eV) FIGURE A5 - 158 -R E . SPECTRA OF THE BISTRIFLUORONITROXIDE FREE RADICAL AND RELATED COMPOUNDS O Xe Xe ( C F 3 ) N O H , (CH^CO IP. (Vert.) (eV) 10 12 14 16 18 20 12 14 16 18 20 12 14 16 18 20 1. 1072 a. 12.04 3. 14.67 4. 16.69 5. 17.34 6. 18.17 1. 11.88 2. 13.20 3. 16.22 4. 16.75 5. 17.44 6. 17.90 1. 12.08 2. 15.98 3. 16.54 4. 17.11 5. 17.86 6. 18.45 CF,C-=CH IP. (Vert.) (eV) 7. 18.28 12 14 16 18 C F 3 N O 11.07 12.80 3. 16J2 4. 17.1 5. 17.5 6. 18J9 C F 3 C N I 1. 2. 3. 4. 14.25 16.49 16.86 1846 14 16 18 20 C H X N C H 3 C i C H 12 14 16 18 10 12 14 16 18 ( C r O N H 8 10 12 14 16 18 20 10 12 16 18 20 IONIZATION POTENTIAL leV) FIGURE A6 IONIZATION POTENTIAL (eV) PU8LISHED; D.C.FROST ET J*. UNPUBLISHED ; u O W. R.LEEOER, PhD. THESIS - 159 -TABLE A(4) (CF 3) 2NO (CF 3) 2NOH (CF 3> ^0 CF NO CF^O (CNDO) 1st A d i a . 9.98 11.18 11.56 10.3 Vert. 10.72 11.80 12.08 11.07 7.8 2nd A d i a . 11.63 12.82 12.1 Ve r t . 12.04 13.20 12.80 14.2 (12.84) (13.79) 3rd A d i a . 13.87 V e r t . 14.67 4th A d i a . 15.68 15.20 15.08 15.6 (16.18) (15.81) Ve r t . 16.69 16.22 15.98 16.12 15.0 5th Adia. V e r t . 17.34 16.75 16.54 17.1 16.3 6th A d i a . 17.65 (17.10) V e r t . 18.17 17.44 17.11 17.5 16.4 7th A d i a . — — (17.46) V e r t . 19.16 17.90 17.86 18.19 17.2 (?) (18.04) 8th A d i a . V e r t . 19.82 19.98 18.45 (19.39) - 160 -TABLE A(5) C H 3 C H C H C F 3 C = C H ' C H 3 C N C H 3 C N C F 3 C E C C F 3 1st Adia. 10.37 11.83 12.12 Vert. V i b . Freq. 2nd Adia. Ve r t . 3rd. 4 th 5 th 6th 7th Adia. V e r t . A d i a . Vert. A d i a . Ve r t . Adia. V e r t . V e r t . 10.37 13.93 14.14 15.13 15.5 16.97 17.2 12.12 (2015) (1080) (1905) 13.76 13.76 (1090) (1615) 14.67 15.22 (15.62) 15.96 1 7 . 1 3 1 7 . 1 3 (1250) C>750) (1075) 1 8 . 0 7 1 8 . 0 7 1 8 . 2 8 12.18 13.11 13.11 15.12 16.98 17.4 13.71(0) 13.82(6) 13.93(8) 05(9) 15(9) 24(9) 12.31(0) 14 14 14 (650) 15.87(2) 16.43(0) 16.48(8) 16.58(1) 16.68(2) (945) 16.02(7) 16.85(6) (16.99(2)) (--) 17.72(4) 17.82(9) 17.91(8) 18.00(0) 18.06(2) 18.14(7) (770) 12.83(5) 12.96(8) 13.22(6) 13.48(4) (2100) (1075)? 14.88 15.48 (—) 15.7 (990) 16.42 (—) 17.49 (1250) (735)? R E . SPECTRA OF SOME SMALL POLYATOMIC MOLECULES IONIZATION POTENTIAL (ev) FIGURE A7 TABLE A ( 6 ) HNSO DNSO ( S 0 2 ) S 0 2 I . P . V i b . F r e q . I . P . V i b . F r e q . I . P . V i b . F r e q . I . P . V i b . F r e q . (+0 .02 eV) (+40 c m " 1 ) (+0 .02eV) (+40 c m " l ) ( + 0 . 