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Unconventional photoelectron spectroscopic studies : PES of some transient species and reactive molecules Lee, Shuit-Tong 1974

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UNCONVENTIONAL PHOTOELECTRON SPECTROSCOPIC STUDIES: PES OF SOME TRANSIENT SPECIES AND REACTIVE MOLECULES by SHUIT-TONG LEE B . S c , The Chinese University of Hong Kong, 1969 M.Sc,, University of Rochester, New York, 1972 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY 1n the Department of CHEMISTRY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October, 1974 In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l m e n t o f the r equ i r emen ts f o r an advanced degree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s tudy . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y pu rposes may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . It i s u n d e r s t o o d tha t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l owed w i thou t my w r i t t e n p e r m i s s i o n . The U n i v e r s i t y o f B r i t i s h Co lumbia Vancouver 8, Canada Department o f - i -ABSTRACT A fast pumping, v e r s a t i l e photoelectron spectrometer su i table for the study of t rans ient species and react ive molecules has been designed, constructed and tes ted . The photoelectron spectra of seventeen such compounds (CS, SO, S 2 0 , HN-NH, DN-MD, CHgN = NH, HCP, OCSe, SCSe, CSe 2 > Og, FNO, CINO, BrNO, HMOg, C1M0 2 and FNOg) have been obtained. The indiv idual spectra have been assigned by using information supplied from v ibrat iona l ana lys i s , by comparison with mole-cular o rb i ta l ca lcu lat ions and with related molecules. The experimental Franck-Condon factors have been u t i l i z e d to e s t i -mate the geometries of the ion ic s ta tes , and the procedure has been found to be quite r e l i a b l e . The study has shown the potential of photoelectron spectroscopy in providing i n f o r -mation on the e lec t ron ic structure of t rans ient species and react ive molecules, information that is d i f f i c u l t to obtain by other spectroscopic techniques. - i i T A B L E OF C O N T E N T S P a g e C H A P T E R I : I N T R O D U C T I O N .. 1 CHAPTER I I : THEORY OF 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 ... 6 2.1 C h a r a c t e r i s t i c o f a P h o t o e l e c t r o n S p e c t r u m 6 2.2 T h e I n t e r p r e t a t i o n o f M o l e c u l a r PE S p e c t r a 9 2.2.1 A B r i e f S u r v e y o f A s s i g n m e n t C r i t e r i a 9 2.2.2 K o o p m a n s ' T h e o r e m a n d t h e C a l c u l a t i o n o f V e r t i c a l I P ' s ... 12 ( a ) K o o p m a n s ' T h e o r e m a n d i t s I m p l i c a t i o n s 12 ( b ) A p p l i c a t i o n o f K o o p m a n s ' T h e o r e m i n PES 16 ( c ) O t h e r M e t h o d s f o r C a l c u l a t i n g V e r t i c a l I P ' s 18 ( d ) O p e n S h e l l M o l e c u l e s 19 2 . 2 . 3 F r a n c k - C o n d o n P r i n c i p l e i n PES . 21 ( a ) Wave M e c h a n i c a l F o r m u l a t i o n o f t h e F r a n c k - C o n d o n ( F C ) P r i n c i -p l e 21 ( b ) Q u a l i t a t i v e A p p l i c a t i o n o f t h e FC P r i n c i p l e i n PES 24 ( c ) Q u a n t i t a t i v e A p p l i c a t i o n o f t h e FC P r i n c i p l e i n PES 26 2.2.4 S e l e c t i o n R u l e s i n PES 33 ( a ) S e l e c t i o n R u l e s i n E l e c t r o n i c E x c i t a t i o n s , 33 ( b ) S e l e c t i o n R u l e s i n V i b r a t i o n a l E x c i t a t i o n s 3 6 - i i i -P a g e 2.3 L i n e w i d t h s a n d R e s o l u t i o n i n PES 37 C H A P T E R I I I : THE S P E C T R O M E T E R AMD PERFORMANCE 42 3.1 I n t r o d u c t i o n , 42 3.2 G e n e r a l D e s i g n C o n s i d e r a t i o n s 42 3.3 T h e S p e c t r o m e t e r 45 3.3.1 T h e V a c u u m S y s t e m 4 5 3.3.2 T h e E l e c t r o n E n e r g y A n a l y s e r .... 4 9 3 . 3 . 3 T h e L i g h t S o u r c e U n i t 54 3.3.4 T h e S a m p l e S y s t e m 57 ( a ) G e n e r a l S a m p l e I n l e t S y s t e m 57 (b) D i r e c t I n l e t s f o r R e a c t i v e M o l e c u l e s 57 ( c ) M i c r o w a v e D i s c h a r g e U n i t f o r P r o d u c i n g T r a n s i e n t s 58 ( d ) H i g h T e m p e r a t u r e P y r o l y s i s U n i t f o r P r o d u c i n g T r a n s i e n t s 61 ( e ) V a r i a b l e T e m p e r a t u r e U n i t f o r E q u i l i b r i u m S t u d i e s 53 3 . 3 . 5 T h e S c a n n i n g a n d D e t e c t i o n S y s t e m 55 3.4 O p e r a t i o n a n d P e r f o r m a n c e o f t h e S p e c t r o m e t e r 67 C H A P T E R I V : 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 OF SOME T R A N S I E N T S P E C I E S 7 6 4.1 C a r b o n M o n o s u l f i d e ( C S ) 7 6 4.1.1 I n t r o d u c t i o n 7 6 4.1.2 E x p e r i m e n t a l 7 7 4.1.3 R e s u l t s a n d D i s c u s s i o n 7 8 4.2 S u l f u r M o n o x i d e ( S O ) 8 4 - i v -P a g e 4.2.1 I n t r o d u c t i o n 84 4 . 2 . 2 E x p e r i m e n t a l , 8 5 4 . 2 . 3 R e s u l t s a n d D i s c u s s i o n 85 4.3 D i s u l f u r M o n o x i d e ( S 2 0 ) 94 4.3.1 I n t r o d u c t i o n 94 4.3.2 E x p e r i m e n t a l 95 4 . 3 . 3 R e s u l t s a n d D i s c u s s i o n 96 4.4 D i a z e n e (HN = NH) a n d D i a z e n e - d 2 • (DN = ND) .. 1 0 4 4.4.1 I n t r o d u c t i o n 104 4 . 4 . 2 E x p e r i m e n t a l 1 0 5 4 . 4 . 3 R e s u l t s 1 0 7 4.4.4 D i s c u s s i o n 1 1 8 4.5 T r a n s - m e t h y l d i a z e n e (CH^N = MH) 1 2 3 CHAPTER V: P H 0 T 0 E L E C T R 0 N S P E C T R O S C O P Y OF SOME R E A C T I V E M O L E C U L E S 1 2 8 5.1 M e t h i o p h o s p h i d e ( H C P ) 1 2 8 5.1 .1 I n t r o d u c t i o n 1 2 8 5.1.2 E x p e r i m e n t a l 1 2 3 5.1.3 R e s u l t s a n d D i s c u s s i o n 1 3 0 5.2 C S e 2 , OCSe a n d S C S e 1 3 3 5.2.1 I n t r o d u c t i o n 1 3 3 5.2.2 E x p e r i m e n t a l 1 3 5 5.2.3 R e s u l t s 1 3 6 5.2.4 D i s c u s s i o n 1 4 7 5.3 O z o n e ( 0 , ) 1 6 3 - V -P a g e 5.3.1 I n t r o d u c t i o n 1 6 3 5.3.2 E x p e r i m e n t a l • • « • 5 . 3 . 3 R e s u l t s a n d D i s c u s s i o n 164 5.4 M i t r o s y l H a l i d e s ( X N O , X = F , C l , B r ) ... 174 5.4.1 I n t r o d u c t i o n 174 5.4.2 E x p e r i m e n t a l 1 7 5 5.4.3 R e s u l t s 175 5.4.4 D i s c u s s i o n 1 7 9 5.5 N i t r i c A c i d (HNO,) a n d M i t r y l H a l i d e s ( X N 0 2 , X = F , C l f 184 5.5.1 I n t r o d u c t i o n 1 8 4 5.5.2 E x p e r i m e n t a l 1 8 5 5.5.3 R e s u l t s 1 8 5 5.5.4 D i s c u s s i o n 1 9 3 C H A P T E R V I : C O N C L U S I O N 202 R E F E R E N C E S 2 0 6 A P P E N D I X 221 - vi -r L I S T OF F I G U R E S F i g u r e P a g e 1. C o r r e l a t i o n B e t v / e e n t h e F r a n c k - C o n d o n P r i n c i p l e a n d t h e S h a p e o f PE B a n d s f o r t h e R e m o v a l o f E l e c t r o n s f r o m M o l e c u l a r O r b i t a l s o f D i f f e r e n t B o n d i n g C h a r a c t e r 25 2. D o p p l e r B r o a d e n i n g ( a ) As a F u n c t i o n o f E l e c t r o n K i n e t i c E n e r g y ( E . . ) f o r D i f f e r e n t M o l e c u l a r W e i g h t s ( M ) ; ( b ) As a F u n c t i o n o f T e m p e r a t u r e f o r D i f f e r e n t E k i /M V a l u e s 39 3. P l a t e 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 4 6 4. V e r t i c a l C r o s s - S e c t i o n o f t h e S p e c t r o m e t e r T h r o u g h t h e E n t r a n c e a n d E x i t A p e r t u r e s o f t h e E l e c t r o n E n e r g y A n a l y z e r 4 7 5. P l a t e o f t h e S p e c t r o m e t e r S h o w i n g t h e G e n e r a l S a m p l e I n l e t s , t h e P u m p i n g P o r t s f o r t h e I o n i z a t i o n C h a m b e r , t h e A n a l y z e r a n d t h e E l e c t r o n M u l t i p l i e r .... 48 6. S c h e m a t i c D i a g r a m o f 1 8 0 ° 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 A n a l y z e r U n i t 51 7. V e r t i c a l C r o s s - S e c t i o n o f t h e L i g h t S o u r c e U n i t 56 8. P l a t e o f t h e K e l - F a n d P y r e x D i r e c t S a m p l e I n l e t s 59 9. P l a t e o f t h e 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 60 1 0 . P l a t e o f t h e H i g h T e m p e r a t u r e P y r o l y s i s U n i t .... 62 1 1 . P l a t e o f t h e V a r i a b l e T e m p e r a t u r e U n i t 64 1 2 . D i a g r a m s o f t h e C o n t r o l C i r c u i t r y 66 1 3 . PE S p e c t r a o f O x y g e n R e c o r d e d b y D i f f e r e n t S c a n n i n g M o d e s 69 1 4 . PE S p e c t r a o f ( a ) A t o m i c H y d r o g e n a n d ( b ) A t o m i c N i t r o g e n a n d V i b r a t i o n a l E x c i t e d N i t r o g e n 73 - v i i -Fi gure Page 15. PE Spectra of Atomic Fluorine 74 15. PE Spectrum of 0 2 ^ A q ^ a n d A t o m i c ° x y s e n •• 7 5 17. PE Spectra of (a) CS ? and (b) The Products of a MW Discharge in CS 2 7 9 18. The Individual PE Bands of CS 82 19. PE Spectra of SO in the Presence of S0 9 and S,,0 . 86 20. The F i r s t PE Band of SO at High Resolution 88 21. Parts of the PE Spectra of S0 ? (A ) and the Products of a MW Discharge in S0 2 (B) at High Resolution . 91 22. The Second, Third and Fourth PE Bands of SO at high Resolution , 93* 23. PE Spectra of (a) S0 o , (b) S0C1 0 After Being Passed Over Hot Ag2S and (cf S0C1 2 . . . f 97 24. Corre lat ion Diaaram for the IP's of S0 o , S o 0 , NSC1 and NSF : ?...? 100 25. The F i r s t , Second and Third PE Bands of S 20 102 26. PE Spectra of (a) NpH-, (b) Vo la t i l e Products of a MW Discharqe in N?H», (c) , (d) and (e) Mixture (b) After Addition Of Increasing Amount of HCl . . 108 27. PE Spectra of (a) N~D. and (b) The Products of a MW Discharge in NgD^ 7 . 109 28. PE Spectra of the F i rs t Bands of N ? D ? and UD~ After Addition of Increasing Amount Of DC1 110 29. NH3.-Free PE Spectrum of N 2 H 2 113 30. The Individual PE Bands of N 9 H 2 115 31. The Individual PE Bands of N 2 D 2 117 32. PE Spectra of (a) CH3N=NH and (b) CH3N=MCH3 125 33. Carbon Arc Reactor for the Synthesis of HCP 129 34. PE Spectrum of HCP in the Presence of C ? H ? 131 - v i i i F i g u r e P a g e 3 5 . PE S p e c t r a o f ( a ) C 0 7 , ( b ) C S 9 , ( c ) CSe,,, ( d ) OCS, ( e ) OCSe a n d ( f ) S C S e ..: 1 3 7 3 6 . T h e S e c o n d a n d T h i r d PE B a n d s o f C S e 2 1 4 2 3 7 . T h e I n d i v i d u a l PE B a n d s o f O C S e ; ( a ) F i r s t , S e c o n d , ( c ) T h i r d a n d ( d ) F o u r t h B a n d s 1 4 4 3 8 . T h e I n d i v i d u a l PE B a n d s o f S C S e ; ( a ) F i r s t , ( b ) S e c o n d , ( c ) T h i r d a n d ( d ) F o u r t h B a n d s 1 4 6 3 9 . C o r r e l a t i o n D i a g r a m f o r t h e I P ' s o f C 0 ? , C S „ , C S e 2 , OCS, OCSe a n d S C S e : . . 1 4 9 4 0 . H e l PE S p e c t r u m o f 0, 1 6 5 4 1 . T h e F i r s t , S e c o n d a n d T h i r d PE B a n d s o f 0., u s i n q ( a ) H e l a n d (b ) N e l L i g h t S o u r c e s . *. . 1 6 6 4 2 . PE S p e c t r a o f ( a ) FMO, ( b ) CINQ a n d ( c ) B r N O ... 1 7 6 4 3 . PE S p e c t r a o f ( a ) H N O g , ( b ) C1HC> 2 a n d ( c ) F N 0 2 . 1 8 6 4 4 . T h e L o w e r E n e r g y PE B a n d s o f ( a ) HNO,, ( b ) C 1 N 0 2 a n d ( c ) F N 0 2 a t H i g h R e s o l u t i o K 1 9 4 4 5 . C o r r e l a t i o n D i a g r a m f o r t h e I P ' s o f N 0 7 , C H - N 0 9 , UNO,, C l M 0 o a n d F H 0 o ..... 2 0 0 - i x -L I S T OF T A B L E S T a b l e P a g e 1. M o n o c h r o m a t i c P h o t o n S o u r c e s f o r U s e i 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 y 2 2. E x p e r i m e n t a l H a l f - W i d t h a n d I n t e n s i t y o f t h e A r g o n 2P3/2 P e a k a t D i f f e r e n t F o c u s i n g C o n d i t i o n s o f t h e A n a l y z e r . 71 3. PES a n d T h e o r e t i c a l D a t a f o r CS 80 4. PES D a t a f o r SO 89 5. PES a n d T h e o r e t i c a l D a t a f o r S^O 98 6. PES D a t a f o r'NgHg a n d ^ 2D2 1 1 2 7. PES a n d T h e o r e t i c a l D a t a f o r C H 3 N = NH 1 2 7 8. PES a n d T h e o r e t i c a l D a t a f o r HCP 132 9. PES D a t a f o r C 0 2 > C S 2 , C S e 2 > OCS, OCSe a n d S C S e . 1 3 8 1 0 . ( a ) E x p e r i m e n t a l a n d C a l c u l a t e d F r a n c k - C o n d o n F a c t o r s i n t h e I n d i v i d u a l S t a t e s o f CO,, CS? a n d C S e * 1 5 2 ( b ) E x p e r i m e n t a l a n d C a l c u l a t e d Franck - C o n d o n F a c t o r s i n t h e , I n d i v i d u a l S t a t e s o f OCS , OCSe a n d S C S e 1 5 4 11 . E s t i m a t e d B o n d L e n q t h s . o f t h e . I n d i v i d u a l S t a t e s o f CO*, C S 2 , C S e 2 , " 0 C S , OCSe , S C S e a n d N 2 0 .. 1 5 9 12 . PES a n d T h e o r e t i c a l D a t a f o r 0 3 1 6 9 13 . PES D a t a f o r t h e M i t r o s y l H a l i d e s (XMO) 1 7 7 14. SCF-MO R e s u l t s f o r t h e N i t r o s y l H a l i d e s 1 8 0 15 . PES a n d T h e o r e t i c a l D a t a f o r HNO,, C 1 M 0 9 a n d F N 0 2 ? . . . . . . ? 1 8 8 - X -ACKNOWLEDGEMENTS I w o u l d l i k e t o e x p r e s s my s i n c e r e a p p r e c i a t i o n a n d g r a t i t u d e t o D r . D. C. F r o s t a n d D r . C. A. M c D o w e l l f o r t h e i r c o n t i n u e d s u p p o r t , u n d e r s t a n d i n g a n d g u i d a n c e t h r o u g h o u t t h e c o u r s e o f t h i s w o r k . I w o u l d a l s o l i k e t o t h a n k my many c o l l e a g u e s , M r . A. B a i n , D r . J . C. B u n z l i , D r . M. C h i a n g , M r . T. C h a u , D r . L . S. C h i a , D r . A. 8. C o r n f o r d , D r . A. K a t r i b , D r . D. J . S m i t h , D r . N. P. C. W e s t w o o d , D r . G. W i g h t a n d M r . D. Y e e f o r t h e i r a s s i s t a n c e on many o c c a s i o n s . T h a n k s a r e a l s o d u e t o D r . R. B o y d , D r . C. E . B r i o n , D r . F. G. H e r r i n g , D r . H. K r o t o , D r . A . J . M e r e r a n d D r . R. T a p p i n g f o r h e l p f u l d i s c u s s i o n s . I w i s h t o a c k n o w l e d g e g r a t e f u l l y t h e s t a f f o f t h e E l e c t r o n i c , G l a s s , I l l u s t r a t i o n a n d M e c h a n i c a l W o r k s h o p s . I am s p e c i a l l y g r a t e f u l t o M r . E . M a t t e r f o r h i s p r e c i s i o n c o n s t r u c t i o n o f t h e s p e c t r o m e t e r a n d t o M r . C. M c C a f f e r t y f o r h i s s k i l l f u l t e c h n i c a l a s s i s t a n c e . I am t h a n k f u l t o D r . N. P. C. W e s t w o o d f o r a c a r e f u l - x i -r e a d i n g o f t h i s m a n u s c r i p t , t o M r . P a u l T a n g f o r t y p i n g p a r t o f t h e f i r s t d r a f t a n d t o M i s s P a t t i Hon f o r t y p i n g t h e t h e s i s . I w i s h t o e x t e n d my a p p r e c i a t i o n t o a l l o u r f r i e n d s who made o u r r e s i d e n c e i n V a n c o u v e r m o s t e n j o y a b l e . I am m o s t g r a t e f u l t o my w i f e f o r h e r l o v e , u n d e r -s t a n d i n g a n d e n c o u r a g e m e n t . I am i n d e b t e d t o my f a t h e r f o r u n d e r g o i n g a l o t o f h a r d s h i p i n b r i n g i n g me u p . O u r s o n S t a n l e y s h o u l d a l s o be t h a n k e d f o r b e h a v i n g h i m s e l f d u r i n g t h e w r i t i n g o f t h i s t h e s i s . - 1 -CHAPTER ONE INTRODUCTION 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 i s , e s s e n t i a l l y , a t e c h n i q u e f o r m e a s u r i n g t h e b i n d i n g e n e r g i e s o r i o n i z a t i o n p o t e n t i a l s o f e l e c t r o n s i n m o l e c u l e s o r atoms, by e x p o s i n g the sample t o a s o u r c e o f monochromatic photons o f known e n e r g y hv and a n a l y z i n g the k i n e t i c e n e r g i e s o f the p h o t o e j e c t e d e l e c t r o n s . The b i n d i n g e n e r g i e s E b o f t h e d i f f e r e n t groups o f e l e c t r o n s can be o b t a i n e d from the measured k i n e t i c e n e r g i e s E ^ ^ n u t i l i z i n g the E i n s t e i n p h o t o e l e c t r i c e q u a t i o n , E b - hv - E k j n . ( 1 ) The e s s e n t i a l r e q u i r e m e n t s f o r a photon s o u r c e which i s t o be used i 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 y a r e t h a t i t be mono-c h r o m a t i c , o f u s a b l e i n t e n s i t y , and have s u f f i c i e n t e n e r g y t o i o n i z e t h e e l e c t r o n s o f i n t e r e s t . A l i s t o f such photon s o u r c e s , t o g e t h e r w i t h t h e i r c h a r a c t e r i s t i c s i s shown i n T a b l e I . Depending upon the e n e r g y o f the photon s o u r c e , the o v e r a l l - 2 -T A B L E 1. M o n o c h r o m a t i c P h o t o n S o u r c e s F o r U s e i 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 y X - R a y P h o t o n S o u r c e E n e r g y ( e V ) Wave L e n g t h ( A ) Ag K a 2 2 1 6 2 . 9 0 . 5 5 9 4 1 Mo K a 1 7 4 7 9 . 3 0 . 7 0 9 3 0 Cu K a 8048 1 . 5 4 0 5 C r K a 5415 2 . 2 8 9 6 A l K a 1 4 8 6 . 6 8 . 3 9 9 Mg K a 1 2 5 3 . 6 1 0 . 0 3 4 Na K a 1 0 4 1 . 0 1 1 . 9 1 0 o VUV P h o t o n S o u r c e E n e r g y ( e V ) Wave L e n g t h ( A ) H e l l 40 . 3 1 2 6 30 3 .731 H e l 21 . 2 1 7 5 584 . 3 3 4 M e l d o u b l e t 16 . 8 4 7 7 / 1 6 . 6 7 0 4 735 . 8 9 5 / 7 4 3 . 7 1 8 A r l d o u b l e t 11 . 8 2 7 3 / 1 1 . 6 2 3 3 1048 . 2 1 9 / 1 0 0 6 6 . 6 5 9 K r I d o u b l e t 10 . 6 4 3 4 / 1 0 . 0 3 2 1 * 1164 . 3 6 7 / 1 2 3 5 . 8 3 8 * H Lyman a 10 . 1 9 8 6 1215 . 6 6 8 X e l d o u b l e t 9 . 5 6 9 7 / 8 . 4 3 6 3 * 1295 . 5 8 6 / 1 4 6 9 . 6 1 0 * * i n d i c a t e s t h e m o r e i n t e n s e l i n e o f t h e d o u b l e t . - 3 -f i e l d of photoelectron spectroscopy f a l l s into two branches. One branch uses higher energy X-ray sources, and is termed 1 2 Electron Spectroscopy for Chemical Analysis (ESCA) ' or X-ray Photoelectron Spectroscopy (XPS). The other uses vacuum u l t r a v i o l e t (VUV) r a d i a t i o n , and is ca l led Molecular Photoelectron Spectroscopy , UV Photoelectron Spectroscopy (UPS), or simply Photoelectron Spectroscopy (PES). In ESCA, Aluminum K a (1486.6 eV) and Magnesium K a (1253.6 eV) are by far the most commonly used sources due to t h e i r narrower l ine widths, higher intens i ty and ease of operat ion, while the Hel resonance l ine (21.2175 eV) is most popularly used in PES for the same reasons. ESCA, because of i t s more energetic source and lower l i m i t i n g resolut ion (>0.2 eV) , is mainly concerned with the study of inner or core she l l e lect rons . PES is exc lus ive ly concerned with outer or valence shel l (up to about 40 eV) e lec t rons . This leaves a gap between 40 eV and 1,000 eV where su i tab le photon sources are lack ing , although recent developments indicate that this gap may be f i l l e d by the use of synchrotron rad iat ion 4-6 in conjunction with a monochromator .. Yttrium X-ray l ines at about 130 eV developed by Krause^ are also a promising source, although s a t e l l i t e l ines are a complicating factor here. This thesis i s concerned with PES using mainly the Hel resonance 1 i ne. In view of the fact that the basic p r inc ip les of PES " 8 were well understood as ear ly as 1905, when E inste in success-f u l l y explained the .photoe lect r i c e f f e c t , i t is surpr i s ing that the f i r s t set of such experiments appeared only as late as the - 4 -9-12 ear ly 1960's . Since then there has been an exponential growth in the f i e l d of PES as evidenced by the appearance of 1 - 3 1 3 - 3 7 numerous extensive reviews ' , and the gathering of several internat ional conferences** 8"^ . Within the past ten years or so, PES has developed from i t s exploratory stage to become one of the standard techniques of modern chemistry. Although the f i e l d is a young and developing one, i t is general ly acknowledged that 42 i t has at least reached i t s f i r s t stage of maturity . The primary goal of PES, which is to obtain molecular ion izat ion potent ia ls with high precis ion and r e s o l u t i o n , has c e r t a i n l y been met judging from the fact that hundreds of spectra have been recorded and discussed in b o o k s ^ ' ^ and in the l i t e r a t u r e . Alerted by i t s p o t e n t i a l , and with the a v a i l a b i l i t y of commercial 4 3 photoelectron spectrometers , more and more researchers have begun to apply PES as a routine tool in molecular spectroscopy. In this respect , PES is e s s e n t i a l l y l i k e any of the standard spectroscopic techniques, such as nmr, mass spectrometry, e tc . To date, most of published PES work has been concerned with compounds that are stable under normal condi t ions , and have appreciable v o l a t i l i t y (> .01 Torr) below 200° C. This kind of PES work w i l l be ca l led 'conventional ' photoelectron spectroscopic studies in th is t h e s i s . On the other hand, PES studies of t ransient species and react ive molecules are rare because of the experimental d i f f i c u l t i e s involved. During the 4 4 - 4 8 i n i t i a l stages of th is study, a few exploratory attempts to study such systems with the technique of PES had revealed i t s great potential in providing information on the chemical - 5 -b o n d i n g o f t r a n s i e n t s p e c i e s , w h i c h w o u l d n o r m a l l y be v e r y d i f f i c u l t t o o b t a i n b y o t h e r m e t h o d s . T h e e x t e n s i o n o f PES t o s t u d y r e a c t i v e m o l e c u l e s , w h i c h c a n be h a n d l e d a n d s t u d i e d o n l y u n d e r a p p r o p r i a t e l y c o n t r o l l e d c o n d i t i o n s , w o u l d b e v e r y f r u i t f u l s i n c e t h e s e m o l e c u l e s p l a y a s i g n i f i c a n t r o l e i n c h e m i s t r y . T h i s t h e s i s w i l l d e s c r i b e an a t t e m p t t o e x t e n d P E S i n t o t h e r e a l m s o f t h e s e ' u n c o n v e n t i o n a l ' 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 . A 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 s u i t a b l e f o r t h i s k i n d o f w o r k h a s b e e n d e s i g n e d a n d c o n s t r u c t e d . W i t h t h i s s p e c t r o m e t e r , some t r a n s i e n t s p e c i e s a n d r e a c t i v e m o l e c u l e s h a v e b e e n s t u d i e d . In a d d i t i o n , i t h a s b e e n p o s s i b l e t o v a r y t h e t e m p e r a t u r e o f t h e s a m p l e b e t w e e n - 1 8 0 ° C a n d + 1 , 2 0 0 ° C , w h e r e t e m p e r a t u r e d e p e n d e n t e q u i l i b r i u m s y s t e m s may be s t u d i e d . - 6 -CHAPTER TWO THEORY OF PHOTOELECTRON SPECTROSCOPY Although photoelectron spectroscopy (PES) is a r e l a t i v e l y new technique, i t s theory is well documented and has 1-3 13-37 been discussed in great deta i l in numerous reviews * . Therefore, except for those aspects which are more pert inent to the study in th is t h e s i s , only a b r i e f summary of the basic p r inc ip les of the theory w i l l be presented here. Greater attention w i l l be paid to those areas where new developments have been achieved. 2.1 Character is t i cs Of A Photoelectron Spectrum When a photoelectron is ejected by a photon of energy hv from a molecule or atom, by the conservation of energy the fol lowing energy re la t ion holds. h v = E k i n + E r + I 0 - + A E v i b + A E r o t + A E t r a n > ( 2 ' 1 ' 1 ) - 7 -w h e r e E ^ . i s t h e k i n e t i c e n e r g y o f t h e p h o t o e l e c t r o n , E ^ i s t h e r e c o i l e n e r g y o f t h e i o n , I Q i s t h e d i f f e r e n c e i n t h e e l e c t r o n i c e n e r g y o f t h e i o n a n d m o l e c u l e , a n d i s t e r m e d t h e 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 , a n d A E . . . , AE a n d A E . a r e r e s p e c t i v e l y t h e d i f f e r e n c e v i D r o t t r a n i n t h e v i b r a t i o n a l , r o t a t i o n a l a n d t r a n s l a t i o n a l e n e r g y o f t h e i o n a n d t h e p a r e n t m o l e c u l e . B e c a u s e o f t h e l a r g e d i f f e r e n c e i n t h e m a s s o f t h e i o n a n d t h e e j e c t e d e l e c t r o n , E r i s n e g l e c t e d t h r o u g h c o n s i d e r a t i o n o f t h e c o n s e r v a t i o n o f momentum. U n d e r t h e p r e s e n t l e v e l o f e x p e r i m e n t a l r e s o l u t i o n (>10 mev, s e e s e c t i o n 2 . 3 ) , r o t a t i o n a l a n d t r a n s 1 a t i o n a l e n e r g y l e v e l a r e u s u a l l y n o t r e s o l v e d , a n d t h u s A E r o t a n c' A E t r a n a r e a l s o n e g l e c t e d . U s u a l l y t h e m o l e c u l e i s i n t h e g r o u n d v i b r a t i o n a l s t a t e , t h u s e q . ( 2 . 1 . 1 ) r e d u c e s t o h* = Ek1n + ! 0 + E v i b < 2 - K 2 ' w h e r e E .? i s t h e v i b r a t i o n a l e n e r g y o f t h e i o n . T h e v i b m e a s u r e m e n t o f t h e k i n e t i c e n e r g y o f t h e e l e c t r o n t h e n g i v e s t h e i o n i z a t i o n p o t e n t i a l o f t h e m o l e c u l e . T h e p h o t o e l e c t r o n ( P E ) s p e c t r u m o f a m o l e c u l e t h u s c o n s i s t s o f s e v e r a l PE b a n d s , e a c h o f w h i c h c o r r e s p o n d s t o an i o n i z a t i o n p o t e n t i a l ( I P ) o f t h e m o l e c u l e , a l l t h e way up t o t h e e n e r g y o f t h e i n c i d e n t p h o t o n s . T h e i n t e g r a t e d a r e a o r t h e i n t e n s i t y o f t h e i n d i v i d u a l b a n d i s r e l a t e d t o t h e d i f f e r e n t i a l p h o t o i o n i z a t i o n c r o s s s e c t i o n c o r r e s p o n d i n g t o t h a t - 8 -i o n i c s t a t e , a n d t h e s h a p e o f t h e b a n d i s an i n d i c a t i o n o f t h e g e o m e t r y c h a n g e u p o n i o n i z a t i o n . E a c h o r some o f t h e s e b a n d s may show a s e r i e s o f r e s o l v e d p e a k s c o r r e s p o n d i n g t o t h e d i f f e r e n t v i b r o n i c s t a t e s . T h e s e p a r a t i o n s b e t w e e n t h e s e p e a k s a r e t h e n t h e v i b r a t i o n a l f r e q u e n c i e s o f t h e i o n . T h e l o w e s t e n e r g y ( f i r s t ) p e a k , o r t h e o n s e t o f t h e PE b a n d i s n o r m a l l y t h e a d i a b a t i c I P , w h i l e t h e m o s t i n t e n s e p e a k o r t h e maximum o f t h e b a n d i s r e f e r r e d t o a s t h e v e r t i c a l I P . I f t h e m o l e c u l e i s v i b r a t i o n a l l y e x c i t e d o r t h e r e i s a l a r g e c h a n g e i n g e o m e t r y d u r i n g i o n i z a t i o n , t h e n c a r e s h o u l d be e x e r c i s e d i n l o c a t i n g t h e e x a c t p o s i t i o n o f t h e a d i a b a t i c I P . T h e i n t e n s i t y o f e a c h i n d i v i d u a l p e a k i n a PE b a n d i s p r o p o r t i o n a l t o t h e p r o b a b i l i t y o f p r o d u c i n g t h a t p a r t i c u l a r v i b r o n i c s t a t e , g i v i n g t h e s o - c a l l e d F r a n c k - C o n . d o n F a c t o r . I n a d d i t i o n t o t h e a b o v e - m e n t i o n e d v i b r a t i o n a l f i n e s t r u c t u r e , f u r t h e r b a n d s t r u c t u r e may be d u e t o s p i n - o r b i t c o u p l i n g , J a h n - T e l l e r d i s t o r t i o n , o r a m i x t u r e o f t h e t w o , i . e . t h e s p i n - v i b r o n i c i n t e r a c t i o n s . F o r q u a s i - l i n e a r i o n i c s t a t e s t h e p o s s i b i l i t y o f t h e R e n n e r - T e l 1 e r e f f e c t e x i s t s . C o m p l i c a t i o n s t o s i m p l e b a n d s h a p e s may a r i s e t h r o u g h a u t o -i o n i z a t i o n o r p r e d i s s o c i a t i o n , a n d m u s t be c o n s i d e r e d i n a r i g o r o u s a n a l y s i s . F o r m o l e c u l e s h a v i n g I P ' s l e s s t h a n ^ 1 1 e V , a d d i t i o n a l b a n d s c o r r e s p o n d i n g t o l o w e n e r g y p h o t o -e l e c t r o n s may a r i s e d u e t o t h e i m p u r i t y l i n e s i n t h e l i g h t s o u r c e ( s e e s e c t i o n 3 . 3 . 3 ) . - 9 -2.2 T h e I n t e r p r e t a t i o n O f M o l e c u l a r PE S p e c t r a 2.2.1 A B r i e f S u r v e y O f A s s i g n m e n t C r i t e r i a A s s u m i n g a n o r b i t a l d e s c r i p t i o n o f a m o l e c u l e , t h e n a t a c e r t a i n l e v e l o f a p p r o x i m a t i o n t h e m o l e c u l a r PE s p e c t r u m i s a d i r e c t d i s p l a y o f t h e e n e r g i e s o f t h e v a r i o u s m o l e c u l a r o r b i t a l s ( M O ' s ) . T h e i n d i v i d u a l PE b a n d s t h e n c o r r e s p o n d t o t h e r e m o v a l o f a n e l e c t r o n f r o m e a c h MO l e a v i n g t h e i o n i n d i f f e r e n t i o n i c s t a t e s . T h e l o w e s t e n e r g y b a n d i s u s u a l l y a s s o c i a t e d w i t h t h e g r o u n d i o n i c s t a t e , w h i l e t h e o t h e r s c o r r e s p o n d t o e x c i t e d i o n i c s t a t e s . I n t h i s w a y , t h e PE s p e c t r u m o f a m o l e c u l e i s s i m i l a r t o t h e e l e c t r o n i c s p e c t r u m o f t h e i o n w i t h t h e t r a n s i t i o n s o c c u r i n g o n l y b e t w e e n i t s o c c u p i e d MO's. T h e q u a n t i t i v e c o r r e s p o n d e n c e b e t w e e n t h e e n e r g i e s o f t h e b a n d s o r I P ' s a n d t h a t o f t h e o r b i t a l s f r o m w h i c h t h e e l e c t r o n s a r e 49 r e m o v e d i s e x p r e s s e d i n K o o p m a n s ' t h e o r e m ( s e e s e c t i o n 2 . 2 . 2 ) j. U w h i c h s t a t e s t h a t t h e n I P c a n be a p p r o x i m a t e d t o t h e n e g a t i v e o f t h e n o r b i t a l e n e r g y . W i t h t h i s t h e o r e m , t h e c o m p a r i s o n o f t h e c a l c u l a t e d o r b i t a l e n e r g i e s a n d t h e e x p e r i m e n t a l I P ' s i s w i d e l y u s e d i n t h e i n t e r p r e t a t i o n o f PE s p e c t r a . T h e v i b r a t i o n a l f i n e s t r u c t u r e s a n d t h e s h a p e s o f t h e i n d i v i d u a l PE b a n d s f r e q u e n t l y c o n t a i n a g r e a t d e a l o f i n f o r m a t i o n on t h e b o n d i n g c h a r a c t e r i s t i c s o f t h e o r b i t a l t o w h i c h t h e y r e l a t e . T h e r e l a t i o n b e t w e e n t h e b a n d s t r u c t u r e s a n d o r b i t a l c h a r a c t e r i s t i c s a r e d e r i v e d f r o m t h e F r a n c k - C o n d o n 50 51 p r i n c i p l e ' , w h i c h w i l l be d i s c u s s e d i n s e c t i o n 2 . 2 . 3 . A f u r t h e r i n d i c a t i o n o f t h e b o n d i n g c h a r a c t e r o f t h e o r b i t a l may - 1 0 -be o b t a i n e d from the change o f the v i b r a t i o n a l f r e q u e n c i e s on g o i n g from the m o l e c u l e t o t h e i o n . T h i s makes use o f t h e c l o s e r e l a t i o n s h i p between the v i b r a t i o n a l f r e q u e n c y , the f o r c e c o n s t a n t and the bond o r d e r . T h u s , i f a n o n b o n d i n g e l e c t r o n i s removed, t h e r e w i l l be p r a c t i c a l l y no change i n v i b r a t i o n a l f r e q u e n c y , whereas i f a b o n d i n g or a n t i b o n d i n g e l e c t r o n i s removed, the v i b r a t i o n a l f r e q u e n c y o f t h e i o n w i l l be l e s s o r g r e a t e r r e s p e c t i v e l y t h an t h a t o f the n e u t r a l m o l e c u l e . In l a r g e r p o l y a t o m i c s , s e v e r a l v i b r a t i o n a l modes may be e x c i t e d upon i o n i z a t i o n , thus making v i b r a t i o n a l a s s i g n m e n t s more d i f -f i c u l t . O c c a s i o n a l l y , t h e a n a l y s i s o f the s p i n o r b i t J a h n - T e l I e r 5 4 " 5 7 and R e n n e r - T e l 1 e r 5 a » 5 3 s p l i t t i n g s can be f r u i t f u l and l e a d to a b e t t e r u n d e r s t a n d i n g o f the s t r u c t u r e o f t h e i o n . A u s e f u l a p p r o a c h , e s p e c i a l l y f o r complex m o l e c u l e s , i s t o s t u d y a l a r g e number o f homologous c o m p o u n d s ^ ' ^ ' By s t u d y i n g many su c h r e l a t e d compounds, and c o r r e l a t i n g and c o m p a r i n g the s p e c t r a down a s e r i e s , one can u s u a l l y g a i n some i n f o r m a t i o n t h a t would n o t be o b v i o u s o t h e r w i s e . In such s t u d i e s , one may a l s o make use o f t h e r e s o n a n c e and i n d u c t i v e e f f e c t s o f t h e s u b s t i t u e n t g r o u p s . F o r p l a n a r 62 6 3 m o l e c u l e s where a p p l i c a b l e , t h e p e r f l u o r o e f f e c t ' may sometimes be used t o d i f f e r e n t i a t e the * and a o r b i t a l s . Sometimes, the r e l a t i v e i n t e n s i t i e s o f the bands may be used as an i n d i c a t i o n o f the d e g e n e r a c y o f t h e s t a t e s , i . e . t h e d e g e n e r a t e s t a t e s have g r e a t e r i n t e n s i t i e s than t h e n o n - d e g e n e r a t e s t a t e s . T h i s g u i d e l i n e must be used w i t h - 11 -caution since i t has been found i n v a l i d in many c a s e s ^ . Quite recently the study of photoionizat ion cross sections as a function of ion iz ing energy has shown high potent ia l in providing information on the bonding character of o r b i t a l s . Both experimental '^' '^ and t h e o r e t i c a l ^ studies have indicated that usual ly the i n t e n s i t i e s of the o r b i t a l s mainly of s character tend to increase with respect to that of the o r b i t a l s mainly of p character when using higher energy l i g h t sources. The theoret ica l studi e s ^ ~ 6 9 have indicated some in te res t ing re lat ionsh ips between the energy dependence of photoionizat ion cross sections and the bonding character of o r b i t a l s . In view of the success of these s tud ies , the comparison of measured and calculated values for r e l a t i v e band i n t e n s i t i e s may become an addit ional useful assignment c r i t e r i o n in PES. Another re lated and promising technique is the study of the in tens i ty of the ejected photoelectrons as a function of the angle between the d i rect ion of the incoming photon and the outgoing photo-e l e c t r o n . This may also shed l i g h t on the nature of the molecular o r b i t a l , through the determination of the B f a c t o r ^ 0 , ^ PES is not a threshold technique, consequently the spectra are generally not complicated by autoionizat ion pro-cesses. On the contrary, because of i t s resonant nature, auto-ion izat ion in PES can usual ly be i d e n t i f i e d by using d i f f e r e n t l i g h t sources, and often leads to useful information about the autoionized s ta te . - 12 -2 . 2 . 2 K o o p m a n s ' T h e o r e m A n d T h e C a l c u l a t i o n o f V e r t i c a l I P ' s 49 ( a ) K o o p m a n s ' T h e o r e m A n d I t s I m p l i c a t i o n s C o n s i d e r a t w o - s t e p p r o c e s s : (1) C a l c u l a t i o n o f a H a r t r e e - F o c k ( H F ) S e l f - C o n s i s t e n t -F i e l d ( S C F ) w a v e f u n c t i o n f o r a m o l e c u l e w i t h n d o u b l y o c c u p i e d m o l e c u l a r o r b i t a l s a n d e v a l u a t i o n o f i t s t o t a l e n e r g y E . ( 2 ) E v a l u a t i o n o f t h e s i n g l e d e t e r m i n a n t e n e r g y o f t h e i o n E + r e s u l t i n g f r o m t h e r e m o v a l o f a n e l e c t r o n f r o m t h e k m o l e c u l a r o r b i t a l , u s i n g t h e H F - S C F o r b i t a l s f r o m t h e a b o v e c a l c u l a t i o n o f t h e m o l e c u l e f o r t h e r e m a i n i n g e l e c t r o n s . 49 " ,1 K o o p m a n s s h o w e d t h a t t h e e n e r g y d i f f e r e n c e o r I P c a l c u l a t e d h i n t h i s way i s j u s t t h e n e g a t i v e o f t h e k o r b i t a l e n e r g y . q u i t e s t r a i g h t f o r w a r d . F o r a m o l e c u l e w i t h n d o u b l y o c c u p i e d * = r « 1 ( l ) * 1 ( 2 ) - - - - * k ( 2 k - l ) * k ( 2 k ) . . . * n ( 2 n - l ) J n ( 2 n ) | , ( 2 . 2 . 1 ) a n d t h e H a m i l t o n i a n o f t h e m o l e c u l e i s T h e m a t h e m a t i c a l p r o o f o f K o o p m a n s ' t h e o r e m ( K T ) i s MO ' s <f>.j , t h e s i n g l e d e t e r m i n a n t w a v e f u n c t i o n i s 2n i < i r • . i j ( 2 . 2 . 2 ) T h e c o r r e s p o n d i n g w a v e f u n c t i o n a n d t h e H a m i l t o n i a n f o r t h e i o n t. X. L. w i t h a n e l e c t r o n r e m o v e d f r o m t h e k MO a r e - 13 -• = 1 ^ ( 1 ) ^ ( 2 ) - . . |ikk(2k.1} j - - - * n ( 2 n - l ) * n ( 2 n ) | ( 2 . 2 . 3 ) a n d 2 n - l 2 K 2 ^ H + = £ ( I* - £ fa. ) + 12 -L- . ( 2 . 2 . 4 ) i = l \ L a=l K a i / i < j r i j In t h e e x p r e s s i o n f o r t h e Hami1 t o n i a n s , Z a i s t h e c h a r g e on t h e n u c l e u s o, r . i s t h e d i s t a n c e o f t h e e l e c t r o n i f r o m t h e a l n u c l e u s a , a n d i s t h e d i s t a n c e b e t w e e n e l e c t r o n i a n d j . I t c a n b e s h o w n 7 / t h a t t h e t o t a l e n e r g y o f t h e m o l e c u l e a n d t h e i o n a r e n n S h , + E = 2 Z-f h . Z^t ( 2 J . . - K. .) ( 2 . 2 . 5 ) •1 = 1 1 i , j = l 1 J 1 J a n d w h e r e n E + = 2 S h . + WJ ( 2 0 . . - K. .) + h. i = l 1 i , j = l 1 J 1 J K i=fk i . j f k n > E ( 2 J . k - K . k ) , ( 2 . 2 . 6 ) * 1+1 2 K '1 mfa™ * i ( 1 ) d r l ( 2 - 2 " 7 ) - 14 -and = energy of an electron alone in the f i e l d of the bare n u c l e i ; comprising the k inet i c and e lectron-nuclear a t t rac t ion energy, J i j = j (/* i ( 1 ) *j ( 2 ) r ~ 7 * 1 ( 1 ) * J ( 2 ) d r 1 d r 2 (2.2.8) = coulombic repulsion between any electron p a i r , K i j = j ( / * 1 ( 1 ) * J ( 2 ) F ~ " • 1 ( 2 ) * j " ( D d r l d r 2 (2 .2.9) = exchange interact ion between any electron pair of the same sp in . Noting J . . = K.. , one obtains n E+ - E - - h k - S ( 2 J i k - K . k ) . (2.2.10) A. L Since , however, the k orb i ta l energy e k i s given by the solut ion pf the SCF equation, i . e . 2 K n (- "4 - X) + £ (20 i - K, \ 2 a = l R a l / i = l 1 1 ) *k = • E k*k ' ( 2 - 2 • 1 1 ) then one has n (2J (2.2.12) i = l ik and hence (2.2.13) This thus i l l u s t r a t e s the v a l i d i t y of KT. There are three kinds of errors in the IP's ca lculated 0 7 by th is method due to : (1) The relaxat ion of the o r b i t a l s in the ion due to the perturbation caused by i o n i z a t i o n . (2) The d i f ference in e lectron c o r r e l a t i o n energy in the ion and the molecule. (3) The d i f ference in r e l a t i v i s t i c energy in the ion and the molecule. The neglect of (1) is inherent in KT and resul ts in the ca lculated IP's being too large . Because of th is neglect , the ca lcu lated IP should correspond to the v e r t i c a l IP. (2) and (3) are introduced in the HF-SCF-MO method which is used to ca lcu late the molecular wave function and the orb i ta l energies. This method makes no attempt to ca lcu late r e l a t i -v i s t i c energy in both the ion and the molecule, with the IP being given only by the di f ference in e l e c t r o s t a t i c energy. Since the r e l a t i v i s t i c e f f e c t increases with the k inet i c energy of the e lec t ron , the correct ion for this e f fec t is usual ly neg l ig ib le for outer shel l e lectrons but is appreciable for - 16 -i n n e r s h e l l e l e c t r o n s . The HF t h e o r y i n c l u d e s some e l e c t r o n c o r r e l a t i o n by assuming t h a t e l e c t r o n e x p e r i e n c e s an a v e r a g e p o t e n t i a l c r e a t e d by t h e r e m a i n i n g ( 2 n - l ) e l e c t r o n s and t h r o u g h i m p o s i n g th e P a u l i e x c l u s i o n p r i n c i p l e on t h e wave f u n c t i o n . As s u c h , t h e HF t h e o r y n e g l e c t s t h e i n s t a n t a n e o u s ( r a t h e r t h an a v e r a g e ) r e p u l s i o n s between p a i r s o f e l e c t r o n s . The c o n t r i b u t i o n t o the t o t a l e n e r g y due t o i n s t a n t a n e o u s 7 3 r e p u l s i o n s i s c a l l e d the c o r r e l a t i o n e n e r g y , s i n c e the m o t i o n o f t h e e l e c t r o n s i s c o r r e l a t e d such t h a t two e l e c t r o n s a r e u n l i k e l y t o g e t v e r y c l o s e t o e a c h o t h e r due t o t h e c o u l o m b i c r e p u l s i o n . W h i l e t h e P a u l i p r i n c i p l e e n s u r e s th e a v o i d a n c e o f e l e c t r o n s w i t h p a r a l l e l s p i n s , i n the i n d e p e n d e n t - p a r t i c l e (one o r b i t a l p e r e l e c t r o n ) HF scheme, the major p a r t o f t h e c o r r e l a t i o n e r r o r comes from the n e g l e c t o f s p a t i a l c o r r e l a t i o n between the e l e c t r o n s w i t h a n t i p a r a l l e l s p i n s . S i n c e c o r r e -l a t i o n e f f e c t s a r i s e t o a l a r g e e x t e n t f r o m 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 , t h e c o r r e l a t i o n e n e r g y i s u s u a l l y l e s s i n t h e i o n than th e m o l e c u l e . As a r e s u l t , g e n e r a l l y t o o s m a l l an IP w i l l be g i v e n by the d i f f e r e n c e o f t h e good HF e n e r g i e s o f t h e i o n and m o l e c u l e c a l c u l a t e d i n d e p e n d e n t l y . (b) A p p l i c a t i o n o f Koopmans' Theorem i n PES F o r t u n a t e l y t h e e r r o r s i n t r o d u c e d by t h e n e g l e c t o f t h e r e l a x a t i o n and c o r r e l a t i o n e f f e c t s a r e o f o p p o s i t e s i g n and v e r y o f t e n t e n d to c a n c e l e ach o t h e r . T h e r e f o r e , IP's c a l c u l a t e d by a ssuming KT f r e q u e n t l y p r e d i c t the e x p e r i m e n t a l - 1 7 -v a l u e s w i t h i n a r e a s o n a b l e d e g r e e o f a c c u r a c y . I t h a s b e e n s h o w n 7 4 ' 7 , 5 t h a t t h e a p p l i c a t i o n o f KT t o c a l c u l a t i o n s n e a r HF a c c u r a c y o f o u t e r s h e l l o r b i t a l e n e r g i e s o f t e n g i v e s I P ' s t o o h i g h b y ^ 8 % . T h i s t e n d s t o i n d i c a t e t h a t , i n . g e n e r a l , t h e e r r o r d u e t o r e l a x a t i o n e n e r g y i s l a r g e r t h a n t h e c o r r e l a t i o n e n e r g y . T h e ^ 9 2 % a g r e e m e n t b e t w e e n e x p e r i m e n t a l a n d c a l c u l a t e d I P ' s i s o f t e n s u f f i c i e n t t o a s s i g n t h e p h o t o -e l e c t r o n s p e c t r a u n a m b i g u o u s l y . B e c a u s e o f i t s s i m p l i c i t y a n d e c o n o m y , KT I P ' s c a l c u l a t e d b y s e m i - e m p i r i c a l SCF-MO s c h e m e s s u c h a s t h e C N D O 7 6 , I N D O 7 6 , H I N D U 7 7 a n d e x t e n d e d H u c k e l 7 8 m e t h o d s a r e m ore w i d e l y u s e d t h a n ab i n i t i o c a l c u l a t i o n s . I n a d d i t i o n t o t h e a s s u m p t i o n s o f KT, t h e s u c c e s s o f t h e s e m e t h o d s d e p e n d s on i t s i n h e r e n t a p p r o x i m a t i o n s . V e r y o f t e n , t h e r e l a t i v e o r d e r i n g o f t h e o r b i t a l s d e r i v e d f r o m t h e s e m i - e m p i r i -c a l m e t h o d s a r e t h e same a s t h e ab i n i t i o r e s u l t , a l t h o u g h t h e o r b i t a l e n e r g i e s may be q u i t e d i f f e r e n t . I n s u c h c a s e s , t h e a s s i g n m e n t o f t h e PE s p e c t r u m w o u l d be t h e s a m e . E x p e r -i e n c e 7 9 h a s s h o w n t h e I P ' s c a l c u l a t e d by C N D O / 2 7 6 a n d I N D O 7 6 m e t h o d s a r e g e n e r a l l y ^4 eV l a r g e r t h a n t h e e x p e r i m e n t a l v a l u e s . D e s p i t e t h e g e n e r a l s u c c e s s o f KT f o r c l o s e d - s h e l l m o l e c u l e s , i t s b r e a k d o w n i n p r e d i c t i n g t h e r i g h t o r d e r i n g o f 80 t h e i o n i c s t a t e s i s n o t u ncommon. T h e c l a s s i c a l c a s e i s N 2 l , 2 2 + w h e r e KT p r e d i c t s r a t h e r t h a n l,^ b e i n g t h e g r o u n d i o n i c 3 1 3 3 s t a t e . O t h e r n o t a b l e e x a m p l e s a r e i n F 2 , C 0 2 , C S 2 , a n d O C S 3 . An e x t r e m e c a s e i s p r o v i d e d by b i s ( i r - a l l y l ) n i c k e l , - 1 8 -82 whose f i r s t IP at 7.85 eV corresponds to the almost pure Ni-3d o o orb i ta l ( 9 0 o f the neutral molecules. Th is , in f a c t , i s the 13 t - 1 o rb i ta l counted from the highest occupied o rb i ta l (7a„) . Ab i n i t i o c a l c u l a t i o n s 4 4 showed that e(9a ) = -18.3 u 9 8 4 eV, whereas separate ca lcu lat ions of the ion and molecule gave the same IP at 7.92 eV after re laxat ion of the e lec t rons . This value is then in good agreement with experiment. Some guidel ines in the safe use of KT and in forecast ing the s i tuat ions where KT is mostly l i k e l y to f a i l have been given 35-88 1n the l i t e r a t u r e . In general , KT is mostly l i k e l y to f a i l when the ion izat ion perturbation gives r i se to large orb i ta l reorganizat ion. In p a r t i c u l a r , this includes the cases when the electron being removed is l o c a l i z e d in one 88 part of the molecule , e i ther due to symmetry reasons or because i t is a ' lone p a i r ' e l e c t r o n . For the same reason, molecules containing e lec t ron- r i ch atoms l i k e F tend to deviate from KT. Par t i cu lar care must be exercised in the case where two or more o rb i ta l s l i e c lose together, and where more often than not the r e l a t i v e ordering is problemat ica l , leading to doubts in assigning the experimental I P ' s . (c) Other Methods For Ca lcu lat ing Ver t i ca l IP's It is obvious that the correct way of gett ing accurate IP's is to perform independent ca lcu la t ions for the molecule and the indiv idual state of the ion , inc luding the c o r r e l a t i o n energy and, perhaps, r e l a t i vist.ic energy. Except for simple - 19 -m o l e c u l e s , t h i s p r o c e s s i s t e d i o u s a n d e x p e n s i v e , i f f e a s i b l e , a n d i s n o t e x p e c t e d t o be p o p u l a r i n t h e n e a r f u t u r e . P a r t l y b e c a u s e o f t h i s , c o n s i d e r a b l e e f f o r t h a s b e e n r e c e n t l y a t t e m p t e d t o c o r r e c t t h e i n e f f i c i e n c y o f K T . One a p p r o a c h 8 9 " 9 6 9 7 9 P h a s i n v o k e d t h e m a n y - b o d y t e c h n i q u e s o f s e c o n d q u a n t i z a t i o n b y w h i c h t h e v e r t i c a l I P i s d i r e c t l y r e l a t e d t o t h e p o l e s o f 9 9 t h e o n e p a r t i c l e G r e e n ' s f u n c t i o n . A p p l i c a t i o n o f t h i s 92 93 m e t h o d t o Ng a n d s h o w e d v e r y g o o d a g r e e m e n t w i t h e x p e r i -m e n t . T h i s m e t h o d a p p e a r s t o be v e r y p r o m i s i n g i n s p i t e o f i t s r a t h e r e l a b o r a t e c o m p u t a t i o n s . A n o t h e r a p p r o a c h 1 0 0 ' 1 0 ^ u s e d t h e R a y l e i g h t S c h r o d i n g e r p e r t u r b a t i o n t h e o r y t o c o r r e c t t h e w a v e f u n c t i o n s a n d e n e r g i e s o f t h e i o n a n d m o l e c u l e . W i t h f i r s t o r d e r c o r r e c t i o n s t o t h e w a v e f u n c t i o n , t h i r d o r d e r c o r r e c t i o n t o t h e e n e r g y , a n d u s i n g a b a s i s s e t o f d o u b l e o r o n e a n d h a l f z e t a S l a t e r o r b i t a l s f o r t h e c a l c u l a t i o n , t h i s m e t h o d was a b l e t o r e p r o d u c e e x p e r i m e n t a l I P ' s w i t h a n a b s o l u t e d e v i a t i o n o f l e s s t h a n 0.5 e V ^ . ( d ) O p e n S h e l l M o l e c u l e s F o r o p e n s h e l l m o l e c u l e s , t h e r e a r e two m o r e d i f f i -c u l t i e s i n a p p l y i n g KT i n a d d i t i o n t o t h e t h r e e l i m i t a t i o n s m e n t i o n e d i n t h e c a s e o f c l o s e d s h e l l m o l e c u l e s . F i r s t , i f T O ? ind t h e r e s t r i c t e d S C F f o r m a l i s m ' i s u s e d , t h e o f f - d i a g o n a l m u l t i p l i e r s e ^ ' s , w h i c h a r e i n t r o d u c e d a s t h e L a g r a n g i a n m u l t i p l i e r s 1 0 5 by t h e c o n s t r a i n t s o f t h e o r t h o g o n a l i t y o f o r b i t a l s , c a n n o t a l w a y s be e l i m i n a t e d a s t h e y a r e i n t h e - 20 -c l o s e d s h e l l c o u n t e r p a r t s . U n d e r s u c h c i r c u m s t a n c e s , a s a r e s u l t , n o p h y s i c a l m e a n i n g c a n b e a t t a c h e d t o t h e d i a g o n a l e ^ - j ' s , w h i c h c a n be i n t e r p r e t e d a s o r b i t a l e n e r g i e s w hen E .• j ' s h a v e b e e n d i a g o n a l i z e d . T h e r e f o r e , i n e s s e n c e , KT d o e s n o t h o l d . E v e n i f t h e e ^ . ' s c a n be e l i m i n a t e d , KT a p p l i e s o n l y i n a l i m i t e d s e n s e . G e n e r a l l y , i t h o l d s f o r t h e o p e n s h e l l o r b i t a l s a n d n o t f o r t h e c l o s e d s h e l l o r b i t a l s i n t h e same m o l e c u l e . I f t h e u n r e s t r i c t e d S C F f o r m a l i s m 1 0 3 , 1 0 6 i s u s e d , t h e n e . . ' s c a n be d i a g o n a l i z e d , a n d t h e p h y s i c a l i n t e r p r e t a -t i o n o f o r b i t a l e n e r g i e s i s p o s s i b l e . H o w e v e r , d u e t o t h e f a c t t h a t o r b i t a l s o f d i f f e r e n t s p i n s a r e d i f f e r e n t a n d t h u s h a v e d i f f e r e n t e n e r g i e s , t h e o n e - t o - o n e c o r r e s p o n d e n c e b e t w e e n t h e o r b i t a l e n e r g i e s a n d t h e I P ' s i s l o s t . S e c o n d l y , i n o r d e r t h a t t h e y be t h e e i g e n f u n c t i o n s o f t h e s p i n o p e r a t o r s 2 S a n d S z > t h e w a v e f u n c t i o n s o f t h e o p e n s h e l l s t a t e s may n o t be s i n g l e S l a t e r d e t e r m i n a n t s . As a r e s u l t , t h e e n e r g y e x p r e s s i o n f o r s u c h s t a t e s c o n t a i n s c r o s s t e r m s d u e t o t h e m a t r i x e l e m e n t s b e t w e e n t h e c o m p o n e n t d e t e r m i n a n t s , a n d t h u s KT u s u a l l y d o e s n o t a p p l y . I n p r a c t i c e , t h e f i r s t d i f f i c u l t y i s n o t a s b a d , a s i t l o o k s s i n c e t h e o r b i t a l e n e r g i e s c a l c u l a t e d i n b o t h f o r m a l i s m s o f t e n s h ow q u a l i t a t i v e a g r e e m e n t s w i t h t h e e x p e r i m e n t a l I P ' s . An a p p r o x i m a t e way o f e s t i m a t i n g I P ' s f o r c l o s e d s h e l l o r b i t a l s i s t o c a l c u l a t e t h e m as e x c i t a t i o n e n e r g i e s f r o m t h e g r o u n d i o n i c s t a t e u s i n g i t s w a v e f u n c t i o n ( f r o z e n o r b i t a l a p p r o x i m a t i o n ) . T h i s was i n f a c t t h e c a s e i n t h e s t u d y o f t h e r a d i c a l s N F 2 1 0 7 , C I O , , 1 0 8 a n d N 0 2 1 0 9 , w i t h - 21 -l imited success. Natura l ly , the proper way would again be to carry out independent ca lcu lat ions for the molecule and the indiv idual ion ic s ta tes . But then, besides being tedious and expensive, the ca lcu la t ion may have the added problem of convergence. This d i f f i c u l t y was exemplif ied in the study of N O , , 1 0 9 . 2.2.3 Franck-Condon Pr inc ip le in PES (a) Wave Mechanical Formulation of The Franck-Condon (FC)  P r i n c i p l e 5 0 , 5 1 . The probab i l i ty of a t r a n s i t i o n between two states is proportional to the square of the e l e c t r i c dipole t r a n s i t i o n integral R given by R = y i ' ( q , Q ) ( R n + R e)^"(q,Q)dqdQ . (2.2.14) In this integral i^'(q,Q) and V ( q , Q ) are the tota l wavefunctions describing the upper and lower quantum states , respect ive ly , indicatedias functions o f the t o t a l i t y of e lec t ron ic coordinates q and the t o t a l i t y of nuclear coordinates Q, and Rn and Rg are the nuclear and e lect ron ic dipole moment operators r e s p e c t i v e l y . Using the Born-Oppenheimer a p p r o x i m a t i o n 1 1 0 , the wavefunction can be separated into a product of e lec t ron ic and nuclear functions giving - 22 -* ( q , Q ) = '!> ( q , Q ) * n ( Q ) . ( 2 . 2 . 1 5 ) S e p a r a t i n g t h e n u c l e a r f u n c t i o n i n t o t h e v i b r a t i o n a l a n d r o t a t i o n a l p a r t s , t h e e x p r e s s i o n t h e n b e c o m e s * ( q , Q ) = * e ( q , Q ) * v ( Q ) * r o t ( Q ) , ( 2 . 2 . 1 6 ) w h e r e i n t e r a c t i o n s b e t w e e n e l e c t r o n i c , v i b r a t i o n a l a n d r o t a t i o n a l m o t i o n h a v e b e e n n e g l e c t e d . T h e r o t a t i o n a l w a v e f u n c t i o n c a n be n e g l e c t e d ^ 1 , s i n c e t h e r e l a t i v e i n t e n s i t i e s o f t h e v a r i o u s v i b r a -t i o n a l p e a k s a r e o f i n t e r e s t h e r e . T h u s we h a v e R• = fye ( q , Q ) * ; . ( Q ) ( R n + R e ) « ; ( q . Q ) * 1 ; ( Q ) d q d Q , ( 2 . 2 . 1 7 ) w h i c h u p o n e x p a n d i n g g i v e s R = 7*; ( Q ) R n * v ( Q ) d Q ^ e ( q . Q ) ^ e + ' . y * v ( Q ) * v ( Q ) d Q / * e ( q , Q ) R e * ^ ( q , Q ) d q . ( 2 . 2 . 1 6 ) By t h e o r t h o g o n a l i t y o f ^ a n d ^ , t h e f i r s t t e r m v a n i s h e s . D e n o t i n g t h e e l e c t r o n i c i n t e g r a l ( t h e e l e c t r o n i c t r a n s i t i o n m o m e n t ) i n t h e s e c o n d t e r m - 23 -R e ( Q ) f^'e ^»Q) R e*e ^»Q) d ( l» (2.2.19) and expanding i t in a Taylor ser ies about the equi l ibr ium nuclear posit ion QQ gives • ( - ) VdQ / w o R e(Q) - R e (Q 0 ) + — ( Q " Q o ) + (2.2.20) 51 The FC p r i n c i p l e is based on the assumption that the var iat ion of Rg with Q is slow, and thus a l l except the f i r s t term can be dropped, giving R - R e ( Q 0 ) / i < ; dQ. (2.2.21) The v ibrat iona l t rans i t ion probab i l i t y or in tens i ty is then V-HV" " 17* i *vdQ|2 • (2.2.22) Equation (2.2.22) embodies the wave mechnical descr ipt ion of the FC p r i n c i p l e : The in tens i ty of a v ibrat iona l peak in an e l e c t r o n i c a l l y allowed t rans i t ion is proportional to the absolute square of the overlap i n t e g r a l , i . e . the FC f a c t o r , of the corresponding v ibrat ional wavefunctions of the i n i t i a l and f i n a l s ta tes . The FC factor i s , in general , appreciable only when the maxima or minima of the two wavefunctions l i e on top - 24 -o f each o t h e r , i . e . a t the same Q, and t h e s e maxima o r minima f o r t h e h i g h e r v i b r a t i o n a l l e v e l s a p p e a r n e a r c l a s s i c a l t u r n i n g p o i n t s o f the m o t i o n , and thus v e r t i c a l e l e c t r o n i c t r a n s i t i o n s a r e u s u a l l y the most f a v o r a b l e . T h i s i s i n agreement w i t h t h e 50 r e s u l t o f the p r i m i t i v e f o r m o f t h e FC p r i n c i p l e , s t a t i n g t h a t t h e e l e c t r o n i c t r a n s i t i o n i s so r a p i d w i t h r e s p e c t to the v i b r a -t i o n a l m o t i o n t h a t t h e m o l e c u l e m a i n t a i n s a f i x e d geometry i m m e d i a t e l y a f t e r t h e t r a n s i t i o n . (b) q u a l i t a t i v e A p p l i c a t i o n Of The FC P r i n c i p l e In PES The i n f l u e n c e o f t h i s p r i n c i p l e on t h e v i b r a t i o n a l p a t t e r n s and band shapes o b s e r v e d i n t h e PE s p e c t r u m o f a d i a t o m i c m o l e c u l e i s i l l u s t r a t e d i n F i g . 1. The shaded a r e a , f r e q u e n t l y c a l l e d t h e FC r e g i o n , r e p r e s e n t s t h e v e r t i c a l t r a n -s i t i o n s a l l o w e d by t h e FC p r i n c i p l e . The d i f f e r e n t band s t r u c t u r e s are d e t e r m i n e d by t h e p o t e n t i a l c u r v e o f the r e s u l t i n g i o n , which i s i n t u r n d e t e r m i n e d by the e q u i l i b r i u m i n t e r n u c l e a r d i s t a n c e and v i b r a t i o n a l f r e q u e n c y o f the i o n . Depending upon w h e t h e r t h e e j e c t e d p h o t o e l e c t r o n came from a b o n d i n g , a n t i b o n d i n g o r n o n b o n d i n g o r b i t a l , the e q u i l i b r i u m i n t e r n u c l e a r d i s t a n c e i n the i o n may be g r e a t e r , s m a l l e r o r a p p r o x i m a t e l y t h e same as t h a t i n t h e p a r e n t m o l e c u l e , w h i l e t h e o p p o s i t e i s t r u e f o r t h e v i b r a t i o n a l f r e q u e n c y due t o t h e c o r r e s p o n d i n g change i n f o r c e c o n s t a n t . These changes i n geometry and v i b r a t i o n a l f r e q u e n c y w i l l t h e n d e t e r m i n e , t h r o u g h t h e FC p r i n c i p l e ( s e e a l s o the f o l l o w i n g s e c t i o n ) , where t h e - S2 -- 26 -ion w i l l be formed on the potential curve, giv ing r i s e to the various PE band structures as depicted in F i g . 1. Thus from the PE band shape and the v ibrat iona l frequency, the bonding char-acter of the corresponding orb i ta l from which the photoelectron is removed can frequently be deduced. In the case of polyatomic molecules, the c o r r e l a t i o n between band shapes, v ibrat ional f requencies, and bonding character becomes somewhat more complicated since the t rans i t ions now take place between multidimensional potential sur faces , and changes in bond lengths and bond angles are poss ib le . Several v ibrat iona l modes may therefore be excited simultaneously. In a d d i t i o n , the bonding character of the MO's are now only meaningful between neighboring atom pairs in a molecule. Indeed, the same o r b i t a l can have d i f f e r e n t bonding character between d i f fe rent atom p a i r s , which means that , within the same band, one p a r t i c u l a r v ibrat ional frequency may increase and another decrease, and the band structures of d i f f e r e n t v ibrat iona l progressions may be d i f f e r e n t . In general , however, the same pr inc ip le appl icable to the diatomic case may be used as a useful guide to the orb i ta l nature in polyatomics. (°) Quantitative Appl icat ion Of The FC P r i n c i p l e In PES According to the equation ( 2 . 2 . 2 2 ) , the r e l a t i v e v ibrat iona l t rans i t ion p r o b a b i l i t i e s or i n t e n s i t i e s within a PE band are proportional to the corresponding FC f a c t o r s . - 27 -These FC factors can be evaluated provided the appropriate v ibrat iona l wavefunctions are a v a i l a b l e . By c a l c u l a t i n g and f i t t i n g FC factors to the experimentally determined values, the geometry of the upper state can be estimated. This p r i n c i p l e has, in f a c t , been frequently used to determine the geometries 112-11 c of excited e lect ron ic states observed in e lect ron ic spectra The same c o n c e p t 1 1 6 is also appl icable in PES for estimating the geometry of the ionic s ta tes . This is p a r t i c u l a r l y a t t r a c -t ive since the structures of ions are normally d i f f i c u l t to obta in , espec ia l l y for polyatomi'cs. The main task in such a quant i tat ive invest igat ion is the evaluation of the overlap integral where Q' and Q" denote the t o t a l i t y of normal coordinates in the ionic and molecular states respect ive ly . Assuming that the v ibrat iona l wavefunctipn of each state has the form ( 2 . 2 . 2 3 ) n <J>V = n * i ( Q . ) , ( 2 . 2 . 2 4 ) then the overlap integral becomes n ( 2 . 2 . 2 5 ) - 28 -I n o r d e r t o e v a l u a t e t h i s i n t e g r a l i t i s n e c e s s a r y t o s p e c i f y t h e t r a n s f o r m a t i o n b e t w e e n Q'.'s a n d O V ' s , w h i c h a r e r e l a t e d b y 1 1 7 Q' = AQ" + d ( 2 . 2 . 2 6 ) w h e r e A i s an nXn s q u a r e m a t r i x a n d d i s a c o l u m n v e c t o r o f o r d e r n . N o r m a l l y A . , i s d i f f e r e n t f r o m z e r o o n l y when t h e i * * 1 a n d j * * 1 n o r m a l c o o r d i n a t e s h a v e t h e same s y m m e t r y . F o r n o n - t o t a l l y s y m m e t r i c Q'. , d . m u s t be z e r o b y s y m m e t r y . S i n c e V ' i s u s u a l l y a v i b r a t i o n l e s s g r o u n d s t a t e a n d i s l a r g e l y c o n c e n t r a t e d a r o u n d t h e o r i g i n o f t h e n o r m a l c o o r d i n a t e s , t h e o v e r l a p i n t e g r a l i s d o m i n a t e d b y t h e c o n t r i -b u t i o n i n t h e r e g i o n o f Q" = 0 a n d Q' = d . I n t h i s c a s e t h e i n t e g r a l w i l l b e i n s e n s i t i v e t o t h e n a t u r e o f A, s i n c e t h e w a v e f u n c t i o n s a r e t h e p r o d u c t s o f e x p o n e n t i a l t e r m s . T h e i n t e g r a l may t h e n be a p p r o x i m a t e d b y c h o o s i n g A t o be a u n i t m a t r i x , g i v i n g Qj = QJ + d . . ( 2 . 2 . 2 7 ) C o n s e q u e n t l y , M s e p a r a t e s i n t o n c o m p o n e n t o v e r l a p i n t e g r a l s n = n M . ( v ! , v; 1) , ( 2 . 2 . 2 3 ) . 1 = 1 1 1 J - 29 -w h e r e M i ( v i ' v j ) = ( Q ; - . ) * v » ( Q; - d i ) D Q ; • ( 2 . 2 . 2 9 ) T h e o b s e r v e d FC f a c t o r s i n t h e PE b a n d may t h e n be a n a l y z e d i n t e r m s o f t h e s q u a r e s o f t h e s e o n e - d i m e n s i o n a l i n t e g r a l s t o g i v e d . ' s . 118 F o r s y m m e t r y c o o r d i n a t e s , we h a v e S 1 = S" + D , ( 2 . 2 . 3 0 ) a n d i f S' = L ' Q ; , S" = L " Q " , ( 2 . 2 . 3 1 ) t h e n Q ' = (LTV'-Q" + ( L 1 ) D ( 2 . 2 . 3 2 ) h e n c e A = ( L ' J ^ L " ( 2 . 2 . 3 3 ) i. d = ( L ' ) _ 1 D o r D = L ' d . ( 2 . 2 . 3 4 ) T h e r e f o r e , a k n o w l e d g e o f t h e d ^ ' s c a n be r e l a t e d t o t h e c h a n g e i n t h e s y m m e t r y c o o r d i n a t e s D i ' s , f r o m w h i c h t h e c h a n g e - 30 -in internal coordinates can in turn be c a l c u l a t e d . The L matrix, which is the eigenfunction of the FG 118 secular equations , is a function of the v ibrat iona l f r e -quencies and force f i e l d s , which are normally not ava i lab le for the ion ic s tates . However, i t has been found that D is quite insens i t ive to L ' . Therefore, for the approximate work undertaken here, the L' matrix for the ion i s approximated by that of the neutral molecule. In add i t ion , for s i m p l i c i t y , the v ibrat iona l wave-functions are approximated by that of a harmonic o s c i l l a t o r , i . e . *1 = N v . H v . ( a i ° i ) expC-1/2 a 2 Q 2 ] , (2.2.35) in which N y > = ( ^ / i r 1 * 2 V i ^ \ ) h , and 2 = 4TT cv^/h The H y (c^Q..) are Hermite polynomials, the v ibrat iona l i -1 quantum numbers, v ibrat ional frequencies in cm , and c is the speed of light-. The component integral can then be 119 expressed in the closed form , - 31 -/, ,2\(V + v " ) / 2 C v ^ v y y V ' -A u , < : l ) i ( v - - t ) i H y . . i [ i » 2 T ( i - » 4 ) - ' ' ] H V „ . J I [ - 6 2 T ( 1 - 8 4 ) " ' 5 ] , ( 2 . 2 . 3 6 ) w h e r e K W ,.,„ = (VIVO* 2 - ( v ' + v " - ^ / 2 ( 3 / ( H 3 2 ) ) ^ e x p [ ( - i / 2 ) e 2 Y 2 / ( i + e 2 ) ] [ v ' , v " ] d e s i g n a t e s t h e s m a l l e r o f t h e two i n t e g e r s v ' a n d v " , a n d 6 = o ' V o ' . Y = - a ' d . ( 2 . 2 . 3 7 ) E q . ( 2 . 2 . 3 6 ) r e d u c e s , when v " * 0, t o , . \ v ' ( l - v ' ) / 2 / \% 2 2 M ( v . > 0 ) = k l l _ 2 _ P _ \ e x p [ . ^ 1 ^ ] . ( v ' ! ) 1 \ i + r / 1 + e 2 . v ' / 2 ^ 1 ^ . ^ . H v , [ i B 2 Y ( l - 3 4 ) ' % ] . ( 2 . 2 . 3 8 ) - 32 -The main ca l cu la t ion is carr ied out by a computer program ( Appendix ) and the procedure i s summarized:-(1) d.. is set equal to zero for the v ibrat iona l mode not excited in the ion . (2) The FC factors (FCF) for any combination level are ca lculated as the product of those for the corresponding overtone l e v e l s . (3) For each excited v i b r a t i o n , the FCF are ca lcu lated at a ser ies of d. values. The best d. value is then chosen on the basis of two c r i t e r i a : - (a) The overal l q u a l i t a t i v e features of the experimental FCF are followed in the cal -culated FCF. (b) The standard deviat ion of the ca lcu lated FCF from the experimental FCF is a minimum. (4) D^'s are evaluated from the d. ' s . ( 5 ) The change in internal coordinates are ca lculated from the D i ' s . N (6) Since the sign of each d.. is undetermined there are 2 possible ionic state iwodels consistent with the observed FCF, where N is the number of v ibrat iona l modes of the same symmetry exc i ted. The choice of the model is based on the bonding, character of the MC from which the e lectron 1s removed. The method outl ined above has been used wherever appl icable 1n this study to estimate the geometry of the ions . By comparing with the experimental data, i t w i l l be shown (section 5.2.4) that this method is quite r e l i a b l e in deter-- 3 3 -o mining the ionic geometries, with a maximum deviat ion of 0 . 0 0 6 A in bond lengths. 2 . 2 . 4 Select ion Rules In PES (a) Select ion Rules In E lectron ic Exc i tat ions It is usual ly assumed that the i n i t i a l (neutral) and f ina l ( ion ic) states are represented by s ingle S later determinants. The i photoionization process can then be described as the exc i tat ion of an e lectron from the o rb i ta l <J>J to a continum photoelectron o rb i ta l <j>p^. The t r a n s i t i o n probab i l i ty for the process i s proportional to the square of the e lec t ron ic t r a n s i t i o n moment R& (eq. ( 2 . 2 . 1 9 ) ) , and using the above assumption, we have, Trans i t ion probab i l i ty « [ / i g - . W ^ e ( N ) d q | 2 • l / * i R e * P h d q | 2 | / * e ( N - 1 ^ ! ( N - D d q | 2 ( 2 . 2 . 3 9 ) = i y*lVPh d q| 2* M.J.L/*J*J d q | 2 * ( 2 - 2 ' 4 0 ) where ^ ( N - 1 ) is the N - 1 e lectron remainder of the i n i t i a l state function ii*1 ( N ) of an N e lectron molecule and $ ( N - 1 ) e e is the wavefunction of the ion . The properties of the i n t e -grals in equations ( 2 . 2 . 3 9 ) and ( 2 . 2 . 4 0 ) d ic tate the fol lowing - 34 -se lect ion rules on the d i f ference between ^ (N) (molecule) and *e ( N - U ( i o n ) . ( 1 ) The change in the total spin angular momentum must be ±h due to the dipole matrix. ( 2 ) The overlap integral in eq. ( 2 . 2 . 3 9 ) requires that the 'pass ive ' o r b i t a l s have the same symmetry. ( 3 ) In the frozen orb i ta l approximation (KT), the overlap Integral in eq. ( 2 . 2 . 4 0 ) requires that the ion and molecule d i f f e r only in one o r b i t a l . In short , under the assumptions of the frozen orb i ta l approximation and that the wavefunctions are described by s ingle S later determinants of one electron o r b i t a l s , photo-ion lzat ion is a s ingle step one e lectron process, and the PE spectrum of a closed shel l molecule may be assigned on the basis of one IP per orb i ta l . However, mult i -e lectron ( i . e . two or more) processes may be allowed i f e lectron cor re la t ion is taken into account in the wavefunctions. The inc lus ion of e lectron c o r r e l a t i o n is usually accomplished by the method of conf igurat ion i n t e r -120 - 1 ? S action (CI) ~ * in that the ionic wavefunction is construc-ted as a l inear combination of the conf igurat ions corresponding to the possible excited ionic states of the same symmetry, i . e . ^ ( N - l ) = £ C. ( N - 1 ) . ( 2 . 2 . 4 1 ) J 4 1 J I - 35 -The t r a n s i t i o n p r o b a b i l i t y t o a CI s t a t e i s g i v e n by * y * 1 R e * P h d q l 2 l / * e ( N - 1 > ' B :ij*i ) d q | 2 . ( 2 . 2 . 4 2 ) The o v e r l a p i n t e g r a l , and thu s t h e t r a n s i t i o n p r o b a b i l i t y w i l l n o t be z e r o i f t h e CI w a v e f u n c t i o n c o n t a i n s a t l e a s t one i o n i c s t a t e w hich i s formed by a s i n g l e e l e c t r o n p r o c e s s . For such a CI s t a t e t h e p r o b a b i l i t y then r e d u c e s to ' y * 1 R e * P h d q ' 2 l / * e ( N - l ) * " ^ ( N - l ) d q | 2 c f 0 , ( 2 . 2 . 4 3 ) where ^ i s t h e w a v e f u n c t i o n o f the a l l o w e d i o n i c s t a t e , and i t s v a l u e r e l a t i v e t o t h a t o f the l a t t e r ^ Q i s g i v e n by t h e s q u a r e o f t h e i r o v e r l a p i n t e g r a l , i . e . l / * o ( N - D ^ i ^ N - D d q l 2 = C 2 Q ( 2 . 2 . 4 4 ) Q u a l i t a t i v e l y , m u l t i - e l e c t r o n p r o c e s s e s can be t h o u g h t t o o c c u r due t o p e r t u r b a t i o n s i n d u c e d by the p h o t o -i o n i z a t i o n p r o c e s s . T h e r e f o r e , one would e x p e c t t h e s e p r o c e s s e s t o become more p r o b a b l e when more e n e r g e t i c i o n i c s t a t e s a r e formed by u s i n g more e n e r g e t i c l i g h t s o u r c e s . In p r a c t i c e t h i s i s what i s o b s e r v e d . Thus m u l t i - e l e c t r o n p r o c e s -1 2 ses a r e q u i t e common i n ESCA * (>1000 e V ) , but e x c e p t i o n a l l y few have been c l a i m e d 1 2 5 " 1 2 7 t o o c c u r i n H e l ( 2 1 . 2 2 eV) PES. However, two e l e c t r o n p r o c e s s e s have r e c e n t l y been f o u n d - 36 -to occur quite frequently in Hell (40.8 eV) PES 1 ^ ° . (b) Selection Rules In Vibrational Excitations The selection rules in vibrational excitations are determined by the vibrational overlap integral (eq. (2.2.25)), In order for M not be zero, the integral must be totally symmetric. Usually the initial state is a vibrationl ess ground state, whose vibrational wavefunction is totally symmetric. Hence the vibrational wavefunction of the ion must be totally symmetric as well. This requirement leads to the following 129 selection rules . (1) Totally symmetric vibrations may assume any quantum number. (2) Non-totally symmetric vibrations can only assume zero or even quantum numbers, whose wavefunctions are then totally symmetric. (3) For combination bands, i .e . simultaneous excitation of two or more vibrational modes, of non-totally symmetric modes, the transition is allowed i f the total wavefunction is totally symmetric. requirement that the overlap integral must be totally symmetric can be used to select the allowed transitions. n For a vibrationally excited init ial state, the - 37 -2- 3 L i n e w i d t h s A n d R e s o l u t i o n In PES L i k e a n y o t h e r k i n d o f s p e c t r o s c o p i c m e a s u r e m e n t s , t h e o b s e r v e d l i n e w i d t h o f a p e a k i n PES i s a c o n v o l u t i o n o f t h e i n s t r u m e n t a l f u n c t i o n a n d t h e i n h e r e n t l i n e w i d t h o f t h e t r a n s i t i o n . T h e f o r m e r i s a m e a s u r e o f t h e i n s t r u m e n t a l r e s o -l u t i o n , w h i l e t h e l a t t e r i s a f u n c t i o n o f t h e n a t u r e o f t h e t r a n s i t i o n a n d t h e m o l e c u l e u n d e r c o n s i d e r a t i o n . I t i s i m m e d i a t e l y o b v i o u s t h a t t h e o b s e r v e d l i n e w i t h c a n o n l y be r e d u c e d a t t h e e x p e n s e o f t h e i n s t r u m e n t a l r e s o l u t i o n , a n d i f t h e l a t t e r h a s b e e n r e d u c e d t o t h e n e g l i g i b l e l i m i t , t h e o b s e r v e d l i n e w i d t h w i l l t h e n be t h e i n h e r e n t l i n e w i d t h . T h e i n h e r e n t l i n e w i d t h i s d e t e r m i n e d t h r o u g h t h e H e i s e n b e r g u n c e r t a i n t y p r i n c i p l e by t h e l i f e t i m e o f t h e i o n i c 1 30 s t a t e . D e p e n d i n g u p o n t h e l i f e t i m e s e n c o u n t e r e d , t h i s may r a n g e f r o m a l i n e b r o a d e n i n g o f 1 0 ~ 7 eV f o r a f l u o r e s c e n t i o n i c s t a t e , up t o 0.4 eV f o r a r a p i d l y d i s s o c i a t i n g i o n i c s t a t e . B e c a u s e t h e u l t i m a t e t h e o r e t i c a l i n s t r u m e n t a l r e s o -l u t i o n o b t a i n a b l e i s g r e a t e r t h a n 1 meV ( s e e b e l o w ) , o n e may a l s o c o n s i d e r t h e l i n e s t r u c t u r e s w h i c h a r e c o m p a r a b l e t o t h i s l i m i t t o be i n h e r e n t l i n e w i d t h s . F o r c o m p l e x m o l e c u l e s , t h e i n h e r e n t l i n e w i d t h may t h e n i n c l u d e r o t a t i o n a l e n v e l o p e s , c l o s e l y s p a c e d u n r e s o l v e d v i b r a t i o n a l s t r u c t u r e s , s p i n - o r b i t s p l i t t i n g s , J a h n - T e l l e r a n d R e n n e r - T e l 1 e r s p l i t t i n g s . I n P E S , t h e i n s t r u m e n t a l r e s o l u t i o n i s u s u a l l y d e f i n e d a s A E - j ^ / E , w h e r e A E ^ 2 a n c i E a r e r e s p e c t i v e l y t h e f u l l w i d t h a t h a l f maximum (FWHM) a n d t h e k i n e t i c e n e r g y o f t h e - 38 -e l e c t r o n s , b o t h m e a s u r e d i n e V . H o w e v e r , i t i s c u s t o m a r y t o u s e s i m p l y A E - J ^ a s a m e a s u r e o f r e s o l u t i o n , a n d i n g e n e r a l 2 ? t h e FWHM o f t h e a r g o n d o u b l e t ( P 3 / 2 ' 1/2^ ^ s u s e d a s a u n i v e r s a l t e s t . T h e v a r i o u s p o s s i b l e f a c t o r s t h a t l i m i t t h e u l t i m a t e I n s t r u m e n t a l r e s o l u t i o n h a v e b e e n d i s c u s s e d i n d e t a i l 1 3 0 131 b y T u r n e r a n d S a m s o n . I n p r a c t i c e , t h e f o l l o w i n g a r e t h e m o s t i n f l u e n t i a l a n d p o s e t h e m a j o r p r o b l e m s . ( 1 ) T h e D o p p l e r e f f e c t d u e t o t h e t h e r m a l m o t i o n o f t h e m o l e c u l e a n d t h e v e l o c i t y o f t h e p h o t o e l e c t r o n - T h e s e two p a r a m e t e r s c o u p l e t o g i v e t h e D o p p l e r w i d t h a c c o r d i n g t o t h e 131 f o r m u l a A E ] / 2 = 0 . 7 2 3 ( E T / M ) ' 5 meV , ( 2 . 3 . 1 ) w h e r e M i s t h e m o l e c u l a r w e i g h t o f t h e i o n , T i s t h e a b s o l u t e t e m p e r a t u r e , a n d E t h e k i n e t i c e n e r g y o f t h e p h o t o e l e c t r o n . T o g i v e a b e t t e r f e e l i n g f o r t h e m a g n i t u d e o f t h i s b r o a d e n i n g , A E ^ g i s p l o t t e d i n F i g . 2 f o r v a r i o u s v a l u e s o f M a s a f u n c t i o n o f e l e c t r o n k i n e t i c e n e r g y a n d t e m p e r a t u r e . T h i s b r o a d e n i n g c a n b e r e d u c e d b y c o o l i n g t h e s a m p l e o r u s i n g l e s s e n e r g e t i c l i g h t s o u r c e s , e . g . N e l i n s t e a d o f H e l t o p r o d u c e p h o t o e l e c t r o n s o f l o w e r e n e r g y . T h e D o p p l e r w i d t h c a n , i n p r i n c i p l e , be c o m p l e t e l y e l i m i n a t e d b y f o r m i n g t h e s a m p l e i n t o m o l e c u l a r beam a n d a n a l y z i n g t h e p h o t o e l e c t r o n s e j e c t e d p e r p e n d i c u l a r t o i t . - 39 -TEMPERATURE ( ° K ) FIGURE 2 - 40 -( 2 ) L i n e w i d t h o f t h e l i g h t s o u r c e d u e t o s e l f - a b s o r p t i o n -131 I t h a s b e e n s h o w n t h a t t h e common t y p e s o f l i n e b r o a d e n i n g e n c o u n t e r e d i n a l i g h t s o u r c e s u c h a s n a t u r a l , D o p p l e r , p r e s s u r e a n d s t a r k b r o a d e n i n g s a r e f a r t o o s m a l l ( < 1 0 e V ) c o m p a r e d t o t h a t d u e t o s e l f - a b s o r p t i o n f o r t h e H e l r e s o n a n c e l i n e . T h e l i n e w i d t h d u e t o t h e l a t t e r i n c r e a s e s w i t h t h e p r e s s u r e , t h e l e n g t h o f t h e d i s c h a r g e r e g i o n , a n d t h a t o f 1 31 t h e u n d i s c h a r g e d g a s c o l u m n . S u r p r i s i n g l y , t h e b r o a d e n i n g d o e s n o t s e e m t o d e p e n d on t h e n a t u r e o f t h e l i g h t s o u r c e n o r t h e d i a m e t e r o f t h e d i s c h a r g e c a p i l l a r y . D e p e n d e n t u p o n t h e a b o v e f a c t o r s , l i n e w i d t h c a n v a r y f r o m 1 meV up t o 7 meV. T h e l o w e r l i m i t c a n be o b t a i n e d a t t h e l o w e s t p o s s i b l e p r e s s u r e b y a m i c r o w a v e d i s c h a r g e , a l t h o u g h a t t h e e x p e n s e o f t h e a v a i l a b l e l i g h t i n t e n s i t y . ( 3 ) L i n e b r o a d e n i n g d u e t o t h e i n h o m o g e n e i t y o f t h e e l e c t r o -s t a t i c a n d m a g n e t i c f i e l d s - T h i s i n c l u d e s t h e e f f e c t s d u e t o t h e r e s i d u a l e a r t h m a g n e t i c f i e l d , i n h o m o g e n e o u s a n a l z e r s u r f a c e p o t e n t i a l , a n d t h e m i c r o p o t e n t i a l s o n t h e a p e r t u r e s . T h e l a s t e f f e c t i s p a r t i c u l a r l y i m p o r t a n t i n t h e c a s e o f t r a n s i e n t s p e c i e s , a n d i s d i f f i c u l t t o r e m e d y . H o w e v e r , i f c a r e f u l p r e c a u t i o n s a r e t a k e n t o e l i m i n -a t e o r r e d u c e t h e s e l i m i t i n g f a c t o r s , a n u l t i m a t e r e s o l u t i o n o f ~1 meV i s p o s s i b l e , w h i c h w o u l d be s u f f i c i e n t t o r e s o l v e r o t a t i o n a l s t r u c t u r e i n some c a s e s . To d a t e , t h e b e s t i n s t r u -m e n t a l r e s o l u t i o n a c h i e v e d h a s b e e n b e t w e e n 4-7 meV w i t h t h e 13 2 a r g o n d o u b l e t ' , h o w e v e r , a n o m i n a l r e s o l u t i o n o f 1 0 - 2 5 meV i s - 41 -c o n v e n i e n t l y o b t a i n a b l e i n m o s t l a b o r a t o r i e s , t h i s b e i n g c o n s i d e r e d a s ' h i g h r e s o l u t i o n ' PES ( m u c h a b o v e 3 0 - 4 0 meV i s c o n s i d e r e d m e d i u m r e s o l u t i o n ) . B e s i d e s t h e e x p e r i m e n t a l c o n s i d e r a t i o n s , t h e w o r k i n g r e s o l u t i o n c a n be i m p r o v e d b y t h e a n a l y t i c a l a n a l y s i s o f t h e r a w d a t a . T h e m e t h o d i s t o e l i m i n a t e t h e l i n e w i d t h 1 3 3 - 1 3 7 d u e t o i n s t r u m e n t a l f u n c t i o n b y d e c o n v o l u t i o n . T h e s u c c e s s o f d e c o n v o l u t i o n r e q u i r e s a n a c c u r a t e e s t i m a t i o n o f t h e i n s t r u m e n t a l f u n c t i o n , a n d t h a t t h e d a t a h a s a f a v o r a b l e s i g n a l t o n o i s e r a t i o . T h i s t e c h n i q u e h a s b e e n 1 3 7 u s e d s u c c e s s f u l l y t o r e c o v e r t h e s p i n - o r b i t s p l i t t i n g 2 + i n t h e X K s t a t e o f 0 o , a n d t h e r e s u l t i s c o m p a r a b l e t o g 2' 1 3 2 t h e v e r y h i g h r e s o l u t i o n s t u d y - 42 -C H A P T E R T H R E E THE S P E C T R O M E T E R AND PERFORMANCE 3 .1 I n t r o d u c t i on D u r i n g t h e c o u r s e o f t h i s s t u d y , two s p e c t r o m e t e r s h a v e b e e n e m p l o y e d . T h e f i r s t s y s t e m was u s e d f o r t h e i n i t i a l e x p l o r a t o r y w o r k a n d e x c e p t f o r some n o t a b l e m o d i f i c a t i o n s , i t s d e t a i l e d f e a t u r e s w e r e d e s c r i b e d i n C o r n f o r d ' s t h e s i s T h e e x p e r i e n c e g a i n e d t h r o u g h e x h a u s t i n g t h e c a p a b i l i t i e s o f t h a t u n i t h a s s u b s e q u e n t l y p l a y e d a s i g n i f i c a n t r o l e i n t h e d e v e l o p m e n t o f t h e s e c o n d s y s t e m , w h i c h i s t h e f o c u s o f t h i s c h a p t e r . 