0 2 eV) (+40 c m - 1 ) V e r t i c a l 1 1 . 6 0 ( 0 ) V e r t i c a l V e r t i c a l 1 1 . 6 5 ( 4 ) 1 1 . 7 4 ( 5 ) 1 1 . 7 8 ( 8 ) 1 1 . 8 6 ( 3 ) 1 1 . 9 3 ( 9 ) 1 2 . 0 1 ( 4 ) 1 2 . 0 6 ( 8 ) 1 2 . 2 7 ( 2 ) 1 2 . 3 6 ( 9 ) 1 2 . 4 5 ( 5 ) 1 2 . 5 4 ( 6 ) 1 2 . 6 4 ( 3 ) 1 2 . 7 4 ( 5 ) ( 1 4 . 7 6 ( 0 ) ) 1 4 . 8 7 ( 9 ) 1 4 . 9 9 ( 1 ) 1 5 . 1 1 ( 5 ) 1 5 . 2 1 ( 7 ) 1 5 . 3 2 ( 5 ) ( 1 5 . 4 4 ( 8 ) ) ( 1 5 . 5 9 ( 3 ) ) ( 1 6 . 2 7 ( 6 ) ) 1 6 . 4 5 ( 3 ) 1 6 . 5 6 ( 1 ) 1 6 . 6 9 ( 5 ) (1115) (1115) (460) (790) (910) (1080) ( 1 1 . 4 9 0 ) ( 1 1 . 5 5 3 ) 1 1 . 6 0 ( 0 ) 1 1 . 6 7 ( 2 ) 1 1 . 7 4 ( 1 ) 1 1 . 8 0 ( 7 ) 1 1 . 8 7 ( 6 ) 1 1 . 9 4 ( 2 ) 1 2 . 0 1 ( 1 ) 1 2 . 2 6 1 2 . 5 2 ( 6 ) 1 4 . 8 7 ( 7 ) 1 4 . 9 5 ( 3 ) 1 4 . 9 8 ( 9 ) 1 5 . 0 4 ( 9 ) 1 5 . 1 0 ( 1 ) 1 5 . 1 6 ( 1 ) 1 5 . 2 1 ( 0 ) ( 1 5 . 3 0 ( 3 ) ) 1 6 . 2 9 ( 9 ) 1 6 . 4 2 ( 7 ) 1 6 . 5 3 ( 3 ) 1 6 . 5 6 ( 2 ) ( 1 6 . 6 6 ( 8 ) ) (1090) (1090) (550) ( 9 0 5 ) ? (555) (905) (950) 1 2 . 2 9 ( 0 ) 1 2 . 5 0 ( 2 ) (400) 1 2 . 9 8 ( 0 ) 1 3 . 2 ( 3 8 0 ) ( 9 3 0 ) 1 3 . 5 (500) 1 5 . 9 7 ( 2 ) 1 6 . 3 3 ( 5 ) 1 2 . 3 2 ( 9 ) 1 2 . 3 7 ( 8 ) 1 2 . 4 2 ( 7 ) 1 2 . 4 7 ( 3 ) 1 2 . 5 2 ( 0 ) 1 2 . 5 6 ( 4 ) 1 2 . 6 1 ( 4 ) 1 2 . 6 7 ( 1 ) (385) to 1 3 . 2 3 ( 3 )  1 3 . 4 7 ( 6 ) (more t h a n 30 i n d i v i d u a l p e a k s o b s e r v e d ) TABLE A ( 6 ) C o n t i n u e d HNSO DNSO ( S 0 2 ) S 0 2 I . P . V i b . F r e g . I . P . V i b . F r e g . I . P . V i b . F r e q . I . P . V i b . F r e q . (+0 .02 eV) (+40 cm-1) (+0 .02 eV) (+40 c i r r i ) (+0 .02 eV) (+40 c m - ! ) V e r t i c a l 1 6 . 6 7 ( 4 ) (880) 1 6 . 6 9 ( 1 ) (1075) 1 6 . 6 ( 8 5 0 ) ( 1 2 4 0 ) 1 6 . 6 5 1 6 . 7 8 ( 1 ) 1 6 . 7 8 ( 0 ) 1 6 . 6 (900) 1 6 . 6 5 1 6 . 8 8 ( 9 ) ~ (18 p e a k s 1 6 . 9 2 ( 6 ) , 1 6 . 9 6 ( 2 ) o b s e r v e d ) 1 7 . 0 3 ( 9 ) 1 7 . 1 7 ( 3 ) (1080) - 164 -TABLE A ( 7 ) V e r t i c a l I . P . ' s and V i b r a t i o n a l F r e q u e n c i e s HNSO SO, HNCO CO, HNCS COS HN, N 2 0 1 s t 2nd 3 r d 4 t h 1 1 . 6 0 (1115) 1 2 . 5 5 (790) 1 5 . 1 2 (910) ( 1 0 8 0 ) ? 1 6 . 6 7 (880) ( 1 0 8 0 ) ? 1 2 . 5 0 (400) 1 3 . 2 (380) ( 9 3 0 ) ? 1 3 . 5 (500) 1 6 . 6 (850) (1240) 5 t h 1 6 . 6 (900) A d i a b a t i c I . P . ' s 1 1 . 6 0 (1050) (2020) ( 3 2 2 2 ) 1 2 . 