3- 2 G e n e r a l D e s i g n C o n s i d e r a t i o n s One o f t h e p u r p o s e s o f t h i s s t u d y was t o d e s i g n a n d c o n s t r u c t a 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 t h a t w o u l d be s u i t a b l e f o r t h e s t u d y o f t r a n s i e n t s p e c i e s a n d r e a c t i v e m o l e c u l e s . In t h e d e s i g n o f s u c h a s p e c t r o m e t e r , t h e r e a r e s e v e r a l s i g n i f i c a n t c o n s i d e r a t i o n s : - 43 -(1) T h e s p e c t r o m e t e r m u s t h a v e a f a s t p u m p i n g s y s t e m . T h i s i s e s s e n t i a l s i n c e t h e s p e c i e s o f i n t e r e s t h e r e a r e e i t h e r s h o r t l i v e d o r t e n d t o d e c o m p o s e e a s i l y on c o n t a c t w i t h s u r f a c e s . 3y f a s t e r p u m p i n g a n d h e n c e r e d u c i n g t h e t i m e b e t w e e n t h e p h o t o i o n i z a t i o n r e g i o n ( i o n i z a t i o n c h a m b e r ) a n d t h e r e g i o n w h e r e t h e y a r e p r o d u c e d ( t r a n s i e n t s p e c i e s ) o r s t o r e d ( r e a c t i v e m o l e c u l e s ) , t h e g r e a t e r t h e c h a n c e o f s u c c e s s f u l l y r e c o r d i n g a PE s p e c t r u m . A u s e f u l a d d i t i o n a l f e a t u r e i s t h e s h o r t pump-down c y c l e o f t h e s p e c t r o m e t e r , w h i c h i n v o l v e s a s h o r t e r t i m e e l a p s e b e f o r e t h e u n i t i s a g a i n i n o p e r a t i n g c o n d i t i o n a f t e r v e n t i n g a n d s e r v i c i n g . A f u r t h e r b o n u s o f t h e f a s t p u m p i n g s y s t e m i s a r e d u c t i o n i n t h e b u i l d - u p o f d e p o s i t s w h i c h may d e c r e a s e t h e e f f i c i e n c y o f t h e a n a l y s i s s y s t e m . (2) T h e s p e c t r o m e t e r s h o u l d h a v e a v e r s a t i l e s a m p l e i n l e t s y s t e m a n d an e a s i l y a c c e s s i b l e i o n i z a t i o n c h a m b e r . T h e i n d i v i d u a l t r a n s i e n t s p e c i e s t h a t a r e o f i n t e r e s t may be p r o d u c e d b y o n e o f s e v e r a l c o m p l e t e l y d i f f e r e n t m e t h o d s , s u c h a s m i c r o w a v e d i s c h a r g e , p y r o l y s i s e t c , o r may, f o r r e a s o n s o f o f c h e m i c a l r e a c t i v i t y , r e q u i r e t h e u s e o f a s p e c i f i c m a t e r i a l f o r t h e i n l e t s y s t e m a n d s a m p l e h o l d e r . I t i s t h e r e f o r e o f t h e e s s e n c e t h a t t h e s p e c t r o m e t e r c a n h a n d l e i t a l l t h e s e d i f f e r i n g s i t u a t i o n s ^ E a s y a c c e s s t o t h e i o n i z a t i o n c h a m b e r r e g i o n i s o f p a r t i c u l a r i m p o r t a n c e , s i n c e m o s t s a m p l e s , by v i r t u e o f t h e i r s h o r t l i f e t i m e s o r r e a c t i v i t y , m u s t be g e n e r a t e d as c l o s e t o t h e c h a m b e r a s p o s s i b l e . - 44 -( 3 ) T h e s p e c t r o m e t e r s h o u l d h a v e h i g h s e n s i t i v i t y a n d t h e c a p a b i l i t y o f h i g h r e s o l u t i o n ( b e t t e r t h a n 25 m e V ) . I t i s a b s o l u t e l y e s s e n t i a l t o h a v e h i g h s e n s i t i v i t y s i n c e m o r e o f t e n t h a n n o t t h e t r a n s i e n t s p e c i e s a r e p r o d u c e d o n l y w i t h l o w p a r t i a l v a p o r p r e s s u r e , e . g . s o m e t i m e s a s l o w a s 1% o f t h e p a r e n t m o l e c u l e . H i g h r e s o l u t i o n i s n e c e s s a r y t o r e s o l v e a n y f i n e b a n d s t r u c t u r e w h i c h i s n e c e s s a r y f o r a r i g o r o u s a n a l y s i s o f t h e s p e c t r u m . ( 4 ) T h e s p e c t r o m e t e r s h o u l d be as d u r a b l e a s p o s s i b l e w i t h r e s p e c t t o c o r r o s i v e v a p o r s a n d y e t a s s i m p l e a s p o s s i b l e i n c o n s t r u c t i o n . I t s h o u l d a l s o be e a s i l y a s s e m b l e d a n d d i s s e m b l e d s o t h a t c l e a n i n g , r e p a i r i n g a n d m o d i f y i n g b e c o m e s t r a i g h t f o r w a r d p r o c e d u r e s . ( 5 ) In a d d i t i o n t o t h e e a r t h ' s m a g n e t i c f i e l d , e t h e r m a g n e t i c f i e l d s may be i n t r o d u c e d b y c e r t a i n e x p e r i m e n t a l a r r a n g e m e n t s , e . g . by t h e c u r r e n t g e n e r a t e d i n a w i r e - w o u n d f u r n a c e . T h e s o u r c e o f t h i s f i e l d i s u s u a l l y s o c l o s e t o t h e e l e c t r o n t r a j e c t o r y i n t h e a n a l y z e r , t h a t i t m a k e s t h e m a g n e t i c s h i e l d i n g b y M u - m e t a l i n f e a s i b l e . C o n s e q u e n t l y , t h e e l i m i n a t i o n o f s t r a y m a g n e t i c f i e l d s i s b e s t a c h i e v e d b y t h r e e p a i r s o f H e l m h o l t z c o i l s m o u n t e d a t r i g h t a n g l e s t o e a c h o t h e r . T h e s e may be i n d i v i d u a l l y a d j u s t e d t o c o m p e n s a t e f o r a n y s u c h f i e l d e f f e c t s . ( 6 ) A m i n o r , b u t e q u a l l y i m p o r t a n t c o n s i d e r a t i o n i s t h e s i t i n g o f t h e s p e c t r o m e t e r i n a r e l a t i v e l y f i e l d - f r e e - 4 5 -r o o m , w i t h i n d e p e n d e n t p o w e r s u p p l i e s t h u s l i m i t i n g t h e i n t e r f e r e n c e o f o t h e r e l e c t r i c a l a p p a r a t u s . 3.3 T h e S p e c t r o m e t e r 3.3.1 T h e V a c u u m S y s t e m T h e v e r t i c a l c r o s s - s e c t i o n o f t h e s p e c t r o m e t e r t h r o u g h t h e e n t r a n c e a n d e x i t a p e r t u r e s o f t h e e l e c t r o n a n a l y z e r i s s h o w n i n F i g . 4. One d i s t i n c t f e a t u r e o f t h e v a c u u m s y s t e m i s t h a t i t d o e s n o t h a v e a s e p a r a t e v a c u u m h o u s i n g f o r t h e a n a l y z e r , i n c o n t r a s t t o o t h e r c o n v e n t i o n a l s p e c t r o m e t e r s . As a r e s u l t , t h e i o n i z a t i o n c h a m b e r i s w i t h i n e a s y a c c e s s a n d t h e p u m p i n g v o l u m e i s r e d u c e d c o n s i d e r a b l e , a f a v o r a b l e c o n d i t i o n f o r f a s t a n d e f f i c i e n t p u m p i n g . T h e i o n i z a t i o n c h a m b e r a n d t h e a n a l y z e r a r e s e p a r a t e d b y t h e c i r c u l a r a p e r t u r e s i n t h e i o n i z a t i o n c h a m b e r , l e n s a n d e n t r a n c e s l i t a s shown i n F i g . 4. T h e d i a m e t e r s o f t h e a p e r t u r e s i n t h e a b o v e - m e n t i o n e d o r d e r a r e r e s p e c t i v e l y 0 . 0 3 0 " , 0 . 1 0 0 " a n d 0 . 0 2 0 " . T h e i o n i z a t i o n c h a m b e r i s c o n n e c t e d t h r o u g h a 2" i . d . v a c u u m t h r o t t l e v a l v e ( V a r i a n A s s o c i a t e s ) t o a 4" o i l d i f f u s i o n pump ( C o o k e V a c u u m P r o d . , DP0 4 - 1 0 0 0 B ) w i t h a p u m p i n g s p e e d o f 1 0 0 0 1 / s e c ( a i r ) . T h e a n a l y z e r a n d t h e h o u s i n g f o r t h e c h a n n e l e l e c t r o n m u l t i p l i e r a r e pumped t h r o u g h a 1 5 / 3 " i . d . v a c u u m t h r o t t l e v a l v e ( V e e c o f o r g e d b r a s s a n g l e v a l v e , t y p e SL 1 5 0 S ) b y a 2" o i l d i f f u s i o n pump ('ARC IIS2) w i t h THE PHOTOELECTRON SPECTROMETER - 47 -VERTICAL CROSS-SECTION OF PE SPECTROMETER Discharge tube materials wwm brass sapphire balls ® O-rings m m teflon electron multiplier mylar lens system 2"Diffusion pump FIGURE 4 - 4fi -P U M P I N G P O R T S F O R T H E A N A L Y Z E R U N I T A N D G E N E R A L S A M P L E I N L E T S PUMPING PORTS VACUUM SEAL SAMPLE INLET - 49 -a p u m p i n g s p e e d o f 2 8 5 1 / s e c ( a i r ) . T h e s e d i f f u s i o n pumps a r e r e s p e c t i v e l y b a c k e d b y D u o - S e a l r o t a r y pumps ( W e l c h S c i e n t i f i c , m o d e l 1 3 9 7 a n d 1 4 0 2 ) . T h e s i x p u m p i n g p o r t s f o r t h e a n a l y z e r a r e s h o w n i n F i g . 5 a n d a r e a r r a n g e d s o a s t o h a v e t h e l e a s t p o s s i b l e d i s t u r b a n c e o n t h e e l e c t r o n t r a j e c t o r y . T h e s e p o r t s a n d t h e o n e f o r t h e m u l t i p l i e r h o u s i n g a r e l o n g c y l i n d e r s o f 3 / 8 " i n d i a m e t e r d r i l l e d a t an a n g l e t h r o u g h t h e s o l i d b r a s s b l o c k w h i c h i s a l s o a n i n t e g r a l p a r t o f t h e a n a l y z e r . An i o n i z a t i o n g a u g e (NRC 5 3 8 P ) s i t u a t e d b e l o w t h e s e p o r t s i s u s e d t o m e a s u r e t h e p r e s s u r e o f t h e s y s t e m . T h e v a c u u m s e a l s a r e e f f e c t e d b y V i t o n O - r i n g s , w i t h t h e h e l p o f b o l t s w h e r e v e r n e c e s s a r y . 3.3.2 T h e E l e c t r o n E n e r g y A n a l y z e r A 1 8 0 ° h e m i S D h e r i c a l e l e c t r o s t a t i c a n a l y z e r was s e l e c t e d f o r t h e s p e c t r o m e t e r f o r t h e f o l l o w i n g r e a s o n s : ( 1 ) T h e d e s i g n o f t h e v a c u u m s y s t e m d e p e n d s u p o n i t . ( 2 ) T h e i o n i z a t i o n c h a m b e r c a n b e r e a d i l y a c c e s s i b l e . ( 3 ) An e l e c t r o s t a t i c a n a l y z e r was c h o s e n o v e r a m a g n e t i c a n a l y z e r b e c a u s e t h e e n e r g y ( e l e c t r o s t a t i c ) a n a l y s i s i s m o r e s t r a i g h t f o r w a r d t h a n t h e momentum ( m a g n e t i c ) a n a l y s i s i n t h a t t h e d e f l e c t i n g e l e c t r i c p o t e n t i a l i s a l i n e a r f u n c t i o n o f t h e e l e c t r o n e n e r g y . H o w e v e r , t h e m a g n e t i c a n a l y s i s i s c o n v e n i e n t t o u s e a t r e l a t i v e l y - 50 -h i g h e l e c t r o n e n e r g i e s (^10* e V , t h u s t h e i r u s e i n corn • l x139,140 E S C A i s m o r e p o p u l a r ) ' . . • Th e e n e r g y d i s p e r s i o n p r o p e r t i e s a n d f o c u s i n g a c t i o n o f 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 a n a l y z e r s ' was f i r s t 141 d i s c u s s e d b y P u r c e l l . I t h a s s u b s e q u e n t l y b e e n d e v e l o p e d a n d i t s p r o p e r t i e s d i s c u s s e d b y a n u m b e r o f r e s e a r c h e r s 1 4 2 " ^ * * . A s c h e m a t i c d i a g r a m o f t h e a n a l y z e r u n i t i s s h o w n i n F i g . 6. T h e p o t e n t i a l a p p l i e d b e t w e e n t h e two h e m i s p h e r i c a l s u r f a c e s o f r a d i i R2>R.j , p r o d u c e s a t r a d i u s r a r a d i a l e l e c t r o s t a t i c f i e l d E ( r ) = A / r 2 , (3.3.1 ) a n d a p o t e n t i a l V ( r ) = A / r + B , (3.3.2) w h e r e A a n d B a r e c o n s t a n t s . F o r an e l e c t r o n w i t h a v e l o c i t y v Q w h i c h e n t e r s t h e a n a l y z e r a t x-j = 0 a n d a t l a u n c h a n g l e a = 0°, a n d e x i t s a f t e r b e i n g d e f l e c t e d b y 180°, t h e f o l l o w i n g r e l a t i o n s h i p h o l d s , C e n t r i f u g a l f o r c e = E l e c t r i c f o r c e V o , e A . ( 3 . 3 . 3 ) 0 - 51 -IONIZATION X 2 180° HEMISPHERICAL ANALYZER UNIT FIGURE 6 - 52 -I f e V Q i n e l e c t r o n v o l t s ( e V ) i s s e t e q u a l t o t h e k i n e t i c 2 e n e r g y o f t h e e l e c t r o n , E n = m e v Q / 2 , t h e n A = 2 R 0 V Q = ( R 1 + R 2 ) V Q , ( 3 . 3 . 4 ) w h e r e R Q i s t h e r a d i u s o f t h e c i r c l e t r a v e l l e d b y t h e e l e c t r o n . F r o m e q . ( 3 . 3 . 2 ) , t h e p o t e n t i a l s o f t h e i n n e r a n d o u t e r h e m i s p h e r e s w i t h r e s p e c t t o R Q , w h i c h i s a l s o t h e m i d p o i n t , a r e t h e n V ( R 1 ) - V ( R Q ) = A / R 1 - A / R Q , ( 3 . 3 . 5 ) V ( R Q ) r - V ( R 2 ) = A / R Q - A / R 2 , ( 3 . 3 . 6 ) a n d c o n s e q u e n t l y V 1 2 i s g i v e n b y V 1 2 = V ^ ) - V ( R 2 ) = A / R 1 -..A/R 2 = V Q [ ( R 2 / R 1 - ( R 1 / R 2 ) ] .. ( 3 . 3 . 7 ) I n a d d i t i o n , f o r t h e h e m i s p h e r i c a l a n a l y z e r , o t h e r p r o p e r t i e s t h a t a r e w o r t h y o f n o t e i n c l u d e ; - 53 -( 1 ) T h e a n a l y z e r h a s f i r s t o r d e r a n g l e ( « ) f o c u s i n g . ( 2 ) T h e a n a l y z e r h a s two d i m e n s i o n a l f o c u s i n g b e c a u s e o f s p h e r i c a l s y m m e t r y . 2 ( 3 ) N e g l e c t i n g t h e a t e r m , t h e e n e r g y r e s o l u t i o n o f t h e a n a l y z e r c a n be a p p r o x i m a t e d b y w h e r e w i s t h e d i a m e t e r o f t h e e n t r a n c e a n d e x i t a p e r t u r e s . T h e p a r t i c u l a r a n a l y z e r e m p l o y e d f o r t h e p r e s e n t s p e c t r o m e t e r h a s t h e f o l l o w i n g p a r a m e t e r s ; R-j = 1 " , R. = 1 . 2 5 " , R 2 = 1 . 5 " a n d w = 0 . 0 2 0 " . T h e r e f o r e t h e w o r k i n g c o n d i t i o n s o f t h e a n a l y z e r a r e 1/2 ' E o = w / 2 R o • ( 3 . 3 . 8 ) V-j / V 2 = ( + 3 ) / ( - 2 ) , ( 3 . 3 . 9 ) ( e q . ( 3 . 3 . 5 ) a n d ( 3 . 3 . 6 ) ) V 1 2 = ( 5 / 6 ^ V 0 ' ( e q ' ( 3 - 3 - 7 ) ) ( 3 . 3 . 1 0 ) a n d AE l / 2 " - o /E = 0 . 8 % ( e q . ( 3 . 3 . 8 ) ) ( 3 . 3 . 1 1 ) T h e o u t e r a n d i n n e r h e m i s p h e r e s o f t h e a n a l y z e r w e r e b o t h m a c h i n e d f r o m s o l i d b r a s s . T h e o u t e r h e m i s p h e r e i s s c r e w e d t o t h e s u p p o r t i n g p l a t e by f o u r n y l o n s c r e w s a n d - 54 -i s i n s u l a t e d by m y l a r a n d t e f l o n s p a c e r s ( F i g . 4 ) , T h e i n n e r h e m i s p h e r e i s h e l d i n p o s i t i o n by t h r e e s a p p h i r e b a l l s a n d i s a l s o s c r e w e d t o t h e s u p p o r t i n g p l a t e , w h i l e i t s i n s u l a t i o n i s e f f e c t e d b y t h e V i t o n 0 - r i n g a n d t h e t e f l o n s h e a t h i n g ( F i g , 4 ) , T h e h e m i s p h e r i c a l s u r f a c e s a r e g o l d p l a t e d a n d t h e n c o a t e d w i t h c o l l o i d a l g r a p h i t e ( C r o w n , 8 0 7 8 D r y G r a p h i t e L u b r i c a n t ) , T h i s c o a t i n g i m p r o v e s t h e e x p e r i m e n t a l r e s o l u t i o n c o n s i d e r a b l y a n d a l s o g r e a t l y r e d u c e s t h e b a c k g r o u n d d u e t d s c a t t e r e d e l e c t r o n s . M o r e o v e r , i t a c t s a s a p r o t e c t i v e c o a t i n g f o r t h e h e m i s p h e r e s , s i n c e t h e a n a l y z e r e f f i c i e n c y c a n be c o m p l e t e l y r e s t o r e d a f t e r s a m p l e p o i s o n i n g b y r e m o v a l o f t h e o l d c o a t i n g w i t h a n o r g a n i c s o l v e n t a n d a p p l i c a t i o n o f a new c o a t i n g , 3,3,3 T h e L i g h t S o u r c e U n i t o T h e 584 A H e l r e s o n a n c e l i n e i s p r o d u c e d by a l o w 6 p r e s s u r e m i c r o w a v e d i s c h a r g e i n u n p u r i f i e d h e l i u m ( C a n a d i a n L i q u i d A i r ) p o w e r e d b y a M i c r o t r o n - 2 0 0 G e n e r a t o r ( E l e c t r o -M e d i c a l S u p p l i e s ) w i t h a maximum o u t p u t o f 200 w a t t s a t 2 4 5 0 MHz. T h e m i c r o w a v e r e s o n a n t c a v i t y i s o f t h e t y p e d e s -c r i b e d by Z e l i k o f f e t a l 1 4 7 , a n d i t s d e s i g n s p e c i f i c a t i o n s 1 48 w e r e f u l l y r e p o r t e d i n V r o o m ' s t h e s i s . T h e c h a r a c t e r i s t i c s 1 4 9 o f s u c h a s o u r c e h a v e b e e n d e s c r i b e d by S a m s o n . W h i l e t h e o o 584 A l i n e i s t h e m o s t i n t e n s e , s m a l l a m o u n t s o f 537 A ( 2 3 . 0 9 e V ) , 522 A ( 2 3 . 3 e V ) * a n d 304 A ( 4 0 . 8 e V ) a r e a l s o p r e s e n t i n - 55 -t h e l i g h t s o u r c e . I n a d d i t i o n , t h e p r e s e n c e o f i m p u r i t y l i n e s d u e t o H ^ ( 1 2 1 5 A ) , 1-^(1 0 2 5 A ) , MI ( 1 1 3 4 , 1 2 0 0 A ) a n d o 01 ( 9 3 9 , 1 304 A) i s q i r i t e common, a n d f r e q u e n t l y g i v e s r i s e t o e x t r a b a n d s i n t h e PE s p e c t r a . T h e v e r t i c a l c r o s s - s e c t i o n o f t h e l i g h t s o u r c e u n i t i s s h o w n i n F i g . 7. T h e d i s c h a r g e i s p r o d u c e d i n a q u a r t z t u b e ( 5 mm o . d , ) , t h e r a d i a t i o n e m e r g i n g t h r o u g h a b o r o n n i t r i d e c o n s t r i c t i o n , i n t o t h e c o l l i m a t i n g c a p i l l a r y t u b e (1 mm i . d . ) , a n d t h e n c e i n t o t h e i o n i z a t i o n c h a m b e r i n a d i r e c t i o n p e r p e n d i c u l a r t o a l i n e j o i n i n g t h e e n t r a n c e a n d e x i t a p e r t u r e s o f t h e a n a l y z e r . T h e p u m p i n q p o r t f o r t h e u n w a n t e d h e l i u m i s a l s o u s e d f o r a l i g n i n g t h e d i s c h a r g e t u b e a n d t h e c a p i l l a r y . T h e a l i g n m e n t i s e f f e c t e d b y c o n s t r a i n -i n g t h e two b r a s s s h e a t h i n g s , o n e o f w h i c h f i t s s n u g l y o v e r t h e d i s c h a r g e t u b e a n d t h e o t h e r o v e r t h e c a p i l l a r y , i n t o a l i n e a r c o n f i g u r a t i o n by t h e s o l i d b r a s s h o u s i n g , a n d i s f u r -t h e r a s s i s t e d by t h e V i t o n 0 - r i n g s ( F i g , 7 ) , w h i c h a l s o s e r v e a s v a c u u m s e a l s . T h e u n d i s c h a r g e d h e l i u m i s pumped e i t h e r b y a r o t a r y o r a 2" o i l d i f f u s i o n pump. H o w e v e r , t h e f o r m e r h a s b e e n f o u n d t o be s u f f i c i e n t f o r t h e r e m o v a l o f e x c e s s H e , t h e r e b y g i v i n g a n e f f i c i e n t a n d i n t e n s e s o u r c e o f H e l r a d i a t i o n . T h e m o r e e f f i c i e n t d i f f u s i o n pump i s a n t i c i p a t e d t o be m o r e e f f e c t i v e f o r t h e p r o d u c t i o n o f t h e H e l l l i n e . V E R T I C A L C R O S S - S E C T I O N O F T H E L I G H T S O U R C E U N I T scale A. 180° HEMISPHERICAL ANALYSER B. SUPPORTING PLATE C . IONIZATION CHAMBER PUMPING PORT D. IONIZATION CHAMBER E. TEFLON F. LENS G. ENTRANCE APERTURE H. O-RING SEAL I. ALIGNMENT SHEATHING J . COLLIMATING CAPILLARY (BRASS) K. BORON NITRIDE CONSTRICTION L. HELIUM PUMP OFF M. SEALING FLANGE N. TUNING STUB O. FORCED AIR COOLING P. MICROWAVE CAVITY Q. POWER INPUT R. QUARTZ DISCHARGE TUBE S . QUARTER WAVE ADJUSTER T. HELIUM INLET FIGURE 7 - 57 -3 .3 .4 T h e S a m p l e S y s t e m ( a ) G e n e r a l S a m p l e I n l e t S y s t e m S t a b l e s a m p l e s a r e h a n d l e d a n d s t o r e d i n two s e p a r a t e b u t s i m i l a r l y c o n s t r u c t e d v a c u u m l i n e s . ' E x c e p t f o r t h e o n e - l i t r e g l a s s s t o r a g e f l a s k , o n e o f t h e m i s made o f s t a i n l e s s s t e e l a n d t h e o t h e r i s o f b r a s s . S t a i n l e s s s t e e l o r b r a s s v e e c o a n g l e v a l v e s a r e u s e d i n t h e s e l i n e s f o r i s o l a t i o n p u r p o s e s . f\ t h e r m o c o u p l e ( V a r i a n ilRC t y p e 5 0 1 ) a n d a p r e s s u r e g a u g e ( M a r s h t y p e 3 2" C o m p o u n d G a u g e ) i n e a c h l i n e m e a s u r e t h e v a c u u m a n d t h e s a m p l e p r e s s u r e r e s p e c t i v e l y . T h e c o n t r o l o f t h e s a m p l e p r e s s u r e i n t o t h e i o n i z a t i o n c h a m b e r i s r e g u l a t e d b y a v a r i a b l e l e a k v a l v e ( G r a n v i l l e P h i l i p s S e r i e s 2 0 3 ) . T h e c o n n e c t i o n b e t w e e n t h e l e a k v a l v e a n d t h e i o n i z a t i o n c h a m b e r i s made v e r y f l e x i b l e f o r s i m p l e d i s c o n n e c t i o n by u s i n g 1 / 8 " o . d . s t a i n l e s s s t e e l o r b r a s s t u b i n g a n d s c r e w - o n t y p e 9 - r i n g v a c u u m s e a l s ( F i g . 5 ) . T h i s f l e x i b i l i t y e n s u r e s l a c k o f s t r a i n , a n d e n a b l e s t h e v a r i o u s i n l e t s t o be s e t i n t o a n y d e s i r e d c o n f i g u r a t i o n . (b ) D i r e c t I n ! e_t_s_ F o r R e a c t i v e M o l e c u l e s T h e c o n s i d e r a t i o n s f o r r e d u c i n g t h e d e c o m p o s i t i o n o f r e a c t i v e m o l e c u l e s b e f o r e t h e y a r e p h o t o i o m ' z e d i n c l u d e : ( 1 ) C h o i c e o f t h e r i g h t k i n d o f m a t e r i a l f o r t h e s a m p l e h o l d e r , (2) R e d u c t i o n i n c o l l i s i o n s w i t h t h e w a l l s , a n d h e n c e - 53 -r e d u c t i o n i n t i m e b e t w e e n s o u r c e a n d p h o t o i o n i z a t i o n , a n d ( 3 ) C o o l i n g o f t h e s a m p l e . T h r e e d i r e c t i n l e t s , w h i c h a r e made o f K e l - F , p y r e x , a n d s t a i n l e s s s t e e l r e s p e c t i v e l y , h a v e b e e n c o n -s t r u c t e d a n d u s e d i n t h i s s t u d y , a n d t h e y h a v e b e e n s u c c e s s -f u l on a l l t h e o c c a s i o n s s o f a r e n c o u n t e r e d . T h e c o n f i g u r -a t i o n o f t h e s p e c t r o m e t e r when t h e s e u n i t s a r e i n u s e i s e x e m p l i f i e d i n t h e c a s e s o f t h e p y r e x a n d K e l - F u n i t s , b o t h s h o w n i n F i g . 8. When t h e s e u n i t s a r e u s e d , t h e s a m p l e i s h e l d i n t h e l i q u i d o r s o l i d p h a s e by i m m e r s i n g i t i n a s u i t a b l e s l u s h b a t h , a n d t h e o p e r a t i n g s a m p l e p r e s s u r e i n t h e i o n i z a t i o n c h a m b e r i s c o n t r o l l e d b y t h e c o m b i n e d u s e o f t h e 4" o i l d i f f u s i o n pump a n d t h e r e g u l a t i n g v a l v e ( K e l - F , T e f l o n o r s t a i n l e s s s t e e l ) ( n o t e - u n d e r s u c h c o n d i t i o n s t h e m o s t v o l a t i l e c o m p o n e n t c o m e s o f f f i r s t , b u t by c a r e f u l m o n i t o r i n g o f t h e s p e c t r u m t h e c o u r s e o f t h e e x p e r i m e n t c a n be f o l 1 owed .) ( c ) M i c r o w a v e D i s c h a r g e U n i t F o r P r o d u c i n g T r a n s i e n t s T h e c o n f i g u r a t i o n o f t h e d i s c h a r g e a p p a r a t u s i s s h o w n i n F i g . 3. I f n e c e s s a r y , t h e d i s c h a r g e c a v i t y ( E v e n s o n t y p e c a n be b r o u g h t c l o s e r t h a n 5 cm t o t h e p h o t o i o n i z a t i o n r e g i o n ( F i g . 9 ) . T h e s c r e w - o n V i t o n 0 - r i n g v a c u u m s e a l i s s o d e s i g n e d t h a t t h e d i s c h a r g e t u b e c a n be t i l t e d a s much a s 1 5 ° t o p r e v e n t s t r a y e l e c t r o n s p r o d u c e d i n t h e d i s c h a r g e - 5 9 -DIRECT SAMPLE INLETS K E L - F INLET - 60 -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 D I S C H A R G E T U B E D I S C H A R G E T U B E FIGURE 9 - 61 -f r o m g o i n g d i r e c t l y i n t o t h e a n a l y z e r a n d c a u s i n g h i g h b a c k -g r o u n d c o u n t s . T h e s e s t r a y e l e c t r o n s c a n f u r t h e r be r e d u c e d t h r o u g h c o l l i m a t i o n a n d e n h a n c e d w a l l c o l l i s i o n s b y i n s e r t i n g t h e d i s c h a r g e t u b e c l o s e r t o t h e i o n i z a t i o n c h a m b e r . By u s e o f a n o z z l e ( 0 . 5 min) i n t h e d i s c h a r g e t u b e i m m e d i a t e l y , a b o v e o r b e l o w t h e d i s c h a r g e c a v i t y , t h e p r o d u c t i o n o f t h e t r a n s i e n t s p e c i e s c a n f r e q u e n t l y b e d r a m a t i c a l l y i n c r e a s e d , a s a r e s u l t o f t h e f o r m a t i o n o f a n o z z l e t y p e m o l e c u l a r b e a m ( d ) H i g h T e m p e r a t u r e P y r o l y s i s U n i t F o r P r o d u c i n g T r a n s i e n t s T h e c o m p o n e n t s ( t h e h e a t e r , t h e q u a r t z a n d c o o l i n g u n i t s ) o f t h e p y r o l y s i s u n i t a r e s h o w n i n F i g . 1 0 . T h e 7 cm h e a t e r i s n a d e o f 0 . 1 0 " t h i c k p l a t i n u m w i r e n o n i n d u c t i v e l y w o und on a g r o o v e d b o r o n n i t r i d e ( B N ) t h i n - w a l l e d f o r m e r , w h i c h f i t s s n u g l y i n t o a n o t h e r BN s h e a t h i n g f o r p o s i t i o n i n g t h e w i r e s a n d r e d u c i n g h e a t l o s s . T h e q u a r t z u n i t i s made f r o m two q u a r t z t u b e s o f 3 mm o . d . a n d 20 rnm i . d . r e s p e c t i v e l y , by s e a l i n g t h e m a t o n e e n d t o f o r m a n o z z l e . T h e o t h e r e n d o f t h e o u t e r t u b e i s j o i n e d t h r o u g h a K o v a r s e a l t o t h e s t a i n l e s s s t e e l v a c u u m - s e a l i n g f l a n g e w h i c h h a s t h r e e a d j u s t a b l e s c r e w s r e s t i n g on t h e c o o l i n g u n i t f o r r a i s i n g , l o w e r i n g o r t i l t i n g t h e q u a r t z u n i t . T h e c o o l i n g u n i t i s made o f c o p p e r w i t h a n o p e n i n g f o r d i f f e r e n t i a l p u m p i n g , a n d t h e c o o l i n g i s e f f e c t e d by f l o w i n g c o l d w a t e r t h r o u g h i t . T h e h e a t e r s l i d e s o v e r t h e i n n e r t u b e t o s i t a t t h e b o t t o m o f t h e q u a r t z u n i t , w h i c h i n t u r n f i t s i n t o t h e c o o l i n g u n i t . T h e w h o l e a s s e m b l y s i t s o n t h e p u m p i n g p o r t f o r t h e i o n i z a t i o n c h a m b e r . A l l v a c u u m s e a l s 62 -- 63 -a r e a c c o m p l i s h e d b y V i t o n O - r i n g s . T h e l e n g t h s o f t h e q u a r t z a n d c o o l i n g u n i t s a r e s o c h o s e n t h a t t h e n o z z l e i s j u s t a f e w mm a b o v e t h e p h o t o i o n i z a t i o n p o i n t . An A l u m e l - C h r o m e l t h e r m o -c o u p l e f o r t e m p e r a t u r e m e a s u r e m e n t i s i n s e r t e d h a l f w a y up t h e BN h e a t i n g u n i t . T h e p o w e r f o r t h e h e a t e r i s s u p p l i e d t h r o u g h a V a r i a c f r o m a n o r d i n a r y 1 1 0 v o l t AC l i n e . A s t e a d y t e m p e r a t u r e o f 1 2 0 0 ° C i s o b t a i n e d w i t h 2 5 0 w a t t s a t 3 a m p s . U n d e r t h e s e c o n d i t i o n s , t h e t e m p e r a t u r e o f t h e s p e c t r o m e t e r i s m e r e l y warm t o t h e t o u c h , i n d i c a t i n g t h a t t h e c o o l i n g i s s u f f i c i e n t . A s l i g h t a d j u s t m e n t o f t h e H e l m h o l t z c o i l s e t t i n g s i s a l l t h a t i s n e c e s s a r y t o r e t u n e t h e s p e c t r o m e t e r . H o w e v e r , i t h a s b e e n f o u n d n e c e s s a r y t o t i l t t h e h e a t e r a l i t t l e b y m e a n s o f t h e 3 a d j u s t i n g s c r e w s i n o r d e r t o r e d u c e t h e l a r g e b a c k g r o u n d c o u n t s w h i c h p r e s u m a b l y a r i s e f r o m t h e t h e r m a l l y e m i t t e d e l e c t r o n s e m e r g i n g d i r e c t l y i n t o t h e a n a l y z e r . ( e ) V a r i a b l e T e m p e r a t u r e U n i t F o r E q u i l i b r i u m S t u d i e s T h e c o m p o n e n t s ( b e f o r e b e i n g w e l d e d t o g e t h e r ) o f t h e v a r i a b l e t e m p e r a t u r e u n i t a r e s h o w n i n F i g . 1 1 . T h e h e a t e r , w h i c h i s w o u n d n o n i n d u c t i v e l y a r o u n d t h e c o i l e d s t a i n l e s s s t e e l ( s . s . ) i n l e t t u b i n g , i s made o f 0 . 0 1 3 " n o n - m a g n e t i c c h r o m e l - P w i r e s . T h e s a m p l e g a s f l o w i n g t h r o u g h t h e s . s . t u b i n g i s c o o l e d b y c o l d n i t r o g e n g a s g e n e r a t e d by v a p o r i z i n g l i q u i d n i t r o g e n f r o m a d e w a r , o r h e a t e d b y t h e h e a t e r w i r e s , a n d i t s t e m p e r a t u r e i s m e a s u r e d b y an A l u m e l - C h r o m e l t h e r m o c o u p l e s p o t - w e l d e d o n t o t h e t u b i n g . By c o n t r o l l i n g t h e c o l d n i t r o g e n f l o w r a t e o r t h e p o w e r s u p p l i e d t o t h e h e a t e r , t h e t e m p e r a t u r e o f t h e s a m p l e c a n b e v a r i e d b e t w e e n - 1 8 0 ° C a n d 5 0 0 ° C , w i t h a f l u c t u a t i o n o f ± 5 ° C . T h e F I G U R E 11 - 65 -d e m o u n t a b l e n o z z l e i s d e s i g n e d f o r t h e s t u d y o f s u p e r c o o l e d 1 50 m o l e c u l a r j e t s , T h e u n i t s i t s o n t h e p u m p i n g p o r t o f t h e i o n i z a t i o n c h a m b e r , w i t h t h e n o z z l e b e i n g a b o u t 2 mm a b o v e t h e p h o t o i o n i z a t i o n p o i n t . T h e v a c u u m s e a l a n d i n s u l a t i o n a r e a g a i n o b t a i n e d b y t h e V i t o n O - r i n g s a n d t h e p l e x i g l a s s w a s h e r . 3.3 . 5 T h e S c a n n i n g A n d D e t e c t i n g S y s t e m T h e c o n t r o l c i r c u i t r y f o r t h e s p e c t r o m e t e r i s s h o w n i n F i g . 1 2 . F o r r e a s o n s o f s i m p l i c i t y a n d e c o n o m y , a l l v o l t a g e s a r e s u p p l i e d b y t h e u s e o f b a t t e r i e s , w h i c h a r e g r o u n d e d on o n e s i d e ( e x c e p t t h a t f o r t h e a n a l y z e r ) . V o l t a g e a d j u s t m e n t s a r e a c c o m p l i s h e d u s i n g v e r n i e r - d r i v e p o t e n t i o m e t e r s (TRW T r i m m e r s -P o t e n t i o m e t e r s ) . T h e p o w e r f o r e a c h H e l m h o l t z c o i l i s s u p p l i e d by a D C . p o w e r s u p p l y ( L a m b d a L H 1 2 2 A F M ) . T h e a n a l y z e r c a n be o p e r a t e d i n o n e o f two p o s s i b l e s c a n n i n g m o d e s : ( 1 ) T h e p o t e n t i a l d i f f e r e n c e ( V ^ ) b e t w e e n t h e h e m i s p h e r e s i s s c a n n e d , w h i l e k e e p i n g t h e i o n i z a t i o n c h a m b e r a t g r o u n d p o t e n t i a l . ( 2 ) T h e p o t e n t i a l d i f f e r e n c e ( V ^ ) b e t w e e n t h e i o n i z a t i o n c h a m b e r a n d t h e g r o u n d e d e n t r a n c e a p e r t u r e o f t h e a n a l y z e r i s s c a n n e d , w h i l e k e e p i n g t h a t b e t w e e n t h e h e m i -s p h e r e s a t a s e l e c t e d v a l u e . T h i s s e l e c t e d v a l u e d e p e n d s u p o n t h e d e s i r e d r e s o l u t i o n a n d i n t e n s i t y . T h e s c a n n i n g p o t e n t i a l s a r e r e s p e c t i v e l y s u p p l i e d by a p r o g r a m m a b l e s w e e p ( f i r s t m o d e ) o r b y a m p l i f y i n g a 4 v o l t ramp o r i g i n a t i n g f r o m a m u l t i c h a n n e l a n a l y z e r ( F a b r i t e k 1 0 7 2 ) ( s e c o n d m o d e ) , a n d t h e i r r a n g e s c a n be r e s p e c t i v e l y v a r i e d i n s t e p s o f 0.1 v o l t s o r l i n e a r l y - u p - 66 -PE SPECTROMETER CONTROL + INNER© @ OUTER. AV RLTER I hi/our /MA«/$CA • A / ^ W loo K-O-@ RETARD IMXX i I00KO-BAlANCst*——— loo /MA M/W/.5CAN J2-2.5V 0N/0Ff LEWS® 1 X L l-AV :±r "PVM 3-LEKJS RE TftRT> RAnp ON/OFF 100 K.0-I 0 T 5V ± 10°AF 100 K-fl-loT 90 V INPUT ® 7mr RAMP AMPLIFIER 6URR 6R0WW I547//5 O p . AlP nzsri^-oFpsET +1N » + POLARITY" RAMP L E V E L V250K j j 6.8 K (OK D-CLEVEt. "5oiT HEU frf f OUTPUT 33K133K TlCOPlANJ 24E 051>- -24-E05P DUAL POIAJEK SUPPLY llov 6 o H z F IGURE 1 2 - 67 -t o 20 v o l t s . I n t h e f i r s t s c a n n i n g m o d e , e l e c t r o n s o f d i f f e r e n t e n e r g i e s a r e s u c c e s s i v e l y f o c u s e d a n d t h u s a r e a n a l y z e d a t d i f f e r e n t r e s o l u t i o n s ( r e f e r e q . ( 3 . 3 . 8 ) ) , w h i l e i n t h e s e c o n d m ode, t h e y a r e p r e - a c c e l e r a t e d o r p r e - d e c e l e r a t e d t o t h e s e l e c t e d f o c u s i n g e n e r g y o f t h e a n a l y z e r , a n d t h u s a r e a n a l y z e d a t c o n s t a n t r e s o l u t i o n . A c h a n n e l e l e c t r o n m u l t i p l i e r ( M u l l a r d B 3 1 9 A L ) o p e r a t i n g i n s a t u r a t e d mode i s u s e d f o r e l e c t r o n d e t e c t i o n . A p o s i t i v e v o l t a g e o f A,300Q v o l t s ( H e w l e t t P a c k a r d 6 5 2 5 DC P o w e r S u p p l y ) i s a p p l i e d t o i t s o u t p u t w h i l e t h e i n p u t i s h e l d a t 3 0 0 V a b o v e g r o u n d ( b y b a t t e r y ) t o a c c e l e r a t e t h e e l e c t r o n s . P u l s e c o u n t i n g i s u s e d f o r d a t a a c q u i s i t i o n a n d c o n v e n t i o n a l p u l s e c o u n t i n g e q u i p m e n t i s u s e d f o r t h i s p u r p o s e , n a m e l y , a p r e a m p l i f i e r , an a m p l i f i e r a n d a d i s c r i m i n a t o r ( H a r s h a w N A - 1 5 , N H - 3 4 A ) . T h e d a t a r e a d o u t s y s t e m may i n v o l v e a m u l t i c h a n n e l a n a l y z e r ( F a b r i t e k 1 0 7 2 ) , r a t e m e t e r ( H a r s h a w N R - 1 0 ) , c h a r t r e c o r d e r ( H e w l e t t P a c k a r d 6 8 0 S t r i p C h a r t R e c o r d e r ) a n d X-Y p l o t t e r (MFE P l o t a m a t i c 7 1 5 ) . 3.4 O p e r a t i o n A n d P e r f o r m a n c e O f T h e S p e c t r o m e t e r T h e b a s e v a c u u m i s i n t h e r e g i o n o f 1-3 x 1 0 " 6 mm H g , w h i c h c a n n o r m a l l y be o b t a i n e d s t a r t i n g f r o m a t m o s p h e r i c p r e s s u r e i n a b o u t o n e h o u r , p r o v i d e d t h a t t h e s y s t e m h a s n o t b e e n e x p o s e d t o t h e a i r t o o l o n g f o r a p p r e c i a b l e a d s o r p t i o n t o h a v e o c c u r r e d . T h e h e l i u m g a s i n t h e l i g h t s o u r c e h a r d l y - 68 -i n c r e a s e s t h e b a s e p r e s s u r e , s i n c e i t i s b e i n g d i f f e r e n t i a l l y p umped ( s e c t i o n 3 . 3 , 3 ) . T h e p r e s s u r e o f t h e s a m p l e a t t h e i o n i z a t i o n c h a m b e r i s n o t m e a s u r e d d i r e c t l y , b u t i s i n s t e a d m o n i t o r e d b y t h e i o n i z a t i o n g a u g e ( s e c t i o n 3 . 3 . 1 ) . T h e o p e r a t i n g p r e s s u r e a s r e a d a t t h e i o n i z a t i o n g a u g e i s b e t w e e n 0.4-3 x 1 0 ~ 5 mm Hg ( c a . 0.01-1 T o r r i n t h e i o n i z a t i o n c h a m b e r ) . T h e d i f f e r e n t i a l pump on t h e i o n i z a t i o n c h a m b e r i s i s o l a t e d when s t a b l e s a m p l e s a r e b e i n g r u n s i n c e l a r g e q u a n t i t i e s o f s a m p l e w o u l d b e c o n s u m e d , a n d i s p a r t i a l l y o r c o m p l e t e l y o p e n e d f o r r u n n i n g r e a c t i v e m o l e c u l e s o r t r a n s i e n t s . T h e a d v a n t a g e s a n d d i s a d v a n t a g e s o f o p e r a t i n g t h e a n a l y z e r i n t h e two s c a n n i n g m o d e s ( s e c t i o n 3.3.5) a r e s h o w n b y r e c o r d i n g t h e PE s p e c t r u m o f o x y g e n i n t h e r e s p e c t i v e mode ( F i g . 1 3 ) . I n t h e f i r s t mode ( s c a n V ^ ) » v e r y l o w e n e r g y e l e c t r o n s ( 0 - 2 e V ) a r e b a d l y d i s c r i m i n a t e d a g a i n s t , w i t h t h e r e s u l t t h a t t h e f o u r t h b a n d o f 0 2 c a n o n l y be d e t e c t e d a f t e r a p p l y i n g a p o s i t i v e p o t e n t i a l t o t h e l e n s ( F i g . 1 3 ) , a n d v a r -i a b l e r e s o l u t i o n ( l i n e w i d t h s ) i s o b t a i n e d i n t h e same s p e c t r u m b e c a u s e o f t h e c o n s t a n t /E ( e q . ( 3 . 3 . 3 ) ) . I n t h e s e c o n d mode ( s c a n V D ) , a r i s i n g b a c k g r o u n d n o r m a l l y o c c u r s o n t h e l o w k i n e t i c e n e r g y s i d e , a n d o c c a s i o n a l l y i s s o s t r o n g t o o b s c u r e a n y b a n d s i n t h i s r e g i o n . T h e s e c o n d mode i s s u p e r i o r i n t h a t v e r y h i g h r e s o l u t i o n c a n be o b t a i n e d b y d e c r e a s i n g V 1 2 , a l t h o u g h a t t h e e x p e n s e o f s i g n a l i n t e n s i t y , w h i l e t h e s e c o n d mode may be u s e d a s a c h e c k t h a t a l l b a n d s a r e d e t e c t e d a t f a i r l y l o w e l e c t r o n e n e r g y r e g i o n ( 1 - 4 e V ) . C o n s e q u e n t l y , i n 1^ 1 3 14 ife iS 1*7 f 8 3) S I O N I Z A T I O N P O T E N T I A L ( e V ) F I G U R E 13 - 7 0 -p r a c t i c e , t h e a n a l y z e r i s a l w a y s o p e r a t e d i n t h e s e c o n d m o de, w i t h t h e e n e r g y o f t h e f o c u s e d e l e c t r o n s c h o s e n t o g i v e t h e b e s t c o m p r o m i s e b e t w e e n r e s o l u t i o n a n d i n t e n s i t y , a n d w h e n e v e r n e c e s s a r y t h e s e c o n d mode i s u s e d a s a c r o s s c h e c k . In t h e s e c o n d s c a n n i n g mode a n e g a t i v e l e n s a t a s e l e c t e d p o t e n t i a l , p r e s u m a b l y d u e t o i t s f o c u s s i n g a c t i o n on t h e e l e c t r o n s , h a s b e e n f o u n d t o i n c r e a s e t h e i n t e n s i t y d r a s t i c a l l y a n d i s t h u s a l w a y s u s e d . A s d i s c u s s e d i n s e c t i o n 2 . 3 , t h e m a j o r s t u m b l i n g b l o c k t o v e r y h i g h r e s o l u t i o n i s t h e i n h o m o g e n e o u s s u r f a c e p o t e n t i a l a n d m i c r o p d t e n t i a l s a t t h e a p e r t u r e s . T h i s i s e s p e c i a l l y d e t r i m e n t a l i n t h e p r e s e n t s t u d y s i n c e t h e s p e c t r o -m e t e r i s c o n s t a n t l y e x p o s e d t o c o r r o s i v e o r r e a c t i v e v a p o r s , m a k i n g i t e x t r e m e l y d i f f i c u l t t o o b t a i n t h e t h e o r e t i c a l r e s o l u t i o n ( 0 . 8 % ) . T h e k i n d o f r e s o l u t i o n a n d s e n s i t i v i t y w h i c h i s t y p i c a l o f t h e p r e s e n t s p e c t r o m e t e r when i t i s r e l a t i v e l y c l e a n i s g i v e n i n T a b l e 2 , w h e r e i s s h o w n t h e 2 o b s e r v e d h a l f w i d t h o f t h e A r P 3 ^ p e a k a n d i t s i n t e n s i t y a t d i f f e r e n t f o c u s i n g c o n d i t i o n s . T h e b e s t r e s o l u t i o n o b t a i n e d h a s b e e n 1 5 meV w i t h an i n t e n s i t y o f 1 0 , 0 0 0 c o u n t s / s e c . N o r m a l l y t h e s p e c t r o m e t e r i s o p e r a t e d a t a r e s o l u t i o n o f 2 0 - 3 0 meV w i t h c o u n t s o f ^ 3 0 , 0 0 0 p e r s e c . T h e p o t e n t i a l o f t h e p r e s e n t s y s t e m f o r s t u d y i n g t r a n s i e n t s p e c i e s i s t e s t e d on some a t o m i c s p e c i e s , e . g . H,N a n d F , a n d 0 ? ( ^ A N ) , w h i c h h a v e b e e n r e p o r t e d b y J o n a t h a n - 71 -T A B L E 2* FWHM a n d F o c u s i n g I n t e n s i t y C o n d i t i o n s O f T h e A r g o n 2 P 3 P e a k a t D i f f e r e n t ' 2 (meV) I ( c o u n t s / s e c ) E ( e V ) V 1 2 (volt) V R ( v o l t ) V L ( v o l t) 19 2 , 0 0 0 0.251 0 . 2 0 9 5 .01 5 - 0 . 0 6 5 20 6 , 0 0 0 0 . 3 7 0 0 . 3 0 3 4 . 3 2 6 - 0 . 2 9 9 20 9 ,00 0 0 . 4 9 0 0 . 4 0 8 4 . 6 7 3 - 0 . 4 2 8 20 11 ,000 0.607 0 . 5 0 6 4 . 5 2 3 - 0 . 6 5 0 21 15 ,000 0 . 727 0 . 6 0 6 4 . 3 6 9 - 0 . 7 5 4 22 19 ,000 0 . 847 0 . 7 0 6 4 . 2 2 0 - 0 . 9 0 4 . 23 25 ,000 0 . 9 6 6 0 . 8 0 5 4 . 0 4 6 -1 .124 24 2 9 , 5 0 0 1 .085 0. 9 0 4 3 . 9 0 7 - 1 . 2 7 3 25 3 8 , 0 0 0 1.201 1 .001 3 . 7 4 6 - 1 . 4 2 6 30 4 3 , 0 0 0 1.321 1 .101 3.597 - 1 . 5 8 6 31 4 8 , 0 0 0 1 .441 1 .200 3.441 -1 ,726 * AE, : FWHM o f t h e p e a k ; I : I n t e n s i t y ; E : E n e r g y o f t h e f o c u s e d e l e c t r o n s ; V ^ : P o t e n t i a l a c r o s s t h e a n a l y z e r h e m i s p h e r e s ; V R : R e t a r d i n g p o t e n t i a l a n d V L : P o t e n t i a l o f t h e l e n s . - 72 -e t a l . " . T h e P E s p e c t r a o f t h e s e s p e c i e s a r e s h o w n i n F i g s . 1 4 , 15 a n d 1 6 , a n d w e r e o b t a i n e d b y a m i c r o w a v e d i s -c h a r g e i n t h e i n d i c a t e d m o l e c u l e s w i t h o u t t h e u s e o f a n y d i l u e n t g a s . I n a l l c a s e s , o u r r e s u l t s a r e a t l e a s t c o m p a r a b l e 4 4 - 4 7 t o t h o s e o f J o n a t h a n e t a l , , b u t w i t h a s i g n i f i c a n t i m p r o v e m e n t i n r e s o l u t i o n . T h i s m e r i t h a s e n a b l e d u s t o d e t e c t some new f e a t u r e s i n t h e P E s p e c t r a o f C S ( s e c t i o n 4 . 1 ) a n d S 0 ( 3 E " ) ( s e c t i o n 4 , 2 ) . (A) i 1 I 1 i 1 —~i 1 1 r 14 15 16 17 18 19 IONIZATION POTENTIAL (eV) F I G U R E 11 - 74 -14 15 16 17 18 19 20 F^P) 13 14~ 15 ~16 17 18 J9 20 IONIZATION POTENTIAL (eV) F I G U R E 15 0 2 ( 3 Sg) I I I I I I I I I 16 17 18 DISCHARGE IN 0 2 0 2 ( 3 2i ) 19 02( 1 Ag) c£( ng) I I I I I 0 9( 3Zg-) I I ' I I 1— 1 1 1 : 1 1 — 10 11 12 13 14 15 16 1 7 - 1 8 19 20 IONIZATION POTENTIAL (eV) F I G U R E IC - 76 -CHAPTER FOUR PHOTOELECTRON SPECTROSCOPY OF SOME TRANSIENT SPECIES 4.1 Carbon Monosulfide (CS) 4.1.1 Introduction Carbon monosulfide (CS) has been extensively 151 studied by spectroscopic methods . The species can be produced in good y i e l d by e i ther the discharge or photolys is of C S 2 1 5 1 g . The f i r s t i on izat ion potential (IP) of CS was 1 52a reported by several groups using e i ther electron impact , mass-s l r o u 151i pectrometry or conventional opt ica l methods " I C O Very recent ly , two groups have obtained the photoelectron (PE) spectrum of CS giving the f i r s t two I P ' s . The f i r s t IP 153 2 was i d e n t i f i e d as re fer r ing to the ground ionic state (X ; and the second was tentat ive ly a s s i g n e d 1 5 3 9 to the ionic 2 n s tate . Considering the s imi la r e lec t ron ic structure of CS to CO, we should expect each IP of CS to be lower than the - 77 -c o r r e s p o n d i n g o n e o f CO i n v i e w o f t h e l o w e r I P ' s o f S t h a n t h a t o f 0. T h e r e f o r e i t i s s u r p r i s i n g t h a t a t h i r d I P was n o t o b t a 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 . T h e PE s p e c t r u m o f a n e u t r a l m o l e c u l e r e s e m b l e s c l o s e l y t h e e l e c t r o n i c s p e c t r u m o f t h e i o n . I t i s i n s t r u c t i v e t o c o m p a r e t h e e l e c t r o n i c s t a t e s o f C P , a n i s o e l e c t r o n i c s p e c i e s o f C S + , w i t h t h a t o f C S + . Two c o n c l u s i o n s may be s a f e l y d r a w n . F i r s t , f o r C P 1 1 1 , 2 2 e x c i t e d s t a t e s A n a n d B z a r e r e s p e c t i v e l y 0 . 8 5 5 a n d 3 . 6 0 8 2 + ? 1 5 3 a eV a b o v e t h e g r o u n d s t a t e X £ , w h i l e a s f o r CS , A n i s 2 1.46 eV a b o v e X z, T h e o r d e r i n g o f t h e f i r s t two e l e c t r o n i c s t a t e s o f t h e two m o l e c u l e s a r e t h e s a m e , a n d t h e s e p a r a t i o n s a r e r e a s o n a b l y c l o s e . H e n c e i t i s r e a s o n a b l e t o e x p e c t CS t o h a v e a t l e a s t a n o t h e r I P b e l o w 2 1 . 2 2 e V . S e c o n d l y , t h e s p i n r o r b i t s p l i t t i n g f o r t h e 2 n s t a t e o f CP i s 1 5 3 . 3 c m " 1 1 1 1 , 2 + a n d s o t h e n s t a t e o f CS s h o u l d h a v e a s p i n - o r b i t s p l i t t i n g o f a s i m i l a r o r d e r o f m a g n i t u d e , p r o b a b l y l a r g e r . In t h e 1 5 3 a l i g h t o f t h i s , t h e r e p o r t e d s p i n - o r b i t s p l i t t i n g f o r t h e 2 + -1 n s t a t e o f CS o f ^ 3 0 cm s e e m s r a t h e r s m a l l . I n t h e f o l l o w i n g , i t w i l l be s h o wn t h a t t h e s e e x p e c t a t i o n s a r e i n d e e d w e l l b o r n e o u t , a n d t h a t t h e s e c o n d I P c a n be u n a m b i g u o u s l y a s s i g n e d t o t h e n s t a t e o f t h e . 154 i o n . 4.1.2 E x p e r i m e n t a l CS was p r o d u c e d by m i c r o w a v e d i s c h a r g e o f CS,,. No a t t e m p t was made t o r e m o v e C S 2 b y t r a p p i n g b e c a u s e no c o m p l i - 7 3 -c a t i o n s o c c u r r e d d u e t o i t s p r e s e n c e . P r e v i o u s l y , we n o t -i c e d t h e d i s c h a r g e i n C S 2 was r a t h e r u n s t a b l e a n d CS was p r o d u c e d i n a m e a s u r a b l e a m o u n t o n l y f o r a b o u t a n h o u r . 1 55 A m o d i f i e d d i s c h a r g e c a v i t y w h i c h h a s b e e n c l a i m e d t o g i v e b e t t e r p e r f o r m a n c e , was u s e d i n s t e a d . G r e a t c a r e was t a k e n t o e n s u r e a v e r y s t a b l e C S 2 p r e s s u r e . P o w e r a s l o w a s 5 0 W was f o u n d s u f f i c i e n t t o p r o d u c e maximum y i e l d o f C S . A q u a r t z t u b e o f 5 mm i . d . was u s e d a n d t h e d i s c h a r g e c a v i t y p l a c e d 27 cm f r o m t h e i o n i z a t i o n r e g i o n . U n d e r t h e s e c o n d i t i o n s , a s t a b l e s u p p l y o f CS w i t h i t s f i r s t p e a k m o r e i n t e n s e t h a n t h a t o f C S 2 was o b t a i n e d c o n t i n u o u s l y f o r s i x h o u r s . M u l t i p l e s c a n s f o r e a c h IP o f CS w e r e p o s s i b l e . T h e s p e c t r a w e r e r e c o r d e d a t a r e s o l u t i o n b e t w e e n 2 5 - 3 0 meV ( m e a s u r e d by FWHM o f A r p e a k s ) . C a l i b r a t i o n was e f f e c t e d u s i n g t h e v a l u e s f o r C S 2 w h i c h a r e i n t u r n c a l i b r a t e d u s i n g a r g o n . 4 . 1 . 3 R e s u l t s a n d D i s c u s s i o n T h e PE s p e c t r a o f C S 2 w i t h a n d w i t h o u t t h e d i s -c h a r g e a r e d e p i c t e d i n F i g s . 1 7 a a n d 1 7 b r e s p e c t i v e l y . I n 1 5 3 a d d i t i o n t o t h e two b a n d s o b s e r v e d b e f o r e , two m o r e b a n d s d u e t o C S + a r e o b t a i n e d . T h e I P ' s a n d t h e v i b r a t i o n a l f r e q u e n c i e s a r e c o l l e c t e d i n T a b l e 3. O u r v a l u e s a r e i n 1 5 0 g o o d a g r e e m e n t w i t h t h e p r e v i o u s o n e s J. I f an o r b i t a l d e s c r i p t i o n o f t h e e l e c t r o n i c s t r u c t u r e i s a s s u m e d , CS h a s a c o n f i g u r a t i o n - 79 -1b. M.W. DISCHARGE IN - i r T — 1 1 1 1 r 1a. Ar - r ~ 11 10 — i — 12 T — i 1 r 13 14 15 16 17 IONIZATION POTENTIAL (eV) i — 18 FIGURE 17 A B L E 3 PES a n d T h e o r e t i c a l D a t a o f CS I o n i z a t i o n p o t e n t i a l s ( e V ) V i b r a t i o n a l f r e q u e n c y ( c m " 1 ) I on i c s t a t e P E S a ) . T h e o r e t i c a l PES T h i s w o r k b ^ R e f . 1 5 3 a R e f . 1 5 3 6 CNDO/2 * R e f . 158 T h i s w o r k c ^ R e f . 1 5 3 a R e f . 1 5 3 b X 2 E A 2 * 11 .34 1 1 . 3 3 11 .33 1 0 . 1 0 1 2 . 3 0 1 3 3 0 1 3 2 5 1 2 9 0 1 2 . 7 3 ( 1 2 . 9 0 ) 1 2 . 7 9 ( 1 2 . 9 2 ) 1 2 . 7 6 1 1 . 2 4 12.61 9 7 0 1 0 5 0 9 8 0 ' CO 8 7 0 i n 2 z 1 5 . 3 3 ( 1 6 . 0 6 ) 1 6 . 5 6 1 3 . 3 3 c 2 t 1 3 . 0 3 2 4 . 6 4 3 0 . 1 2 1 0 6 0 a ) a d i a b a t i c I P ' s a r e g i v e n , v e r t i c a l IP i n p a r e n t h e s e s b) e s t i m a t e d e x p e r i m e n t a l e r r o r ± 0.02 e V . c ) e s t i m a t e d e x p e r i m e n t a l e r r o r ± 30 c m " 1 . d ) w i t h 4 eV a d j u s t m e n t . - 81 -( l 0 ) 2 ( 2 o ) 2 ( 3 a ) 2 ( 4 a ) 2 ( l 1 r ) 4 ( 5 a ) 2 ( 6 a ) 2 ( 2 1 r ) 4 ( 7 o ) 2 . T h e f o u r o b s e r v e d b a n d s w i l l t h e n c o r r e s p o n d t o t h e r e m o v a l o f e l e c t r o n s f r o m t h r e e a a n d o n e v m o l e c u l a r o r b i t a l s ( M O ) . T h e s p i n - o r b i t s p l i t t i n g f o r t h e s e c o n d b a n d i s r e s o l v e d c l e a r l y ( F i g . 1 8 b ) a n d h a s a v a l u e ( A ) o f 2 8 0 ± 30 c m " 1 . T h i s v a l u e i s i n e x c e l l e n t a g r e e m e n t w i t h a v e r y r e c e n t r e s u l t ( 3 0 1 . 2 8 3 cm" ) o b t a i n e d f r o m t h e e l e c t r o n i c s p e c -t r u m o f C S + . T h i s t h e n l e a v e s no d o u b t t h a t t h e s e c o n d 2 b a n d c o r r e s p o n d s t o t h e n s t a t e a n d t h e o t h e r t h r e e t o t h e ? + "z s t a t e s o f CS . T h e f i r s t b a n d , g i v i n g a c o i n c i d e n t a d i a b a t i c a n d v e r t i c a l I P a t 1 1 . 3 4 e V , h a s a t l e a s t t h r e e o b s e r v a b l e v i b r a t i o n a l c o m p o n e n t s ( F i g . 1 8 a ) . T h e s h a r p n e s s o f t h e f i r s t p e a k , t o g e t h e r w i t h t h e i n c r e a s e i n t h e v i b r a t i o n a l s p a c i n g c o m p a r e d t o t h e g r o u n d m o l e c u l a r s t a t e , i . e . f r o m 1 2 7 5 t o 1 3 8 0 c m " 1 , i n d i c a t e s i o n i z a t i o n o f an a n t i b o n d i n g a MO. T h e seco-nd b a n d ( F i g . . 1 8 b ) o v e r l a p s w i t h o n e f r o m C S g . B e c a u s e o f t h e d i f f e r e n c e i n v i b r a t i o n a l s p a c i n g s a n d t h e s p l i t t i n g o f f i n e s t r u c t u r e f o r C S , we a r e a b l e t o d e t e r m i n e t h e a d i a b a t i c a n d v e r t i c a l I P ' s a c c u r a t e l y . T h i s b a n d h a s a v i b r a t i o n a l s e r i e s o f a t l e a s t s i x c o m p o n e n t s , a n d t h e d e c r e a s e i n t h e v i b r a t i o n a l s p a c i n g , t o g e t h e r w i t h t h e l e n g t h o f t h e s e r i e s c l e a r l y i n d i c a t e s t h e r e m o v a l o f a co =1055 cm"' i — i — i — - | — i — i — i — i — | — i — i — i — i — j — i 12.5 13.0 13.5 eV F I G U R E 18 - 83 -s t r o n g l y b o n d i n g (TT) e l e c t r o n . T h e t h i r d b a n d ( F i g . 1 8 c ) i s a l s o o v e r l a p p e d by o n e f r o m C S g . T h e r e s o l u t i o n i s g o o d e n o u g h t o e n a b l e an a c c u r a t e d e t e r m i n a t i o n o f t h e a d i a b a t i c a n d v e r t i c a l I P ' s a t 1 5 . 8 3 a n d 1 6 . 0 6 eV r e s p e c t i v e l y . T h e b a n d s h o w s a t l e a s t s i x v i b r a t i o n a l c o m p o n e n t s a n d a l a r g e d e c r e a s e i n v i b r a -t i o n a l s p a c i n g . T h e MO i n v o l v e d a p p e a r s t o be e v e n m o r e b o n d i n g t h a n t h e s e c o n d . T h e d e c r e a s e i n t h e v i b r a t i o n a l s p a c i n g o f t h e f o u r t h b a n d i s i n l i n e w i t h t h e r e m o v a l o f a b o n d i n g e l e c t r o n , y e t t h e s h a r p n e s s o f t h e f i r s t p e a k ( F i g . 1 8 d ) i s u n e x p e c t e d . T h e c o r r e s p o n d i n g IP h a s n o t b e e n o b t a i n e d f o r C O , b u t a f o u r t h I P a t 2 3 . 6 0 eV f o r Ng h a s r e c e n t l y b e e n o b t a i n e d 157 f r o m R y d b e r g s e r i e s . H e n c e by c o m p a r i n g w i t h N g . i t i s r e a s o n a b l e t o a s s i g n t h e f o u r t h b a n d , w i t h a n i d e n t i c a l 2 a d i a b a t i c a n d v e r t i c a l IP a t 1 8 . 0 3 e V , t o t h e C i s t a t e o f t h e i o n . I n o r d e r t o o b t a i n w o r e c o n f i r m a t i o n f o r t h e a s s i g n m e n t , we r e s o r t t o t h e o r e t i c a l c a l c u l a t i o n s . An ab i n i t i o S C F - L C A O - M O c a l c u l a t i o n u s i n g a n e x t e n d e d b a s i s s e t 158 o f S l a t e r t y p e o r b i t a l s h a s b e e n c a r r i e d o u t f o r C S . 49 By a p p l y i n g Koo>pmans' t t i e o r e m , t h e I P ' s a r e c a l c u l a t e d ( T a b l e 3 ) . T h e a g r e e m e n t b e t w e e n t h e e x p e r i m e n t a l a n d t h e o r e t i c a l r e s u l t s i s n o t s a t i s f a c t o r y , n o t a b l y i n t h e o r d e r i n g o f t h e i o n i c n s t a t e . T h i s p o i n t s t o t h e f a c t t h a t K o o p m a n s ' t h e o r e m m u s t be a p p l i e d c a u t i o u s l y . N e v e r -- 84 -t h e l e s s , t h e b o n d i n g c h a r a c t e r o f i n d i v i d u a l MO's a g r e e s w i t h t h e e x p e r i m e n t a l f i n d i n g . We h a v e p e r f o r m e d a C M D O / 2 7 6 c a l c u l a t i o n a n d s u r p r i s i n g l y t h e p r e d i c t e d I P ' s a r e i n b e t t e r a g r e e m e n t w i t h e x p e r i m e n t , e x c e p t t h a t a b o n d i n g e l e c t r o n i s i n d i c a t e d t o be i n v o l v e d i n t h e f i r s t I P . 4.2 S u l f u r M o n o x i d e ( S O ) 4.2.1 I n t r o d u c t i o n T h e PE s p e c t r u m o f S 0 ( z ) h a s b e e n s t u d i e d by 47 1 59 J o n a t h a n e t a l . ' , who i d e n t i f i e d t h r e e I P ' s b e l o w 21.2 e V . I t i s c l e a r t h a t t h e s p e c t r u m o f SO i s f a r f r o m c o m p l e t e a n d r a i s e s s e v e r a l p o i n t s t h a t n e e d t o be c l a r i f i e d . F i r s t , c o n s i d e r i n g t h e s i m i l a r i t y i n e l e c t r o n i c s t r u c t u r e b e t w e e n 0 2 a n d SO, o n e n o t e s t h a t t h e r e p o r t e d s p e c t r u m o f 1 59 SO i s f a r t o o s i m p l e i n t h a t some i o n i c s t a t e s a c c e s s i b l e 2 i n H e l PES may h a v e b e e n u n d e t e c t e d . S e c o n d l y , t h e X n i o n i c s t a t e s h o u l d h a v e a s p i n - o r b i t s p l i t t i n g g r e a t e r t h a n t h a t o f 0 2 X 2 M g s t a t e (A = +195 cm" 1 1 1 1 ) a n d a t l e a s t c o m -p a r a b l e t o t h a t ©tf £ S + A 2 M s t a t e 1 6 0 (A = - 2 8 0 c m " 1 , s e c t i o n 4 . 1 . 3 ) . T h e r e f o r e , w i t h b e t t e r r e s o l u t i o n o n e s h o u l d be a b l e t o r e s o l v e t h i s s p l i t t i n g . T h i r d l y , i n t h e s t u d y o f S - , 0 1 6 1 ( s e c t i o n 4 . 3 . 3 ) , we h a v e r a i s e d t h e p o i n t t h a t t h e ' s e c o n d I P ' o f SO a t 1 1 . 3 eV may be d u e i n s t e a d t o t h e s e c o n d PE b a n d o f S 9 0 . T h e p r e s e n t s e c t i o n i s a i m e d a t - 85 -a t t e m p t i n g t o a n s w e r t h e s e i m p o r t a n t q u e s t i o n s . 4.2.2 E x p e r i m e n t a l S 0 ( i ) was p r o d u c e d b y a MW d i s c h a r g e ( 1 0 0 W ) i n S 0 9 w i t h o u t o r w i t h t h e a d d i t i o n o f v a r y i n g a m o u n t s o f s u l f u r v a p o r . T h e a m o u n t o f t h e l a t t e r was c o n t r o l l e d by a h e a t i n g t a p e w o u n d a r o u n d a s i d e - a r m o f t h o d i s c h a r g e t u b e i n t o w h i c h some ( s u b l i m e d ) s u l f u r p o w d e r was p l a c e d . T h e PE s p e c t r u m was c a l i b r a t e d u s i n g t h e s p e c t r o s c o p i c I P ' s o f t h e o x y g e n a t o m s 0 p r o d u c e d i n t h e d i s c h a r g e . 4 . 2 . 3 R e s u l t s a n d D i s c u s s i o n T h e PE s p e c t r a o f S 0 2 ( A ) a n d t h e p r o d u c t s o f t h e d i s c h a r g e i n S 0 ? w i t h d i f f e r e n t a m o u n t s o f S v a p o r i s s h o w n i n F i g . 1 9 , w i t h t h e a m o u n t o f S i n c r e a s i n g f r o m B t o J. T h e d i s c h a r g e i n S0. 2 a l o n e g a v e a PE s p e c t r u m ( F i g . 1 9 B ) w h i c h s h o w s , i n a d d i t i o n t o t h e 0 + p e a k s , two a d d i t i o n a l b a n d s d u e t o S O 4 7 ' 1 5 9 a t 1 0 . 3 3 eV a n d 1 4 . 9 4 e V . The s p e c t r u m i s i d e n t i c a l t o t h a t o f J o n a t h a n e t a l , e x c e p t t h a t t h e PE b a n d a t 1 1 . 3 eV i s h a r d l y d e t e c t a b l e i n F i g . 1 9 3 . W i t h a l i t t l e S v a p o r a d d e d i n t o t h e d i s c h a r g e ( F i g . 1 9 C ) , t h e p e a k s d u e t o 0 + d i s a p p e a r e d . I n a d d i t i o n , two new b a n d s a t 1 0 . 6 eV a n d 1 1 . 3 eV b e g a n t o a p p e a r , a n d t h e y i n c r e a s e d d r a m a t i c a l l y w i t h r e s p e c t t o t h o s e o f S 0 + w i t h i n c r e a s i n g S v a p o r p r e s s u r e ( F i n . 1 9 D ) . M o r e o v e r , a t h i g h S v a p o r p r e s s u r e two m o r e b a n d s a t 1 4 . 7 e V ' a n d 1 6 . 0 eV ( F i g . 19D) c o u l d be FIGURE 19 - 87 -i d e n t i f i e d . C o m p a r i n g w i t h t h e PE s p e c t r u m o f S 2 0 1 6 1 ( s e c t i o n 4 . 3 . 3 ) , t h e s e f o u r b a n d s c a n b e a s s i g n e d u n a m b i g u o u s l y t o t h o s e o f S g O . - T h i s t h u s c o n f i r m s o u r p r e v i o u s c l a i m 1 6 1 t h a t t h e 2 n d b a n d ( 1 1 . 3 e V ) o f SO o b s e r v e d b y J o n a t h a n e t 159 nd a l was a c t u a l l y t h e 2 b a n d o f S 2 0 , w h i c h was f o r m e d by t h e r e a c t i o n o f SO w i t h S d e p o s i t e d on t h e w a l l s . T h e p a r t i a l o v e r l a p o f t h e f i r s t b a n d o f SO by t h a t o f S 9 0 e x p l a i n s t h e 159 u n e x p e c t e d b r o a d e n i n g o f v ' = 1 a n d 2 p e a k s o f t h e f o r m e r . T h e o b s e r v a t i o n t h a t t h e i n t e n s i t i e s o f t h e b a n d s o f S ? 0 g r o w w i t h i n c r e a s i n g S c o n c e n t r a t i o n i s i n c o m p l e t e a c c o r d w i t h t h e f a c t t h a t SO ( z ~ ) f o r m e d i n t h e d i s c h a r g e r e a c t s r a p i d l y w i t h S t o f o r m S 2 0 . I n d e e d , w i t h l e s s e f f i c i e n t p u m p i n g SO c a n be e l i m i n a t e d f r o m t h e s p e c t r u m as a r e s u l t o f i t s c o m p l e t e r e a c t i o n w i t h S. s t T h e 1 b a n d o f SO was r e c o r d e d a t h i g h r e s o l u t i o n a n d i s s h o w n i n F i g . 2 0 . T h e e x p e c t e d s p i n - o r b i t c o m p o n e n t s a r e c l e a r l y r e s o l v e d , w i t h a n a v e r a g e s p l i t t i n g o f 320 + -1 s t 20 cm ( 4 0 meV) w h i c h t h e n l e a v e s no d o u b t t h a t t h e 1 I P i s d u e t o t h e n i o n i c s t a t e . T h e I P ' s a n d v i b r a t i o n a l f r e q u e n c i e s c o r r e s p o n d i n g t o t h e t w o d i s t i n c t b a n d s a t 1 0 . 3 3 eV a n d 1 4 . 9 4 eV a r e c o l l e c t e d i n T a b l e 4, a n d t h e s e 47 1 5 9 v a l u e s a r e i n g o o d a g r e e m e n t w i t h t h e p r e v i o u s r e s u l t s ' T h e a v e r a g e t o t a l h a l f - w i d t h s o f t h e o v e r l a p p e d 2 2 2 ( n ^ , n 3 ^ ) p e a k s i n t h e 0-0 a n d 0-1 c o m p o n e n t s o f t h e X n s t a t e a r e 85 + 3 meV. U s i n g t h e o b s e r v e d s p i n - o r b i t s p l i t t i n g 2 2 a n d a s s u m i n g t h e p e a k s o f t h e two s p i n s t a t e s a n d n 3 ^ a r e e q u a l l y i n t e n s e , h a v e t h e same h a l f - w i d t h s , a n d a r e o f > / - 88 -112 114 VL6~ 118 12.0 I P ( e V ) FIGURE 20 - 89 -TABLE 4 PES Data for SO Ionic state Vert ica l IP (eV) This work (adiabat ic) Vibrat ional^ frequency (cm" ) Ref. 159 10.33±0.01 1 0 . 3 7 ± 0 . 01 10.32 This work Ref. 159 1270±30 1270±30 1 210 a 4n A 2 n bV 13.90 (13.39)±0.10 <\,14.6 (^14.0) 14.94+0.01 14.96 750+1 00 700±100 920+30 970 90 -g a u s s i a n t y p e , t h e h a l f - w i d t h s o f t h e s e i n d i v i d u a l p e a k s 164 c a n t h e n be e s t i m a t e d t o be 54 + 4 meV. T h i s v a l u e i s s i g n i f i c a n t l y l a r g e r t h a n t h e o b s e r v e d h a l f - w i d t h s o f t h e p e a k s i n t h e b a n d a t 1 4 . 9 4 e V , w h i c h a r e 4 5 + 3 meV. T h e s m a l l e r h a l f - w i d t h s o f t h e l a t t e r t e n d t o s u g g e s t t h a t t h i s b a n d i s p r o b a b l y d u e t o a s t a t e w i t h no s p i n - o r b i t s p l i t t i n g , i . e . a £ s t a t e ( o r a n s t a t e w i t h a v e r y s m a l l s p i n - o r b i t s p l i t t i n g w h i c h i s u n l i k e l y i n v i e w o f t h e 40 meV s p l i t t i n g f o r t h e 2 + 1 3 ? X n s t a t e ) . C o m p a r i n g w i t h 0^ » t h e a s s i g n m e n t o f t h i s b a n d t o t h e b ^ z " s t a t e o f S 0 + i s m o s t p r o b a b l e , a s s u g g e s t e d , 1 5 9 p r e v i o u s l y C o m p a r i n g t h e 1 3 . 5 . - 1 5 . 0 eV r e g i o n o f t h e PE s p e c t r u m ( F i g . 2 1 ) r e c o r d e d w i t h (B ) a n d w i t h o u t ( A ) t h e MW d i s c h a r g e i n SO,, a l o n e , we h a v e o b s e r v e d t h a t t h e r e a r e e x t r a p e a k i n t e n s i t i e s a n d some a d d i t i o n a l p e a k s i n t h e f o r m e r . T h e s e f e a t u r e s w e r e a l s o e v i d e n t i n t h e s p e c t r a 159 m e a s u r e d by J o n a t h a n e t a l b u t h a d n o t b e e n m e n t i o n e d i n t h e i r d i s c u s s i o n . W h i l e t h e 2 n d b a n d o f S o J ( F i g . 2 1 A ) d r o p s t o t h e b a s e l i n e b e y o n d 14.1 e V , t h e r e a r e , h o w e v e r , some d i s t i n c t p e a k s ( F i g . 2 1 B ) i n t h e same r e g i o n w h i c h s e e m t o c o r r e l a t e w i t h t h e p r e s e n c e o f SO i n t h e s p e c t r u m . By m a n u a l l y s u b t r a c t i n g t h e i n t e n s i t y o f t h e 2 b a n d o f S 0 2 f r o m t h e s p e c t r u m o f t h e d i s c h a r g e p r o d u c t s , t h e e x t r a p e a k i n t e n s i t i . e s a r e s h o w n a s v e r t i c a l l i n e s i n F i g . 2 2 . T h e s e p e a k s m u s t t h e n be d u e t o SO s i n c e t h e i r i n t e n s i t i e s f o l l o w t h e o t h e r b a n d s o f SO. T h e s p a c i n g b e t w e e n p e a k s v a r i e s b e t w e e n 8 5 0 cm" 1 a n d 7 0 0 c m " 1 w i t h a n a v e r a g e Ionization Potential (eV) F I G U R E 21 - 92 -D f 750 c m " 1 c o m p a r e d w i t h t h e g r o u n d s t a t e v a l u e o f -1 111 1111 cm . T h e d r a s t i c r e d u c t i o n i n v i b r a t i o n a l f r e q u e n c y a n d t h e l e n g t h o f t h e p r o g r e s s i o n ( a t l e a s t c a . 12 c o m p o n e n t s ) i s i n d i c a t i v e o f t h e r e m o v a l o f + 1 32 a s t r o n g l y b o n d i n g e l e c t r o n . By c o m p a r i n g w i t h Og , i t 4 s e e m s r e a s o n a b l e t o a s s i g n t h i s b a n d t o be d u e t o t h e a n s t a t e , s i n c e t h e c o r r e s p o n d i n g t r a n s i t i o n i n 0^ a l s o s h o w s an e x t e n s i v e v i b r a t i o n a l p r o g r e s s i o n w i t h a l a r g e d e c r e a s e i n t h e v i b r a t i o n a l f r e q u e n c y , f r o m 1 7 8 0 cm" 1 t o ^ 9 0 0 c m " 1 1 1 1 ( f o r t h e m o l e c u l e a n d i o n ) . T h e a d i a b a t i c a n d v e r t i c a l I P ' s o f t h i s b a n d a r e e s t i m a t e d t o b e a t 1 3 . 3 9 eV a n d 1 3 . 9 0 eV ( v ' = 5) r e s p e c t i v e l y . B e c a u s e o f t h e o v e r l a p f r o m t h e s e c o n d b a n d o f S O g . t h e s e q u o t e d IP v a l u e s may be i n e r r o r by a s much a s o n e v i b r a t i o n a l q u a n t u m . U p o n c l o s e e x a m i n a t i o n , i t i s n o t e d that p e a k s o f 4 t h e a n s t a t e b r o a d e n a t a n d b e y o n d v ' = 6 , a n d i r r e g u l a r s t r u c t u r e a p p e a r s a r o u n d 1 4 . 6 eV ( F i g . 2 2 ) . I n v i e w o f t h e f a c t t h a t t h i s i s r e p r o d u c e a b l e a n d f o l o w s t h e SO b a n d i n t e n -+ 1 32 s i t i e s , a n d g u i d e d by t h e s p e c t r u m o f 0^ , w h e r e a s i m i l a r p h e n o m e n o n o c c u r s on t h e h i g h e n e r g y s i d e o f t h e a % u b a n d , we a r e i n c l i n e d t o s u g g e s t t h e r e may be a n a d d i t i o n a l b a n d i n t h e m e n t i o n e d r e g i o n , w h i c h o v e r l a p s s t r o n g l y w i t h t h e 4 ' + 2 a n s t a t e o f SO:. I f t h i s i s g e n u i n e , i t w i l l t h e n be t h e A n i o n i c s t a t e . T h e c o r r e s p o n d i n g a d i a b a t i c a n d v e r t i c a l I P ' s c a n n o t be d e t e r m i n e d w i t h c e r t a i n t y b u t a r e e s t i m a t e d t o be a t 1 4 . 0 eV a n d 1 4 . 6 eV r e s p e c t i v e l y ( F i g . 2 2 ) a n d t h e 13 -i I . r-14 15 Ionization Potential ( eV ) F I G U R E ZZ - 94 -v i b r a t i o n a l s p a c i n g i s a p p r o x i m a t e l y 7 0 0 cm" 1 c o m p a r e d w i t h 1111 cm" 1 1 1 1 f o r t h e g r o u n d s t a t e . T h i s c o m p a r e s w i t h t h e v a l u e s o f 1 7 8 0 c m " 1 a n d ^ 8 0 0 c m " 1 f o r t h e A 2 i r s t a t e o f o 2 . 4 2 W i t h t h e i d e n t i f i c a t i o n o f t h e a n a n d A n s t a t e s , t h e PE s p e c t r u m o f S 0 + t h e n , a s e x p e c t e d , c l o s e l y r e s e m b l e s + 2 -t h a t o f 0 2 e x c e p t f o r t h e a b s e n c e o f t h e B £ ^ j s t a t e i n t h e f o r m e r ( 0 2 , 2 0 . 4 e V ) . We s u s p e c t t h i s s t a t e t o be b u r i e d u n d e r n e a t h t h e i n t e n s e t h i r d b a n d o f S 0 2 ( a t 1 6 . 6 e V ) . I n c o n c l u s i o n , i t may be a d d e d t h a t a t t e m p t s t o r e m o v e S 0 2 f r o m t h e d i s c h a r g e p r o d u c t s e i t h e r b y s e l e c t i v e c o l d r t r a p p i n g o r b y u s i n g an e x c e s s o f S v a p o r r e s u l t e d i n t h e c o m p l e t e d i s a p p e a r a n c e o f SO as w e l l . 4.3 D i s u l f u r M o n o x i d e ( S g O ) 4.3.1 I n t r o d u c t i on T h e c h e m i s t r y o f S 2 0 h a s b e e n r e v i e w e d b y S c h e n k 16 3 a n d S t e u d e l ,. In t h e v a p o r p h a s e a n d a t l o w p r e s s u r e s o f c a . o n e t o r r o r l e s s , S 2 0 c a n be s t a b l e f o r a f e w h o u r s , a n d SO f o r m e d a s an i n t e r m e d i a t e i n a r e a c t i o n i s a l m o s t a l w a y s t h e p r e c u r s o r o f S 2 0 . H i s t o r i c a l l y , S 2 0 was a c t u a l l y m i s t a k e n f o r SO f o r m o r e t h a n a d e c a d e u n t i l i t s i d e n t i t y was f i n a l l y e s t a b l i s h e d i n 1 9 5 6 . T h e PE s p e c t r u m o f S 2 0 i s , m o r e o v e r , o f a d d i t i o n a l i n t e r e s t b e c a u s e i t i s i s o e l e c t r o n i c w i t h t h e m o l e c u l e s NSF a n d S 0 2 , t h e PE s p e c t r a o f w h i c h h a v e r e c e n t l y b e e n t h e - 95 -1 6 7 - 1 7 0 s u b j e c t o f e x t e n s i v e s t u d i e s " . T h e r e l a t e d c o m p o u n d 1 5 7 USC1 h a s a l s o b e e n i n v e s t i g a t e d b e c a u s e o f d i s a g r e e m e n t s r e g a r d i n g t h e a s s i g n m e n t o f v a r i o u s i o n i z a t i o n p o t e n t i a l s 1 6 8 ' 1 * ' 9 . I t w i l l be s h o wn t h a t o u r r e s u l t s a n d a s s i g n m e n t s l e a d t o a r e s o l u t i o n a n d d e f i n i t e c o n f i r m a t i o n o f t h e o r d e r o f t h e o r b i t a l s i n t h i s s e r i e s o f m o l e c u l e s . 4.3.2 E x p e r i m e n t a l SgO was p r o d u c e d b y two s e p a r a t e m e t h o d s . ( 1 ) By t h e m i c r o w a v e d i s c h a r g e o f s u l f u r d i o x i d e (SO,,) i n t h e p r e s e n c e o f s u l f u r , w h i c h was c o a t e d on t h e w a l l o f t h e d i s c h a r g e t u b e . D e p e n d i n g o n t h e p r e s s u r e o f SO^, v a r i o u s p r o p o r t i o n s o f S g O / S O g c o u l d be o b t a i n e d . A t a c e r t a i n p r e s s u r e , t h e PE s p e c t r u m o f SOg d i s a p p e a r e d c o m p l e t e l y y e t when t h e s u p p l y o f SO^ was t u r n e d o f f , t h e b a n d s d u e t o SgO a l s o d i s a p p e a r e d . ( 2 ) By p a s s i n g t h i o n y l c h l o r i d e ( S 0 C 1 , ) v a p o r t h r o u g h a 4 cm c o l u m n o f s i l v e r s u l f i d e ( A g g S ) p o w d e r h e a t e d t o a b o u t 1 5 0 ° C 1 7 1 . T h e h e a t e r was made by w i n d i n g c h r o m e l - P w i r e s n o n - i n d u c t i v e l y a r o u n d a q u a r t z t u b e . T h e d i r e c t i n l e t s y s t e m made o f p y r e x ( s e c t i o n 3 . 3 . 4 . b ) was u s e d f o r S O C l ^ b e c a u s e o f i t s r e a d y d e c o m p o s i t i o n . T h e PE s p e c t r u m o f S 0 C 1 2 d i s a p p e a r e d c o m p l e t e l y when A g 0 S was h e a t e d a n d t h e new b a n d s a p p e a r e d c o u l d b e i d e n t i f i e d a s d u e t o SgO a n d SO,,. T h e b e s t y i e l d was o b t a i n e d a t a r o u n d 1 5 0 ° C . T h e PE s p e c t r a o b t a i n e d f r o m t h e two m e t h o d s w e r e - 96 -i d e n t i c a l e x c e p t f o r a d i f f e r e n c e i n t h e S 2 0 / S 0 2 i n t e n s i t y r a t i o , w i t h t h e s e c o n d m e t h o d g i v i n g c o n s i s t e n t l y b e t t e r • i c q y i e l d s o f S 2 0 as e x p e c t e d . T h i s l e a v e s no d o u b t t h a t t h e new s p e c i e s we o b s e r v e d i s S 2 0 . T h e r e s o l u t i o n a s m e a s u r e d b y t h e FWHM o f t h e a r g o n P 3 p e a k i s b e t w e e n 2 5 - 3 0 meV. C a l i b r a t i o n o f t h e s p e c t r u m was e f f e c t e d u s i n g a r g o n a n d o x y g e n . T h e e s t i m a t e d e r r o r i s ± 20 meV f o r i o n i z a t i o n p o t e n t i a l s ( I P ' s ) a n d ± 30 c m " 1 f o r v i b r a t i o n a l f r e q u e n c i e s . 4.3.3 R e s u l t s a n d D i s c u s s i o n T h e PE s p e c t r u m o b t a i n e d by p a s s i n g S 0 C 1 2 o v e r f i e a t e d A g 2 S i s s h o w n i n F i g . 2 3 , w h e r e t h e PE s p e c t r a o f p u r e S 0 2 a n d S 0 C 1 2 a r e a l s o i n c l u d e d f o r c o m p a r i s o n . A l t h o u g h i t was n o t p o s s i b l e t o g e t r i d o f S 0 2 f r o m t h e s p e c t r u m o f S 2 0 , f o r t u n a t e l y i t s e x i s t e n c e c a u s e d l i t t l e p r o b l e m . T h e PE s p e c t r u m o f S 2 0 ( F i g . 2 3 b ) c o n s i s t s o f f o u r d i s t i n g u i s h a b l e b a n d s w i t h v i b r a t i o n a l f i n e s t r u c t u r e r e s o l v e d i n two o f t h e m . T h e I P ' s a n d v i b r a t i o n a l f r e q u e n c i e s a r e c o l l e c t e d i n T a b l e 5. T h e i n t e r p r e t a t i o n o f t h e PE s p e c t r u m o f S 2 0 i s a i d e d by c o m p a r i s o n w i t h t h o s e o f N S F 1 6 7 " 1 7 0 a n d S O ^ ^ 2 , w h i c h a r e i s o e l e c t r o n i c t o S 2 0 . C o m p a r i s o n w i t h t h e PE s p e c t r a o f N S C l 1 6 7 i s a l s o h e l p f u l , s i n c e t h i s h a s s i m i l a r v a l e n c e e l e c t r o n i c s t r u c t u r e t o $ 2 0. T h e d e d u c t i o n s d r a w n f r o m t h e s e i n t e r m o l e c u ! a r c o m p a r i s o n s a n d t h e MO c a l c u l a t i o n s lO 11 12 13 14 15 16 . 17 18 19 Ionization Potential (eV) F I G U R E 23 - 98 -T A B L E 5 PES a n d T h e o r e t i c a l D a t a f o r S o 0 CNDO/2 P E S MO s y m m e t r y l p a r ( e V ) I P b ) ( e V ) v c ) ( c m _ 1 ) 7a ' 9.68 1 0 . 5 3 (1 0 . 5 9 ) 4 8 0 ( 6 7 9 ) 6a ' 1 0 . 3 6 11 .25 9 3 0 ( 1 1 6 5 ) 2 a " 10.71 11.31 ( 1 1 . 4 2 ) 9 4 0 ( 1 1 6 5 ) 5a ' 14.11 1 4 . 3 ( 1 4 . 7 ) l a " 1 4 . 1 3 .14.9 4a ' 1 5 . 4 4 1 5 . 5 ( 1 6 . 0 ) 3 a 1 1 8 . 3 9 1 8 . 7 ( ? ) a ) I P ' s o b t a i n e d by s u b t r a c t i n g 4 eV f r o m t h e c o r r e s p o n d i n g a b s o l u t e MO e n e r g y . b) v e r t i c a l I P ' s a r e i n p a r e n t h e s e s , c ) v a l u e s i n p a r e n t h e s e s a r e t h e c o r r e s p o n d i n g v i b r a t i o n a l f r e q u e n c i e s i n t h e n e u t r a l m o l e c u l e f r o m r e f . 1 6 6 . - 99 -( s e e b e l o w ) s u g g e s t s i x t o s e v e n a c c e s s i b l e I P ' s a n d t h a t t h e o r b i t a l c o n f i g u r a t i o n o f S 2 0 i s K K K L L ( l a ' ) 2 ( 2 a ' ) 2 ( 3 a ' ) 2 ( 4 a ' ) 2 ( l a " ) 2 ( 5 a ' ) 2 ( 2 a " ) 2 2 2 ( 6 a ' ) ( 7 a 1 ) . In a d d i t i o n , t h e c a l c u l a t i o n s h o w s , p r o v i d e d K o o p m a n s ' t h e o r e m h o l d s , f o r S 2 0 t h e s e c o n d a n d t h i r d I P ' s a r e c l o s e , a n d t h e f o u r t h a n d f i f t h I P ' s a r e d e g e n e r a t e w i t h i n e x p e r i m e n t a l a c c u r a c y . T h e f a c t t h a t t h e s e c o n d a n d t h i r d I P ' s o f S 0 2 1 7 2 , M S F 1 6 8 " 1 6 9 a n d N S C 1 1 5 7 a r e c l o s e a n d t h e f o u r t h a n d f i f t h I P ' s o f S 0 2 a r e a l s o c l o s e l e a d s o n e t o e x p e c t s i m i l a r b e h a v i o r f r o m S 2 0 , a n d l e n d s s u p p o r t t o t h e MO c a l c u l a t i o n s . I n f a c t , t h e PE s p e c t r a o f t h e s e c o m p o u n d s a r e r e m a r k a b l y s i m i l a r , a s e x e m p l i f i e d i n t h e i o n i z a t i o n p o t e n t i a l c o r r e l a t i o n d i a g r a m d e p i c t e d i n F i g . 2 4 . N a m e l y , t h e PE b a n d s o f e a c h c o m p o u n d c a n be g r o u p e d i n t o t h r e e r e g i o n s , i . e . t h e f i r s t I P , t h e s e c o n d a n d t h i r d I P ' s , a n d t h e h i g h e r I P ' s . C o n s e q u e n t l y , t h e a n a l y s i s f o r t h e s p e c t r u m o f S 2 0 a p p e a r s t o be f a i r l y s t r a i g h t f o r w a r d u s i n g t h i s i n t e r m o l e c u l a r c o r r e l a t i o n . I n o r d e r t o s t r e n g t h e n t h e b a s i s o f t h e a b o v e e x p e c t a t i o n a n d t o o b t a i n m o r e i n f o r m a t i o n f o r t h e a n a l y s i s , we t u r n t o MO c a l c u l a t i o n s . A CNDO/2 MO c a l c u l a t i o n 7 6 was p e r f o r m e d on S o 0 w i t h t h e e x p e r i m e n t a l " I C C g e o m e t r y a n d t h e r e s u l t s c o l e c t e d i n T a b l e 5. T h e d e d u c t i o n s f r o m t h e i n t e r m o l e c u l a r c o m p a r i s o n s a n d t h e MO c a l c u l a t i o n s a r e e n c o u r a g i n g l y c o n s i s t e n t , a n d t h u s l e n d s u p p o r t t o t h e MO c a l c u l a t i o n . F u r t h e r m o r e , t h e s u c c e s s w i t h w h i c h t h e e a r l i e r a p p r o x i m a t e MO c a l c u l a t i o n s p r e d i c t e d t h e - 100 -C Q) O Q_ c O o N c Q 18-1 > o> 16 JS) D 12-1 0 J OSO SSO NSCl NSF 4a , 3 a, l b , 3b ; J 1 a 2 4 a T \ 4 a ' l a 5 a ' '-. 2 a " 6 a 7 7 a 7 1a 5 a ' / 6a:2a? 7a ' l a " 5 a ' 2 a " j; 6 a ' 7a' F I G U R E 24 - 1 0 1 " I P ' s o f N S F 1 6 8 " 1 7 0 , N S C 1 1 6 7 a n d S O g 1 7 3 m a k e s i t e v i d e n t t h a t we c a n a f f e c t s i m i l a r s u c c e s s i n t h e c a s e o f S g O . T h e f i r s t b a n d i n t h e PE s p e c t r u m o f S^O ( F i g . 2 5 ) c o n s i s t s o f o n e I P , w i t h t h e a d i a b a t i c a n d v e r t i c a l v a l u e s a t 1 0 , 5 3 a n d 1 0 . 5 9 eV r e s p e c t i v e l y . T h i s i s c o m p a r e d t o 1 0 . 3 ± 0 . 1 e V , t h e o n l y o b s e r v e d IP o f S g O , o b t a i n e d f r o m m a s s 1 74 s p e c t r o m e t r y . T h e f o u r c l e a r l y d i s c e r n i b l e v i b r a t i o n a l c o m p o n e n t s w i t h an a v e r a g e s p a c i n g o f 4 3 0 c m " 1 a r e a s s i g n e d t o w h i c h i s m a i n l y t h e S-S s t r e t c h . T h e c a l c u l a t i o n i n d i c a t e s t h e 7 a ' MO i s S-S b o n d i n g i n a c c o r d w i t h t h e d e c r e a s e o f t h e v i b r a t i o n a l f r e q u e n c e f r o m 679 c m - 1 1 6 6 -1 2 i n t h e n e u t r a l m o l e c u l e t o 4 8 0 cm i n t h e X A' i o n i c s t a t e . T h e a s s i g n m e n t o f t h e v i b r a t i o n 4 8 0 cm" 1 t o t h e b e n d i n g mode ( 3 3 8 c m " 1 1 6 ^ i n t h e n e u t r a l m o l e c u l e ) i s r u l e d o u t b e c a u s e t h e W a l s h r u l e s p r e d i c t an i n c r e a s e i n b o n d a n g l e a c c o m p a n y i n g t h e r e m o v a l o f o r b i t a l 7 a ' a n d 1 72 h e n c e a d e c r e a s e i n b e n d i n g f r e q u e n c y ( a s o c c u r r e d i n SO,, a n d N S F 1 7 ^ ) . H o w e v e r , t h e s h a p e o f t h e h i g h e n e r g y e n d o f t h e b a n d may i n d i c a t e a n a d d i t i o n a l , b u t v e r y w eak v i b r a t i o n a l p r o g r e s s i o n o f s p a c i n g c a . 2 0 0 c m " 1 , w h i c h i f r e a l s h o u l d be t h e i o n i c b e n d i n g f r e q u e n c y . I t i s d i f f i c u l t t o d e c i d e w h e t h e r o n e o r two I P ' s a r e i n v o l v e d i n t h e s e c o n d PE b a n d ( F i g . 25),, As d i s c u s s e d a b o v e , we p r e f e r t o a s s i g n two I P ' s t o t h i s b a n d a n d p r o p o s e , t h a t e l e c t r o n s f r o m t h e M 0 1 s 6 a ' a n d 2 a " r e s p e c t i v e l y a r e r e m o v e d . T h e c o r r e s p o n d i n g I P ' s a n d v i b r a t i o n a l f r e q u e n c i e s a r e s u m m a r i z e d i n T a b l e 5. T h e v i b r a t i o n a l s t r u c t u r e on H = 930 cm-' r— I 1 f i i i 1 — — — i 1 — , — 10.6 R O 11.4 11.8 Ionization Potential (eV) F I G U R E 25 - 1 0 3 -b o t h b a n d s s e e m s t o be a s i m p l e s e r i e s i n , w h i c h i s m a i n l y S-0 s t r e t c h , i n d i c a t i n g a d e c r e a s e i n f r e q u e n c y i n t h e i o n i c s t a t e . T h i s i s s u p p o r t e d b y t h e c a l c u l a t i o n , w h i c h p r e d i c t s t h a t t h e 6 a 1 a n d 2 a " o r b i t a l s a r e , r e s p e c t i v e l y , b o n d i n g a n d s l i g h t l y b o n d i n g b e t w e e n t h e c e n t r a l S a n d t h e 0 a t o m s , w h i l e t h e y a r e b o t h b o n d i n g b e t w e e n t h e S a t o m s . T h e t h i r d a n d f o u r t h b a n d s a r e b o t h b r o a d a n d f e a t u r e l e s s . T h e f o r m e r s t a r t s a t 1 4 . 3 eV a n d h a s i t s maximum a t 1 4 . 7 e V , w h i l e t h e c o r r e s p o n d i n g v a l u e s f o r t h e l a t t e r 1 74 a r e 1 5 . 5 a n d 1 6 . 0 eV r e s p e c t i v e l y . H a g e m a n n g a v e t h e a p p e a r -a n c e p o t e n t i a l o f SO f r o m SgO t o be < 1 4 . 5 ± 0.2 e V . T h i s i s c o n s i s t e n t w i t h t h e f e a t u r e l e s s c h a r a c t e r o f t h e t h i r d b a n d . U n e q u i v o c a l a s s i g n m e n t o f t h e s e two b a n d s i s d i f f i c u l t . T h e CNDO/2 r e s u l t i s u s e d f o r t h e t e n t a t i v e a s s i g n m e n t , i . e . t h e t h i r d b a n d a r i s e s f r o m two I P ' s (MO's 5 a ' a n d l a " ) a n d t h e f o u r t h f r o m o n e IP (HO 4 a ' ) . T h e s h a p e o f t h e t h i r d b a n d ( F i g . 2 3 b ) s u g g e s t s a f o u r t h IP a t 1 4 . 3 eV a n d a f i f t h a t 14.9 e V . T h i s t h e n l e a v e s o n e IP ( p r e d i c t e d by c a l c u l a t i o n ) u n a c c o u n t e d f o r , w h i c h we s u s p e c t t o be p a r t o f t h e f o u r t h b a n d o r a t ^ 1 8 . 7 e V , w h e r e t h e r e i s some i n d i c a t i o n o f an a d d i t i o n a l b a n d s i t t i n g on t h e r i s i n g b a c k g r o u n d . In t h e s p e c t r u m o f SgO o b t a i n e d f r o m t h e MW d i s c h a r g e o f S O g , i n a d d i t i o n t o t h e b a n d s d e s c r i b e d a b o v e , we o b s e r v e d a c l e a r l y d i s c e r n i b l e v i b r a t i o n a l s e r i e s o f t h r e e c o m p o n e n t s w i t h an a v e r a g i n g s p a c i n g o f 9 2 0 c m " 1 , c o r r e s p o n d i n g t o a n - 104 -a d i a b a t i c a n d v e r t i c a l I P a t 1 4 . 9 5 e V . T h i s b a n d c a n be a s s i g n e d u n a m b i g u o u s l y t o be t h e t h i r d b a n d o f S 0 + b e c a u s e o f t h e c l o s e a g r e e m e n t o f IP a n d f r e q u e n c y v a l u e s w i t h t h o s e f o u n d b y J o n a t h a n e t a l 1 5 9 f o r S 0 + a s w e l l a s t h e s i m i l a r e x p e r i m e n t a l c o n d i t i o n s . F u r t h e r m o r e we w i s h t o p o i n t o u t t h a t t h e a s s i g n m e n t o f t h e weak b a n d a t 1 1 . 3 eV b y J o n a t h a n 159 e t a l t o SO may n o t be c e r t a i n . C o n s i d e r i n g t h a t SgO i s i n v a r i a b l y f o r m e d » i n t h e d i s c h a r g e o f S 0 2 a n d t h e 159 i n t e n s i t y o f t h e s e c o n d b a n d o f SO i s s e e m i n g l y i n c o m p a t i b l e w i t h t h a t o f t h e o t h e r two b a n d s , we w o u l d s u g g e s t t h a t t h i s i s a c t u a l l y t h e s e c o n d b a n d o f S 2 0 , w h i c h we f i n d t o h a v e i t s o r i g i n a t v e r y n e a r l y t h e same e n e r g y , i . e . 