3 9 (565) 1 5 . 5 4 (1113) ( 1 5 . 2 ) ? (450) 1 3 . 7 9 (2110) (1420) 1 7 . 6 0 (1100) 1 8 . 0 8 (1270) ( 1 4 0 0 ) ? 1 9 . 4 0 (1390) (1470) 1 0 . 0 5 1 0 . 3 5 (655) 1 3 . 3 3 (856) ( 1 5 . 3 ) (445) 1 1 . 1 9 (650) (2000) 1 5 . 5 3 (790) ( 2 0 5 0 ) 1 6 . 0 4 1 7 . 9 6 (970) ( 4 1 0 ) ? (2170) 1 0 . 7 4 1 2 . 2 (445) 1 5 . 4 7 1 6 . 8 1 2 . 8 9 ( 1 1 4 0 ) (1750) 1 6 . 3 9 (1350) ( 6 0 0 ) ? ( 2 4 6 0 ) 1 8 . 2 4 ( 9 0 0 ) ? (2300) 2 0 . 1 1 (1280) (2300) G r o u n d S t a t e V i b r a t i o n a l F r e q u e n c i e s 1 1 5 1 3 5 3 1 1 3 8 8 859 518 2274 667 520 1362 1327 2349 2062 797 816 1285 589 2224 1152 '6 R E . SPECTRA OF S O M E S M A L L POLYATOMIC M O L E C U L E S 0 2 4 NOa {)7 *Uctrontj HNSO |]£ •Uctron»j NOCI f 18 eleitronf) CF, 1^8 •UcfroniJ NF, |l9 •l«<troni) CIO, 119 «Uct rons j O C I , OF, (aO •l>c>rom}| UL. . IH111. , i l prodlcted .'.', i: predicted lli.l,. ill .1 II, ,11,1,!,. / A . . . . ill,, ,llll>. LL. / ,ii„ J /V\. 10 12 14 16 18 20 I O N I Z A T I O N P O T E N T I A L (ev) FIGURE A8 10 12 14 16 18 20 IONIZATION POTENTIAL (eV> ON Cn - 166 -RE. SPECTRA OF SOME MICROWAVE DISCHARGE SPECIES 1Z07 12.31 12.55 13.60 13 1Z77 13j00  J Vi 7 8 Ice 14.01 1 brationally Hot arbon Monoxide '•  [ CF3NO D I S C H A R G E P R O D U C T S 10 12 14 16 18 20 [ Vibrationally Hot ;15J9 1 15.77 \ Hydrogen Fluoride 16J5 1631 CH 2 F 2 D I S C H A R G P R O D U C T S 16 18 20 CH 2F 2 12 14 16 18 20 IONIZATION POTENTIAL ( « V ) IONIZATION POTENTIAL (eV) F I G U R E A 9 - 167 -R E . SPECTRAL SIMILARITIES IN TRIPLE BONDED TRIFLUOROMETHYL COMPOUNDS IP. (Vert.) Vib. Freq. (eV) (em-') 1. 12.83 2100 2. 15.48 3. 15.7 990 4. 16.42 5. 17.49 1250 12 14 16 18 20 IP. (Vert.) Vlb. Freq. (eV) (em"') 1. 14.25 650 2. 16.49 945 3. 16.86 4. 18.06 770 14 16 18 20 IONIZATION POTENTIAL (eV) FIGURE A10 - 168 -R E . SPECTRA or METHYL SILOXANE POLYMER COMPLEXES AND RELATED COMPOUNDS IP. ( V « t . ) (aV) (MeaSi)<0 , 9. 9.90 2. 10.8 3. 12.5 4. U.4 16 18 ( M e 2 S i O ) 3 10 12 14 16 18 20 1. 9.77 2. 10.12 3. 11.02 4. 11.84 5. 14.21 6. ^ 7. 15.90 17.72 10 12 U 16 '8 20 ( M e 3 G e ) 2 S 10 12 14 16 18 20 M e . S i - S M e 1. 9.9 2. 10.8 3. 12.4 4. 13.5 5. 14.37 10 12 14 16 IB 20 IONIZATION POTENTIAL l » V ) IONIZATION POTENTIAL UV ) F I G U R E A l l - 169 -FIGURE A12 

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