1 1 . 2 3 e V . T h e f i r s t b a n d o f S^O may v e r y w e l l be b u r i e d u n d e r n e a t h t h e f i r s t b a n d o f SO i n t h e s p e c t r u m o f J o n a t h a n e t a l ( s e e a l s o s e c t i o n . 4 .2 . 3) . 4.4 D i a z e n e (HN = N H ) - A n d 0 i a z e n e - d 2 (DN = ND) 4 •4•1 I n t r o d u c t i o n T h e s h o r t l i v e d s p e c i e s HN = ;'IH ( o r N,>H9) h a s r e c e n t l y p r o v o k e d a g r e a t d e a l o f i n t e r e s t w i t h r e g a r d t o i t s c h e m i s t r y 1 7 5 " 1 7 7 a n d i t s s p e c t r o s c o p i c p r o p e r t i e s 1 7 8 " 1 8 5 l 2 f l ? , a s t h e f i r s t member o f t h e a z o s e r i e s , h a s a l s o a t t r a c t e d t h e a t t e n t i o n o f many t h e o r e t i c i a n s 1 8 6 - 1 8 9 . W i t h t h i s w e a l t h o f ^ i n t e r e s t on t h e m o l e c u l e , i t t h u s s e e m s S u r p r i s i n g t h a t N 9 H 7 r e m a i n s a s p e c t r o s c o p i c e n i g m a i n many r e s p e c t s , e . g . t h e i n t e n s i t y a l t e r n a t i o n i n t h e v a c u u m - 1 0 5 -u l t r a v i o l e t s p e c t r u m * 8 1 , t h e u n c e r t a i n t y i n t h e p r e c i s e 1 ^ 4 i n t e r p r e t a t i o n o f n e a r u l t r a v i o l e t s p e c t r u m , t h e g e o m e t r y o f t h e e x c i t e d s t a t e s , a n d t h e p o s s i b i l i t y o f c i s / t r a n s i s o m e r i s m i n t h e g a s p h a s e . In v i e w o f t h e s e c o n t r o v e r s i e s a n d t h e p o t e n t i a l o f PES i n p r o v i d i n g i n f o r m a t i o n o n t h e e l e c t r o n i c s t r u c t u r e o f m o l e c u l e s , i t t h e r e f o r e s e e m e d a p p r o p r i a t e t o u n d e r t a k e a PE s t u d y o f T h e p r o p e r t i e s o f N 0 H 0 w h i c h a r e r e l e v a n t t o o u r s t u d i e s h e r e may be s u m m a r i z e d a s f o l l o w s . I t i s g e n e r a l l y a c c e p t e d t h a t t h e m o l e c u l e i n i t s g r o u n d s t a t e a s s u m e s a t r a n s f o r m a n d i s a s i n g l e t e l e c t r o n i c s t a t e , a l t h o u g h a t r i p l e t g r o u n d s t a t e h a s b e e n s u g g e s t e d 1 8 1 . T h e m o l e c u l e h a s a l i f e t i m e o f t h e o r d e r o f m i n u t e s i n t h e v a p o r p h a s e a t l o w p r e s s u r e , a n d d e c o m p o s e s r a p i d l y t o q i v e N 0 a n d H 0 a s m a j o r p r o d u c t s . 4.4.2 E x p e r i men t a i • N 2-K 9 was g e n e r a t e d b y two d i f f e r e n t m e t h o d s . 177 ( 1 ) A v e r y w e ak m i c r o w a v e d i s c h a r g e i n t h e e q u i l i b r i u m v a p o r o f , f J2 H4 ( E a s t m a n , 9 9 % p u r i t y ) a t r o o m t e m p e r a t u r e . T h e PE s p e c t r u m o f NpH^ ( F i g . 2 6 a ) was e s s e n t i a l l y t h a t o f 190 p r e v i o u s w o r k e r s . U n d i s s o c i a t e d N 2 H ^ a n d i n v o l a t i l e p r o d u c t s w e r e t r a p p e d a t - 7 8 ° C , t h e o n l y r e m a i n i n g s p e c i e s i n t h e g a s p h a s e b e i n g N 2 H 4 » M H 3 > ^ a n c l H 2 ^ F i g . 26b) • G r e a t t r o u b l e was e x p e r i e n c e d b y o t h e r w o r k e r s d u e t o t h e - 106 -p r e s e n c e o f t h e NH^, w h i c h may c o n f u s e a n y s p e c t r o s c o p i c m e a s u r e m e n t , a n d i n d e e d p a r t l y o b s c u r e s t h e PE s p e c t r u m o f N 2 H 2 ( F i g . 2 6 b ) . D i s t i l l a t i o n , o r t r a p p i n g a n d r e v o l a t i l i -z a t i o n o f t h e N H 3 / N 2 H 2 m i x t u r e d o e s n o t a l l e v i a t e t h i s p r o b l e m . H o w e v e r , we h a v e e l i m i n a t e d t h i s p r o b l e m b y t i t r a t i n g t h e g a s p h a s e p r o d u c t s w i t h H C l a f t e r p a s s a g e t h r o u g h t h e t r a p a t - 7 8 ° C . ( 2 ) T h e s e c o n d m e t h o d i n v o l v e d p r e p a r a t i o n o f s o d i u m b i s ( t r i m e t h y l s i l y l ) a m i d e f r o m h e x a m e t h y l d i s i l a z a n e 1 9 1 » 1 9 2 ^ T h i s , i n t u r n , was c o n v e r t e d i n t o s o d i u m t o s y l h y d r a z i d e by 17 5 r e a c t i o n w i t h t o s y l h y d r a z i n e . T h e s o d i u m t o s y l h y d r a z i d e was t h e n p y r o l y z e d a t 5 5 - 6 0 ° C ( a s r e c o m m e n d e d by W i b e r g e t a l 1 7 5 , 1 7 6 ) t o g i v e p u r e N 2 H 2 p l u s N 2 a n d a l i t t l e H 2 i n t h e g a s p h a s e . H o w e v e r , we h a v e f o u n d t h a t b y r a i s i n g t h e t e m p -e r a t u r e t o 9 0 - 1 0 0 ° C a n d u s i n g f a s t e r d i f f e r e n t i a l p u m p i n g , t h e y i e l d o f N ? H 2 c a n be i m p r o v e d s e v e r a l f o l d w i t h r e s p e c t t o N 2 . In b o t h c a s e s ( 1 ) a n d ( 2 ) , t h e g a s p h a s e m i x t u r e o f N 2 H 2 / N 2 / H 2 was l e d d i r e c t l y i n t o t h e i o n i z a t i o n c h a m b e r , w h c i h was d i f f e r e n t i a l l y p u m p e d . T h e p r e s s u r e s r e g i s t e r e d - 5 - 5 a t t h e i o n i z a t i o n g a u g e w e r e 2 x 1 0 a n d 0 . 3 x 1 0 mm Hg r e s p e c t i v e l y i n c a s e s ( 1 ) a n d ( 2 ) . B e c a u s e o f t h e much l o w e r s a m p l e p r e s s u r e i n t h e l a t t e r , a l o n g s c a n n i n g t i m e (^1 h r ) was n e c e s s a r y t o r e c o r d a d e c e n t s p e c t r u m . T h e PE s p e c t r a w e r e i d e n t i c a l i n b o t h c a s e s e x c e p t f o r a b e t t e r ^2 H2^'*2 r a t i o i n t h e 2 n d m e t h o d . N 2 D 2 was g e n e r a t e d f r o m N 2 D 4 ( M e r c k , S h a r p a n d - 1 0 7 -Dohme) by m e t h o d ( 1 ) a b o v e . N D 3 was e l i m i n a t e d b y g a s p h a s e t i t r a t i o n w i t h DC1 . I n t h e d i s c h a r g e m e t h o d o u t l i n e d a b o v e t h e e f f i c i e n c y o f t h e d i s c h a r g e i n s e l e c t i v e l y p r o d u c i n g N ^ l i , , / ^ ^ c o u l d be m o n i t o r e d b y t r a p p i n g a l l t h e p r o d u c t s a t - 1 9 6 ° C , a n d o b s e r v i n g t h e y e l l o w c o l o r o f t h e d i a z e n e . I n t h i s r e s p e c t , we h a v e c o n f i r m a t i o n o f t h e p r o d u c t i o n o f t h e d e s i r e d s p e c i e s , t h e a l t e r n a t i v e p r e p a r a t i v e m e t h o d f o r N g l l g p r o v i d i n g us w i t h f u r t h e r e v i d e n c e f o r c o r r e c t i d e n t i f i c a t i o n . T h e s p e c t r a w e r e c a l i b r a t e d u s i n g t h e I P ' s o f t h e ; l 2 , H ? / D 2 o r H C 1 / D C 1 , w i t h a n a c c u r a c y o f ± 0.01 eV f o r t h e I P ' s o f s h a r p p e a k s a n d ± 30 cm" 1 f o r t h e v i b r a t i o n a l s p a c i n g s . R e s o l u t i o n i n a l l c a s e s was b e t t e r t h a n 30 meV. 4.4.3 R e s u l t s -F i g . 26 d e p i c t s t h e s p e c t r a o f HgH^ w i t h o u t ( a ) a n d w i t h ( b ) t h e d i s c h a r g e , a n d , i n a d d i t i o n , i l l u s t r a t e s t h e e f f e c t o f a d d i n g i n c r e a s i n g a m o u n t s o f H Cl t o t h e d i s c h a r g e p r o d u c t s ( c , d a n d e ) . F i g . 27 a n d 28 i l l u s t r a t e t h e a n a l o g o u s p r o c e d u r e f o r t h e d e u t e r a t e d s y s t e m , e x c e p t h e r e t h e e f f e c t o f DC1 was m o n i t o r e d u s i n g o n l y t h e f i r s t b a n d s o f MD^ a n d N ? D 2 i n F i g . 2 3 . In b o t h c a s e s , i t h a s b e e n f o u n d t h a t t h e g a s p h a s e r e a c t i o n s o f H C l ( D C ! ) w i t h M H 3 ( N D 3 ) i s i n s t a n t a n e o u s , g i v i n g a w h i t e d e p o s i t o f N H ^ C l ( N O ^ C l ) on t h e w a l l s , w h e r e a s t h e r e a c t i o n o f M C l ( D C l ) w i t h 112 H 2 (N 2 ^  2 ^  i s v e r y s l o w , l i e h a v e d o n e f u r t h e r e x p e r i m e n t s on t h e r e a c t i o n o f H C l w i t h p u r e NgH?* g e n e r a t e d b y t h e 2 n d m e t h o d , a n d h a v e - 108 -F I G U R E 26 - 109 -— i 1 1 1—i i i i i i i r (b) DISCHARGE IN N2D4 i 1 I I I I I 1 1 1 1 1 I T 8 9 10 II 12 13 14 15 16 17 18 19 20 IONIZATION POTENTIAL (eV) FIGURE 27 - n o -i — i — i — | — r (b) DCI ADDED i — i — i — i — T ' I I I (d) EXCESS DCI D20/537A "i i |—i—r (c) MORE DCI (a) NO DCI 1 I r 10.0 1—I—f T l r T ~ r -11.0 10.0 IONIZATION POTENTIAL (eV) I I I I 11.0 F I G U R E 28 - I l l -o b s e r v e d t h e same b e h a v i o r . By u s i n g t h e r i g h t a m o u n t o f M C l ( D C l ) , t h e r e s u l t i n g s p e c t r u m o f *'J2H2^f*2^2^ m a y t l l e n b e o b t a i n e d f r e e f r o m H H ^ O ^ ) i n t e r f e r e n c e ( F i g . 2 6 d a n d F i g . 2 8 c ) , a n d , i n some c a s e s , c o n t a i n s a d d i t i o n a l b a n d s d u e t o . l - I C l ( D C l ) ( F i g . 2 6 e a n d 2 8 d ) , w h i c h p r o v e u s e f u l f o r c a l i b r a t i o n p u r p o s e s . T h e e x p e r i m e n t a l l y d e t e r m i n e d I P ' s a n d v i b r a t i o n a l f r e q u e n c i e s a r e c o l l e c t e d i n T a b l e 6, w h e r e t h e v i b r a t i o n a l f r e q u e n c i e s o f t h e g r o u n d s t a t e m o l e c u l e s a r e a l s o l i s t e d f o r c o m p a r i s o n . T h e N H ^ - f r e e s p e c t r u m o f i l g ' ^ o b t a i n e d b y t h e s e c o n d m e t h o d , w h i c h b e c a u s e o f i t s f a v o r a b l e ^2^2^''2 r a t i o g i v e s a b e t t e r p r e s e n t a t i o n o f t h e MgHg b a n d s , i s s h o w n i n F i g . 2 9 . HN = NH T h e PE s p e c t r u m o f H ? H 2 ( F i o . 2 9 ) e x h i b i t s f o u r d i s t i n g u i s h a b l e b a n d s , w i t h e x t e n s i v e v i b r a t i o n a l s t r u c t u r e on a l l o f t h e m . T h e f i r s t b a n d ( F i g . 3 0) h a s an a d i a b a t i c I P o f 9.59 e V , a n d a v e r t i c a l I P o f 1 0 . 0 2 e V . T h i s c o m p a r e s f a v o r a b l y w i t h t h e o n l y p r e v i o u s l y d e t e r m i n e d v a l u e o f 1 7 8 9.85 ± 0.1 eV f r o m m ass s p e c t r o m e t r y ' . T h e b a n d e n v e l o p e e x t e n d s o v e r c a . 1.3 e V , w i t h a t l e a s t 12 c o m p o n e n t s , t h e v e r t i c a l t r a n s i t i o n o c c u r r i n g a t v ' = 3. T h e v i b r a t i o n a l i n t e r v a l b e t w e e n t h e 0 a n d 1 v i b r a t i o n a l q u a n t a i s 1 1 8 0 c m - 1 h o w e v e r , t h i s t e n d s t o d e c r e a s e t o w a r d s h i g h v i b r a t i o n a l q u a n t a , t h e v a l u e a v e r a g e d o v e r 0 t o 9 b e i n g 1 0 5 0 cm 1. T h e r e i s a l s o t h e s u g g e s t i o n o f a n o t h e r s e r i e s , p o s s i b l y s t a r t i n g a t ^ 1 8 0 0 c m - 1 a b o v e t h e v ' = 0 p e a k b e t w e e n t h e v ' = 1 a n d 2 v i b r a t i o n a l c o m p o n e n t s o f t h e p r i m a r y s e r i e s . H e r e t h e v i b r a t i o n a l s p a c i n g b e t w e e n m e m b e r s i s a c o n s t a n t 1 1 8 0 cm 1 . TABLE 6 PES Data for HN = NH and flN = MD Ionic state X2A • 9 A 2 A u B2B u 9 Vert ica l IP (adiabat ic) (eV) v ( c n ~ 1 ) 10.02 (9.59) 14.39 (14.10) 15.03 (14.71 ) 16.9 (16.19) 1180 1110 11 70 940 1 583 1 529 1 583 1 529 N 2D 2 Vert ica l IP ^(cm" 1 ) fori Mol ecu! e* (adiabatic) (eV) Ion Mol ecul e* 10.10 (9.61) 1020 1215 14.39 (14.11) 1110 1539 15.17 (14.69) 960 1215 * from r e f . 182. CJ I T 1 1 r 9 10 11 12 13 14 15 16 17 18 19 2 0 I O N I Z A T I O N P O T E N T I A L (eV) FIGURE 29 - 114 -T o w a r d s h i g h e r v i b r a t i o n a l q u a n t u m n u m b e r s t h e two s e r i e s a p p e a r t o m e r g e , m a k i n g p r e c i s e m e a s u r e m e n t s d i f f i c u l t . T h e s e c o n d b a n d ( F i g . 3 0 ) h a s a n a d i a b a t i c I P o f 1 4 . 1 0 e V , a n d a v e r t i c a l IP o f 1 4 . 3 9 e V . H e r e t h e b a n d e n v e l o p e c o n t a i n s a t l e a s t 7 m e m b e r s s p r e a d i n g o v e r a b o u t -1 0.8 e V , w i t h a f a i r l y e v e n v i b r a t i o n a l s p a c i n g o f 1 1 1 0 cm T h e v e r t i c a l t r a n s i t i o n o c c u r s a t v ' = 2. T h e t h i r d b a n d ( F i g . 3 0 ) i s o v e r l a p p e d by t h e s e c o n d b a n d , b u t t h e a d i a b a t i c a n d v e r t i c a l I P ' s c a n be c o n f i d e n t l y a s s i g n e d a t 1 4 . 7 1 eV a n d 1 5 . 0 3 e V , r e s p e c t i v e l y . H e r e t h e b a n d s h o w s a t l e a s t f i v e d i s c e r n i b l e p e a k s w i t h t h e v e r t i c a l t r a n s i t i o n o c c u r i n g a t v ' = 2. T h e f i r s t f i v e v i b r a t i o n a l c o m p o n e n t s o f t h e f o u r t h b a n d a r e c l e a r l y d i s t i n g u i s h a b l e , b u t t h e h i g h e r e n e r g y s i d e o f t h e b a n d i s c o n c e a l e d b y t h e i n t e n s e s e c o n d b a n d o f N ^ F i g . 3 0 ) . J u d g i n g f r o m t h e i n t e n s i t y u n d e r t h e l a t t e r , we e x p e c t t h e p r o g r e s s i o n o f t h e f o u r t h b a n d t o be q u i t e e x t e n s i v e , s p r e a d i n g o v e r a b o u t 1.5 e V , a n d e s t i m a t e t h e v e r t i c a l IP a t 1 5 . 9 e V . T h e a d i a b a t i c I P c a n be a c c u r a t e l y a s s i g n e d a t 1 6 . 1 9 eV a n d t h e a v e r a g e v i b r a t i o n a l s p a c i n g o v e r t h e f i r s t f i v e c o m p o n e n t s i s 940 c m - 1 . DN = ND T h e PE s p e c t r u m o f ^^2 ^ F i g* 2 7 ) 1 s i n m a n y r e s p e c t s s i m i l a r t o t h a t o f H^H0. S u b t l e d i f f e r e n c e s i n d u c e d by d e u t e r a t i o n , h o w e v e r , e n a b l e us t o be m o r e c o n f i d e n t a b o u t o u r a s s i g n m e n t s ( s e e b e l o w ) . T h e f i r s t b a n d ( F i g . 3 1 ) h a s an a d i a b a t i c IP o f - 1 1 5 -< ( A d V d l i a d V ) A1ISN31NI - 116 -9.61 e V , a n d a v e r t i c a l IP o f 1 0 . 1 0 e V . I n t h i s c a s e , t h e v e r t i c a l t r a n s i t i o n o c c u r s a t v 1 = 4, w i t h t h e 0-0 t r a n s i t i o n h a v i n g a much l o w e r p r o b a b i l i t y . T h e f i r s t v i b r a t i o n a l s p a c i n g i s 1 0 2 0 c m " 1 , t h i s d e c r e a s i n g t o an a v e r a g e o f 9 3 0 c m " 1 o v e r t h e f i r s t 4 i n t e r v a l s , a n d t o 8 8 0 c m " 1 o v e r t h e f i r s t 9 i n t e r v a l s . A g a i n t h e r e i s t h e s u g g e s t i o n o f a n o t h e r v i b r a -t i o n a l s e r i e s , b u t i n v i e w o f t h e e x p e r i m e n t a l d i f f i c u l t i e s , a n d n a r r o w e r v i b r a t i o n a l s p a c i n g i t i s i m p o s s i b l e t o be s p e c i f i c . T h e s e c o n d b a n d ( F i g . 3 1 ) h a s a n a d i a b a t i c IP o f 14.11 eV a n d a v e r t i c a l IP o f 1 4 . 3 9 e V . T h i s o c c u r s a t v 1 = 2, t h e b a n d e n v e l o p e s p r e a d i n g o v e r 8 d i s t i n g u i s h a b l e c o m p o n e n t s . T h e v i b r a t i o n a l s p a c i n g i s f a i r l y c o n s t a n t a t 1 1 1 0 c m " 1 , a n d s o i n a l l r e s p e c t s i t i s i d e n t i c a l t o t h e s e c o n d b a n d o f N 2 H 2 . T h e t h i r d b a n d ( F i g . 3 1 ) a g a i n o v e r l a p s w i t h t h e s e c o n d b a n d , b u t we a r e c o n f i d e n t i n a s s i g n i n g t h e a d i a b a t i c IP t o 1 4 . 6 9 e V . ( T h e a g r e e m e n t w i t h t h e a n a l o g o u s v a l u e o f ^2^2 tends t o s u p p o r t t h e c o r r e c t a s s i g n m e n t o f t h e a d i a b a t i c t r a n s i t i o n ) . T h e v e r t i c a l t r a n s i t i o n p o s e s m o r e o o f a p r o b l e m , d u e t o o v e r l a p p i n g w i t h t h e 5 3 7 A s h a d o w o f t h e s e c o n d b a n d o f N 2 . H o w e v e r , t h e g e n e r a l s h a p e o f t h e FC e n v e l o p e , w h i c h e x t e n d s o v e r a t l e a s t f i v e v i b r a t i o n a l c o m p o n e n t s , s u g g e s t s t h e v e r t i c a l t r a n s i t i o n t o o c c u r a r o u n d v ' = 3 + 1, i . e . a t 1 5 . T 7 ± 0 . 1 2 e V . T h i s a s s i g n m e n t i s c o n s i s t e n t w i t h t h e c o r r e s p o n d i n g v e r t i c a l IP o f N 9 H 9 , ( w h i c h - 1 1 7 . -- 1 1 8 -o c c u r s a t v ' = 2) s i n c e t h e v i b r a t i o n a l s p a c i n g f o r N 0 D 2 , w h i c h a v e r a g e s 9 6 0 c m " 1 ( o v e r t h e f i r s t 4 i n t e r v a l s ) , i s 2 1 0 c m " 1 s m a l l e r t h a n t h a t f o r N 2 H 2 , a n d t h u s a m o r e e x t e n s i v e v i b r a t i o n a l p r o g r e s s i o n i s e x p e c t e d f o r N 2 D 2 < T h e f o u r t h b a n d i s c o m p l e t e l y c o n c e a l e d b y D 2 a n d t h e s e c o n d b a n d o f N 2 ( F i g . 2 7 b ) , a n d t h u s c a n n o t be. o b s e r v e d . 4.4.4 D i s c u s s i o n T h e a s s i g n m e n t s f o r t h e PE s p e c t r a a r e a s s i s t e d b y t h e a n a l y s i s o f t h e v i b r a t i o n a l s t r u c t u r e , t h e e f f e c t o f d e u t e r a t i o n , c o m p a r i s o n w i t h o t h e r a z o c o m p o u n d s a n d w i t h MO c a l c u l a t i o n s . . 1 q 3 - l 9 5 By c o m p a r i s o n w i t h o t h e r a z o c o m p o u n d s " , t h e f i r s t b a n d c a n be a s s i g n e d t o t h e n + ( a ^ ) o r b i t a l ( N 2 H 2 a s s u m e s C 2 ^ s y m m e t r y ) , i . e . t h e s y m m e t r i c c o m b i n a t i o n o f N l o n e p a i r s . T h i s a s s i g n m e n t i s c o r r o b o r a t e d by t h e v i b r a t i o n a l s t r u c t u r e w h i c h s t a r t s a t 1 1 8 0 c m - 1 ( f o r M 2 H 2 ) a n d w h i c h s t e a d i l y d e c r e a s e s t o c a . 980 cm" a r o u n d t h e 10 v i b r a t i o n a l q u a n t u m n u m b e r . F o r t h e n e u t r a l m o l e c u l e v 9 , 1 8 2 c • ( 6 N N H ) i s 1 5 8 3 cm" , a n d i f t h i s i s t h e mode e x c i t e d we o b s e r v e a d e c r e a s e o f a t l e a s t 4 0 0 c m " 1 . T h i s i s t o be a n t i -c i p a t e d i f t h e r e i s a l a r g e o p e n i n g o f t h e NNH a n g l e , w h i c h i s e x p e c t e d i f a n e l e c t r o n f r o m t h e n + o r b i t a l i s r e m o v e d . T h e PE s p e c t r u m o f t h e d e u t e r a t e d s p e c i e s c o n f i r m s t h i s a s s i g n m e n t s i n c e h e r e t h e v i b r a t i o n a l s t r u c t u r e on t h e f i r s t b a n d d e c r e a s e s f r o m 1 0 2 0 cm" 1 t o c a . 8 0 0 cm' 1 ( a t t h e 9 t h - 1 1 9 -v ibrat ional quantum number). This compares with the value of 1 2 1 5 cm" 1 for in the neutral molecule ti2®2' ^ e ^ i r s t components of both NgHg.and ^2^2 a r e w i t h i n t n e 1 irni ts of the zero point energy giving us confidence regarding the value of the adiabatic IP. The other v ibrat ional structure with an average spacing of ca . 1 8 0 0 c m - 1 , which i f genuine, may be assigned to the symmetric N - N s t retch N3), which is 1 5 2 9 cm" 1 in the neutral molecule. The re la t i ve ordering of the bands corresponding to the n_(b u) o r b i t a l , i . e . the antisymmetric combination of the N lone pa i r s , and the N - N ir (a u ) o rb i ta l cannot be unequivocally assigned on the basis of the v ibra t iona l structure in N ^ H 2 alone. Thus, both the second and th i rd bands have v ibrat ional structure in the range 1 1 0 0 to 1 2 0 0 cm" 1 , and in the neutral molecule a n c * v 3 a r e 1 5 8 3 and 1 5 2 9 cm" 1 respec t i ve ly . However, upon deuteration the v i b r a -t ional structure on the second band remains exactly the same ( 1 1 1 0 cm" 1 ) , whereas that on the th i rd band drops to 9 6 0 c m " 1 . Thus we can conf ident ly assign the second band to the TT ion izat ion and the th i rd band to the n_ o r b i t a l . The fourth band can be assigned to the strongly bonding N - N cr (a^) o r b i t a l . This assignment is in l i ne with the large decrease, i . e . ca . 6 0 0 cm" , in the v ? ( M - M stretch) frequency of the ion . The above assignments for the bands and v ibra t iona l structure are wholly supported by ab i n i t i o c a l c u l a t i o n s 6 2 ' 1 8 6 1 8 ^ - 1 2 0 -In a d d i t i o n , t h e s e c a l c u l a t i o n s c o n s i s t e n t l y p r e d i c t e d t h a t o n l y f o u r I P ' s a r e b e l o w 2 1 . 2 2 e V , t h u s s u g g e s t i n g t h a t t h e H e l PE s p e c t r u m o f N 2 H 2 P r e s e n t e d h e r e i s c o m p l e t e . As t h e p r o t o t y p e o f t h e a z o c o m p o u n d s , ' 2 ^ 2 ^ a s by f a r t h e l a r g e s t n + - n _ s e p a r a t i o n (A E=5.1 e V ) y e t o b s e r v e d f o r a d j a c e n t n i t r o g e n a t o m s , To d a t e t h e l a r g e s t 1 9 5 o b s e r v e d v a l u e i s t o be f o u n d i n 3,3 d i m e t h y l d i a z i r i n e ( A E = 3 . 5 5 e V ) . T h e l a r g e s p l i t t i n g i n N 2 H 9 may be a t t r i b u t e d t o t h e d u a l f a c t s t h a t f o r t h i s m o l e c u l e t h e n i t r o g e n ' l o n e ' p a i r s h a v e t h e l a r g e s t c o n t r i b u t i o n i n n + a n d n_ o r b i t a l s . T h i s , c o u p l e d w i t h t h e 1 8 0 ° d i h e d r a l a n g l e g i v e s maximum o v e r l a p a n d t h u s i n t e r a c t i o n f o r t h e N l o n e p a i r s . I t i s o f i n t e r e s t t o c o m p a r e t h e PE s p e c t r u m o f d i f 1 u o r o d i a z e n e , N 2 F 2 ' w ^ t n t h a t , o f N ? H 9 . Due t o t h e u n c e r t a i n t y i n t h e h i g h e r I P ' s , o n l y t h e n + a n d T T l e v e l s i n N 9 F 9 h a v e b e e n a s s i g n e d , a n d a r e a t 1 3 . 4 a n d 14.1 eV r e s p e c t i v e l y . T h e s e c o m p a r e w i t h t h e n + a n d TT i o n i z a t i o n s o f 1 2 ! ' 2 a t a n d 14.4 eV r e s p e c t i v e l y , i n d i c a t i n g a p o s i t i v e p e r f l u o r o s h i f t o f 3.4 eV on t h e n + i o n i z a t i o n a n d a n e g a t i v e p e r f l u o r o s h i f t o f 0.3 eV on t h e T T l e v e l . S u c h a l a r g e p o s i t i v e s h i f t f o r t h e n + l e v e l i s i n d i c a t i v e o f t h e l a r g e c o n t r i b u t i o n o f t h e F a t o m s t o t h i s o r b i t a l i n N 2 F 2 , a n d 6 2 p a r a l l e l s t h e v a l u e o f +3.6 eV o b t a i n e d f r o m t h e c a l c u l a t i o n T h e n e g a t i v e v a l u e f o r t h e TT i o n i z a t i o n i s u n u s u a l , t h e o n l y o t h e r n e g a t i v e v a l u e f o r s u c h an i o n i z a t i o n b e i n g o b s e r v e d 62 on t h e C 2 H 9 a n d C 2 F 2 s y s t e m (-0.14 e V ) . I t i s m o s t - 121 -p r o b a b l e t h a t t h i s e f f e c t a r i s e s f r o m a s t a b i l i z a t i o n o f t h e i o n i c s t a t e b y d e l o c a l i z a t i o n o v e r t h e F a t o m s . T h e o v e r a l l r e s u l t i s t h u s i n a c c o r d w i t h t h e c o n c l u s i o n s o f t h e p e r f 1 u o r o e f f e c t , i . e . t h a t the* T T - I o n i z a t i o n s h i f t i s s m a l l i n c o m p a r -i s o n t o t h e a i o n i z a t i o n s h i f t . T h e F r a n c k - C o n d o n e n v e l o p e s f o r t h e u i o n i z a t i o n i n M 2 F 2 a n d ^2^2 a r e s ^ m ^ a r » a g a i n i l l u s t r a t i n g t h e r e l a t i v e l y s m a l l e f f e c t o f t h e F a t o m s on t h e TT i o n i z a t i o n . o In c o m p a r i n g t h e v a c u u m u l t r a v i o l e t ( 1 7 0 0 A ) 181 a b s o r p t i o n b a n d s o f • N 2 H 2 a n d N 2 D 2 w 1 t n t h e i r r e s p e c t i v e f i r s t PE b a n d s , we n o t i c e a n u m b e r o f s i m i l a r i t i e s , w h i c h t e n d t o s u g g e s t t h a t t h e a b s o r p t i o n b a n d i s p r o b a b l y a R y d b e r g t r a n s i t i o n l e a d i n g t o t h e f i r s t I P o f t h e m o l e c u l e . F i r s t , b o t h t h e a b s o r p t i o n a n d PE b a n d s s how a n e x t e n s i v e p r o g r e s s i o n i n v 2 (<5NNH). S i g n i f i c a n t l y , t h e v i b r a t i o n a l f r e q u e n c i e s a r e q u i t e s i m i l a r i n t h e two c a s e s , b e i n g 1192 a n d 1180 c m - 1 (0-1 s p a c i n g ) f o r t h e a b s o r p t i o n a n d PE b a n d s o f i l 2 H 2 > a n d 955 a n d 1 020 c m " 1 f o r t h e c o r r e s p o n d i n g d e u t e r a t e d c a s e s . S e c o n d l y , t h e s i n g l e q u a n t u m e x c i t a t i o n i n N-N s t r e t c h i n g mode i n t h e a b s o r p t i o n b a n d a l s o a p p e a r s i n t h e PE b a n d a n d h a s s i m i l a r f r e q u e n c i e s , i . e . 1374 cm a n d A - I S O O cm" 1 f o r t h e a b s o r p t i o n a n d i o n i z a t i o n r e s p e c t i v e l y . T h i r d l y , t h e FC e n v e l o p e s o f t h e two b a n d s a r e s i m i l a r . 181 A l t h o u g h t h e v e r t i c a l t r a n s i t i o n s i n t h e a b s o r p t i o n b a n d s o f N ? H 2 a n d N 2 D 2 a P P e a r t 0 o c c u r o n e Q u a n t u m n u m b e r l o w e r t h a n t h a t i n t h e i r r e s p e c t i v e PE b a n d s , t h i s d i f f e r e n c e - 122 -may n o t be c e r t a i n i n v i e w o f t h e o v e r l a p o f t h e a b s o r p t i o n b a n d b y t h e s t r o n g b a n d s o f NH^, w h i c h i n e v i t a b l y c a u s e s d i f f i c u l t y i n a s s i g n i n g t h e v e r t i c a l t r a n s i t i o n . F u r t h e r m o r e , u s i n g t h e e x p e r i m e n t a l F C F , we h a v e e s t i m a t e d ( s e c t i o n 2 . 2 . 3 . c ) an i n c r e a s e o f 2 0 ° i n t h e N-N-H a n g l e , w h i c h , q u i t e s i g n i f i -c a n t l y , i s a l s o t h e a n g l e i n c r e a s e e s t i m a t e d f o r t h e e x c i t e d s t a t e 1 8 1 . T r e a t i n g t h e a b s o r p t i o n b a n d a s a R y d b e r g t r a n s i t i o n a n d u s i n g t h e e x p e r i m e n t a l l y d e t e r m i n e d IP v a l u e , t h e q u a n t u m d e f e c t i s c a l c u l a t e d t o be 0.62 f o r t h e R y d b e r g s t a t e w i t h a p r i n c i p a l q u a n t u m n u m b e r N=3. T h i s v a l u e f o r t h e q u a n t u m 196 d e f e c t i s i n t h e e x p e c t e d r e g i o n f o r M 3P R y d b e r g o r b i t a l s , a n d t h u s t e n d s t o s u p p o r t o u r a s s e r t i o n . R e c e n t l y , t h e n e a r u l t r a v i o l e t a b s o r p t i o n s p e c t r a ( 3 0 0 0 - 4 3 0 0 A, i r * ^ n + t r a n s i t i o n ) o f t h e W^W^y a n d NgDg h a v e 18 5 b e e n s t u d i e d i n g r e a t d e t a i l b y B a c k e t a l , H o w e v e r , t h e y c o u l d n o t r u l e o u t t h e p o s s i b i l i t y t h a t a s m a l l f r a c t i o n o f t h e c i s i s o m e r , w i t h an e x t i n c t i o n c o e f f i c i e n t 10 o r 20 t i m e s t h a t o f t h e t r a n s i s o m e r , i s r e s p o n s i b l e f o r t h e s p e c t r a . In t h e l i g h t o f o u r r e s u l t s , i t i s o f i n t e r e s t t o i n v e s t i g a t e t h i s p o s s i b i l i t y f u r t h e r . 3 o t h t h e a b s o r p t i o n a n d t h e f i r s t i o n i z a t i o n i n v o l v e t h e h i g h e s t o c c u p i e d n + o r b i t a l , a n d c o n s i s t o f an e x t e n s i v 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 ^ . l i e n o t e i n p a r t i c u l a r , h o w e v e r , t h a t t h e FC e n v e l o p e i s m o r e e x t e n s i v e i n t h e a b s o r p t i o n c a s e , i n t h a t f o r N 2 H 2 t* 1 G v e r t'" c a^ t r a n s i t i o n i n a b s o r p t i o n . o c c u r s a t v'=6 c o m p a r e d t o v'=3 i n i o n i z a t i o n , a n d f o r U ? D ? t h e y a r e r e s p e c t i v e l y a t v'=3 a n d 4. - 1 2 3 -M o r e o v e r , t h e i n c r e a s e i n t h e N-N-H a n g l e i s e s t i m a t e d t o be 185 2 5 ° i n t h e e x c i t e d s t a t e w h e r e a s i t i s o n l y 2 0 ° i n t h e g r o u n d i o n i c s t a t e . T h e s e t r e n d s i n v i b r a t i o n a l p r o g r e s s i o n a n d a n g l e i n c r e a s e a p p e a r t o be i n c o n f l i c t w i t h o u r e x p e c t a -t i o n , s i n c e t h e o p p o s i t e i s t o b e a n t i c i p a t e d i n v i e w o f t h e l a r g e r p e r t u r b a t i o n c a u s e d b y t h e i o n i z a t i o n p r o c e s s , a n d t h e ir* o r b i t a l ( i n v o l v e d i n t h e a b s o r p t i o n ) h a s h a r d l y a n y i n f l u e n c e on t h e N-N-H a n g l e . T h e r e f o r e , on t h i s b a s i s we w o u l d n o t r u l e o u t t h e p o s s i b i l i t y o f t h e c i s i s o m e r p l a y i n g a r o l e i n t h e a b s o r p t i o n s p e c t r u m . F r o m t h e PE s p e c t r a , i t s e e m s p e r t i n e n t t o make a c o m m e n t r e g a r d i n g t h e p o s s i b i l i t y o f t h e g r o u n d s t a t e o f 181 d i a z e n e b e i n g a t r i p l e t a s s u g g e s t e d by T r o m b e t t i b a s e d o u p o n t h e a n a l y s i s o f t h e r o t a t i o n a l s t r u c t u r e i n t h e 1 7 0 0 A a b s o r p t i o n b a n d . T h e s i m p l i c i t y o f t h e PE s p e c t r u m o f d i a z e n e a n d t h e f a c t t h a t t h e s p e c t r u m c a n be c o n v i n c i n g l y a s s i g n e d on t h e b a s i s o f a s i n g l e t g r o u n d s t a t e s t r o n g l y a r g u e s a g a i n s t a t r i p l e t g r o u n d s t a t e . 4 • 5 T r a n s - m e t h y l d i a z e n e (CH^N = NH) T h e m o n o m e t h y l s u b s t i t u t e d d e r i v a t i v e o f f ^ ^ ' 19 CH^NNH, h a s a l s o a t t r a c t e d a g r e a t d e a l o f a t t e n t i o n r e c e n t l y D e s p i t e i t s r e l a t i v e s t a b i l i t y c o m p a r e d t o N 2 H 2 , t h e s t u d y a n d u n d e r s t a n d i n g o f CHgNNH i s f a r l e s s c o m p l e t e t h a n t h a t o f t h e f o r m e r . I n t h e f o l l o w i n g , p r e l i m i n a r y r e s u l t s on t h e PE s p e c t r u m o f t h i s s p e c i e s a r e p r e s e n t e d . - 1 2 4 -CI! 3rjMH was p r e p a r e d i n a weak m i c r o w a v e d i s c h a r g e i n t h e e q u i l i b r i u m v a p o r o f m e t h y l h y d r a z i n e a t r o o m t e m p -e r a t u r e . U n d i s c h a r g e d m e t h y l h y d r a z i n e a n d i n v o l a t i l e p r o d u c t s w e r e t r a p p e d a t - 7 8 ° C l e a v i n g o n l y C H 3 N N H / M H 3 / N 9 / C M 4 i n t h e g a s p h a s e . E l i m i n a t i o n o f ! i H 3 was a c h i e v e d b y t r a p p i n g t h e s e g a s p h a s e p r o d u c t s a t - 1 9 6 ° C , a n d h o l d i n g t h e p r o d u c t s a t - 1 2 G ° C ( m e t h y l c y c l o h e * a n e s l u s h b a t h ) . P u m p i n g t h e p r o d u c t s t h r o u g h t h e i o n i z a t i o n c h a m b e r , we o b s e r v e d t h e s p e c t r a o f N H 3 a n d C H ^ , a n d t r a c e s o f C ^ l l g , u n t i l f i n a l l y t h e s p e c t r u m o f C H 3 ' ! N H was o b s e r v e d , f r e e o f a n y o t h e r s p e c i e s . T h e e f f i c i e n c y o f t h e d i s c h a r g e i n s e l e c t i v e l y 20 p r o d u c i n g C H^Mi ' H was m o n i t o r e d b y o b s e r v i n g t h e y e l l o w c o l o r o f 'CMgNMH i n t h e - 1 9 6 ° C t r a p . I n t h i s r e s p e c t , we a l s o h a v e c o n f i r m a t i o n o f t h e p r o d u c t i o n o f t h e d e s i r e d s p e c i e s . T h e r e s e m b l a n c e o f t h e PE s p e c t r u m o f C M ^ N i i l ! ( F i g . 3 2 a ) w i t h t h a t o f C H 3 N N G H 3 ( F i g . 3 2 b ) a n d N ? H 9 ( F i g . 2 9 , n e g l e c t i n g t h e v i b r a t i o n a l f i n e s t r u c t u r e o f t h e i n d i v i d u a l b a n d s ) p r o v i d e s f u r t h e r e v i d e n c e f o r t h e c o r r e c t s p e c t r u m . T h e i d e n t i f i c a t i o n o f t h e s p e c t r u m i s , m o r e o v e r , s u b s t a n t i a t e d b y t h e o b s e r v a t i o n o f r i 9 b a n d s i n t h e s u b s e q u e n t s p e c t r u m o f C H 3 ' K I M ( F i g . 3 2 a ) , w h i c h c a n o n l y r e s u l t f r o m t h e d e c o m p o s i t i o n o f t h i s s p e c i e s . An a d d i t i o n a l p r o o f o f i t s p o s i t i v e i d e n t i f i c a t i o n b y p r e p a r -2 00 i n g t h e s p e c i e s f r o m an a l t e r n a t i v e r o u t e ^ i s p r e s e n t l y i n p r o g r e s s . ' T h e PE s p e c t r u m o f C H ^ i N H ( F i g , 3 2 a ) c o n s i s t s o f f i v e d i s t i n c t , f e a t u r e l e s s b a n d s . T h e a b s e n c e o f v i b r a t i o n a l s t r u c t u r e on a n y o f t h e m i s u n d e r s t a n d a b l e i n v i e w o f t h e l o w s y m m e t r y ( C c ) o f t h e m o l e c u l e . T h e i n t e n s i t y a n d t h e - 125 -- 1 2 6 -s h a p e o f t h e s e c o n d b a n d s t r o n g l y s u g g e s t t h a t i t c o n s i s t s o f t w o o v e r l a p p i n g I P ' s , w h i c h i s f u r t h e r s u p p o r t e d by c o m p a r i s o n w i t h t h e s p e c t r a o f N 2 H 2 a n d C H 3 N N C H 3 ( s e e b e l o w ) . T h e e x p e r i -m e n t a l r e s u l t s a r e g a t h e r e d i n T a b l e 7. T h e a s s i g n m e n t o f t h e s p e c t r u m o f CHgNNH i s f a i r l y s t r a i g h t f o r w a r d by c o m p a r i s o n w i t h t h e c l o s e l y r e l a t e d m o l e c u l e s . \ ! 2 H 9 a n d C H ' 3 N N C H 3 . C o n s i d e r a t i o n o f t h e e f f e c t o f i n t r o d u c i n g a C H 3 g r o u p i n t o N 2 H 2 l e a d s u s t o e x p e c t t h a t t h e i n d i v i d u a l b a n d s o f CH 3NNH w o u l d be s h i f t e d t o l o w e r I P v a l u e c o m p a r e d t o t h e c o r r e s p o n d i n g b a n d s o f f^2^2 " ^^1S t r e n d i s e x p e c t e d t o c o n t i n u e on g o i n g f r o m CHgNNH t o CH-jiNNCM.^. T h i s i s , i n d e e d , w h a t i s o b s e r v e d ( s e e F i g . 32 a n d F i g . 2 9 ) , i f t h e 2 n d b a n d o f CH. 3NNH i s d u e t o two I P ' s . T h e r e f o r e , t h e same r e l a t i v e o r d e r i n g n + , 7 r , ^ _ ; . , n_ a n d cr:^  _ ^ c a n b e a s s i g n e d t o CHjNNH a s i n t h e c a s e o f NgH, ( s e c t i o n 4 . 4 . 4 ) a n d C h ^ N N C H g 1 9 5 ' T h e two h i g h e s t I P ' s a r e t e n t a t i v e l y a s s i g n e d on t h e b a s i s o f CNDO/2 a n d INDO c a l c u l a t i o n s ( T a b l e 7 ) . - 1 2 7 -T A B L E 7 PES a n d T h e o r e 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 ( i n e V ) o f C HjNNH I o n i c s t a t e PES T h e o r e t i c a l V e r t i c a l IP ( a d i a b a t i c ) O r b i t a l C N D 0 / 2 * INDO* V 9.57 ( 3 . 8 ) 7 a ' ( n + ) 9.3 7.5 2 A " 1 2 . 9 ( 1 2 . 0 5 ) 2 a " ( n N . f | ) 1 1 . 4 1 1 . 3 2A- 1 3.4 6 a ' ( n _ ) 1 6 . 3 1 4 . 8 2 A ' 1 4 . 7 4 ( 1 4 . 4 ) 5a ' (or.j ) 1 2 . 9 1 2 . 5 2 A " 1 5 .58 ( 1 5 . 2 ) l a " ( n c . H ) 18.2 1 8 . 0 2 A ' 1 6 . 7 2 ( 1 6 . 3 ) 4 a ' 2 0 . 3 1 9 . 3 w i t h 4 eV a d j u s t m e n t . - 128 -C H A P T E R F I V E 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 OF SOME R E A C T I V E M O L E C U L E S 5.1 M e t h i o p h o s p h i d e ( H C P ) 5.1.1 I n t r o d u c t i pn T h e p h o s p h o r o u s a n a l o g o f h y d r o g e n c y a n i d e ( H C N ) , m e t h i o p h o s p h i d e ( H C P ) , i s a n u n u s u a l a n d i n t e r e s t i n g m o l e c u l e i n t h a t i t i s t h e o n l y i s o l a t e d c o m p o u n d w i t h a p h o s p h o r o u s a t o m b o n d e d t o o n l y o n e n e i g h b o r i n g a t o m , p r e s u m a b l y t h r o u g h a c a r b o n - t o - p h o s p h o r o u s t r i p l e b o n d . B e c a u s e o f t h i s u n u s u a l f e a t u r e a n d i t s s i m i l a r i t y t o HCN, a 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 y o f HCP was c a r r i e d o u t . I n a d d i t i o n , t h e r e s u l t s t h r e w 2 0 5 l i g h t o n t h e a s y e t u n r e s o l v e d f i r s t PE b a n d o f HCN 5.1.2 E x p e r i m e n t a l O A £ HCP was f i r s t p r e p a r e d i n 1961 by G i e r a n d o u r m e t h o d o f s y n t h e s i s f o l l o w e d h i s , i n w h i c h p h o s p h i n e was p a s s e d t h r o u g h a l o w i n t e n s i t y ( 7 0 - 8 0 a m p e r e s , 20 v o l t s ) Carbon Arc Reactor WATER OUT -PLEXIGLAS r7////////A WATER IN VACUUM PORT ' 0 ' R I N G S - # 4 3 0 WATER IN TEFLON WATER IN 4 GAS IN ELECTRODE CONNECTION } WATER OUT PLEXIGLAS -MATERIALS-STAINLESS STEEL (304) OTHERS UNSPECIFIED-BRASS, COPPER WATER OUT FIGURE 33 - 1 3 0 -r o t a t i n g a r c s t r u c k b e t w e e n a p a i r o f s p e c i a l l y c o n s t r u c t e d c a r b o n e l e c t r o d e s ( F i g . 3 3 ) . A b o u t f o u r p a r t s o f a c e t y l e n e 2 n S t o o n e o f HCP w w e r e f o r m e d i n t h e r e a c t i o n , b u t b e c a u s e o f 9 0 fi t h e v e r y r e a c t i v e a n d u n s t a b l e ( s t a b l e f o r a f e w h o u r s 2 0 7 a t r o o m t e m p e r a t u r e ) c h a r a c t e r o f HCP, no a t t e m p t was made t o s e p a r a t e t h e two c o m p o u n d s . T h e PE s p e c t r u m o f HCP was r e c o r d e d i m m e d i a t e l y a f t e r i t s s y n t h e s i s a n d c a l i b r a t e d a g a i n s t t h e known i o n i z a t i o n p o t e n t i a l s ( I P ' s ) o f x e n o n a n d a c e t y l e n e . T h e s a m p l e was k e p t a t - 1 6 0 ° C ( i s o p e n t e n e s l u s h b a t h ) i n t h e K e l - F c o n t a i n e r ( s e c t i o n 3 . 3 . 4 . b ) a n d t h e p r e s s u r e was f u r t h e r c o n t r o l l e d b y t h e K e l - F v a l v e . 5.1.3 R e s u l t s A n d D i s c u s s i o n D e s p i t e t h e p r e s e n c e o f a c e t y l e n e a n d t r a c e s o f p h o s p h i n e a n d e t h y l e n e i t was p o s s i b l e t o d e t e r m i n e t h e f i r s t two o f t h e t h r e e e x p e c t e d I P ' s o f HCP u n d e r h i g h r e s o l u t i o n ( F i g . 3 4 ) . T h e t h i r d IP e v a d e d d e t e c t i o n , p r e s u m a b l y b e c a u s e o f i t s w e ak c r o s s s e c t i o n a s i s t h e c a s e f o r t h e c o r r e s p o n d i n g I P o f HCN, a n d we s u s p e c t i t may v e r y w e l l be b u r i e d u n d e r n e a t h t h e 2 n d o r 3 r d b a n d s f r o m a c e t y l e n e . T h e e x p e r i m e n t a l r e s u l t s s u m m a r i z e d i n T a b l e 8 c o m p a r e f a v o r a b l y w i t h a r e c e n t e x t e n s i v e ab i n i t i o HO c a l c u -l a t i o n on H C P 2 0 0 . T h i s a g r e e m e n t t o g e t h e r w i t h t h e e x p e r i -m e n t a l o b s e r v a t i o n o f t h e v i b r a t i o n a l f i n e s t r u c t u r e i n e a c h -131 -1110 cm-i 1250cm"-1 i — i — i — i r—i i 1 ~~i r- 1 r 1 1 r 11 13 15 17 19 IONIZATION POTENTIAL (eV) FIGURE 3 4 - 132 -T A B L E 8 PES a n d T h e o r e t i c a l D a t a o f HCP O r b i t a l a s s i g n m e n t * I o n i z a t i o n p o t e n t i a l s ( e V ) v i b r a t i o n a l a n d c h a r a c t e r i s t i c s m c a l c u l a t i o n * P E S frequences 1 TT ( C - P b o n d i n g o n l y ) 3 a ( l o n e p a i r e l e c t r o n s on P a t o m ) 2o ( m a i n l y C-H b o n d i n g ) * f r o m R e f . 2 0 3 . 1 0 . 0 1 0 . 7 9 3 + 0 . 0 0 5 v 3 = 1 1 1 0 ± 3 0 1 2 . 5 1 2 . 3 5 5 + 0 . 0 0 5 v 3 = 1 2 5 0 + 3 0 2 0 . 0 - 1 3 3 -b a n d s h o w s q u i t e c o n c l u s i v e l y t h a t t h e f i r s t a n d s e c o n d I P ' s a r e f r o m t h e TT a n d a MO' s r e s p e c t i v e l y , a n d t h e o r b i t a l c o n f i g u r a t i o n o f HCP f o r t h e v a l e n c e s h e l l i s ( l a ) " ( 2 o ) 2 4 ( 3 a ) ( I n ) . T h e f i r s t b a n d ( F i g . 3 4 ) w i t h a v i b r a t i o n a l s p a c i n g o f 1 1 1 0 c m " 1 ( a d e c r e a s e f r o m 1 2 7 6 c m " 1 i n t h e g r o u n d m o l e c u l a r s t a t e ) c o m e s f r o m t h e C-P b o n d i n g u - o r b i t a l a s 2 0 8 p r e d i c t e d by t h e c a l c u l a t i o n . T h e s e c o n d b a n d ( F i g . 2 ) i s t y p i c a l o f a n o n - b o n d i n g o r b i t a l , b e c a u s e o f i t s s h a r p n e s s a n d a v i b r a t i o n a l f r e q u e n c y l i t t l e c h a n g e d f r o m t h e n e u t r a l m o l e c u l e . 5.2 C S e 2 , OCSe A n d S C S e 5.2.1 I n t r o d u c t i on T h e r e a r e s i x t r i a t o m i c l i n e a r m o l e c u l e s c o n t a i n i n g c a r b o n a n d o x y g e n , s u l f u r o r s e l e n i u m , 0 C 0 , OCS, S C S , O C S e , S C S e a n d S e C S e . T h e f i r s t t h r e e h a v e b e e n s t u d i e d e x t e n -s i v e l y b o t h b y o p t i c a l a n d p h o t o e l e c t r o n 3 ' 1 7 2 s p e c t r o s c o p y ; h o w e v e r , t h e s p e c t r o s c o p i c d a t a f o r t h e o t h e r t h r e e i s v e r y l i m i t e d . T h e t o x i c i t y , u n p l e a s a n t o d o r s ( e v e n i n v e r y l o w c o n c e n t r a t i o n s ) , o t h e r a d v e r s e p h y s i o l o g i c a l e f f e c t s , i n s t a b i l i t y a n d t h e d i f f i c u l t y o f o b t a i n i n g t h e m i n p u r e f o r m make t h e s e m o l e c u l e s u n a p p e a l i n g t o s p e c t r o s c o p i s t s . In t h i s s e c t i o n , t h e PE s p e c t r a o f t h e s e s e l e n i u m c o m p o u n d s a r e p r e s e n t e d a n d d i s c u s s e d , a n d t h e s p e c t r a o f C 0 2 , OCS - 134 -a n d C S 9 h a v e b e e n i n c l u d e d f o r t h e s a k e o f c o m p a r i s o n a n d c o m p l e t e n e s s . PES h a s i n r e c e n t y e a r s s t i m u l a t e d a g r o w i n g i n -t e r e s t i n t h e q u a n t i t a t i v e f o r m u l a t i o n a n d a p p l i c a t i o n o f 21 n F r a n c k - C o n d o n p r i n c i p l e t o p o l y a t o m i c m o l e c u l e s . T h e m a j o r m o t i v e b e h i n d t h e s e s t u d i e s h a s b e e n t o d e t e r m i n e t h e g e o m e t r y o f i o n i z e d m o l e c u l e s f r o m e x p e r i m e n t a l l y d e t e r -m i n e d r e l a t i v e v i b r a t i o n a l t r a n s i t i o n p r o b a b i l i t i e s . T h e g e n e r a l a p p r o a c h i n a l l t h e s e s t u d i e s h a s b e e n t o a s s u m e a c o n s t a n t e l e c t r o n i c t r a n s i t i o n m o ment, t h u s r e d u c i n g t h e p r o b l e m t o an e v a l u a t i o n o f v i b r a t i o n a l o v e r l a p i n t e g r a l s , i . e . t h e F r a n c k - C o n d o n o v e r l a p s , a n d t o u s e t h e h a r m o n i c o s c i l l a t o r a p p r o x i m a t i o n t o s i m p l i f y t h e i n t e g r a t i o n . A l t h o u g h t h e t h e o r e t i c a l F r a n c k - C o n d o n f a c t o r ( F C F ) may n o t r e p r o d u c e t h e e x p e r i m e n t a l o n e q u a n t i t a t i v e l y , q u a l i t a t i v e 2 1 0 a g r e e m e n t i s a l w a y s o b t a i n e d . F r o m p a s t e x p e r i e n c e , i t i s r e a s o n a b l e t o e x p e c t t h a t t h e b e s t g e o m e t r y w i l l g i v e a s e t o f F C F ' s i n b e s t a g r e e m e n t w i t h e x p e r i m e n t . We h a v e c h e c k e d t h i s e x p e c t a t i o n by d e t e r m i n i n g t h e g e o m e t r y o f t h e i o n i c s t a t e s o f t h e m o l e c u l e s s t u d i e d h e r e e m p l o y i n g t h e p r o c e d u r e o u t l i n e d i n s e c t i o n 2 . 2 . 3 . C , a n d c o m p a r i n g t h e m , w h e r e p o s s i b l e , w i t h e x p e r i m e n t a l d a t a , i n t h e h o p e t h a t a s y s t e -m a t i c a n d r e l i a b l e way o f d e t e r m i n i n g i o n i c g e o m e t r y c a n be e s t a b l i s h e d . An a t t r a c t i v e f e a t u r e o f t h e s e s t r u c t u r a l l y a n d c h e m i c a l l y s i m i l a r c o m p o u n d s i s t h a t s e v e r a l " a n a l o g o u s " s e r i e s a r e a v a i l a b l e f o r s t u d y . - 1 3 5 -5.2.2 E x p e r i m e n t a l C 0 9 , OCS a n d C S 2 w e r e o b t a i n e d c o m m e r c i a l l y ( M a t h e s o n ) a n d w e r e u s e d w i t h o u t f u r t h e r p u r i f i c a t i o n . C S e 2 was p r e p a r e d by p a s s i n g m e t h y l e n e c h l o r i d e v a p o r o v e r m o l t e n s e l e n i u m a t 211 a b o u t 5 0 0 ° C . S e l e n i u m was r e m o v e d b y s t e a m d i s t i l l a t i o n a n d t h e m a j o r i t y o f m e t h y l e n e c h l o r i d e by s i m p l e d i s t i l l a t i o n . T h e r e s i d u e f r o m s i m p l e d i s t i l l a t i o n was s u b j e c t e d t o r e p e a t e d p r o c e s s e s o f f r e e z i n g ( - 7 8 ° , d r y i c e - a c e t o n e b a t h ) , t h a w i n g a n d p u m p i n g a t - 7 8 ° C . T h e s p e c t r u m o f C S e 2 was f i n a l l y r e c o r d e d when t h e PE b a n d s o f m e t h y l e n e c h l o r i d e w e r e no l o n g e r i n e v i d e n c e . OCSe was o b t a i n e d i n l o w y i e l d b y p a s s i n g 212 c a r b o n m o n o x i d e g a s o v e r m o l t e n s e l e n i u m a t a b o u t 5 0 0 ° C T h e r e a c t i o n v e s s e l s w e r e k e p t s c r u p u l o u s l y d r y . T r a c e s o f c a r b o n m o n o x i d e w e r e r e m o v e d by p u m p i n g . S C S e was s y n t h e s i z e d b y m i x i n g C S ? v a p o r w i t h s e l e n i u m v a p o r a t a b o u t 5 0 0 ° C a n d 2 1 3 h e a t i n g t h e m i x t u r e a t a b o u t 7 5 0 ° C . S e l e n i u m was r e m o v e d by s i m p l e d i s t i l l a t i o n a n d t h e b u l k o f C S ^ by f r a c t i o n a l d i s t i l l a t i o n . T h e S C S e e n r i c h e d s o l u t i o n o f C S 2 was t h e n s u b j e c t t o t r a p ( - 7 8 ° C ) t o t r a p ( - 1 9 5 ° C ) d i s t i l l a t i o n . T h e l a s t t r a c e o f C S 2 was pumped away a t - 7 8 ° C . T h e IR s p e c t r a o f t h e p r e p a r e d s e l e n i u m c o m p o u n d s 214 a g r e e d v e r y w e l l w i t h p r e v i o u s l y r e p o r t e d w o r k a n d s h o w e d l i t t l e i m p u r i t y . A l l t h e s e l e n i u m c o m p o u n d s w e r e f o u n d t o d e c o m p o s e r e a d i l y i n c o n t a c t w i t h m e t a l , O C S e b e i n g t h e m o s t s e r i o u s . T h e d e p o s i t i o n o f s e l e n i u m i n s i d e t h e i o n i z a t i o n - 1 3 6 -c h a m b e r c a u s e d s e v e r e d e t e r i o r a t i o n i n t h e i n s t r u m e n t a l r e s o l u t i o n . T o a l l e v i a t e t h i s p r o b l e m , two p r o c e d u r e s w e r e t a k e n ; ( 1 ) T h e i o n i z a t i o n c h a m b e r was d i f f e r e n t i a l l y p u m p e d t o s h o r t e n t h e t i m e o f c o n t a c t b e t w e e n t h e s a m p l e a n d t h e m e t a l l i c s u r f a c e s , ( 2 ) A g l a s s s a m p l e i n l e t s y s t e m ( s e c t i o n 3 . 3 . 4 . b ) was a d o p t e d ( f o r O C S e ) , b e c a u s e t h e CO f o r m e d b y d e c o m p o s i t i o n i n t e r f e r e d s t r o n g l y w i t h t h e PE s p e c t r u m o f O C S e . T h e p r e s s u r e o f t h e s a m p l e was c o n t r o l l e d b y a n a p p r o p r i a t e s l u s h b a t h ( i s o p e n t a n e f o r O C S e ) a n d a t e f l o n s t o p c o c k . W i t h t h e s e p r e c a u t i o n s t a k e n , i t was s t i l l n e c e s s a r y t o c l e a n t h e v a c u u m c h a m b e r f r e q u e n t l y , e s p e c i a l l y t h e i o n i z a t i o n c h a m b e r . P r o c e d u r e ( 2 ) was n o t a d o p t e d f o r C S e 2 a n d S C S e , w h e r e t h e d e c o m p o s i t i o n p r o d u c t s d i d n o t i n t e r f e r e w i t h t h e s p e c t r a o f t h e p a r e n t c o m p o u n d s . T h e s p e c t r a w e r e r e c o r d e d w i t h a r e s o l u t i o n b e t w e e n 2 0 - 3 0 meV ( m e a s u r e d b y FWHM o f t h e a r g o n d o u b l e t ) a n d c a l i b r a t e d a g a i n s t a p p r o p r i a t e n o b l e g a s e s . T h e i o n i z a t i o n p o t e n t i a l s ( I P ' s ) w e r e a c c u r a t e t o 0.01 eV a n d v i b r a t i o n a l f r e q u e n c i e s t o 30 c m " 1 , u n l e s s s p e c i f i e d o t h e r w i s e . 5 . 2 .3 R e s u l t s T h e PE s p e c t r u m o f e a c h c o m p o u n d i s s h o w n i n F i g . 3 5 . T h e e x p e r i m e n t a l r e s u l t s a r e t a b u l a t e d i n T a b l e 9, w h e r e t h e v i b r a t i o n a l f r e q u e n c y f o r t h e g r o u n d s t a t e o f e a c h n e u t r a l m o l e c u l e i s a l s o i n c l u d e d f o r c o m p a r i s o n . 137 -Z D tr < cr t m cr < u Ld <5 CO D O o e) OCSe d)OCS ~i r~ Kr Kr - i 1 r -i 1 T 1 1-c) CSe ; 1—! r-b) CS 2 _ A I l i jy ik -i r™ -i r~ a) 0 O 2 J L Ar i t r iW k t J 9 TO H 12 13 14 15 16 T7 18 19~20 I O N I Z A T I O N P O T E N T I A L ( e V ) FIGURE 35 T A B L E 9 PES D a t a f o r C 0 2 , C S 2 , C S e 2 > OCS, OCSe a n d S C S e M o l e c u 1e CO. CO, CS-CS, CSe, )Se, E l e c t r o n i c s t a t e A d i a b a t i c I P ' s a ; ( e V ) V i b r a t i o n a l f r e q u e n c i e s ( c m " 1 )  v 2 v 3 s p i n - o r b i t s p l i t t i n g d ) ( e V ) x V g 1 3 8 8b ^ 6 6 7 b ) 2 34 9 b ) *\ 1 3 . 8 0 1 2 7 0 -- 1 4 4 0 0 . 0 2 0 ° ) u 17 . 3 4 ( 1 7 .60) 1 1 0 0 .' --B V u 1 8 . 0 8 1 3 1 0 --C 2 E + g x V g 19 .39 1 34 0 6 5 8 b ) 3 9 7 b ) 1 4 7 0 1 5 3 3 b ) X 2 n ' g 1 0 .06 -- 0 .055 A 2 n u 1 2 . 6 7 ( 1 2 .81 ) 5 7 0 B V u 1 4 . 4 7 6 4 0 c V g 1 6 . 1 8 6 6 0 81 0 CI s t a t e 16 . 7 0 ( 1 7 .08) -- --x V g 3 6 3 ° ) 3 0 8 ° ) 1 3 0 3 C ) 2 X ^ n g 9 u 9 .27 0 . 265 11 . 4 9 ( 1 1 .69) 3 2 0 1 3 . 6 3 31 0 -- --1 5 . 9 0 -- --CO CO TABLE 9 ( c o n t ' d ) PES Data f o r C 0 o , C S 9 , C S e 0 . OCS, OCSe and SCSe M o l e c u l e E l e c t r o n i c s t a t e Ad i a b a t i c IP's (eV) V i b r a t i o n a l (cm' f r e q u e n c i e s 1 1 V o s p i n - o r b i t s p l i t t i n g (eV) OCS 0CS + OCSe 0 C S e + SCSe SCSe + 2 0 6 2 b ) 5 2 0 b ) 3 5 9 b > x^ n n . i s 2030 -- 700 0 . 04 6 2 A^n 1 5 . 0 9 1990 -- 790 B V 1 6 . 0 5 -- •-c V 17 . 9 6 21 50 -- 9 60 -x V 2 0 2 1 C ^  4 6 6 c ) 6 4 2 C * - -o V ' - r A U 1 0 . 3 6 ( 1 0 . 4 3 ) 2100 ±50 -- 500 0 . 207 A 2 n 1 4 . 5 8 ( 1 4 . 7 3 ) 2 0 0 0 i l 0 0 590 - -1 5 . 7 5 1 930±1 00 - -1 7 . 9 0 2100 720±50 _ _ x V 14 3 5 c ) 355°^ 5 0 6 C ^ - -x 2n 9 . 5 8 1 220±50 490±50 0 . 185 A 2II 1 2 . 1 3 ( 1 2 . 3 5 ) — -- 440 - -B V 1 4 . 0 7 460 - -c V 1 6 . 0 6 930±50 430±50 - 1 4 0 -T A 3 L E 9 ( c o n t ' d ) PES D a t a f o r C 0 2 , C S 2 , C S e 2 , OCS, OCSe a n d S C S e a^ v a l u e s i n p a r e n t h e s e s a r e v e r t i c a l I P ' s b ) f r o m r e f . 209 c^ f r o m r e f . 2 1 4 d^ e x p e r i m e n t a l e r r o r ± 0 . 0 0 4 eV - 141 -C O o , C S Q and PCS O u r r e s u l t s a n d i n t e r p r e t a t i o n o f t h e PE s p e c t r a f o r t h e s e c o m p o u n d s a r e i n c o m p l e t e a g r e e m e n t w i t h t h e p r e v i o u s s t u d i e s ( s e e d i s c u s s i o n b e l o w ) , a c c o r d i n g l y no f u r t h e r d i s c u s s i o n w i l l b e g i v e n h e r e . C_S_e_2 T h e s p e c t r u m o f C S e 2 ( F i g . 3 5 c ) e x h i b i t s f o u r d i s t i n g u i s h a b l e b a n d s , a n d v i b r a t i o n a l s t r u c t u r e i s r e s o l v e d on two o f t h e m . T h e a d i a b a t i c IP o f t h e f i r s t b a n d i s 9.27 e V . T h i s b a n d c o n s i s t s o f two e q u a l l y i n t e n s e p e a k s w h i c h a r e t h e 2 2 d o u b l e t c o m p o n e n t s n , n ^ , w i t h a s p i n - o r b i t s p l i t t i n g o f 0 . 2 6 5 e V . T h e s e c o n d b a n d ( F i g . 3 6 a ) s h o w s an e x t e n s i v e , s i m p l 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-j . I t s p r e a d s o v e r a p p r o x i m a t e l y 0.5 eV a n d c o n s i s t s o f a t l e a s t 12 c o m p o n e n t s w i t h an a v e r a g e s p a c i n g 320 c m " 1 . T h e a d i a b a t i c a n d v e r t i c a l I P ' s a r e 1 1 . 4 9 a n d 1 1 . 6 9 eV r e s p e c t i v e l y . T h e t h i r d b a n d ( F i g . 3 6 b ) h a s an i d e n t i c a l a d i a b a t i c a n d v e r t i c a l IP a t 13.63 e V . I t s h o w s two l e v e l s o f v i b r a t i o n a l e x c i t a t i o n i n v.| w i t h a s p a c i n o o f 310 c m - 1 . T h e f o u r t h b a n d i s a s i n g l e p e a k w i t h an IP a t 1 5 . 9 0 e V . O CSe F o u r bands a r e d i s t i n g u i s h a b l e i n t h e s p e c t r u m ( F i g . 3 5 e ) , a n d v i b r a t i o n a l f i n e s t r u c t u r e i s s e e n on a l l f o u r . T h e s e c o n d a n d t h i r d bands o v e r l a p ( a s i n OCS) a n d IONIZATION POTENTIAL (eV) F I G U R E 36 - 143 -t h e o v e r a l l f o r m o f t h e s p e c t r u m i s v e r y s i m i l a r t o t h a t o f OCS, w i t h t h e b a n d s o f t h e l a t t e r b e i n g s l i g h t l y h i g h e r i n e n e r g y . C o n s i d e r a b l e o v e r l a p o c c u r s i n t h e f i r s t b a n d ( F i g . 3 7 a ) . T h e b a n d , h o w e v e r , c a n b e s a t i s f a c t o r i l y a n a l y z e d i f t h e f i r s t a n d f o u r t h p e a k s a r e t r e a t e d r e s p e c t i v e l y a s 2 2 t h e o r i g i n s o f t h e d o u b l e t c o m p o n e n t s n 3 , Hi, . T h i s a s s i g n m e n t i s s t r o n g l y s u p p o r t e d b y t h e e x p e r i m e n t a l f a c t t h a t t h e a p p e a r a n c e o f t h e f i r s t t h r e e p e a k s i s r e f l e c t e d i n t h e n e x t t h r e e , e x c e p t f o r a d i f f e r e n c e i n i n t e n s i t y ( s e e 2 2 b e l o w ) . T h i s g i v e s a n 3 - n i s p l i t t i n g o f 0 .207 eV a n d t h e a d i a b a t i c a n d v e r t i c a l I P ' s a t 1 0 . 3 6 a n d 1 0 . 4 3 eV r e s p e c t i v e l y . A 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 3 w i t h a n a v e r a g e s p a c i n g o f 500 c m " 1 i s c l e a r l y d i s c e r n i b l e i n e a c h o f t h e d o u b l e t c o m p o n e n t s . J u d g i n g f r o m t h e i n t e n s i t y d i s -t r i b u t i o n o f t h e p r o g r e s s i o n , a t l e a s t f o u r l e v e l s o f e x c i t a -t i o n a r e e x p e c t e d f o r v ^ . T h i s m e a n s t h e ( 0 0 3 ) a n d ( 0 0 4 ) 2 2 c o m p o n e n t s o f t h e n 3 / s t a t e a r e b u r i e d u n d e r n e a t h t h e n X / b a n d . A s i n g l e l e v e l e x c i t a t i o n i n v-| i s a l s o o b s e r v e d w i t h an a v e r a g e s p a c i n g o f 2 1 0 0 c m " 1 . T h e p r o g r e s s i o n 2 ( l O n - j ) o f t h e n \^  s t a t e c a n b e d i s t i n g u i s h e d , w i t h n 3 = 0 , 1 , 2 , r 2 a n d 3, w h i l e t h e c o r r e s p o n d i n g p r o g r e s s i o n o f t h e n 3 s t a t e i s c o m p l e t e l y o v e r l a p p e d by t h e ( 0 0 n 3 ) p r o g r e s s i o n o f t h e f o r m e r s t a t e . T h i s e x p l a i n s t h e g r e a t e r i n t e n s i t y o f t h e 2 n i b a n d . — I 1 1 1— 10.4 U . O IONIZATION POTENTIAL (eV) F I G U R E 37 i on - 1 4 5 -T h e s e c o n d b a n d ( F i g . 3 7 b ) , w i t h a d i a b a t i c I P a t 1 4 . 5 8 eV c o n s i s t s o f a t l e a s t 15 p e a k s s p r e a d i n g o v e r a b o u t 1 e V . T h e r e g u l a r s p a c i n g o f 590 c m - 1 s u g g e s t s a p r o g r e s s i o n i n v 3 , y e t t h e c o m p l e x i t y a n d t h e i r r e g u l a r i n t e n s i t y d i s t r i b u t i o n o f t h e b a n d t e n d t o s u g g e s t m o r e t h a n o n e v i b r a t i o n a l p r o g r e s s i o n . As i n OCS, t h e a d d i t i o n a l e x c i t a t i o f v.j i s m o s t l i k e l y . J u d g i n g f r o m t h e i n t e n s i t y v a r i a t i o n p r o f i l e o f t h e b a n d , a s p a c i n g o f c a . 2 0 0 0 c m " 1 i s a s s i g n e d t o v-| . T h e t h i r d p e a k a t 1 4 . 7 3 eV i s t h e n t a k e n a s t h e v e r t i c a l t r a n s i t i o n . T h i s u n c e r t a i n t y a r i s e s b e c a u s e o f t h e o v e r l a p w i t h t h e much m o r e i n t e n s e t h i r d b a n d . T h e t h i r d b a n d ( F i g . 3 7 b , a d i a b a t i c I P 1 5 . 7 5 e V ) i s a v e r y i n t e n s e s h a r p p e a k w i t h p e r h a p s a n o t h e r p e a k a t 1 9 3 0 c m " 1 t o h i g h e r e n e r g y , a s s i g n e d t o b y c o m p a r i s o n w i t h t h e g r o u n d - s t a t e f r e q u e n c y . T h e f o u r t h b a n d ( F i g . 3 7 c , a d i a b a t i c I P 1 7 . 9 0 e V ) s h o w s tw o i o n i c f r e q u e n c i e s t o be i n v o l v e d , a s i n OCS. T h e y a r e a s s i g n e d t o v-j a n d r e s p e c t i v e l y , b y c o m p a r i s o n w i t h t h e m o l e c u l a r g r o u n d - s t a t e f r e q u e n c i e s . S C S e F o u r b a n d s c a n be d i s t i n g u i s h e d i n t h e s p e c t r u m ( F i g . 3 5 f ) , e a c h s h o w i n g v i b r a t i o n a l f i n e s t r u c t u r e . T h e a d i a b a t i c I P f o r t h e f i r s t b a n d i s 9.58 eV ( F i g . 3 3 a ) . T h e p e a k s o c c u r i n p a i r s i n d i c a t i n g t h e s p l i t t i n g 2 • 2 o f t h e s t a t e i n t o d o u b l e t . c o m p o n e n t s n 3^  , n ^ w i t h a s p i n -o r b i t s p l i t t i n g o f 0 . 1 8 5 e V . A n a l y s i s o f t h e v i b r a t i o n a l f i n e s t r u c t u r e i n d i c a t e s s i n g l e q u a n t u m l e v e l e x c i t a t i o n o f - 146 -a)X n t z D >-cr < cr cr < o LU CO \ CO r-z D o o 1/2 9.5 i 1 1— 10.0 b) A n I I I I I I I I I I —I— I 1 1 1 1 1— 12.1 12.4 12.7 2 T T - + c ) B z 2 2 v + d ) C ^ S 14.0 i — i — 1 4 3 — i 1 1 — i — 1 6 D 16.3 IONIZATION POTENTIAL (eV) F I G U R E 38 - 147 -two ionic frequencies to be involved. One is v 3 , and the other v , . The (100) peak of the n 3 state overlaps with 2 the (000) peak of the 2 n ^ s ta te . The second band ( F ig . 33b, adiabatic IP 12.13 eV, ver t i ca l IP 12.35 eV) consists of a long series of peaks, spreading over approximately 0.6 eV. At least ten peaks are resolved with an average spacing of 440 cm" 1 , which is assigned to , The th i rd band (F ig . 38c, adiabatic IP 14.07 eV) consists of a s ingle quantum of exc i t a -tion in v 3 . The fourth band (F ig . 33d, adiabatic IP 16.06 eV) indicates a s ingle quantum of exc i ta t ion in both and v 3 ' 5.2.4 Discussion Assignment Of The PE Spectra The assignment of the PE spectra of the selenium compounds studied here can eas i l y be accomplished by a simple 3 17 2 corre la t ion with the known ionic states o f - C ^ , OCS and C S 2 ' The i den t i f i c a t i on of v ibrat ional f ine structure of each band can be f a c i l i t a t e d by comparison with the molecular ground-state frequencies and by considering the fol lowing se lect ion rules for v ibrat iona l exc i tat ion (section 2 . 2.4 . b) ; (1) Tota l l y symmetric v ibrat ions may assume any quantum number in the ionic s ta te ; ( 2 ) v ibrat ions antisymmetric with a symmetry species can only assume zero or even quantum number. An examination of F ig . 35 shows the general resem-blance of a l l the PE spectra . Typ i ca l l y , the second band of - 1 4 8 -e a c h m o l e c u l e s h o w s an e x t e n s i v 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 d i c a t i n g a c o n s i d e r a b l e c h a n g e i n g e o m e t r y f r o m t h e m o l e c u l e t o i o n a n d t h e i o n i z e d e l e c t r o n m u s t t h e r e f o r e h a v e come f r o m a s t r o n g l y b o n d i n g o r a n t i b o n d i n g MO. T h e o t h e r t h r e e b a n d s s how l e s s o r l i t t l e v i b r a t i o n a l f i n e s t r u c -t u r e , i n d i c a t i n g l i t t l e c h a n g e i n g e o m e t r y a n d t h e i n v o l v e m e n t o f a m o r e o r l e s s n o n b o n d i n g e l e c t r o n i n e a c h c a s e . I n a d d i t i o n , t h e f i r s t PE b a n d o f e a c h m o l e c u l e s h o w s a r e s o l v a b l e s p i n - o r b i t s p l i t t i n g ( e x c e p t C 0 2 ) . C o n s i d e r i n g t h e s e r e s e m -b l a n c e s a n d t h e c h e m i c a l s i m i l a r i t y b e t w e e n t h e c o m p o u n d s , we c o n c l u d e t h a t t h e o r d e r o f i o n i c s t a t e s i s t h e same t h r o u g h -o u t t h e s e r i e s . A c o r r e l a t i o n d i a g r a m o f i o n i c s t a t e s i s s h o w n i n F i g . 3 9 , a n d i t s h o w s s e v e r a l i n t e r e s t i n g t r e n d s . T h e e n e r g y o f e a c h i o n i c s t a t e i s l o w e r e d i f o n e e n d a t o m i s s u b s t i t u t e d by a h e a v i e r o n e . T h e e f f e c t o f s u b s t i t u t i o n o f S b y Se i s much s m a l l e r t h a n t h a t o f 0 by S o r S e . T h e l a t t e r s u b s t i t u t i o n s r e s u l t i n an e n e r g y l o w e r i n g g r e a t e r t h a n l e V , w h i l e t h e f o r m e r p r o d u c e f a r l e s s t h a n 1 e V . T h i s r e f l e c t s a w e l l e s t a b l i s h e d f a c t t h a t t h e r e s e m b l a n c e b e t w e e n two s u c c e s s i v e . e l e m e n t s i n a p e r i o d i c g r o u p i s c l o s e r a s o n e p r o c e e d s down t h e g r o u p . T h e t r e n d i n t h e m a g n i t u d e o f s p i n - o r b i t s p l i t t i n g A o f t h e g r o u n d i o n i c s t a t e i s a s e x p e c t e d e x c e p t p e r h a p s w h e r e t h a t o f O C S e i s l a r g e r t h a n t h a t o f S C S e + . A c l o s e r a n a l y s i s o f A v a l u e s w i l l e x p l a i n t h i s p a r a d o x a n d g i v e a d e e p e r i n s i g h t i n t o t h e b o n d i n g c h a r a c t e r - 14 9 -CO O CO cn o o + CO o o (D CO o CO o w CM CM CM w c G CM CM CM w CM CM CM w CM / CM CM 4- D> W CM + 3 3 0> w G G CM CM CM Ul OS u. + 3 W w CM CM CM CM o w O CM + 3 w CM G G CM CM i 1 1 r o CM 00 CD CM O (A3) nVLLN3XOd NOI1VZINOI - 1 5 0 -o f t h e h i g h e s t o c c u p i e d n ( g ) f • A s e m i e m p i r i c a l m e t h o d b y 9 1 5 I s h i g u r o a n d K o b o r i " g a v e a v e r y c l o s e e s t i m a t e o f A v a l u e s f o r s i m p l e d i a t o m i c m o l e c u l e s . T h e i r m e t h o d s h o u l d a p p l y e q u a l l y w e l l t o o u r l i n e a r m o l e c u l e s , e s p e c i a l l y i n 2 t h e n ^ g j i o n i c s t a t e , b e c a u s e t h e h a l f f i l l e d MO o f t h e i o n i s n o n b o n d i n g ( o r n e a r l y s o ) a n d h a s l i t t l e c o n t r i b u t i o n f r o m t h e c e n t r a l c a r b o n a t o m t o make t h e two c e n t e r c o n t r i b u t i o n n e g l i g i b l e 2 1 5 T h e s p i n - o r b i t c o u p l i n g c o n s t a n t c a n t h e n be a p p r o x i m a t e d 2 1 5 b y : A = C 2 c + C 2-t + C 2 c ; w h e r e C ' s r J c c x x y y a r e t h e c o e f f i c i e n t s o f t h e c o r r e s p o n d i n g p ^ a t o m i c o r b i t a l i n t h e h i g h e s t o c c u p i e d n ^ ^ M O a n d s ' s a r e t h e c o r r e s p o n d i n g a t o m i c s p i n - o r b i t c o u p l i n g c o n s t a n t s . B e c a u s e C c = ? c = 0, ? 2 we c a n w r i t e A = C"e + C c , U s i n g t h e o b s e r v e d A v a l u e s we x x y y o b t a i n e d ? Q = 0 . 0 2 0 e V , e $ = 0 . 0 5 5 eV a n d c S e = 0 . 2 6 5 e V . W i t h t h e s e ? a n d a p p r o p r i a t e A v a l u e s , we o b t a i n t h e f o l l o w i n g a p p r o x i m a t e w a v e f u n c t i o n f o r t h e n^. jMO; n OCS OCSe v S C S e ' •0.51 p n + 0.86 p c TT , 0 TT , S •0.49 p , n + 0.87 p C a ", 0 IT , S e •0.62 p c + 0 J 9 p c J TT,S rir, Se T h e g r e a t e r c o n t r i b u t i o n o f t h e p ^ S e o r b i t a l , a s i n d i c a t e d b y t h e M0 c o e f f i c i e n t s , i n OCSe t h a n S C S e c a n be i n f e r r e d d i r e c t l y f r o m t h e l a r g e r d i f f e r e n c e i n t h e o r b i t a l e n e r g i e s b e t w e e n 0 a n d S e t h a n b e t w e e n S a n d Se a n d i s t h e e x p l a n a t i o n 2 + f o r t h e l a r g e r A v a l u e o f t h e X n s t a t e o f OCSe . 151 -r In v i e w o f t h e e x i s t e n c e o f t h e f i f t h b a n d i n t h e PE s p e c t r u m o f CS,,, an e f f o r t h a s b e e n made t o f i n d a s i m i l a r b a n d f o r t h e s e l e n i u m c o m p o u n d s . I f t h e p h o t o i o n i z a t i o n c r o s s s e c t i o n i s n o t t o o s m a l l t o e v a d e t h e d e t e c t i o n o f t h e f i f t h b a n d , a s i s m o s t l i k e l y t r u e , we t h e n a r e l e d t o t h e c o n c l u s i o n t h a t t h e f i f t h b a n d o f CS,, i s n o t f o r m e d by a n o n e - e l e c t r o n p r o c e s s , b u t b y a t w o - e l e c t r o n o n e . T h i s r e s u l t s i n a n i o n i c s t a t e w h o s e e l e c t r o n c o n f i g u r a t i o n d i f f e r s f r o m t h a t o f t h e g r o u n d s t a t e o f t h e n e u t r a l m o l e c u l e b y two ( o r m o r e ) MO's. T h i s n o r m a l l y f o r b i d d e n t r a n s i t i o n t o a d o u b l y - e x c i t e d s t a t e o f t h e m o l e c u l a r i o n i s a l l o w e d a s a r e s u l t o f c o n f i g u r a t i o n i n t e r a c t i o n ( s e c t i o n 2 . 2 . 4 . a ) . T h e f u r t h e r c l a r i f i c a t i o n o f t h i s s t a t e m u s t a w a i t f u t u r e t h e o r e t i c a l w o r k . F r a n c k - C o n d o n F a c t o r s a n d T h e G e o m e t r y o f t h e M o l e c u l a r I o n . T h e v i b r a t i o n a l F r a n c k - C o n d o n f a c t o r s ( F C F ) f o r e a c h i o n i c s t a t e h a v e b e e n m e a s u r e d a n d a r e l i s t e d i n T a b l e 1 0 . T h e F C F ' s i n t h e X 2 n a n d A 2n s t a t e s o f 0 C S e + h a v e n o t b e e n m e a s u r e d b e c a u s e o f s e v e r e o v e r l a p . T h e e r r o r s i n t h e s e e x p e r i m e n t a l F C F ' s v a r y c o n s i d e r a b l y , a n d i n c r e a s e w i t h t h e e x t e n t o f o v e r l a p i n t h e b a n d . H o w e v e r , t h e q u a l i t a t i v e f e a t u r e , i . e . , t h e i n t e n s i t y d i s t r i b u t i o n , i s n o t e x p e c t e d t o c h a n g e . T h e m e a s u r e d F C F ' s h a v e b e e n u s e d t o e s t i m a t e t h e g e o m e t r y o f e a c h s t a t e o f t h e m o l e c u l a r i o n u s i n g t h e m e t h o d T A B L E 1 0 ( a ) E x p e r i m e n t a l a n d C a l c u l a t e d F r a n c k - C o n d o n F a c t o r s i n t h e I n d i v i d u a l S t a t e s o f C 0 2 + , C S 2 + a n d C S e 2 + V i b r a t i o n a l F r a n c k - C o n d o n f a c t o r I o n i c s t a t e 1 e v e l n 2 n 3 CO E x p e r i m e n t a l + 2 C a l c u l a t e d CS E x p e r i m e n t a l + 2 C a l c u l a t e d C S e 2 + E x p e r i m e n t a l C a l c u l a t e d 0 0 0 0.80 0.81 1.0 1.0 1 .0 1 .0 100 0.15 0.16 200 -vO.02 0.01 002 % 0 . 0 3 0.02 0 0 0 0 .08 0.06 0.10 0.07 0.01 0 . 0 0 2 100 0.20 0.19 0.20 0.20 0.03 0 . 0 1 3 200 0.23 0 . 2 7 0.24 0.27 0.06 0 . 0 4 3 300 0.21 0.24 0.19 0.23 0.09 0 . 0 9 2 400 0 . 1 3 0.15 0.13 0.14 0.13 0 . 1 4 3 500 0 .08 0.07 0.09 0.06 0.15 0 . 1 7 3 6 0 0 0.04 0.02 0.05 0.02 0.14 0.171 700 0 . 0 3 0.01 0.02 0.01 0.13 0 . 1 4 2 800 0.01 0.001 0.10 0 . 1 0 0 T A B L E 1 0 ( a ) ( c o n t ' d ) E x p e r i m e n t a l a n d C a l c u l a t e d F r a n c k - C o n d o n F a c t o r s i n t h e I n d i v i d u a l S t a t e s o f C 0 2 + , ^ S ^ * a n d C S e 2 + F r a n c k - C o n d o n f a c t o r V i b r a t i o n a l I o n i c l e v e l r n + r c + r c + s t a t e C 0 2 C S 2 C S e 2 u c V g 1 n 2 n 3 E x p e r i men t a i C a l c u l a t e d E x p e r i m e n t a l C a l c u l a t e d E x p e r i m e n t a l C a l c u l a t e i 9 0 0 0.07 0.061 1000 0.05 0 . 0 3 3 1 1 0 0 0.03 0.01 5 1200 0.02 0 . 0 0 6 1 3 0 0 0.01 0.002 000 0.89 0.90 0.87 0.89 0.71 0.73 100 0.10 0.10 0.10 0.10 0.24 0.25 2 0 0 M).01 <0.01 ^0,02 <0.01 ^ 0 . 0 5 0.02 0 0 0 0.88 0.91 0 .37 0.35 1 .0 1 .0 1 00 0.06 0.06 0.11 0.11 002 0.06 0 . 0 3 MD.02 0.04 T A B L E 1 0 ( b ) E x p e r i m e n t a l a n d C a l c u l a t e d F r a n c k - C o n d o n F a c t o r s i n t h e I n d i v i d u a l S t a t e s o f 0 C S + , 0 C S e + a n d S C S e + F r a n c k - C o n d o n f a c t o r V i b r a t i o n a l r 1 z I o n i c l e v e l OCS OCSe S C S e s t a t e n-| n 2 n ? E x p e r i m e n t a l C a l c u l a t e d E x p e r i m e n t a l C a l c u l a t e d E x p e r i m e n t a l C a l c u l a t e d 000 0.45 0.41 a) 0.78 0.76 001 0.27 0.27 0.09 0 .09 002 -^0.09 0.07 0 . 0 0 4 00 3 <0.01 100 0.13 0.12 0.13 0 . 1 3 101 0.07 0.03 0.02 102 0.02 2 0 0 0.02 201 0.01 000 0.05 0 .048 0 . 0 3 b ) 0 0.01 0.01 001 0.08 0 . 1 0 5 0.04 0.004 0.05 0.05 0 0 2 0.10 0 . 1 1 0 0.05 0.02 0.13 0.11 0 0 3 0.07 0.074 0.07 0.04 0.17 0.18 T A B L E 1 0 ( b ) ( c o n t ' d ) E x p e r i m e n t a l a n d C a l c u l a t e d F r a n c k - C o n d o n F a c t o r s i n t h e I n d i v i d u a l S t a t e s o f 0 C S + , 0 C S e + a n d S C S e + F r a n c k - C o n d o n f a c t o r I o n i c s t a t e 1 e v e l n a i - — OCS 0 C S e + S C S e + 1 n 2 n 3 E x p e r i m e n t a l C a l c u l a t e d E x p e r i m e n t a l C a l c u l a t e d E x p e r i m e n t a l C a l c u l a t e d 0 0 4 0.04 0 . 0 3 6 0.10 0.08 0.19 "0.20 0 0 5 O v e r l a p w i t h 2 0 0 0 . 0 1 3 0 . 1 0 0.12 0.16 0.13 0 0 6 0.004 0.11 0.15 0.13 0.13 0 0 7 0.11 0.16 0.08 0.08 0 0 8 0.11 0.14 0.06 0.04 009 0.10 0.11 0.03 0.02 0 0 1 0 0.03 0.03 001 1 0.07 0.05 001 2 0.05 0.03 0 0 1 3 0.01 5 100 0.05 0 . 0 4 7 101 0.10 0 . 1 0 2 102 0 . 1 0 0 . 1 0 7 T A B L E 1 0 ( b ) ( c o n t ' d ) E x p e r i m e n t a l a n d C a l c u l a t e d F r a n c k - C o n d o n F a c t o r s i n t h e I n d i v i d u a l S t a t e s o f 0 C S + , 0 C S e + a n d S C S e + V i b r a t i o n a l F r a n c k - C o n d o n f a c t o r T n n i r n u i o i i u i i o i T — s l l l l l e v e l 0 C S + 0 C S e + S C S e + n 1 n 2 n 3 E x p e r i m e n t a l C a l c u l a t e d E x p e r i m e n t a l C a l c u l a t e d E x p e r i m e n t a l C a l c u l a t e d 103 0.09 0 . 0 7 2 104 0.05 0 . 0 3 5 105 0.02 0.01 3 106 MD.01 0 . 0 0 4 2 0 0 ( 0 . 0 6 ) 0 . 0 2 2 2 01 0.06 0 . 0 4 8 202 0.07 0 . 0 5 0 203 0.05 0.034 20 4 0 . 0 1 6 3 0 0 . 0 . 0 0 2 301 j 0 . 0 1 4 / O v e r l a p w i t h 3 02 > ? + 0.01 5 I B E s t a t e . 3 0 3 ] 0 . 0 1 0 304 ' 0 . 0 0 5 T A B L E 1 0 ( b ) ( c o n t ' d ) E x p e r i m e n t a l a n d C a l c u l a t e d F r a n c k - C o n d o n F a c t o r s i n t h e I n d i v i d u a l S t a t e s o f 0 C S + , 0 C S e + a n d S C S e + V i b r a t i o n a l 1 e v e l F r a n c k - C o n d o n f a c t o r I o n i c s t a t e 0 C S+ OCSe + S C S e + n l n 2 n 3 E x p e r i m e n t a l C a l c u l a t e d E x p e r i m e n t a l C a l c u l a t e d E x p e r i m e n t a l C a l c u l a t e d 0 0 0 1 .0 0 .96 0.96 0.33 0.83 001 ^ 0 . 0 4 0.04 0.14 0.14 002 ^ 0 . 0 3 0.03 c V 0 0 0 0.48 0.43 0.44 0.42 0 . 7 0 0.67 001 0.16 0.15 0.12 0.12 0.17 0.17 002 0.03 0 .02 0.01 100 0.24 0 .22 0.25 0.25 0.13 0.12 101 ~ 0 .03 0 07 0.10 0.07 0.03 102 0.02 0.01 2 0 0 0 . 0 7 0.05 0.09 0.08 201 0 0.02 0.02 300 - 0 . 0 2 0.01 0.02 --a ) b ) F C F was n o t m e a s u r e d b e c a u s e o f s e v e r e o v e r l a p . a s i n g l 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 3 was a s s u m e d . - 1 5 8 o u t l i n e d i n s e c t i o n 2 . 2 . 3 . c , t h e r e s u l t s a r e c o l l e c t e d i n T a b l e F o r c o m p a r i s o n t h e g e o m e t r i e s o f two i o n i c s t a t e s o f NgO h a v e a l s o b e e n e s t i m a t e d u s i n g t h e F C F ' s r e p o r t e d b y T u r n e r 3 e t a l , a n d c o m p a r e d w i t h t h e m e a s u r e d b o n d l e n g t h s . T h e e s t i m a t e d i o n i c g e o m e t r i e s t u r n e d o u t t o a g r e e r e a s o n a b l y w e l l w i t h t h e a v a i l a b l e e x p e r i m e n t a l o n e s , w i t h a maximum o d e v i a t i o n o f 0 . 0 0 6 A ( T a b l e 11). U n f o r t u n a t e l y , t h e c o m p a r i s o n i s l i m i t e d b y a v a i l a b l e e x p e r i m e n t a l d a t a on i o n i c g e o m e t r y . G u i d e d b y t h e i n i t i a l s u c c e s s s t a t e d a b o v e , we m i g h t e x p e c t t h e r e s u l t s f o r t h e t y p e s o f m o l e c u l e s s t u d i e d h e r e , i . e . l i n e a r t r i a t o m i c m o l e c u l e s , w o u l d be r e l i a b l e . T h e m e t h o d u s e d h e r e t o g e t h e r w i t h t h e PES t e c h n i q u e m i g h t p r o v i d e a v e r y g e n e r a l a n d c o n v e n i e n t way o f e s t i m a t i n g i o n i c g e o m e t r i e s . T h e t e s t f o r t h e r e l i a b i 1 i t y o f t h i s m e t h o d i f a p p l i e d t o m o l e c u l e s o f o t h e r s y m m e t r y s p e c i e s h a s t o a w a i t f u t u r e e x p e r i m e n t a l d a t a . S e v e r a l i n s t r u c t i v e c o n c l u s i o n s c a n be d r a w n f r o m t h e s e r i e s o f c a l c u l a t i o n s : (1) B e c a u s e o f t h e c r i t e r i a ( s e c t i o n 2 . 2 . 3 . c ) u s e d i n d e t e r m i n i n g t h e d v a l u e s , we do n o t e x p e c t t h e c a l c u l a t e d i o n i c g e o m e t r y t o be v e r y much d e p e n d e n t o n t h e a c c u r a c y o f i o n i c v i b r a t i o n a l f r e q u e n c i e s . T h i s means t h e f r e q u e n c i e s d e t e r m i n e d b y PES a r e a c c u r a t e e n o u g h f o r t h e p u r p o s e a l t h o u g h t h e y n o r m a l l y h a v e a n e r r o r l i m i t o f 30 c m " 1 ; ( 2 ) T h e m e t h o d o u t ! i n e d i n s e c t i o n 2 . 2 . 3 . C a s s u m e s no m i x i n g o f n o r m a l c o o r d i n a t e s o f t h e same s y m m e t r y i n g o i n g f r o m t h e n e u t r a l t o t h e i o n i c s t a t e , i . e . , A-- = 6.. TABLE 11 E s t i m a t e d Bond L e n g t h s o f t h e I n d i v i d u a l S t a t e s o f C 0 2 + , C S 2 + , C S e 2 + , 0 C S + , SCSe + , and N 2 0 + . M o l e c u l e E l e c t r o n i c d-, XI 0 2 0 d 3 X1 0 2 0 A r ( C - X ) A r ( C - Y ) r ( C - X ) a ^ r ( C - Y ) a ^ XCY S t a t e g ? 2cm . g^cm A A A A oco 1 1 <- 0° ) . 1 o2 1 . 1 6 2 b ) 0 .124 0 .1 24 0.017 0.017 1 .179(1 .177) 1 .179(1 .1 77 ) o c o + ' y% I u 0 0 .503 .095 0 0 .503 .095 0.069 0.013 0.069 0.01 3 1 1 .231 (1 .175(1 .228) .180) 1 1 .231 (1 .175(1 .228) .1 80) * c 2 i + q 0 .073 0 .073 0.01 0 0 .010 1 .172 1 .172 ses x V g 1 . 5 5 5 b ) 1 . 5 5 5 b ) , x 2 n g 0 0 0 0 1 .555(1 .554) 1 .555(1 . 554) s c s + l A 2 n J u B V 1 u o 0 .702 .144 0 0 .702 .144 0.068 0.014 0 .068 0.014 1 1 .623 .569(1 .564) 1 1 .623 .569(1 .564) g 0 .144 0 .144 0 .014 0.014 1 .569 1 .569 SeCSe 1 . 7 1 1 C ) 1 . 7 1 1 C ) 0 0 0 0 1 .711 1 .711 l A 2 n " u 1 .425 1 .425 0 .088 0.088 1 .799 1 .799 T A B L E 11 ( c o n t ' d ) E s t i m a t e d B o n d L e n g t h s o f t h e I n d i v i d u a l S t a t e s o f C 0 p + , CS + 2 • C S e ? + , OCS + + , S C S e , a n d N 2 0 + . M o l e c u l e E l e c t r o n i c d l x i o 2 0 d 3 X 1 0 2 0 A r ( C - X ) A r ( C - Y ) r ( C - X ) a ) r ( C - Y ) a ) XCY S t a t e g 'cm g 2cm o A o A o A o A S e C S e + In2 + fuZ u 0 . 3 2 4 0 . 324 0.020 0.020 1 .731 1 .731 1 + {CUZ + g 0 0 0 0 1.711 1 .711 OCS „1 y + l . ! 5 0 b > 1 . 5 6 0 b ) / A n 0.1 26 0 .281 - 0 . 0 2 4 0.074 1 .136 1 .634 0CS + 1 A 2 n ? + B - i : 0 .230 0 0 .534 0 0 . 0 9 2 0 0. 0 4 6 0 1 .252 1 .160 1 1 .606 .560 1 0 . 1 6 6 0.211 -0.039. 0.069 1 .1 21 1 .629 OCSe x V v2 1 . 1 5 9 ° ) 1 . 7 0 9 ° ) 0 C S e + 1 /X n A 2 n d > B 2 E + 0 0 . 0 4 3 1 .1 70 0 0.031 - 0 . 0 1 4 0.131 0.009 1 .190 1 .145 1 1 .890 .718 C 2 E + 0.131 0 .219 - 0 . 0 4 8 0.067 1.111 1 .776 S C S e X ] E + 1 . 5 5 7 C ) 1 . 7 0 9 ° ) T A B L E 11 ( c o n t ' d ) E s t i m a t e d B o n d L e n g t h s o f t h e I n d i v i d u a l S t a t e s o f C 0 2 , C S 2 ' C S e 2 + , 0 C S + , SCSe + a n d H 2 0 * . M o l e c u l e E l e c t r o n i c d ^ X l O 2 0 d 3 X 1 0 2 0 XCY S t a t e g 2cm g 2cm A r ( C - X ) A r ( C - Y ) r ( C - X ) o o o A A A a) r ( C - Y ) a ) 0 . 1 1 5 0 .1 61 - 0 . 0 2 0 0.04.1 . 1 .537 1 .750 ? A n 0 1 .054 0 . 0 6 3 0 .1 04 •1 .620 1 .813 S C S e 9 + B Z E + 0 0 . 1 1 9 - 0 . 0 0 7 - 0 . 0 1 2 1 .550 1 .738 0 . 1 2 2 0 . 2 3 4 - 0 . 0 1 7 0 . 0 5 0 1 .540 1 .759 NNO x V 1 . 1 2 8 b ) 1 . 1 8 4 b > + I X 2 n 0 . 0 4 6 0 . 0 7 8 0 . 0 2 2 0.007 1 . 1 5 0 ( 1 . 1 5 5 ) 1 .191 (1 . 1 8 5 ) NNO 9 x , A 2 E + 0 . 0 6 3 0.131 0.011 -0.041 1 . 1 3 9 ( 1 .140) 1 . 1 4 3 ( 1 .141 ) v a l u e s i n ( ) a r e e x p e r i m e n t a l b o n d l e n g t h s f r o m r e f . 2 09 f r o m r e f . 2 0 9 f r o m r e f . 2 1 4 a s i n g l 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 _ was a s s u m e d . - 162 -i n t h e e x a c t e x p r e s s i o n Q! = EA..Q!.1 + d . . F o r m o l e c u l e s o f D^CCX,,) s y m m e t r y , no s u c h a s s u m p t i o n i s n e c e s s a r y . T h e r e f o r e , we e x p e c t t h e r e s u l t s f o r t h e s e m o l e c u l e s t o be m o r e r e l i a b l e t h a n t h o s e f o r m o l e c u l e s o f C m V s y m m e t r y . T h i s a r g u m e n t c a n f u r t h e r be e x t e n d e d t o t r i a t o m i c m o l e c u l e s o f o t h e r s y m m e t r i e s . T h e r e l i a b i l i t y o f t h e r e s u l t s w o u l d t h e n be i n t h i s o r d e r D u > C 0 >C >C , b e c a u s e t h e e x t e n t o f c o u p l i n g b e t w e e n °°h 2 v «> v s n o r m a l v i b r a t i o n s o f t h e t o t a l l y s y m m e t r i c s p e c i e s w o u l d n o r m a l l y i n c r e a s e i n t h e same o r d e r ; ( 3 ) I n c o m p a r i n g t h e e x p e r i m e n t a l a n d c a l c u l a t e d F C F ' s ( e v a l u a t e d a t t h e e s t i m a t e d g e o m e t r y ) i n T a b l e 1 0 , we n o t i c e t h e a g r e e m e n t i s much l e s s s a t i s f a c t o r y f o r t h e s e c o n d b a n d s w h e r e h i g h e r l e v e l s o f v i b r a t i o n a l e x c i t a t i o n a r e i n v o l v e d . I n p a r t i c u l a r , t h e c a l c u l a t e d F C F ' s i n d i c a t e a much n a r r o w e r i n t e n s i t y d i s t r i b u t i o n e n v e l o p e . T h e s e d i s c r e p a n c i e s a r e u n d e r s t a n d a b l e i n v i e w o f t h e h a r m o n i c o s c i l l a t o r a s s u m p t i o n . T h e u n s a t i s f a c t o r y a g r e e m e n t i s d u e t o a p o o r e r d e s c r i p t i o n o f t h e v i b r a t i o n a l f u n c t i o n by h a r m o n i c w a v e f u n c t i o n s a t h i g h e r v i b r a t i o n a l q u a n t u m n u m b e r s , w h e r e a s t h e m o r e ' p e a k e d ' i n t e n s i t y d i s t r i -b u t i o n i s d u e t o t h e s t e e p e r s l o p e o f t h e h a r m o n i c p o t e n t i a l c o m p a r e d t o t h e t r u e p o t e n t i a l . An i m p r o v e m e n t i n t h e m e t h o d w o u l d be t h e i n c l u s i o n o f a n h a r m o n i c i t y i n t h e v i b r a t i o n a l w ave f u n c t i o n s . ( 4 ) I f t h e s e c o n d b a n d o f OCSe w e r e t r e a t e d a s a s i n g l e p r o g r e s s i o n i n o n l y , t h e e s t i m a t e d g e o m e t r y o w o u l d g i v e an i n c r e a s e o f 0.181 A i n r ( C - S e ) , w h i c h w o u l d be r a t h e r l a r g e . A s m a l l e r i n c r e a s e i n r ( C - S e ) w o u l d mean a l e s s e x t e n s i v e p r o g r e s s i o n . T h i s r e s u l t i s c o n s i s t e n t w i t h - 163 -o u r a s s i g n m e n t t h a t m o r e t h a n o n e p r o g r e s s i o n i s i n v o l v e d i n t h e b a n d . 5.3 O z o n e ( 0 3 ) 5.3.1 I n t r o d u c t i on T h e PE s p e c t r u m o f 0 3 h a s p r e v i o u s l y b e e n r e c o r d e d 2 1 6 b y Radwan a n d T u r n e r u s i n g a l o w r e s o l u t i o n , r e t a r d i n g g r i d a n a l y z e r . O u r s t u d y d i f f e r s f r o m t h e i r s i n t h a t i t i s o f much h i g h e r r e s o l u t i o n a n d t h e m e a s u r e m e n t s w e r e made on a v e r y p u r e 0 3 s a m p l e (> 9 8 % ) . V i b r a t i o n a l f i n e s t r u c t u r e i s now r e s o l v e d i n some b a n d s a n d s p u r i o u s b a n d s d u e t o t h e p r e s e n c e o f r e s i d u a l o x y g e n c a n be u n a m b i g u o u s l y i d e n t i f i e d , t h u s e n a b l i n g a m o r e a c c u r a t e d e t e r m i n a t i o n o f i o n i z a t i o n p o t e n t i a l s f o r 0^> 5.3.2 E x p e r i m e n t a l T h e PE s p e c t r a w e r e r e c o r d e d w i t h a n o m i n a l r e s o l u -t i o n o f 2 0 - 2 5 meV ( m e a s u r e d a t FWHM o f t h e a r g o n d o u b l e t ) , a n d c a l i b r a t i o n was e f f e c t e d u s i n g b o t h a r g o n a n d o x y g e n p e a k s a s m a r k e r s . A b o u t 8 ml o f o z o n i z e d o x y g e n (^ 4% 0 3 ) p r o d u c e d i n a l a b o r a t o r y o z o n i z e r was c o n d e n s e d , a n d t h e c o n d e n s a t e pumped a t - 1 9 6 ° C down t o a v o l u m e o f a b o u t 1 ml w h i c h was t h e n t r a n s f e r r e d t o t h e K e l - F i n l e t s y s t e m ( s e c t i o n 3 . 3 . 4 . b ) . T h e s p e c t r o m e t e r was t h e n pumped f r o m - 1 6 4 -a t m o s p h e r i c p r e s s u r e w h i l e k e e p i n g t h e 0 3 s a m p l e a t - 1 9 6 ° C w i t h t h e v a l v e c o m p l e t e l y o p e n . T h e o p e r a t i n g v a c u u m was a c h i e v e d i n a b o u t a n h o u r , by w h i c h t i m e t h e a m o u n t o f t h e r e s i d u a l o x y g e n s h o u l d b e n e g l i g i b l e s i n c e a t - 1 9 6 ° C o x y g e n h a s a v a p o u r p r e s s u r e o f ^ 130 mm Hg. T h e PE s p e c t r u m o f o z o n e was t h e n r e c o r d e d w i t h t h e s a m p l e h e l d i n a n i s o p e n t a n e s l u s h b a t h ( - 1 6 0 ° C ) a f t e r r e g u l a t i n g t h e p r e s s u r e w i t h t h e K e l - F v a l v e . T h e h a n d l i n g o f 0 3 a d o p t e d h e r e i n v o l v e d t h e l e a s t d i s t u r b a n c e o f t h e s a m p l e a n d h e n c e g r e a t l y r e d u c e d t h e h a z a r d o f an e x p l o s i o n . F u r t h e r m o r e , w i t h t h e u s e o f t h e d i r e c t i n l e t s y s t e m , t h e d e c o m p o s i t i o n o f 0 3 was f o u n d t o b e n e g l i g i b l e a n d t h u s a s t u d y o f p u r e 0 3 was p o s s i b l e . T h e p u r i t y o f t h e 0 3 s a m p l e was c h e c k e d b y f o c u s s i n g on t h e f i r s t b a n d o f o x y g e n f o r a t l e a s t 5 m i n u t e s a n d n o t i n g no d e t e c t a b l e s i g n a l a b o v e t h e b a c k g r o u n d . By r e p e a t i n g t h i s p r o c e s s s e v e r a l t i m e s , we f i n a l l y e s t i m a t e d t h e o x y g e n i m p u r i t y t o be l e s s t h a n 2% a n d we e x p e c t i t t o d e c r e a s e a s t h e e x p e r i -m e n t p r o c e e d e d . 5.3.3 R e s u l t s a n d D i s c u s s i o n T h e He I PE s p e c t r u m o f 0 3 i s s h o w n i n F i g . 4 0 . T h e f i r s t b a n d h a s a p e c u l i a r c o n t o u r w h i c h c o n s i s t e n t l y s h o w e d up i n two i n d e p e n d e n t m e a s u r e m e n t s s e p a r a t e d i n t i m e b y s e v e r a l d a y s , w i t h b o t h t h e H e l ( F i g . 4 1 a ) a n d N e l ( F i g . 4 1 b ) l i g h t s o u r c e s . T h e p e r s i s t e n c e o f t h e b a n d s t r u c t u r e F I G U R E 40 - 166 -| 1—i—i 1—| i i—i 1—|—i 1 1 — i — | 1 2 . 5 1 3 . 0 1 3 . 5 1 4 . 0 I O N I Z A T I O N P O T E N T I A L ( e V ) F I G U R E 41 - 167 -r u l e s o u t t h e p o s s i b i l i t y o f e i t h e r an a u t o i o n i z a t i o n p r o c e s s o r an e x p e r i m e n t a l a r t i f a c t . I n f a c t , u p o n c l o s e e x a m i n a t i o n t w o b a n d s c a n be i d e n t i f i e d a n d a s s i g n e d t o two i n d e p e n d e n t i o n i z a t i o n p r o c e s s e s . T h e f i r s t I P h a s a d i a b a t i c a n d v e r -t i c a l v a l u e s o f 1 2 . 5 3 ± 0.01 a n d 1 2 . 7 5 ± 0.01 eV r e s p e c t i v e l y , w h i l e t h e s e c o n d IP a p p e a r s t o h a v e an i d e n t i c a l a d i a b a t i c a n d v e r t i c a l v a l u e a t 1 3 . 0 3 ± 0.02 e V . T h e f i r s t b a n d e x h i b i t s s i x c l e a r l y r e s o l v e d v i b r a t i o n a l p e a k s , a n d b e y o n d t h e s i x t h p e a k ( v 1 = 5) o v e r l a p s s t r o n g l y w i t h t h e s e c o n d b a n d . T h e v i b r a t i o n a l s p a c i n g , u n l i k e m o s t c a s e s , i n c r e a s e s t o w a r d s h i g h e r v i b r a t i o n a l l e v e l s i n d i c a t i n g a p o s i t i v e a n h a r m o n i c i t y c o n s t a n t . W i t h e x t e n s i v e a v e r a g i n g , t h e a v e r a g e v i b r a t i o n a l s p a c i n g i s f o u n d t o b e 6 3 0 c m " 1 w i t h t h e f i r s t b e i n g 5 2 0 ± 30 c m " 1 . T h i s v i b r a t i o n a l p r o g r e s s i o n i s - I 2 0 9 a s s i g n e d t o t h e b e n d i n g m o de, v 2 > w h i c h i s 7 0 5 cm" i n t h e m o l e c u l a r g r o u n d s t a t e , s i n c e t h e s y m m e t r i c s t r e t c h , t h e o t h e r p o s s i b l e mode, i s 1 1 1 0 c m " 1 i n t h e n e u t r a l m o l e c u l e a n d w o u l d be e x p e c t e d t o be much l a r g e r . Two a d d i t i o n a l p e a k s a s s o c i a t e d w i t h t h e s e c o n d IP c a n b e i d e n t i f i e d ( F i g . 4 1 a ) w i t h an a v e r a g i n g s p a c i n g o f 7 0 0 ± 50 cm" 1 w h i c h a g a i n c a n b e a s s i g n e d t o t h e b e n d i n g v i b r a t i o n . T h e t h i r d b a n d i s s h a r p a n d i n t e n s e w i t h a n IP a t 1 3 . 5 7 ± 0.01 e V , a n d d e f i n i t e l y i n v o l v e s a r a t h e r e x t e n s i v e p r o g r e s s i o n . I t w a s , h o w e v e r , o n l y p o s s i b l e t o r e s o l v e some v i b r a t i o n a l s t r u c t u r e i n some o f t h e s c a n s , t h u s r e n d e r i n g t h e v i b r a t i o n a l a n a l y -- 1 6 3 -s i s o f t h e b a n d i n c o n c l u s i v e . I n f a v o u r a b l e c a s e s , a weak p r o g r e s s i o n o f ^ 5 00 c m " 1 a p p e a r e d , w h i c h , i f r e a l , w o u l d t h e n p r o b a b l y c o r r e s p o n d t o t h e b e n d i n g v i b r a t i o n . T h e f o u r t h a n d f i f t h b a n d s a r e b o t h w e a k , b r o a d a n d f e a t u r e l e s s c e n t e r i n g a t 1 7 . 8 eV a n d 2 0 . 3 eV r e s p e c t i v e l y . B e c a u s e o f t h e f a s t r i s i n g b a c k g r o u n d i n t h e l o w e l e c t r o n k i n e t i c e n e r g y r e g i o n , t h e e x i s t e n c e o f t h e s e b a n d s may n o t be e n t i r e 1 y c e r t a i n . C o n t r a s t i n g o u r r e s u l t s w i t h t h o s e o f Radwan a n d O "I C T u r n e r ' " ( T a b l e 1 2 ) , we n o t e t h a t t h e a d d i t i o n a l I P ' s g i v e n by t h e m a t 1 2 . 3 , ^ 17 eV a n d ^ 1 9 . 3 eV a r e m o s t p r o b a b l y d u e t o o x y g e n w h i l e t h e o t h e r IP v a l u e s a g r e e v e r y c l o s e l y w i t h o u r s . T h e f i r s t I P f r o m PES i s i n a c c o r d w i t h t h e 2 1 7 v a l u e 1 2 . 8 ± 0.1 eV o b t a i n e d b y e l e c t r o n i m p a c t c o n s i d e r i t h a t t h e v e r t i c a l IP i s u s u a l l y o b t a i n e d f r o m t h e l a t t e r t e c h n i q u e . A r e c e n t p h o t o i o n i z a t i o n s t u d y o f o z o n e w i t h p 1 Q m a s s a n l y s i s g a v e t h e f i r s t I P a t 1 2 . 6 7 ± 0.02 eV w h i c h i s 0.14 eV h i g h e r t h a n o u r a d i a b a t i c v a l u e . T h i s l a r g e d i f f e r e n c e , w h i c h i s f a r b e y o n d t h e e x p e r i m e n t a l e r r o r l i m i t c l a i m e d , i s p e r p l e x i n g s i n c e p h o t o i o n i z a t i o n i s a t h r e s h o l d 2 1 8 p h e n o m e n o n . I n v i e w o f t h e e x p e r i m e n t a l d i f f i c u l t y , we w o u l d be i n c l i n e d t o r e s o l v e t h i s a p p a r e n t d i s c r e p a n c y by s u g g e s t i n g t h a t t h e t r a n s i t i o n t o t h e f i r s t two c a t i o n i c v i b r o n i c l e v e l s may be t o o w e a k t o be d e t e c t e d i n t h e p h o t o i o n i z a t i o n s t u d y a n d t h e v a l u e 1 2 . 6 7 eV a c t u a l l y T A B L E 12 PES a n d T h e o r e t i c a l D a t a f o r 0 V e r t i c a l I P ( e V ) f r o m PES IP ( e V ) f r o m ab i n i t i o S C F MO I o n i c • T h i s w o r k R e f . 2 1 6 , O r b i t a l R e f . 219 , R e f . 221 1 2 . 7 5 ± 0.01 ( 1 2 . 5 3 ) * 1 2 .52 ± 0.05 6a-| 1 5.00 1 5 . 3 9 • V 1 3 . 0 3 t 0.02 1 2 . 3 ± 0.1 ? l a 2 1 3.22 1 3.35 \ 1 3 . 5 7 ± 0.01 1 3 . 5 2 ± 0 . 0 5 4 b 2 1 5.31 1 5 . 8 0 \ 2 B B 2 16.4 • • 1 7 . 4 ? l b l 2 1 . 1 9 2 0 . 6 9 1 7 . 8 ± 0 . 1 1 9 . 2 4 ± 0.1 3 b 2 2 1 . 6 7 2 1 . 2 5 2 A A l 2 0 . 3 ± 0.1 5 a ^ 2 2 . 5 7 22 .23 * a d i a b a t i c I P - 1 7 0 -c o r r e s p o n d s t o t h e v£ = 2 p e a k i n t h e PE s p e c t r u m . F o r t h e f i r s t PE b a n d , we f u r t h e r n o t i c e t h a t t h e p e a k s b e y o n d t h e v £ = 2 l e v e l b r o a d e n c o n s i d e r a b l y . T h e b r o a d e n i n g c a n be e x p l a i n e d a s d u e t o t h e s h o r t l i f e t i m e o f t h e i o n i n t h e 2 1 8 h i g h e r v i b r o n i c l e v e l s a s s u p p o r t e d b y C o o k ' s m e a s u r e m e n t t h a t t h e a p p e a r a n c e p o t e n t i a l s o f t h e 0 2 + a n d 0 5 + i o n s a r e b o t h a t 1 2 . 6 7 e V , w h i c h , a s p o i n t e d o u t a b o v e , c o r r e s p o n d s t o t h e v ' 2 = 2 p e a k . T h e r e a r e a n u m b e r o f a c c u r a t e S C F HO c a l c u l a -2 1 9 - 2 2 1 t i o n s f o r t h e g r o u n d s t a t e o f 0 3 a n d t h e a g r e e m e n t among t h e m i s e x c e l l e n t . I t i s i n t e r e s t i n g t o s e e w h a t u s e f u l i n f o r m a t i o n t h e s e c a l c u l a t i o n s p r o v i d e . A c c o r d i n g 21 9 t o R o t h e n b e r g a n d S c h a e f e r , who e m p l o y e d t h e m o s t e x t e n s i v e b a s i s s e t a n d o b t a i n e d t h e l o w e s t t o t a l e n e r g y , t h e o r b i t a l c o n f i g u r a t i o n o f 0^ i s : ( 2 b 2 ) 2 ( 4 a 1 ) 2 ( 5 a 1 ) 2 ( 3 b 2 ) 2 ( l b 1 ) 2 ( 4 b 2 ) 2 ( 6 a 1 ) 2 ( l a 2 ) 2 , 1 A ] T h e y a l s o s t u d i e d t h e e f f e c t o f d - o r b i t a l c o n t r i b u t i o n a n d f o u n d t h a t t h e o r d e r i n g o f HO e n e r g i e s i s t h e same w i t h a n d 4 9 w i t h o u t t h e m . A s s u m i n g K o o p m a n s ' t h e o r e m ( K T ) , t h e I P ' s a r e c a l c u l a t e d f r o m t h e r e s u l t s o f r e f e r e n c e 219 a n d 221 a n d a r e c o l l e c t e d i n T a b l e 12 f o r c o m p a r i s o n . C o n s i d e r i n g t h a t t h e c a l c u l a t e d I P ' s a r e u s u a l l y h i g h e r t h a n t h e c o r r e s p o n d i n g e x p e r i m e n t a l v a l u e s , we e x p e c t f r o m t h e c a l c u l a t i o n s f i v e t o s i x I P ' s a c c e s s i b l e w i t h H e l e x c i t a t i o n . I t s e e m s t h e n r e a s o n a b l e t o a s s i g n t h r e e I P ' s t o t h e PE b a n d s b e t w e e n 12 a n d 14 eV a n d t h e r e s t t o t h e f o u r t h a n d - 171 -f i f t h b a n d s . D i r e c t a p p l i c a t i o n o f KT t o t h e MO r e s u l t s 2 1 9 ' 2 2 1 p g i v e s t h e o r d e r i n g o f t h e o b s e r v e d i o n i c s t a t e s a s X A,,, A 2 A ] , B 2 B 2 , C 2 B r D 2 B 2 a n d E 2 A ] . T h e l o n g v i b r a t i o n a l p r o g r e s s i o n i n t h e f i r s t b a n d , h o w e v e r , i n d i c a t e s s u b s t a n t i a l g e o m e t r y c h a n g e , a n d i s n o t l i k e l y t o be a s s o c i a t e d w i t h t h e n o n - b o n d i n g l a 2 MO w h i c h i s t h e a n t i s y m m e t r i c c o m b i n a t i o n o f t h e e n d a t o m p ^ f u n c t i o n s . T h i s i s c l e a r l y m a n i f e s t e d 221 i n P e y e r i m h o f f a n d B u e n k e r ' s w o r k i n w h i c h t h e a n g u l a r d e p e n d e n c e o f o r b i t a l e n e r g i e s was s t u d i e d a n d t h e e n e r g y o f t h e l a 2 o r b i t a l was f o u n d t o h a v e l i t t l e a n g u l a r d e p e n -d e n c e . In f a c t , b y f i t t i n g t h e c a l c u l a t e d F r a n c k - C o n d o n f a c t o r s t o t h e e x p e r i m e n t a l o n e s ( s e c t i o n 2 . 2 . 3 . c ) , t h e b o n d a n g l e a n d b o n d l e n g t h o f t h i s i o n i c s t a t e w e r e e s t i m a t e d t o o be 1 2 ° a n d 0 . 0 2 4 A g r e a t e r t h a n t h a t o f t h e m o l e c u l a r g r o u n d s t a t e . C o n s i d e r i n g f u r t h e r t h a t t h e f a i l u r e o f KT i n p r e -d i c t i n g t h e c o r r e c t o r d e r i n g o f s t a t e s i s m o r e f r e q u e n t when t h e s e s t a t e s a r e c l o s e t o g e t h e r , i t w o u l d s e e m r e a s o n a b l e t o b e l i e v e KT i s i n a p p l i c a b l e i n t h e p r e s e n t c a s e a n d we w o u l d p r o v i s i o n a l l y p r o p o s e an i n t e r c h a n g e o f c a t i o n i c s t a t e s r e l a t i v e t o t h e KT o r d e r i n g , i . e . t h e t h r e e l o w e s t 2 2 2 i o n i c s t a t e s w o u l d be X A-j , A A 2 a n d B B,,. T h e a s s i g n m e n t p r o p o s e d h e r e i s c o n s i s t e n t w i t h t h e p r e d i c t i o n s d r a w n f r o m 109 222 c o m p a r i n g t h e PE s p e c t r u m o f 0 3 w i t h t h a t o f M 0 2 ' a n d 3 17 2 S 0 2 ' ; a l l o f t h e s e m o l e c u l e s h a v e s i m i l a r e l e c t r o n i c s t r u c t u r e . T h e b a n d s a s s o c i a t e d w i t h t h e c o r r e s p o n d i n g - 1 7 2 -s t a t e s i n t h e t h r e e m o l e c u l e s show s i m i l a r b e h a v i o u r , n a m e l y , t h e l e n g t h o f t h 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 e a c h b a n d i s c o m p a r a b l e f r o m m o l e c u l e t o m o l e c u l e , a n d d e c r e a s e s i n t h e 2 2 2 o r d e r o f A-, > 3 2 > A 2 , a s e x p e c t e d f r o m t h e r e s p e c t i v e b o n d i n g c h a r a c t e r o f t h e o r b i t a l s i n v o l v e d . T h e l o n g v i b r a t i o n a l p r o g r e s s i o n a s s o c i a t e d w i t h t h e s e c o n d ( B 2 ) a n d t h i r d ( 2 A 2 ) I P ' s i n S 0 2 3 , 1 7 2 i s u n d e r s t a n d a b l e i n v i e w 219 223 o f t h e f a r m o r e i m p o r t a n t d - o r b i t a l p a r t i c i p a t i o n ' i n S 0 2 t h a n i n 0 3 o r M 0 2 . T h e b a n d s e p a r a t i o n s i n t h e PE s p e c t r u m o f a n e u t r a l m o l e c u l e s h o u l d c o r r e l a t e w i t h t h e e l e c t r o n i c s p e c t r u m o f i t s i o n i f d u e c a r e i s g i v e n t o t h e f a c t t h a t t w o - e l e c t r o n p r o c e s s e s a r e n o r m a l l y f o r b i d d e n i n P E S . C o n s e q u e n t l y , some o f t h e i o n i c s t a t e s o b s e r v e d i n t h e e l e c t r o n i c s p e c t r u m may n o t be a c c e s s i b l e i n p h o t o i o n i z a t i o n a n d h e n c e t h e r e may n o t a l w a y s be a o n e - t o - o n e c o r r e s p o n d e n c e b e t w e e n t h e c o r r e s -p o n d i n g PE a n d e l e c t r o n s p e c t r a . W i t h t h i s p r o v i s i o n , i t w o u l d be i n s t r u c t i v e t o c o m p a r e t h e e l e c t r o n i c s p e c t r u m o f N 0 2 > an i s o e l e c t r o n i c s p e c i e s t o 0 3 + , w i t h t h e PE s p e c t r u m o f C U + . C o m p a r i s o n s o f t h i s t y p e h a v e b e e n a t t e m p t e d f o r CP 3 a n d C S + ( s e c t i o n 4 . 1 ) , UH9 a n d H 9 0 a s w e l l a s PH9 a n d + 2 2 4 : H 2 S . I n t h e s e c a s e s , t h e s p e c t r a f o r e a c h p a i r o f m o l e -c u l e s a r e i d e n t i c a l w i t h r e s p e c t t o t h e o r d e r i n g o f e l e c -t r o n i c s t a t e s , a n d a r e r e m a r k a b l y s i m i l a r i n t h e o r d e r o f m a g n i t u d e o f t h e c o r r e s p o n d i n g e x c i t a t i o n e n e r g i e s . I t s e e m s , t h e r e f o r e , r e a s o n a b l e t o e x p e c t a s i m i l a r r e s e m b l a n c e 173 b e t w e e n 0 3 a n d UO^. He n o t e t h a t t h e o r b i t a l c o n f i g u r a t i o n o f M 0 2 2 1 9 i s : ( 3 b 2 ) 2 ( l b 1 ) 2 ( 4 b 2 ) 2 ( l a 2 ) 2 ( 6 a 1 ) 1 ,  2i\} w i t h t h e t h r e e h i g h e s t o c c u p i e d MO's a r r a n g e d i n t h e same o r d e r a s t h e t h r e e l o w e s t s t a t e s o f 0 3 + a n d t h a t t h e g r o u n d s t a t e o f N 0 2 i s X 2 A 1 1 0 9 , 2 2 2 . One 2 B 2 s t a t e o f N 0 2 h a s 29 5 r e c e n t l y b e e n o b s e r v e d b y B r a n d e t a l . " t o l i e a t 1.48 eV a b o v e t h e g r o u n d s t a t e a n d h a s b e e n a s s i g n e d a s t h e l o w e s t 2 e x c i t e d s t a t e . U n f o r t u n a t e l y , no A 2 s t a t e h a s b e e n i d e n t i -f i e d f o r N 0 2 , p r o b a b l y b e c a u s e a f o r b i d d e n t r a n s i t i o n i s i n v o l v e d . A s t r a i g h t f o r w a r d c o m p a r i s o n b e t w e e n 0 3 a n d i J 0 2 i s t h u s i m p o s s i b l e s i n c e t h e p o s i t i o n o f t h e u n o b s e r v e d 2 . 2 i\0 s t a t e o f i ! 0 2 w i t h r e s p e c t t o t h a t o f t h e B 2 s t a t e i s n o t k n o w n . Y e t some i n t e r e s t i n g i n f e r e n c e s c a n be d r a w n b y c o m p a r i n g t h e c o r r e s p o n d i n g e x c i t a t i o n e n e r g i e s i f t h e y c a n be a s s u m e d t o be c o m p a r a b l e . The f i r s t a n d s e c o n d e x c i t a t i o n e n e r g i e s ( a d i a b a t i c ) o f 0 3 + a r e 0.50 a n d 1.04 eV r e s p e c t i v e l y C o m p a r i n g t h e s e two v a l u e s t o 1.48 e V , we t e n d t o b e l i e v e 2 t h a t t h e new B 2 . s t a t e o f N 0 2 i s l i k e l y t o c o r r e s p o n d t o t h e s e c o n d e x c i t e d s t a t e o f 0 , + r a t h e r t h a n t h e f i r s t o n e . I f t h i s i s t h e c a s e , t h e s e c o n d e x c i t e d s t a t e o f 0 3 + w o u l d 9 be t h e "Bp^ s t a t e , a n d f u r t h e r , i f t h e o r d e r i n g o f t h e i r r e s p e c t i v e e l e c t r o n i c s t a t e s i s a s s u m e d t o be t h e s a m e , t h e n 2 t h e f i r s t e x c i t e d s t a t e o f N 0 2 w o u l d be t h e A 2 s t a t e i n s t e a d 9 ' + o f t h e '"B7 s t a t e a s i s t h e c a s e p r o p o s e d f o r 0 3 . - 1 7 4 -In v i e w o f t h e a p p a r e n t f a i l u r e o f KT i n a s s i g n i n g t h e f i r s t IP o f 0^, t h e v a l i d i t y o f KT when a p p l i e d t o h i g h e r I P ' s i s e v e n more d o u b t f u l . T h e r e f o r e , t h e a s s i g n m e n t o f t h e PE b a n d a t 1 7 . 3 eV t o l b ^ a n d 3 b 9 i o n i z a t i o n s , a n d t h a t 2 0 . 3 eV t o 5a-| , b a s e d p u r e l y on MO r e s u l t s , i s o n l y t e n t a t i v e . 0 ° 6 P r e l i m i n a r y c a l c u l a t i o n s by H e r r i n g e t a l . ' " ' ' h a s s u g g e s t e d t h a t t h e b a n d a t 1 7 . 8 eV may be d u e t o C I s t a t e s . I f s u c h i s t h e c a s e , t h e b a n d a t 2 0 . 3 eV w o u l d t h e n be m o s t l i k e l y d u e t o i o n i z a t i o n s f r o m t h e o r b i t a l s l b - j , 3b^ a n d 5a.| . 5.4 N i t r o s y l H a l i d e s (XHO, X = F, C l , B r ) 5.4.1 I n t r o d u c t i on In s p i t e o f t h e f a c t t h a t t h e s e a r e s i m p l e m o l e -c u l e s , t h e r e i s o n l y a m e a g e r u n d e r s t a n d i n g o f t h e e l e c t r o n i c s t r u c t u r e o f t h e n i t r o s y l h a l i d e s . T h i s i s p r e s u m a b l y d u e t o t h e i r i n s t a b i l i t y a n d h i g h r e a c t i v i t y , a s e x e m p l i f i e d b y t h e f a c t t h a t t h e y a r e h i g h l y o x i d i z i n g . T h e s e m o l e c u l e s a r e i n c r e a s i n g l y u n s t a b l e t o w a r d s d e c o m p o s i t i o n i n t h e s e r i e s FNO, Cl.'IO, B r N O , s u c h t h a t BrNO i s v e r y d i f f i c u l t t o o b t a i n i n p u r e f o r m ( e . g . i t i s ^ 7% d e c o m p o s e d i n t h e g a s p h a s e a t r o o m t e m p e r a t u r e ) . A l l o f t h e m a t t a c k many m e t a l s w i t h FNO b e i n g t h e m o s t r e a c t i v e . T h i s s e r i e s o f m o l e c u l e s i s , m o r e o v e r , o f a d d i t i o n a l i n t e r e s t i n v i e w o f t h e weak b o n d i n g b e t w e e n t h e h a l o g e n - 1 7 5 -a t o m a n d t h e MO r a d i c a l , a s r e f l e c t e d i n t h e i r a b n o r m a l l y ? 2 7 l o n g N-X b o n d l e n g t h s " . T h i s a s p e c t h a s , i n f a c t , b e e n 2 23 e x t e n s i v e l y d i s c u s s e d by P e y e r i m h o f f a n d B u e n k e r f o r t h e c a s e o f R I O . 5.4.2 E x p e r i m e n t a l A l l t h r e e m o l e c u l e s w e r e r u n u s i n g t h e s t a i n l e s s s t e e l d i r e c t i n l e t ( s e c t i o n 3 . 3 . 4 . b ) w i t h t h e s a m p l e c o o l e d a n d h e l d i n an a p p r o p r i a t e t e m p e r a t u r e b a t h (FNO a t - 1 1 2 ° , C l H O a t - 7 8 ° a n d B r N O a t - 6 3 ° C ) . I t was f o u n d n e c e s s a r y i n a l l c a s e s t o u s e t h e d i f f e r e n t i a l pump t o a l l e v i a t e t h e p r o b -l e m o f d e c o m p o s i t i o n . C a l i b r a t i o n o f t h e s p e c t r a was e f f e c t e d u s i n g t h e a r g o n d o u b l e t . B r N O was p r e p a r e d b y r e a c t i n g l i q u i d B r ^ w i t h a n e x c e s s o f NO, W h i c h was f i n a l l y pumped a w a y . C1 i 10 a n d FNO w e r e b o t h c o m m e r c i a l p r o d u c t s ( M a t h e s o n a n d O z a r k M a h o n i n g r e s p e c t i v e l y ) . 5.4.3 R e s u l t s T h e PE s p e c t r a o f t h e s e m o l e c u l e s a r e s h o w n i n F i g . 42 a n d t h e I P ' s a r e g a t h e r e d i n T a b l e 1 3 . FNO T h e PE s p e c t r u m o f FNO ( F i g . 4 2 a ) c o n s i s t s o f t h r e e f e a t u r e l e s s b a n d s . T h e f i r s t b a n d s t a r t s a t 1 2 . 6 6 eV a n d h a s i t s v e r t i c a l IP a t 1 2 . 9 4 e V . I t i s r e l a t i v e l y s h a r p c o m p a r e d t o t h e o t h e r two b a n d s . T h e s e c o n d b a n d i s b r o a d e x t e n d i n g o v e r 2.4 e V , a n d h a s an i n t e n s i t y a l m o s t t w i c e - 17 6 -1 \ i i i r i 1 1 1 1 r 10 II 12 13 14 15 16 17 18 19 IONIZATION POTENTIAL (eV) FIGURE 42 - 177 -T A B L E 13 PES D a t a f o r t h e N i t r o s y l H a l i d e s (XNO) F'lO C l NO B r ^ O V e r t i c a l IP V e r t i c a l IP V e r t i c a l I P ( a d i a b a t i c ) ( a d i a b a t i c ) ( a d i a b a t i c ) ( s V ) ( e V ) ( e V ) 1. 1 2 . 9 4 ( 1 2 . 6 6 ) 1 1 . 5 ( 1 0 . 9 4 ) 1 0 . 9 ( 1 0 . 7 ) 2. 1 4 . 5 ( 1 3 . 5 ) 1 1 . 5 1 0 . 9 3. 1 4 . 5 1 1 . 5 1 0 . 9 4. 1 8 . 5 .(1 7 . 3 5 ) 1 6.1 5 ( 1 5 . 6 3 ) 1 5.1 2(1 4.7 ) 5. 1 9 . 0 1 7 . 1 3 ( 1 6 . 9 5 ) 1 6 . 4 2 ( 1 5 . 8 ) 6. 1 9 . 5 ( ? ) 1 8 . 9 7 ( 1 3 . 5 9 ) 1 7 . 4 1 ( 1 6 . 3 ) 7. -- -- 1 8 . 6 6 ( 1 3 . 0 ) - 178 -t h a t o f t h e f i r s t b a n d . I t c o m m e n c e s a t 1 3 . 5 eV a n d r e a c h e s i t s maximum a t 1 4 . 5 e V . T h e t h i r d b a n d i s a l s o b r o a d a n d i n t e n s e , w i t h a s p r e a d o f c a . 2 e V . T h e t h r e s h o l d a n d maximum o f t h i s b a n d a r e a t 1 7 . 8 5 a n d 1 8 . 5 eV r e s p e c t i v e l y . C I NO T h e PE s p e c t r u m o f CINO ( F i g . 4 2 b ) s h o w s f o u r d i s t i n c t b a n d s w i t h v i b r a t i o n a l s t r u c t u r e r e s o l v e d o n t w o o f t h e m . T h e f i r s t b a n d i s by f a r t h e m o s t i n t e n s e , w i t h a d i a b a t i c a n d v e r t i c a l I P ' s o f .10.94 a n d 1 1 . 4 7 eV r e s p e c t i v e l y . T h e s e c o n d d i s t i n c t b a n d s h o w s some v i b r a t i o n a l f i n e s t r u c t u r e , w h i c h d i s a p p e a r s b e y o n d t h e t h i r d c o m p o n e n t p e a k . T h e v i b r a t i o n a l s p a c i n g a v e r a g e s 1 3 8 0 + 100 c m " 1 . T h e a d i a b a t i c I P i s a t 1 5 . 6 3 eV w h i l e t h e v e r t i c a l o c c u r s a t 1 6 . 1 5 e V . T h e t h i r d b a n d e x t e n d i n g f r o m c a . 1 6 . 7 t o 1 8 . 3 eV s h o w s a t l e a s t n i n e v i b r a t i o n a l c o m p o n e n t s , t h e s p a c i n g b e t w e e n t h e m d e c r e a s i n g r a p i d l y f r o m 1 3 7 0 t o 9 3 0 c m " 1 . T h e v e r t i c a l IP i s a t 1 7 . 1 3 eV b u t t h e e x a c t p o s i t i o n o f a d i a b a t i c IP a t 1 6 . 9 5 eV i s t e n t a t i v e b e c a u s e o f a p o s s i b l e p e a k a t 1 6 . 7 6 e V . T h e f o u r t h b a n d i s f e a t u r e l e s s , w i t h a d i a b a t i c a n d v e r t i c a l I P ' s a t 1 8 . 5 9 a n d 1 3 . 9 7 eV r e s p e c t i v e l y . T h e r e i s , i n a d d i t i o n , a s u g g e s t i o n o f a n a d d i t i o n a l b r o a d b a n d a t 1 3 . 8 e V , w h i c h i s e x t r e m e l y w e a k r e l a t i v e t o t h e o t h e r b a n d s . B r N O A l t h o u g h t h e PE s p e c t r u m o f B rNO ( F i g . 4 2 c ) i s c o m p l i -c a t e d b y t h e p r e s e n c e o f t h e b a n d s d u e t o Br,, a n d NO, f i v e b a n d s o f B r N O , a l l s t r u c t u r e l e s s , c a n be d i s t i n c t l y i d e n t i f i e d . T h e f i r s t b a n d i n t h e s p e c t r u m i s d u e t o a s u p e r i m p o s i t i o n - 179 -o f t h e b a n d s o f Cr,, a n d B r N O , w h i c h u p o n n a n u a l d e c o n v o l u t i on g i v e s t h e f i r s t b a n d o f BrNO ( F i g . 4 2 c ) h a v i n g a d i a b a t i c a n d v e r t i c a l I P ' s a t 1 0 . 1 7 a n d 1 0 . 9 e V r e s p e c t i v e l y . I n a l l r e s p e c t s , t h i s b a n d o f BrNO i s s i m i l a r t o t h e c o r r e s p o n d i n g b a n d o f C1M0. T h e s e c o n d b a n d o f BrNO i s p a r t i a l l y o v e r l a p p e d b y t h e t h i r d b a n d o f B ^ , b u t i t s v e r t i c a l IP c a n b e c o n -f i d e n t l y p l a c e d a t 1 5 . 1 2 e V . T h e t h i r d b a n d w h i c h i s p a r t l y c o n c e a l e d b y t h e i n t e n s e p e a k o f NO a t 1 6 . 5 4 eV i s e s t i m a t e d t o h a v e i t s v e r t i c a l a t 1 6 . 4 2 e V . T h e v e r t i c a l I P ' s o f t h e f o u r t h a n d f i f t h b a n d s a r e 17.41 a n d 1 8 . 6 6 eV r e s p e c t i v e l y . 5.4.4 D i s c u s s i o n T o a s s i s t t h e a s s i g n m e n t o f t h e PE s p e c t r a , we h a v e c a r r i e d o u t CNDO t y p e MO c a l c u l a t i o n s f o r t h e n i t r o s y l h a l i d e s . T h e r e s u l t s a r e c o l l e c t e d i n T a b l e 1 4 , w h i c h l i s t s t h e s y m m e t r y , b o n d i n g c h a r a c t e r a n d e n e r g y o f t h e 9?P 2 29 i n d i v i d u a l MO. I n a d d i t i o n , t h e ab i n i t i o c a l c u l a t i o n s ' on FNO a r e a l s o i n c l u d e d . A l l t h e s e c a l c u l a t i o n s c o n s i s t e n t l y i n d i c a t e t h a t t h e i n d i v i d u a l n i t r o s y l h a l i d e s c a n be d e s c r i b e d q u i t e c o n c i s e l y a s a l o o s e l y b o u n d c o m b i n a t i o n o f a h a l o g e n a t o m a n d a NO r a d i c a l . M o r e s p e c i f i c a l l y , t h e XNO m o l e c u l e c a n be t r e a t e d a s a c o l l e c t i o n o f u n p e r t u r b e d d o u b l y o c c u p i e d h a l o g e n a t o m i c o r b i t a l s a n d NO HO's w i t h o n e t r u e MO, i . e . 7 a ' , f o r m e d f r o m an a l m o s t e q u a l a d m i x t u r e o f h a l o g e n np a n d NO 2n o r b i t a l c h a r a c t e r . \BLE 14 SCF-MO R e s u l t s f o r t h e N i t r o s y l i!a 1 i d e s (XMO) IP(eV) =-;iu e n e r g y 'mmetry B o n d i n g C h a r a c t e r FNO C l u 0 B r N O P e r t u r b a t i o n 9 ^ . A b : b ) i n i t i o ' CNDO/p C N J Od ^ C N D O / ^ C N D O d ) C N u U d ) 7a ' 2 H - N 0 ( o u t o f p h a s e ) + p x -X 13.1 14 . 6 11.3 1 4 . 0 3 . 7 12.4 1 1 . 7 2 a " P -X y 1 5 . 6 1 7 . 6 1 5 . 3 1 5 . 6 1 0 . 5 1 3.4 1 2 . 4 6a ' P Z - X 1 5 . 6 1 7 . 8 15.4 1 5.6 1 0 . 5 13.4 1 2 . 4 l a " l n - N 0 ( i n p h a s e ) 19.1 21 .4 1 9 . 2 1 6 . 7 1 8 . 3 1 5 . 7 1 5 . 2 i 5a ' 5 a - N 0 ( i n p h a s e ) 1 9 . 7 21 .6 1 9 . 4 1 6 . 9 1 8 . 4 16.1 1 5 . 6 g 4a ' i n - i i o ( i n p h a s e ) 2 0 . 3 2 3 . 0 21 .7 1 7 . 7 2 0 . 4 1 6 . 9 1 6 . 4 ' 3a ' 4 o - N O ( o » t o f p h a s e ) 2 3 . 5 2 7 . 8 2 4 . 8 2 6 . 9 2 3 . 7 2 4 . 2 2 3 . 3 ) r e f . 2 2 9 . b ) r e f . 2 2 8 . c ) r e f . 7 6 ; a l l I P ' s w i t h 4 eV a d j u s t m e n t , d ) r e f . 232 - 181 -I n a d d i t i o n t o t h e c o r r e l a t i o n s w i t h t h e s e t h e o -r e t i c a l r e s u l t s , t h e a s s i g n m e n t o f t h e PE s p e c t r a s h o u l d a l s o i n v o k e t h e f o l l o w i n g c o n s i d e r a t i o n s : ( 1 ) In v i e w o f t h e l o c a l i z a t i o n o f t h e h a l o g e n l o n e p a i r s , t h e r e o r g a n i z a t i o n e n e r g y i s e x p e c t e d t o be a p p r e c i a b l e u p o n i o n i z a t i o n . T h i s i s e s p e c i a l l y t r u e f o r t h e f l u o r i n e l o n e p a i r s s i n c e a p o s i t i v e l y c h a r g e d f l u o r i n e a t o m i s p a r t i c u l a r l y u n s t a b l e , a n d t h u s t e n d s t o d r a w e l e c t r o n s f r o m t h e NO m o i e t y . C o n s e q u e n t l y , a l l o w a n c e f o r s u c h a n e f f e c t s h o u l d be made when a p p l y i n g K o o p m a n s 1 t h e o r e m t o t h e t h e o r e t i c a l r e s u l t s . ( 2 ) S i n c e t h e e l e c t r o n i c s t r u c t u r e s o f t h e t h r e e m o l e c u l e s a r e c l o s e l y r e l a t e d , q u a l i t a t i v e c o n s i d e r a t i o n s l e a d o n e t o e x p e c t t h a t b a n d s i n t h e s p e c t r u m o f B r NO s h o u l d be s h i f t e d t o l o w e r I P ' s r e l a t i v e t o t h e c o r r e s p o n d i n g b a n d s i n t h e s p e c t r u m o f C I N O , w h i c h s h o u l d , i n t u r n , b e l o w e r t h a n t h o s e o f FNO. B a s e d u p o n t h e s e c o n s i d e r a t i o n s a n d , i n a d d i t i o n , t h e r e l a t i v e b a n d i n t e n s i t i e s , t h e s p e c t r a o f t h e n i t r o s y l h a l i d e s c a n be s a t i s f a c t o r i l y a s s i g n e d a s f o l l o w s : FNO T h e f i r s t b a n d i s t o be a s s o c i a t e d w i t h t h e 7 a ' o r b i t a l ( t h e o n l y t r u e MO), w h e r e a s t h e s e c o n d b a n d i s d u e t o t h e a l m o s t d e g e n e r a t e , h i g h l y r e l a x e d F l o n e p a i r s , 2 a M a n d 6 a ' . W h i l e i t i s d i f f i c u l t t o d e c i d e w h e t h e r two o r t h r e e I P ' s a r e t o be a s s o c i a t e d w i t h t h e t h i r d b a n d , t h e b a n d s h a p e a n d - 182 -r e l a t i v e i n t e n s i t y t e n d t o s u g g e s t t h a t t h r e e I P ' s ( l a " , 5 a ' a n d 4 a ' ) a r e o c c u r r i n g h e r e . T h i s c o n c l u s i o n i s a l s o s u p p o r t e d by t h e ab i n i t i o c a l c u l a t i o n w i t h p e r t u r b a t i o n 229 c o r r e c t i o n s t o K o o p m a n s ' t h e o r e m ( T a b l e 1 4 ) . ClMO T h e i n t e n s e , f i r s t b a n d i s a s s i g n e d t o t h r e e I P ' s , t h e 7 a ' a n d t h e two C l l o n e p a i r s , 2 a " a n d 6 a ' . T h e s e c o n d , t h i r d a n d f o u r t h b a n d s a r e r e s p e c t i v e l y d u e t o l a " , 5 a ' a n d 4 a ' . T h e v i b r a t i o n a l s t r u c t u r e o b s e r v e d i n t h e s e c o n d a n d t h i r d b a n d s ( 1 3 8 0 a n d 1 2 0 0 c m - 1 r e s p e c t i v e l y ) i s a s i m p l e p r o g r e s s i o n o f v,,J0 ( 1 8 0 0 c m " 1 i n t h e n e u t r a l m o l e c u ! e 2 3 0 ) . T h e e x c i t a t i o n o f V,,,Q a n d t h e d e c r e a s e i n f r e q u e n c y a r e t o be e x p e c t e d i n v i e w o f t h e b o n d i n g c h a r a c t e r o f t h e r e s p e c t i v e MO's ( T a b l e 1 4 ) . T h e s u d d e n l o s s o f t h e v i b r a -t i o n a l s t r u c t u r e b e y o n d v' = 2 i n t h e s e c o n d b a n d i s i n d i c a t i v e o f a d i s s o c i a t i o n p r o c e s s . T h e a s s i g n m e n t o f t h e weak b a n d a t 1 3 . 8 eV p o s e s a p r o b l e m . A t t h e moment, we c a n o n l y s u g g e s t s e v e r a l a l t e r n a t i v e s t o e x p l a i n t h i s f e a t u r e : - ( 1 ) T h e p o s s i b i l i t y o f i m p u r i t i e s i s u n l i k e l y s i n c e t h e b a n d e x i s t s w i t h t h e same r e l a t i v e i n t e n s i t y i n s p e c t r a o b t a i n e d w i t h d i f f e r e n t s a m p l e s a n d i t s i n t e n s i t y a p p e a r s t o f o l l o w t h a t o f t h e o t h e r b a n d s . ( 2 ) In v i e w o f t h e e x t r e m e l y w e a k i n t e n s i t y o f t h e b a n d a n d by c o m p a r i n g t h e i n d i v i d u a l s p e c t r a o f t h e t h r e e n i t r o s y l h a l i d e s , we do n o t e x p e c t a b a n d a r i s i n g f r o m an ' a l l o w e d ' s i n g l e - s t e p o n e - e l e c t r o n i o n i z a t i o n t o a p p e a r i n t h i s r e g i o n . ( 3 ) I t s - 1 8 3 -a b n o r m a l i n t e n s i t y a n d p o s i t i o n t e n d s t o s u g g e s t t h a t t h e b a n d may be a s s o c i a t e d w i t h a CI s t a t e . B r N O E x c e p t f o r an a d d i t i o n a l b a n d , t h e s p e c t r u m o f B r N O i s v e r y s i m i l a r t o t h a t o f C 1 N 0 , a n d i n d e e d i t c a n be a n a l y z e d i n a s i m i l a r f a s h i o n . T h e f i r s t b a n d i s a g a i n d u e t o t h r e e I P ' s , o n e f r o m 7 a ' a n d two f r o m t h e C r l o n e p a i r s , 2 a " a n d 6 a 1 , w h i l e o t h e r b a n d s a r e s u c c e s s i v e l y a s s i g n e d t o l a " , 5 a ' , 4 a ' a n d 3 a 1 o r b i t a l s . T h e a s s i g n m e n t o f t h e b a n d s d u e t o t h e two h a l o g e n l o n e p a i r s t o be p a r t o f t h e f i r s t b a n d i n t h e c a s e s o f C1N0 a n d B r NO i s s u p p o r t e d b y t h e f o l l o w i n g o b s e r v a t i o n s . ( 1 ) T h e r e g i o n 1 2 - 1 4 eV i n b o t h c a s e s i s f r e e o f a n y b a n d s t h a t may be a s s o c i a t e d w i t h C l o r B r l o n e p a i r I P ' s . As t h e y a r e n o t a n t i c i p a t e d t o f a l l a t an IP g r e a t e r t h a n 14 e V , t h e y m u s t h a v e m e r g e d w i t h t h e f i r s t ( 7 a ' ) b a n d . ( 2 ) T h i s i s c o r r o b o r a t e d by t h e r e l a t i v e i n t e n s i t y o f t h e f i r s t b a n d i n C l i i O a n d 3 r N 0 , s u g g e s t i n g t h a t t h i s b a n d i s a s s o c i a t e d w i t h m o r e t h a n o n e I P . ( 3 ) I n a d d i t i o n t h e t r e n d o f a l l t h e I P ' s f r o m t h e F t h r o u g h t h e C l t o t h e B r s u b s t i t u t e d c o m p o u n d s ( s e e c o r r e l a t i o n l i n e s i n F i g . 4 2 ) i s w h a t i s a n t i c i p a t e d f o r t h e h a l o g e n s e r i e s on e l e c t r o n e g a t i v i t y g r o u n d s . C o m p a r i n g t h e I P ' s o f t h e h a l o g e n l o n e p a i r s i n t h e s e m o l e c u l e s ( F = 1 4 . 5 e V , C l = 1 1 . 5 eV a n d B r = 1 0 . 9 e V ) t o t h e i r r e s p e c t i v e v a l e n c e o r b i t a l i o n i z a t i o n p o t e n t i a l s - 184 -( F = 13.7 e V , C l = 1 3 . 7 eV a n d B r = 1 2 . 4 e V p , o n e n o t e s t h e l a r g e e f f e c t o f r e o r g a n i z a t i o n u p o n i o n i z a t i o n . T h e m a g n i t u d e o f t h i s r e o r g a n i z a t i o n w h i c h i s i n t h e o r d e r , B r < C l < F, i s u n d e r s t a n d a b l e on t h e b a s i s o f t h e d i f f e r e n c e o f t h e i r e l e c t r o n e g a t i v i t i e s ( B r < C l < F ) , s i n c e t h e m o r e e l e c t r o n e g a t i v e h a l o a e n w i l l t e n d t o w i t h d r a w m o r e e l e c t r o n s f r o m t h e NO m o i e t y , a n d t h u s g i v e s r i s e t o m o r e r e o r g a n i z a -t i o n u p o n i o n i z a t i o n . I n t h i s r e s p e c t , o n e h a s t h e c a s e o f + 2 2 7 FNO a p p r o a c h i n g t h e s i t u a t i o n o f F~N0 , t h u s r e s u l t i n g i n an a b n o r m a l l y l o w IP ( 1 4 . 5 eV) f o r t h e F l o n e p a i r s , w h i c h a r e g e n e r a l l y a r o u n d 16 eV o r h i g h e r ( e . g . H F , t h e f l u o r i n e s u b s t i t u t e d m e t h a n e s , S i F ^ ) . 5 • 5 N i t r i c A c i d ( H H 0 3 ) A n d N i t r . y l H a l i d e s ( X N O Q , X = F, C l ) 5.5.1 In t r o d u c t i on HNO^ a n d i t s h a l o g e n s u b s t i t u t e d d e r i v a t i v e s FNOg a n d C1110g a r e e x t r e m e l y c o r r o s i v e c o m p o u n d s a n d s t r o n g o x i d a n t s . T h e y a t t a c k many m e t a l s a n d d e c o m p o s e r e a d i l y . H e n c e , t h e y a p p e a r t o be a p p r o p r i a t e c o n t e n d e r s f o r t h e p r e s e n t s p e c t r o m e t e r . I n v i e w o f t h e r e m o t e n e s s o f t h e h y d r o g e n a t o m f r o m t h e N 0 2 g r o u p i n H:iO, a n d t h e f r e e r o t a t i o n o f t h e M-O(H) b o n d , t h e OH g r o u p may be q u i t e c o n c i s e l y t r e a t e d a s a p s e u d o - h a l o g e n a t o m when t h e i n t e r a c t i o n b e t w e e n t h e OH a n d - 185 -iiQ,, g r o u p s i s c o n s i d e r e d . T h u s , HNO^ may be a s s u m e d t o h a v e C 0 s y m m e t r y i n o r d e r t o f a c i l i t a t e a c o m p a r i s o n w i t h t h e n i t r y l h a l i d e s . I n a d d i t i o n , t h e s e m o l e c u l e s a r e a l l o f t h e same v a l e n c e e l e c t r o n i c s t r u c t u r e , a n d t h u s a r e c o n v e n -i e n t l y d e a l t w i t h t o g e t h e r . 5.5.2 E x p e r i m e n t a l HNO-j a n d C I N O ^ w e r e r u n u s i n g t h e p y r e x d i r e c t i n l e t , w h i l e t h e s t a i n l e s s s t e e l i n l e t was u s e d f o r FNC^ . A l l s a m p l e s w e r e i m m e r s e d i n a n a p p r o p r i a t e t e m p e r a t u r e b a t h ( H ; i 0 3 a n d C1:30 2 a t - 7 8 ° C , F N 0 2 a t - 1 1 2 ° C ) a n d h a d t o be d i f f e r e n t i a l l y pumped t o r e d u c e d e c o m p o s i t i o n . P u r e H!I03 was p r e p a r e d by t h e r e a c t i o n o f d r y 9 3 3 s o d i u m n i t r a t e a n d ^ 1 0 0 % s u l f u r i c a c i d " . C I N C ^ was p r e p a r e d by r e a c t i n g c h i o r o s u l f o n i c a c i d w i t h a n h y d r o u s 2 3 4 ilN03 ' a v a i l a b l e f r o m t h e a b o v e p r o c e d u r e . F i i O ^ was o f c o m m e r c i a l o r i g i n ( O z a r k M a h o n i n g ) . A l l s p e c t r a w e r e r e c o r d e d a t l e a s t t w i c e on s e p a r a t e d a y s u s i n g f r e s h s a m p l e s t o e n s u r e r e p r o d u c i b i l i t y o f t h e s p e c t r a . C a l i b r a t i o n was e f f e c t e d u s i n g t h e a r g o n d o u b l e t . 5 . 5 . 3 R e s u l t s T h e PE s p e c t r a o f t h e t h r e e m o l e c u l e s a r e s h o w n i n F i g u r e 4 3 , a n d t h e e x p e r i m e n t a l r e s u l t s a r e t a b u l a t e d i n •» 186 «» a. HN0 3 IONIZATION P O T E N T I A L ( e V ) FTSL1RF 43 - 1 8 7 -T a b l e 1 5 . H M 0 3 T h e PE s p e c t r u m o f H M 0 3 ( F i g . 4 3 a ) s h o w s e v i d e n c e f o r a t l e a s t e i g h t I P ' s . T h e r e i s e x t e n s i v e v i b r a t i o n a l s t r u c t u r e i n t h e r e g i o n b e t w e e n 1 2 . 0 a n d 13,1 eV ( F i g . 4 4 a ) , w h i c h c a n be i d e n t i f i e d a s a r i s i n g f r o m t h r e e b a n d s . T h e f i r s t b a n d h a s an a d i a b a t i c I P o f 1 1 . 9 6 eV a n d a v e r t i c a l I P o f 1 2 . 3 9 eV c o r r e s p o n d i n g t o t h e s e v e n t h p e a k . T h e e x t e n s i v e v i b r a t i o n a l p r o g r e s s i o n w i t h an a v e r a g e s p a c i n g o f 5 6 0 cm" 1 p r o b a b l y c o r r e s p o n d s t o t h e i o n i z a t i o n o f a s t r o n g l y b o n d i n g o r a n t i b o n d i n g e l e c t r o n . T h e r e i s a l s o s t r o n g e v i d e n c e f o r a n o t h e r s e r i e s s t a r t i n g a t 1 3 7 0 c m " 1 . H e r e t h e v i b r a t i o n a l s p a c i n g i s a g a i n ^ 5 6 0 c m " 1 . T o w a r d s h i g h e r v i b r a t i o n a l q u a n t u m n u m b e r , t h e two s e r i e s m e r g e , a n d a r e t h e n o v e r l a p p e d b y t h e s e c o n d a n d t h i r d b a n d s . T h e s e c o n d b a n d ( F i g . 4 4 a ) h a s a n i d e n t i c a l a d i a b a t i c a n d v e r -t i c a l I P o f 1 2 . 4 9 eV a n d s h o w s a s i m p l e p r o g r e s s i o n w i t h a f a i r l y e v e n s p a c i n g o f 1 0 6 0 c m " 1 . T h e t h i r d b a n d ( F i g . 4 4 a ) r e s e m b l e s t h e s e c o n d , w i t h c o i n c i d e n t a d i a b a t i c a n d v e r t i c a l I P ' s a t 1 2 . 6 3 e V . H e r e t h e v i b r a t i o n a l s e r i e s s h o w s f o u r d i s c e r n i b l e p e a k s , w i t h o n c e a g a i n a f a i r l y e v e n s p a c i n g o f 1 1 6 0 c m " 1 . T h e f o u r t h b a n d w i t h a c o i n c i d e n t a d i a b a t i c a n d v e r t i c a l I P a t 1 3 . 2 3 eV i s s h a r p a n d i n t e n s e , m o s t l i k e l y c o r r e s p o n d i n g t o t h e i o n i z a t i o n o f a n o n b o n d i n g e l e c t r o n . T h i s b a n d s h o w s a weak v i b r a t i o n a l s e r i e s w i t h a n a v e r a g e s p a c i n g o f 8 4 0 c m " 1 . T h e f i f t h b a n d w i t h a v e r t i c a l T A B L E 15 PES a n d T h e o r e t i c a l D a t a f o r HNOg, C I N O g a n d F N 0 2 H--J0-2 "A A 2 . "A. C l MO 2 •B. 1 PES V e r t i c a l I P ( a d i a b a t i c ) (eV) 1 2 . 3 9 ( 1 1 . 9 6 ) 12 .49 1 2 . 6 3 1 3 . 2 3 1 3 . 7 0 ( 1 3 . 4 ) 1 6 . 7 8 ( 1 6 . 1 0 ) 1 3 . 6 3 ( 1 8 . 3 6 ) 19 . 2 4 ( 1 9 . 0 ) 1 2 . 0 8 ( 1 1 . 8 4 ) V i b r a t i o n a l f r e q u e n c y ( c m - 1 ) 5 6 0 , 1 3 7 0 ( ? ) 1 0 6 0 1 1 6 0 8 4 0 4 2 0 CN30/. O r b i t a l s y m m e t r y O r b i t a i e n e r g y ( e V ) 1 a 2 1 0 . 6 ( 1 0 . 3 ) 5a-| 11 . 7 ( 1 1 .0) 4 b 2 1 2 . 8 ( 1 1 .9) 2 b 1 ( 0 H - p 7 r ) 1 5 . 2 ( 1 3 . 9 ) 3 b 2 ( a Q H ) 1 6 . 5 ( 1 5 . 2 ) l b l 21 . 9 ( 2 1 .4) 4 a . 21 . 3 ( 1 9 . 9 ) 2 b 2 2 0 . 5 ( 1 8 . 7 ) 3a i 2 4 . 1 ( 2 3 . 3 ) 4 b 2 ( C l - P a ) 1 2 . 5 2 b 1 (C I - P T T ) 12.6 T A B L E 15 ( c o n t ' d ) P ES a n d T h e o r e t i c a l D a t a f o r H N 0 3 > C I N O g a n d F N 0 2 PES V e r t i c a l I P ( a d i a b a t i c ) ( e V ) V i b r a t i o n a l f r e q u e n c y ( c m " 1 ) O r b i t a l s y m m e t r y CNDO/, O r b i t a l e n e r g y ( e V ) 'A , '2 "A, 12.81 ( 1 2 . 4 0 ) 1 3 . 2 5 ( 1 3 . 0 1 ? ) 1 3 . 6 2 18.1 - 1 9 . 6 8 5 0 l a , 5a. 3b, l b -4a-2b, 3a-11 .0 10.3 1 3 . 3 21 .0 20 . 9 19.2 2 3 . 0 FNO, "A, 13.51 ( 1 3 . 0 9 ) 13.91 1 4 . 4 0 ( 1 4 . 2 ) 11 70 l a , 5 4b, 11 . 5 ( 1 1 .3) 1 3 . 2 ( 1 2 . 7 ) 1 3 . 8 ( 1 2 . 9 ) T A B L E 15 ( c o n t ' d ) PES a n d T h e o r e t i c a l D a t a f o r HN0-, C l N O , a n d FNO P E S CNDO/. V e r t i c a l IP ( a d i a b a t i c ) ( e V ) V i b r a t i o n a l f r e q u e n c y ( c n r 1 ) O r b i t a i s y m m e t r y O r b i t a i e n e r g y (eV) 1 5 . 0 - 17.1 1 8 . 5 ( 1 8 . 0 ) 1 8 . 9 1 9.60 1 0 0 0 3 b 2 ( F - P a ) 2 b ] ( F - P i r ) l b , 4 a . 2b, 3a, 1 7 . 6 ( 1 6 . 3 ) 1 7 . 8 ( 1 6 . 9 ) 2 3 . 4 ( 2 3 . 0 ) 2 2 . 5 ( 2 1 . 0 ) 21 . 6 ( 2 0 . 0 ) 2 5 . 9 ( 2 4 . 7 ) * V a l u e s i n b r a c k e t s a r e INDO r e s u l t s ; a l l I P ' s w i t h 4 eV a d j u s t m e n t . - 191 -IP a t 1 3 . 7 0 eV i s b r o a d a n d s t r u c t u r e l e s s . T h e n e x t t h r e e b a n d s w i t h v e r t i c a l I P ' s a t 1 6 . 7 8 , 1 8 . 6 3 a n d 1 9 . 2 4 eV r e s p e c t i v e l y a r e a g a i n s t r u c t u r e l e s s . C I N O Q T h e PE s p e c t r u m o f C 1 N 0 2 ( F i g . 4 3 b ) s h o w s f i v e d i s t i n c t b a n d s . T h e f i r s t b a n d ( F i g . 4 4 b ) w i t h a v e r t i c a l I P a t 1 2 . 0 8 eV i s s h a r p , s u g g e s t i n g t h a t t h e i o n i z a t i o n o f n o n b o n d i n g e l e c t r o n s i s i n v o l v e d . T h e r e i s , h o w e v e r , some weak v i b r a t i o n a l s t r u c t u r e on t h i s b a n d , t h e a v e r a g e s p a c i n g b e t w e e n p e a k s b e i n g 4 2 0 c m - 1 . T h e s e c o n d b a n d ( F i g . 4 4 b ) w i t h a v e r t i c a l I P a t 1 2 . 8 1 eV p a r t i a l l y o v e r -l a p s t h e t h i r d b a n d a n d a p p e a r s t o h a v e no s t r u c t u r e . B e c a u s e o f t h i s o v e r l a p , t h e a d i a b a t i c IP o f t h e t h i r d b a n d c a n n o t be d e t e r m i n e d a c c u r a t e l y , b u t i s e s t i m a t e d a t 1 3 . 0 1 e V . T h e v e r t i c a l t r a n s i t i o n o f t h e t h i r d b a n d o c c u r s a t 1 3 . 2 5 e V , a n d t h e v i b r a t i o n a l p r o g r e s s i o n a p p e a r s t o be a s i m p l e s e r i e s w i t h an a v e r a g e s p a c i n g o f 8 5 0 c m " 1 . T h e f o u r t h b a n d a t 1 3 . 6 2 e V , w h i c h p a r t l y o v e r l a p s w i t h t h e t h i r d , a p p e a r s t o be s t r u c t u r e l e s s . T h e f i f t h b a n d ( F i g . 4 3 b ) a t 1 3 . 7 eV s e p a r a t e d by ^ 4 eV f r o m t h e f i r s t f o u r b a n d s , i s f a i r l y b r o a d a n d i n t e n s e , a n d t h u s p r o b a b l y c o r r e s p o n d s t o m o r e t h a n o n e I P . F N O p T h e PE s p e c t r u m o f FN'Og ( F i g . 4 3 c ) e x h i b i t s s i x d i s -t i n c t b a n d s . T h e f i r s t t h r e e b a n d s o v e r l a p o n e a n o t h e r , a n d a r e s h o w n a t h i g h e r r e s o l u t i o n i n F i g . 4 4 c . T h e v e r t i c a l t r a n s i t i o n s o f t h e s e t h r e e b a n d s a r e a t 1 3 . 5 1 , 1 3 . 9 1 a n d - 1 9 2 -a. HN0 3 12 13 14 b. CIN0 2 c. F N 0 2 1 1 i 1 r n — 1 — 1 — 1 — r 12 13 il* V3 1k 1b I O N I Z A T I O N P O T E N T I A L ( e V ) FIGURE 44 - 193 -1 4 . 4 0 eV r e s p e c t i v e l y . T h e o v e r a l l s h a p e o f t h e f i r s t b a n d d e f i n i t e l y s u g g e s t s t h e e x i s t e n c e o f some v i b r a t i o n a l s t r u c t u r e . H o w e v e r , b e c a u s e o f t h e o v e r l a p f r o m t h e s e c o n d b a n d , a n d t h e s e v e r e d e t e r i o r a t i o n o f r e s o l u t i o n ( p r e s u m a b l y r e s u l t i n g f r o m s a m p l e p o i s o n i n g ) , i t i s i m p o s s i b l e t o be s p e c i f i c a b o u t t h e n a t u r e o f t h i s v i b r a t i o n a l s t r u c t u r e . A r o u n d t h e maximum o f t h e s e c o n d b a n d a v i b r a t i o n a l s p a c i n g o f 1 1 7 0 c m " 1 i s o b s e r v e d , b u t d u e t o t h e o v e r l a p t h e 0-0 t r a n s i t i o n f o r t h i s b a n d c a n n o t be m e a s u r e d . J u d g i n g f r o m t h e FC e n v e l o p e , t h i s b a n d a p p e a r s t o be f a i r l y e x t e n s i v e , c o r r e s p o n d i n g t o t h e i o n i z a t i o n o f a f a i r l y b o n d i n g o r a n t i -b o n d i n g e l e c t r o n . T h e t h i r d b a n d i s q u i t e s h a r p y e t s h o w s no s t r u c t u r e . T h e f o u r t h a n d f i f t h b a n d s a r e b o t h b r o a d a n d f e a t u r e l e s s . T h e s h a p e o f t h e f i f t h b a n d s u g g e s t s t h a t i s may a c t u a l l y c o n s i s t o f two b a n d s h a v i n g v e r t i c a l I P ' s o f 1 8 , 5 a n d 1 8 . 9 eV r e s p e c t i v e l y , T h e s i x t h b a n d h a s a n i d e n t i c a l a d i a b a t i c a n d v e r t i c a l IP o f 1 9 , 6 0 eV a n d two h i g h e r v i b r a t i o n a l p e a k s w i t h a n a v e r a g e s p a c i n g o f 1 0 0 0 cm" 5 . 5 . 4 . D i s c u s s i o n T h e a s s i g n m e n t s f o r t h e PE s p e c t r a o f t h e s e m o l e -c u l e s a r e a s s i s t e d by c o r r e l a t i o n w i t h t h e known l e v e l s o f 1 09 222 N 0 2 • , by c o n s i d e r a t i o n o f t h e e l e c t r o n i c e f f e c t s o f t h e X s u b s t i t u e n t , by u s i n g i n f o r m a t i o n f r o m v i b r a t i o n a l a n a l y s i s , a n d b y r e f e r e n c e t o t h e M0 c a l c u l a t i o n s , T h e PE s p e c t r a o f H N 0 3 , C I N O g a n d F N 0 2 ( F i g . 4 3 ) - 1 9 4 -c a n be c o n v e n i e n t l y d i v i d e d i n t o t w o r e g i o n s . T h e f i r s t r e g i o n r e f e r s t o b a n d s l y i n g b e t w e e n 1 2 - 1 6 e V , w h i l e t h e o t h e r r e g i o n c o n s i d e r s b a n d s a b o v e 16 e V . I f t h e N 0 2 g r o u p a n d t h e X s u b s t i t u e n t may be c o n s i d e r e d a s s e p a r a t e m o i e t i e s a s i n t h e c a s e o f XNO ( s e c t i o n 5 . 4 ) , t h e n f r o m a q u a l i t a t i v e p o i n t o f v i e w we e x p e c t t o f i n d i n t h e H e l s p e c t r a o f X N 0 2 a p p r o x i m a t e l y e i g h t I P ' s . T h e y c o r r e s p o n d r o u g h l y t o t h e s i x I P ' s f r o m t h e NO,, g r o u p a n d t h e two I P ' s f r o m t h e X s u b s t i t u e n t , w h i c h a r e r e s p e c t i v e l y t h e n u m b e r o f MO's c a p a b l e o f b e i n g i o n i z e d by H e l r a d i a t i o n ( 2 1 . 2 2 e V ) i n t h e i n d i v i d u a l m o i e t i e s . T h e a c t u a l n u m b e r o f I P ' s o b s e r v e d may d i f f e r f r o m t h e e x p e c t e d e i g h t i f i n t e r a c t i o n s b e t w e e n t h e N 0 2 a n d X m o i e t i e s r e s u l t s i n a l o w e r i n g o r r a i s i n g o f some o f t h e o r i g i n a l l y a c c e s s i b l e I P ' s . F u r t h e r m o r e , u s i n g t h e same k i n d o f q u a l i t a t i v e c o n s i d e r a t i o n i t s e e m s s a f e t o e x p e c t t h a t i n t h e f i r s t r e g i o n o f t h e s p e c t r a t h e r e w i l l be t h r e e I P ' s r e s u l t i n g f r o m t h e i o n i z a t i o n o f t h e h i g h e s t o c c u p i e d ( a - | , a 2 a n d b 2 ) MO's o f N 0 2 , a n d two f u r t h e r I P ' s f r o m t h e X s u b s t i t u e n t , i . e . a t o t a l o f f i v e I P ' s b e l o w c a . 16 e V . T h e two I P ' s c o m i n g f r o m t h e h a l o g e n g r o u p c o r r e s p o n d t o t h e n e a r l y d e g e n e r a t e p ^ ( b 2 ) a n d p ^ b - j ) l o n e p a i r s , a n d t h o s e f o r t h e OH g r o u p c o r r e s p o n d t o t h e s i n g l e 0 l o n e p a i r a n d OH a o r b i t a l . T h e a b o v e e x p e c t a t i o n s a r e c o r r o b o r a t e d by t h e CNDO/2 MO r e s u l t s ( T a b l e 1 5 ) , i n t h a t t h e c a l c u l a t i o n s - 1 9 5 -c o n s i s t e n t l y p r e d i c t t h a t t h e i n d i v i d u a l s p e c t r a i n d e e d s e p -a r a t e i n t o two r e g i o n s w i t h t h e f i r s t r e g i o n h a v i n g e x a c t l y f i v e I P ' s . F u r t h e r m o r e , t w o o f t h e f i r s t f i v e I P ' s a r e m a i n l y l o c a l i z e d on X a n d t h e o t h e r t h r e e a r e p r e d o m i n a n t l y f r o m t h e N 0 2 c o n t r i b u t i o n . C o n s i d e r i n g t h a t t h e I P o f t h e C l l o n e p a i r i n o t h e r C l s u b s t i t u t e d c o m p o u n d s ( C l 2 , CH-jCl , C l NO e t c . ) n o r m a l l y f a l l s b e t w e e n 1 1 - 1 2 e V , we may a s s i g n t h e f i r s t b a n d ( 1 2 . 1 e V ) o f C 1 N 0 2 t o t h e e s s e n t i a l l y d e g e n e r a t e C l l o n e p a i r s ( 4 b 2 a n d 2 b 1 ) . T h e s h a r p n e s s o f t h e b a n d i s c o n s i s -t e n t w i t h t h e n o n b o n d i n g c h a r a c t e r o f t h e l o n e p a i r o r b i t a l s , a n d t h e g e n e r a l s h a p e s u g g e s t s t h e a l m o s t c o m p l e t e s u p e r -i m p o s i t i o n o f two I P ' s . T h e w e a k v i b r a t i o n a l e x c i t a t i o n o f 4 2 0 c m " 1 may be a s s i g n e d t o t h e s y m m e t r i c N 0 2 b e n d i n g -1 2 3 5 mode, w h i c h i s 411 cm i n t h e n e u t r a l m o l e c u l e . S i m i l a r c o n s i d e r a t i o n s l e a d u s t o a s s i g n t h e b r o a d b a n d a t 1 5 . 8 eV i n F M 0 2 t o t h e two a l m o s t d e g e n e r a t e F l o n e p a i r s . T h e w i d t h o f t h i s b a n d c o m p a r e d t o t h a t o f t h e c o r r e s p o n d i n g b a n d o f C 1 N 0 2 may be a c c o u n t e d f o r by t h e d u a l f a c t s t h a t t h e two F l o n e p a i r s may h a v e s l i g h t l y d i f f e r e n t e n e r g i e s , a n d t h a t a p p r e c i a b l e r e o r g a n i z a t i o n o c c u r s u p o n i o n i z a t i o n . I n t h e c a s e o f H M 0 3 > t h e s h a r p b a n d a t 1 3 . 2 3 eV i s m o s t l i k e l y d u e t o t h e 0 l o n e p a i r ( 2 b ^ ) , w h i l e c o m p a r a t i v e e n e r g y c o n -s i d e r a t i o n s l e a d us t o a s s i g n t h e f e a t u r e l e s s b a n d a t 1 3 . 7 0 eV t o t h e OH a o r b i t a l ( 3 b 9 ) . T h e e n e r g y s e p a r a t i o n ( 0 . 5 e V ) - 1 9 6 -b e t w e e n t h e s e two I P ' s i s s u r p r i s i n g l y s m a l l c o m p a r e d t o t h a t o f o t h e r Oil c o n t a i n i n g c o m p o u n d s ( e . g . 2.1 eV f o r H 2 Q ) . T h e l o w v a l u e f o r t h e a OH i o n i z a t i o n may be e x p l a i n e d b y t h e f a c t t h a t t h e b o n d b e t w e e n 0 a n d H i s h i g h l y i o n i c a s r e f l e c t e d i n t h e s t r o n g a c i d i t y o f Hi'-iO-j, a n d t h u s l e s s e n e r g y i s r e q u i r e d t o r e m o v e a n e l e c t r o n d w e l l i n g m a i n l y o n t h e 0 a t o m . On t h e o t h e r h a n d , t h e h i g h IP f o r t h e 0 l o n e p a i r may be a c c o u n t e d f o r b y t h e s t r o n g e l e c t r o n w i t h d r a w i n g a b i l i t y o f t h e M 0 2 g r o u p . T h e v i b r a t i o n a l s t r u c t u r e ( 8 4 0 c m " 1 ) o n t h e f o u r t h b a n d i s m o s t l i k e l y v 2 ( T h e N-OH s t r e t c h ) , w h i c h i s 336 cm" i n t h e n e u t r a l m o l e c u l e \ As d i s c u s s e d a b o v e , t h e t h r e e o t h e r b a n d s i n t h e f i r s t r e g i o n c o r r e s p o n d t o t h e t h r e e l o w e s t o c c u p i e d o r b i t a l s (a-j , a 2 a n d b 2 ) o f NO,,. I n NOg i t s e l f , t h e r e l a t i v e o r d e r i n g o f t h e s e o r b i t a l s i s a^ < a ^ b . g . T h e o r b i t a l a.-|, i s s t r o n g l y b o n d i n g a l l o v e r t h e N 0 2 g r o u p , a 2 i s t h e n o n -b o n d i n g , a n t i s y m m e t r i c c o m b i n a t i o n o f 0 o r b i t a l s , a n d t ? i s a n t i b o n d i n g . As a r e s u l t , t h e PE b a n d c o r r e s p o n d i n g t o t h e i o n i z a t i o n o f a n e l e c t r o n f r o m t h e a-j o r b i t a l u s u a l l y s h o w s e x t e n s i v e v i b r a t i o n a l s t r u c t u r e . T a k i n g t h i s i n t o a c c o u n t we a r e l e d t o a s s i g n t h e f i r s t b a n d ( 1 2 . 3 9 e V ) o f i i i i O - j , t h e t h i r d b a n d ( 1 3 . 2 5 e V ) o f C l i J 0 2 a n d t h e s e c o n d b a n d ( 1 3 . 9 1 e V ) o f FiiO„ t o t h e a, i o n i z a t i o n , s i n c e a l l o f t h e m show e x t e n s i v e v i b r a t i o n a l s t r u c t u r e . In v i e w o f t h e b o n d i n g c h a r a c t e r o f t h e a-, o r b i t a l , t h e 5 6 0 a n d 1 370 c m " 1 - 197 -v i b r a t i o n a l i n t e r v a l s on t h e f i r s t b a n d o f HNO^ a r e t h e n a s s i g n e d t o v 3 ( s y m m e t r i c i-!02 b e n d ) a n d ( s y m m e t r i c N-0 s t r e t c h ) , t h e c o r r e s p o n d i n g f r e q u e n c i e s i n t h e n e u t r a l -I 2 3 6 -1 m o l e c u l e b e i n g 6 7 5 a n d 1 3 2 0 cm . S i m i l a r l y , t h e 8 5 0 cm v i b r a t i o n a l i n t e r v a l on t h e t h i r d b a n d o f C 1 M 0 2 a n d t h e 1 1 7 0 cm' 1 on t h e s e c o n d b a n d o f F N 0 2 a r e i d e n t i f i e d a s v., , w h i c h i s 1 2 9 3 2 3 5 a n d 1 31 2 2 3 7 c m " 1 i n t h e n e u t r a l s t a t e s o f C1'J0 P a n d F N 0 2 r e s p e c t i v e l y . T h e r e l a t i v e o r d e r i n g o f t h e two r e m a i n i n g N 0 2 t y p e l e v e l s a 2 a n d b 2 p r e s e n t s s o m e w h a t o f a p r o b l e m , s i n c e i n t h e s e c a s e s v i b r a t i o n a l a n a l y s i s i s n o t h e l p f u l . H o w e v e r , by r e f e r e n c e t o t h e c a l c u l a t i o n s ( T a b l e 1 5 ) , a n d b y c o m p a r i -s o n w i t h i!0 2 i t s e l f , t h e a 2 l e v e l p r o b a b l y l i e s l o w e r i n e n e r g y t h a n b 2 . C o n s e q u e n t l y , t h e r e l a t i v e o r d e r i n g o f t h e N 0 2 t y p e l e v e l s i s a-| < a 2 < b 2 f o r H N 0 3 a n d a 0 < a^ < b 2 f o r C1N0 2 a n d F N 0 2 . I n HM0 3 t h e v i b r a t i o n a l s t r u c t u r e a s s o c i a t e d w i t h t h e s e c o n d ( a 2 ) a n d t h i r d ( b 2 ) b a n d s may be a s s i g n e d t o v-| (1 3 2 0 c m " 1 i n t h e n e u t r a l m o l e c u l e ) . T h e r e v e r s e o r d e r i n g o f t h e a-j a n d a 2 l e v e l s i n g o i n g f r o m H N 0 3 t o C 1 N 0 2 a n d F N 0 9 i s u n d e r s t a n d a b l e i n v i e w o f t h e e l e c t r o n i c e f f e c t s o f t h e X s u b s t i t u e n t s . C o n s i d e r i n g t h e b o n d i n g c h a r a c t e r o f t h e a-j a n d a 2 o r b i t a l s , we t h e n e x p e c t t h e a 2 o r b i t a l ( w i t h no X c h a r a c t e r ) t o be t h e l e a s t p e r t u r b e d , a n d t h e a^ o r b i t a l t o be a p p r e c i a b l y a f f e c t e d . A s a r e s u l t , t h e a^ o r b i t a l i s m o r e s t a b l e t h a n t h e a 2 o r b i t a l i n C 1 N 0 2 a n d F N 0 2 . - 198 -D e f i n i t i v e a s s i g n m e n t s f o r t h e b a n d s i n t h e s e c o n d r e g i o n (> 16 e V ) o f t h e PE s p e c t r a a r e d i f f i c u l t . By 109 22 2 a n a l o g y w i t h NO,, • , t h e t e n t a t i v e a s s i g n m e n t s a r e g i v e n i n T a b l e 1 5 . I n t h e s e c o n d r e g i o n , t h e r e a r e t h r e e b a n d s i n t h e s p e c t r a o f HNO^ a n d F N 0 2 , w h i l e o n l y o n e b r o a d b a n d i s o b s e r v e d f o r C1N0,,. C o n s i d e r i n g t h a t t h e i n d u c t i v e e f f e c t o f C l i s s o m e w h e r e b e t w e e n t h a t o f OH a n d F , we s h o u l d e x p e c t t h e same n u m b e r o f b a n d s f o r C1 i N 0 2 . T h i s e x p e c t a t i o n i s w e l l s a t i s f i e d i f t h e b a n d o f C l NO,, l y i n g b e t w e e n 18.1 - 1 9 . 6 eV i s a c t u a l l y d u e t o t h r e e o v e r l a p p i n g b a n d s . T h e v i b r a t i o n a l e x c i t a t i o n ( 1 0 0 0 c m - 1 ) on t h e l a s t b a n d ( 1 9 . 6 e V ) o f F N 0 2 i s i d e n t i f i e d a s v ] ( 1 3 1 2 c m " 1 i n t h e n e u t r a l m o l e c u l e ) , a n d i s t h u s c o n s i s t e n t w i t h t h e s t r o n g l y b o n d i n g c h a r a c t e r o f t h e b-j o r b i t a l . Q u a l i t a t i v e c o n s i d e r a t i o n s o f t h e o r d e r i n g o f t h e i n d u c t i v e e f f e c t s o f t h e s u b s t i t u e n t s , i . e . OH < C l < F , l e a d u s t o e x p e c t t h a t t h e b a n d s c o r r e s p o n d i n g t o t h e r e m o v a l o f e l e c t r o n s f r o m o r b i t a l s m a i n l y o f N 0 2 c h a r a c t e r s h o u l d be s h i f t e d t o h i g h e r I P ' s i n t h e o r d e r H N 0 3 < C 1 N 0 2 < F M 0 2 . T h a t t h i s c o n d i t i o n i s s a t i s f i e d i s i l l u s t r a t e d i n t h e c o r r e l a t i o n d i a g r a m s h o wn i n F i g . 4 5 , w h i c h l e n d s f u r t h e r s u p p o r t t o t h e a s s i g n m e n t s p r o p o s e d h e r e . I n t h e l i g h t o f t h e p r e s e n t s t u d y , i t s e e m s p e r t i n e n t t o make a f e w c o m m e n t s on t h e PE s p e c t r a o f O O Q C H , N 0 9 , w h i c h h a s s i m i l a r e l e c t r o n i c s t r u c t u r e t o t h e - 1 9 9 -X r ! 0 2 m o l e c u l e s . F i r s t , i n d r a w i n g a n a n a l o g y b e t w e e n N 0 2 2 3 3 a n d CHgNOg, R a b a l a i s m a k e s no m e n t i o n o f t h e b a n d s t h a t may be d u e t o t h e CH^ g r o u p . I f t h i s i s d o n e , t h e n t h e s i x I P ' s o b s e r v e d w o u l d be a l l t h a t s h o u l d be e x p e c t e d i n t h e PE s p e c t r u m o f CH^'-IO,,. H o w e v e r , by c o n s i d e r i n g t h e s m a l l e r i n d u c t i v e e f f e c t o f t h e CH^ g r o u p c o m p a r e d t o t h a t o f t h e OH g r o u p , a n d f u r t h e r , a s s u m i n g t h e a b o v e c o n s i d e r a t i o n s f o r t h e b a n d p o s i t i o n s t o h o l d , we a r e t e m p t e d t o s u g g e s t t h a t t w o I P ' s h a v e n o t b e e n i d e n t i f i e d i n t h e H e l PE s p e c t r u m o f CH^NO ( F i g . 4 5 ) . M o r e o v e r , t h e e x i s t e n c e o f two a d d i t i o n a l I P ' s 2 33 a r e f u r t h e r s u g g e s t e d by t h e INDO MO c a l c u l a t i o n s , w h i c h p r e d i c t s e i g h t I P ' s b e l o w 21 eV f o r C H j N O g . S e c o n d l y , t h e 2 3P a s s i g n m e n t o f t h e t h i r d b a n d i n t h e s p e c t r u m o f CH^NC^ a t 1 4 . 7 3 eV as due t o t h e 4 b 2 i o n i z a t i o n c o n f l i c t s w i t h o u r q u a l i t a t i v e p i c t u r e , a n d t h u s a p p e a r s t o be u n r e a l i s t i c s i n c e t h e U ? J - s t a t e o f C H , N 0 o l i e s h i g h e r i n e n e r g y t h a n i n a n y o t h e r r e l a t e d X f ! 0 ? m o l e c u l e a s shown i n F i g . 4 5 . I n t h i s r e s p e c t , we a l s o n o t e t h a t t h e c a l c u l a t i o n s ( T a b l e 15 a n d r e f . 2 3 3 ) p r e d i c t t h e N 0 2 t y p e b 2 o r b i t a l i n C i l 3 N 0 2 h a s t h e s m a l l e s t a b s o l u t e e n e r g y i n t h e X N 0 2 s e r i e s . I n v i e w o f t h e s e a p p a r e n t d i s c r e p a n c i e s , t h e f o l l o w i n g a m e n d m e n t s may be p r o p o s e d : - I n o r d e r f o r t h e s p e c t r u m o f CH.jM0 2 t o c o r r e l a t e w e l l w i t h t h o s e o f t h e o t h e r X N 0 „ m o l e c u l e s ( F i g . 4 5 ) , two a d d i t i o n a l I P ' s a r e r e q u i r e d . I n s p e c t i o n o f R a b a l a i s ' s p e c t r u m s u g g e s t s o n e a d d i t i o n a l b a n d may b e IONIZATION POTENTIAL (eVj V 5 t <f ? V ? T 9 , _ — CO > 00 CO > KJ > KJ 00 CD KJ CO CO 1 l KJ ! CD KJ DO KJ' KJ CD KJ \ \ \ 3 \ \ v KJ CD KJ KJ -5° KJ KJl CD KJ CO KJ A * KJ,',' 00 KJ co KJ b KJ KJ 00 KJ •A I KJ KJ > KJ 00 KJ KJ 002 -- 201 -f o u n d u n d e r n e a t h t h e f i r s t o r s e c o n d b a n d , a n d a n o t h e r u n d e r t h e s t r o n g t h i r d o r f o u r t h b a n d . T h e f i r s t o f t h e s e two new b a n d s w o u l d t h e n be i d e n t i f i e d a s d u e t o t h e 4 b 2 l e v e l . T h e a s s i g n m e n t s f o r t h e f i v e b a n d s a b o v e 14 eV w o u l d be f a c i l i t a t e d b y t h e i d e n t i f i c a t i o n o f b a n d s d u e t o t h e CH^ g r o u p a s i n t h e c a s e o f o t h e r XNOg. By c o m p a r i s o n w i t h •3 3 CH^ a n d t h e m e t h y l h a l i d e s , t h e b a n d s c o r r e s p o n d i n g t o i o n i z a t i o n f r o m t h e C H 3 g r o u p p r o b a b l y l i e b e t w e e n 1 4 - 1 6 e V . W i t h t h e s e a m e n d m e n t s t h e r e s u l t i n g a s s i g n m e n t s f o r t h e s p e c t r u m o f C H ^ O g a r e s h o w n i n F i g . 4 5 , a n d i n d i c a t e s a t i s f a c t o r y c o r r e l a t i o n w i t h t h e o t h e r m o l e c u l e s . I t i s f u r t h e r n o t e d t h a t t h e p r o p o s e d a s s i g n m e n t s a r e a l s o i n 2 38 c o m p l e t e a g r e e m e n t w i t h t h e c a l c u l a t i o n s - 2 0 2 -C H A P T E R S I X C O N C L U S I O N T h e r e s u l t s p r e s e n t e d i n t h i s s t u d y h a v e s h o w n t h a t t h e PE s p e c t r o m e t e r c a n b e s u c c e s s f u l l y u s e d t o s t u d y t r a n s i e n t s p e c i e s a n d r e a c t i v e m o l e c u l e s , a n d t h a t P E S p r o v i d e s i n v a l u a b l e i n f o r m a t i o n o n t h e m o l e c u l a r e l e c t r o n i c s t r u c t u r e o f t h e s e s y s t e m s , i n f o r m a t i o n t h a t i s d i f f i c u l t t o o b t a i n b y a n y o t h e r s p e c t r o s c o p i c t e c h n i q u e s . W o r k i s i n p r o g r e s s t o e x t e n d t h e r a n g e o f t r a n s i e n t s p e c i e s t h a t c a n b e s t u d i e d b y t h i s t e c h n i q u e . I n t h i s r e s p e c t , w e n o t e t h a t t h e p r i n c i p a l r e q u i r e -m e n t f o r a s u c c e s s f u l s t u d y i s t h e f o r m a t i o n o f a s u f f i c i e n t c o n c e n t r a t i o n o f t h e s p e c i e s o f i n t e r e s t i n t h e i o n i z a t i o n r e g i o n . T h e u l t i m a t e c o n c e n t r a t i o n o f t h e s p e c i e s i s m a i n l y l i m i t e d b y i t s l i f e t i m e a n d t h e e f f i c i e n c y w i t h w h i c h t h e s p e c i e s i s p r o d u c e d . T h e l i m i t a t i o n i m p o s e d b y t h e l i f e t i m e c a n u s u a l l y b e a l l e v i a t e d b y f a s t p u m p i n g , f o r m i n g t h e s p e c i e s i n t o a m o l e -c u l a r b e a m , o r by p r o d u c i n g t h e s p e c i e s r i g h t i n t h e i o n i z a t i o n r e g i o n . T h i s l e a v e s t h e e f f i c i e n c y o f p r o d u c i n g t h e s p e c i e s a s t h e c r u c i a l f a c t o r f o r a s u c c e s s f u l s t u d y . T h u s , t h e s p e c t r a - 203 -o f t h e a t o m i c s p e c i e s 0, N, H a n d F ( s e c t i o n 3.4) h a v e b e e n o b t a i n e d w h e r e a s , t h o s e o f BF a n d UNO, w h i c h h a v e l o n g e r l i f e t i m e s , h a v e s o f a r e v a d e d o u r d e t e c t i o n . I n a d d i t i o n t o t h e s e n s i t i v i t y o f t h e PE s p e c t r o m e t e r , t h e l o w e s t l i m i t o f t h e n e c e s s a r y c o n c e n t r a t i o n f o r d e t e c t i o n i s f u r t h e r d e t e r m i n e d b y t h e p h o t o i o n i z a t i o n c r o s s s e c t i o n o f t h e s p e c i e s , a n d b y t h e e x t e n t o f o v e r l a p i n t h e PE s p e c t r u m . T h i s l a t t e r p o i n t may be s e r i o u s i n s y s t e m s i n w h i c h we a r e t r y i n g t o p r o d u c e m o d i f i c a t i o n s b y , s a y , d i s c h a r g e o r p y r o l y s i s , a n d w h e r e t h e p r o d u c t s a n d r e a c t a n t s h a v e I P ' s o f s i m i l a r e n e r g i e s . H o w e v e r u n d e r f a v o r a b l e c o n d i t i o n s , c o n c e n t r a t i o n s a s l o w a s a f e w p e r c e n t ( a s i n t h e c a s e o f N a t o m s ) may be d e t e c t e d . W ork i s a l s o i n p r o g r e s s t o u s e t h e h i g h t e m p e r a t u r e p y r o l y s i s u n i t ( s e c t i o n 3 . 3 . 4 . d ) a n d t h e v a r i a b l e t e m p e r a t u r e u n i t ( s e c t i o n 3 . 3 . 4 . e ) , w h i c h h a v e s o f a r b e e n o n l y t e s t e d , t o s t u d y some p r a c t i c a l s y s t e m s . F o r t h e h i g h t e m p e r a t u r e u n i t , t h e f o l l o w i n g p r o j e c t s a r e p r e s e n t l y u n d e r w a y . ( 1 ) S 8 0 0 ° C ( r e f . 2 3 9 ) 8 ( 2 ) cc i 4 8 0 0 ° C ( r e f . 2 4 0 ) ( 3 ) C B r 8 0 0 ° C ( r e f . 2 4 0 ) 1 2 0 0 ° C ( r e f . 2 4 1 ) - 204 -U) 1 2 0 0 ° C ( r e f . 2 4 2 ) p 6 0 0 ° C (5) ( r e f . 2 4 3 ) :0 T h e s t u d y o f t h e e q u i l i b r i u m b e t w e e n N 0 2 a n d I'^O^ a t v a r i a b l e t e m p e r a t u r e i s p r o c e e d i n g , a s w e l l a s w o r k on t h e c o n f o r m a t i o n a l t e m p e r a t u r e u n i t . T h e p r e s e n t s t u d y h a s a l s o r e v e a l e d a f u r t h e r a s p e c t o f PES w h i c h m e r i t s i m m e d i a t e a t t e n t i o n , i . e . t h e u s e o f PES f o r i n - s i t u a n a l y s i s o f d y n a m i c c h e m i c a l s y s t e m s . T h e p o t e n t i a l o f PES f o r s u c h s t u d i e s h a s b e e n e x e m p l i f i e d i n t h e s t u d y o f t h e s y s t e m s o f S 0 - S 2 0 - S 0 2 ( s e c t i o n 4 . 2 ) a n d N 2 H 2 - N H 3 - H C 1 ( s e c t i o n 4 . 4 ) . W i t h a k n o w l e d g e o f t h e ( r e l a t i v e ) p h o t o i o n i z a t i o n c r o s s s e c t i o n s o f m o l e c u l e s ( w h i c h may a l s o be a v a i l a b l e b y s p e c i f i c PE s t u d i e s ) , s u c h a n a l y s i s by PES c a n b e q u a n t i t a t i v e a s w e l l a s q u a l i t a t i v e . T h e c a p a b i l i t y o f c o n t i n u o u s l y m o n i t o r i n g t h e s y s t e m o f i n t e r e s t , t o g e t h e r w i t h t h e s i m p l i c i t y a n d s p e e d o f t h e o p e r a t i o n i s a g r e a t a d v a n t a g e , a n d PES p r o m i s e s t o be a p o t e n t i a l t o o l f o r a n a l y s i s o f s u c h d y n a m i c s y s t e m s . I n t h e s t u d y o f t r a n s i e n t s p e c i e s , t h e p o s i t i v e i d e n t i f i c a t i o n o f t h e b a n d s d u e t o s p e c i e s o f i n t e r e s t i s u s u a l l y a p r o b l e m , s i n c e o t h e r s p e c i e s a r e a l s o p r e s e n t b y v i r t u e o f t h e m e t h o d s o f p r o d u c t i o n . T h e t e c h n i q u e s t h a t c a n be a p p l i e d i n t h i s r e s p e c t i n v o l v e c o m p a r i s o n a n a l y s i s o f o x a l y l h a l i d e s 244 by t h e u s e o f t h e v a r i a b l e - 205 -w i t h m o l e c u l e s o f s i m i l a r e l e c t r o n i c s t r u c t u r e ( e . g . S O ) , w i t h HO c a l c u l a t i o n s , t h e j u d i c i a l u s e o f t h e c h e m i c a l p r o p e r t i e s o f t h e s p e c i e s ( e . g . N 2 H 2 ) , a n d m e t h o d s o f p r o d u c i n g t h e s p e c i e s by a l t e r n a t i v e r o u t e s t o be c e r t a i n o f i d e n t i f i c a t i o n ( e . g . S o 0 , N 2 H 2 ) . E v e n s o , u n e q u i v o c a l i d e n t i f i c a t i o n o f t h e m o l e c u l e i s v e r y o f t e n d i f f i c u l t . A f u r t h e r a d v a n c e w o u l d be t h e u s e o f t h e m a s s s p e c t r o m e t e r i n c o n j u n c t i o n w i t h t h e PE s p e c t r o m e t e r t o d e t e c t t h e m o l e c u l a r i o n s i n c o i n c i d e n c e w i t h t h e p h o t o e l e c t r o n s . T h i s w o u l d g r e a t l y s i m p l i f y t h e p r o c e d u r e , a n d e n a b l e o n e t o be c e r t a i n o f t h e o r i g i n o f a n e l e c t r o n o f a p a r t i c u l a r e n e r g y . - 2 0 6 -R E F E R E N C E S 1. K. S i e g b a h n , C. N o r d l i n g , A. F a h l m a n , R. N o r d b e r g , K. H a r m i n , J . H e d m a n , G. J o h a n s s o n , T . B e r g m a r k , S. K a r l s s o n , I . L i n d g r e n , a n d B. L i n d g r e n , E l e c t r o n S p e c t r o s c o p y f o r C h e m i c a l A n a l y s i s . 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CCCC 2. H. M. ROSENSTOCK JCP 41 , 3454 (1964) ; IBID 44, 1271 (1966) . CCCC J . J . B. COON cT AL. J . MOL. SPECT. 8, 285 (1962) . CCCC PROGRAM WRITTEN BY SHUIT-TONG LEE IN 197? AT THE DEPARTMENT CCCC OF CHEMISTRY, THE UNIVERSITY OF BRITISH COLUMBIA CCCC CCCC INPUT DATA CCCC 1ST CARD IDENTIFICATION (20A4) CCCC 2ND CARD (3 (F15 .7 ,5X ) ,21 2) CCCC 1ST ENTRY VIBRATIONAL FREQUENCY OF THE MOLECULE IN WAVE NO. CCCC 2ND ENTRY VIBRATIONAL FREQUENCY OF THE ION IN WAVE NO. CCCC 3RD ENTRY CHANGE IN NORMAL COORDINATE IN (GM**0.5*CM*1OD20), CCCC IF EQUAL TO 0, IT IS INITIALIZED INTERNALLY, CCCC SEE 5TH ENTRY BELOW CCCC 4TH ENTRY MAX NO. OF VIBRATIONAL PEAKS FOR WHICH CALN IS CCCC DONE MAX NO. = 16 CCCC 5TH ENTRY 10 = 2 RATIO DF V = 2 TO V = 1 IS USED TO CCCC CALCULATE THE CHANGE IN NORMAL COORD. CCCC 10 = 1 RATIO TO V = I TO V = 0 IS USED TO CCCC CALCULATE THE CHANGE IN NORMAL COORD. CCCC 10 = 0 BOTH CASES ABOVE ARE CALCULATED AND THEIR CCCC MEAN IS TAKEN. CCCC 3RD CARD READ IN EXPERIMENTAL FRANCK-CONDON FACTOR (8(1004)) CCCC 4TH CARD (I 2 ,3X , F12 .8 , 2X ,12 ) CCCC 1ST ENTRY NO OF INCREMENT CCCC 2ND ENTRY STEP OF INCREMENT CCCC 3RD ENTRY THE NO OF THE MOST INTENSE PEAK MAXVIB= VIS NO + 1 CCCC IF MAXVIB = 0 FRANCK-CONDON FACTORS ARE CALCULATED, CCCC OTHERWISE RATIO OF FCF ARE CALCULATED DOUBLE PRECISION BETA,GAMA,X,Y,Z,RFCF,FCF,A,TEST,ERROR DIMENSION RFCF(20) ,FCF(20) ,1DEN(20) ,TEST(20) READ(5,10) ( IDENd ), 1 = 1,20) WRITE(6,20) ( IDEN( I ) ,1=1,20) 10 FORMAT(20A4) 20 FORMAT(•1•,' FRANCK-CONDON FACTORS OF THE MOLECULE*»2X,20A4//) READ (5,30) FREQM,FREQI,D,NOVIB,10 WRITE(6,40) FREQM,FREQI,D,NOV IB 30 F0RMAT(3(F15.7,5X) ,212) 40 FORMATC MOLECULAR VIB FREQ = • , F7 .2 , ' CM-1• ,10X , 1'ION VIB FREQ = • , F 7 . 2 , ' C M - 1 • / / • CHANGE IN NORMAL COORDINATE = 2 F 7 . 4 / / 1 MAX VIB PEAK CALCULATED = ' , 12// ) READ(5,600) (TEST( I ) ,1=1,16> 600 F0RMAT(8D10.4) READ(5,80) N,DINC,MAXVIB 80 FORMAT(12,3X,F12.8,2X, I 2) WRITE(6,90) N,DINC 90 FORMAT(* NO OF INCREMENT = •,12/• STEP OF INCREMENT = « , F 7 . 4 / / ) - 222 -B£TA=FREQM/FREQI X=1.-BETA Y = l.+BETA IF (D.GT.O.O) GO IC 159 01=0.0 D2=0.0 TtSTl=TEST(l ) TEST2=TEST(2) TEST3=TEST(3) IF (I0.EQ.2) GO TO 31 R = SQRT(TEST2/TEST1 ) Z=R*Y/SQRT(2.) D1 = Z/(BETA*0.1336466*SQRT(FREGI) ) IF (IO.NE.O) GO TO 61 31 R=SQRT(TEST3/TEST2) S=R**2*Y**2-2*X Z=(R*Y+SQRT(S) )/2. D2=Z/(BETA*0.1336466*SQRT(FREUI) ) 61 D=(Dl+D2)/2. 159 DO 1000 K=1,N IF (K . G T . N) GO TO 1100 IF (K . E Q . 1) GO TO 160 D=D+DINC 160 WRITE(6,95) D 95 FORMAT(* 1•,• CHANGE IN NORMAL COORDINATE = « , F 7 . 4 / / / ) GAMA=0.1336466*SQRT(FREQI)*D Z =BETA*GAMA 1 = 1 R F C F ( l ) = i . 1=1*1 IF (I . G T . NOVIB) GO TO 800 RFCF(2)=2.*Z**2/Y**2 1=1 + 1 IF (I . G T . NOVIB) GO TO 800 RFCF(3)=(2.*Z**2/Y+X)**2/(2.*Y**2) 1=1+1 IF ( I . G T . NOVIB) CiO TO 800 RFCF(4)=(2.*Z**3/Y+3.*X*Z)**2/(3.*Y**4) 1=1 + 1 IF ( I . G T . NOVIB) GO TO 800 RFCF(5) = (16.*Z**4/Y**2+48.*Z**2*X/Y+12.*X**2)**2/(24.* 16.*Y**4) 1=1 + 1 IF (I . G T . NOVIB) GO TO 800 RFCF (6) = { 32.*Z**5/Y**2+160.*Z**3*X/Y+120.*X* ;#2)**2/ 1 (120.*32.*Y**6) 1 = 1 + 1 IF (I . G T . NOV IB) GO TO 800 RFCF(7)=<64.*Z**6/Y**3+480.*Z**4*X/Y**2+720.*Z**2*X**2/Y 1 +120.*X**3)**2/<720.*64<*Y**6) 1 = 1 + 1 IF (I . G T . NOVIB) GO TO 800 RFCF(8)=(128.*Z**7/Y*#3+1344.*Z**5*X/Y**2+3360.*Z**3*X**2/Y 1 +1680.*Z*X**3)**2/(5040.*128.*Y**8) 1 = 1 + 1 IF (I . G T . NOVIB) GO TO 800 - 223 -RFCF(9) = (256.*Z**8/Y**4+3584,*Z**6*X/Y**3+13 440.*Z**4*X**2/Y**2 1 +13440.*Z**2*X**3/Y+1680.*X**4)**2/(40320.*256.*Y**8) 1=1*1 IF (I . G T . NOVIB) GO TO 800 RFCF(10)=(512.*Z**9/Y**4+9216.*Z**7*X/Y**3+4 8384.*Z**5*X**2/Y**2 1 +80640.*Z**3*X**3/Y+30240.*Z*X**4)**2/(362880.*512.*Y**10) 1 = 1 + 1 IF (I . G T . NOVIB) GO TO 800 RFCF(11)=(1024.*Z**10/Y**5+23040.*Z**8*X/Y**4+161280.*Z**6*X**2/ 1 Y**3+403200.*Z**4*X**3/Y*«2+302400.*Z**2*X**4/Y+30240. *X**5 )**2/ 2 (3628800.* 1024.*Y**10) 1 = 1 + 1 IF (I . G T . NOVIB) GO TO 800 RFCF(12)=(2.048D3*Z**11/Y**5+5.632D4*Z**9*X/Y**4+ 1 5.0688D5*Z**7*X**2/Y**3+1.77408D6*Z**5*X** 3/Y**2+ 2 2. 2176D6*Z**3*X**4/Y+6.652 8D5*Z*X**5)**2/ 3 (3.99168D7*2.046D3*Y**12) 1 = 1 + 1 IF (I . G T . NOVIB) GO TO 800 RFCF(13)=(4.096D3*Z**12/Y**6+1.35168D5*Z**10*X/Y**5+ 1 1.5 206406*Z**8*X**2/Y**4+7.09632D6*Z**6*X**3/Y**3+ 2 1.33056D7*Z**4*X*#4/Y**2+7.98336D6*Z**2*X**5/Y+6.652805*X**6)**2 3 /(4.79001608*4.096D3*Y**12) 1 = 1 + 1 IF (I . G T . NOV IE ) GO TO 800 RFCF(14)=(8.192D3*Z**13/Y**6+3.19488D5*Z**U*X/Y**5+ 1 4.39296D6*Z**9*X**2/Y**4+2.635 776D7*Z**7*X**3/Y**3+ 26.918912D7*Z**5*X**4/Y**2+6.918912D7*Z**3*X**5/Y+ 3 1.729728D7*Z*X**6)**2/(6.227020809*8.19203*Y**14) 1 = 1+1 IF (8 . G T . NOV IB) GO TO 800 RFCF(15)=(1.63 84D4*Z**14/Y**7+7.4547 205*Z**12*X/Y**6+ 1 1.2300288D7*Z**10*X**2/Y**5+9. 225216D7*Z**8*X**3/Y**4+ 2 3.22882 56D8*Z**6*X**4/Y**3+4.8432384D8*Z**4*X**5/Y**2+ 3 2.421759208*Z**2*X**6/Y+l.7297 28D7*X**7)**2/ 4 (8.71782912010*1.6384D4*Y**14) 1 = 1 + 1 IF (8 . G T . NOVIB) GO TO 800 RFCF( 16) = ( 3.2766D4*Z**15/Y**7+1.72032D6*Z**13*X/Y**6 + 1 3.354624D7*Z**llf'X**2/Y**5 + 3.075072D8*Z**9*X**3/Y**4+ 2 1. 3837824D9*Z**7*X**4/Y**3 + 2.9059430409*Z**5*X**5/Y**2 + 3 2.4216472D9*Z**3*X**6/Y+5.189184D8*Z*X**7)**2/ 4 (1.307674368012*3.2768D4*Y**16) WRITE (6 f99) 99 FORMAT(• FRANCK-CONDON FACTORS'/) 600 DO 200 1=1,NOVIB J = I-1 WRITE<6,100) J,RFCF(I ) 100 FORMAT ( ' R F C F C l ^ ) = ' ,011.4) 200 CONTINUE IF(MAXVIB.NE.O) GO TO 560 A=O.ODOO DO 400 1=1,NOVIB A=A+RFCF(I) 400 CONTINUE - 224 -WRITE(6,410> A 410 FORMATt//' SUM OF RFCF = ' ,D11 .4 / / ) GO TO 550 560 A=RFCF(MAXVIB) M=MAXVIB-1 WRITE<5,570) M, A 570 FORMAT(/* R F C F l ' , 1 2 , ' ) =•,D11.4,3X,• IS TAKEN AS THE REFERENCE * / ) WRITE(6,520) 520 F0RMAT(6X,'CALCULATI ON*, 16X,'EXPERIMENT'/) 550 DO 500 I=1,N0VIB J=I-1 FCF(I)=RFCF( I)/A WRITE(6,510) J ,FCF<I) ,TEST(I) 510 FORMATC F C F ( ' , I 2 , ' ) = * » D11.4,1 OX,D11•4) 500 CONTINUE CCC STANDARD DEVI ATI0N=ERROR IS USED AS A CRITERION FOR THE BEST FIT ERR0R=0.ODOO DO 900 I=1,N0VIB ERR0R=ERR0R+(FCF(I)-TEST!I))**2 900 CONTINUE WRITE(6,910) ERROR 910 FORMAT!// ' STANDARD DEVIATION = SD11.4) 1000 CONTINUE 1100 STOP E ND P u b l i c a t i o n s 1 . S. T. Lee and K. norokuma, ' A b - i n i t i o a o l e c u l a r O r b i t a l C a l c u l a t i o n o f U i t r e n i u m I o n and I t s - I m p l i c a t i o n s ' , J . Amer. Chem. Soc. _9_3_, 6 8 6 3 ( 1 9 7 1 J . 2. D. C. F r o s t , S. T. Lee and C. A. M c D o w e l l , 'The H i g h R e s o l u t i o n P h o t o e l e c t r o n Spectrum of CS', Chem. Phys. L e t t . r7_, 1 5 3 ( 1 9 7 2 ) . 3 . D. C. F r o s t , S. T. Lee and C. A. M c D o w e l l , The P h o t o e l e c t r o n S p e c t r a o f OCSe, SCSe and CSe 2», J . Chem. Phy s . j£, 5 4 3 4 ( 1 9 7 3 ) . 4 . D. C. F r o s t , S. T. Lee and C. A. M c D o w e l l , 'The P h o t o e l e c t r o n Spectrum o f S O ' , Chem. Phys . L e t t . 2 2 , 2 4 3 U 9 7 3 ) . 5. D. C. F r o s t , S. T. Lee and C. A. Mc D o w e l l , 'The P h o t o e l e c t r o n Spectrum o f HCP and Comments on the F i r s t P h o t o e l e c t r o n J a n d of HCN* , Chem. Phys. L e t t . 2^, 473 U 9 7 3 ) . 6 . D. C. F r o s t , S. T. Lee and C. A. M c D o w e l l , 'The H i g h R e s o l u t i o n P h o t o e l e c t r o n Spectrum Of Ozone', Chem. Phy s . L e t t . 1 4 9 ( 1 9 7 3 ) . 7 . D. C. F r o s t , S. T. Lee, C. A. McDowell and N. P. C. Westwood, 'The P h o t o e l e c t r o n S p ectrum o f D i a z e n e ( D i i m i n e ) ' , t o be p u b l i s h e d . 8. D. C. F r o s t , S. T. Lee and C. A. M c D o w e l l , •The H i g h R e s o l u t i o n H e l P h o t o e l e c t r o n S p ectrum of SO*, t o be p u b l i s h e d . 9. D. C. F r o s t , S. T. Lee and C. A. M c D o w e l l , • 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 o f t h e O x y h a l i d e s o f n i t r o g e n . I The N i t r o s y l H a l i d e s (XNO, X= F, C l , B r ) • , t o be p u b l i s h e d . 1 0 . D. C. F r o s t , S. T. L e e , C. A. McDowell and N« P. C. Westwood, ' 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 o f t h e O x y h a l i d e s o f N i t r o g e n . I I The N i t r y l H a l i d e s IXtfO , X= F, C l ) and N i t r i c A c i d ' , t o be p u b l i s h e d . 1 1 . D. C. F r o s t , S. T. Lee and C. A. n c D o w e l l , 'A F a s t Pumping, v e r s a t i l 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 * , t o be p u b l i s h e d . 

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