UBC Theses and Dissertations

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UBC Theses and Dissertations

K-shell excitation of molecules by fast electron impact 1974

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K-SHELL EXCITATION OF MOLECULES BY FAST ELECTRON IMPACT by GORDON ROBERT WIGHT B.Sc. Hons., Memorial U n i v e r s i t y o f Newfoundland, 1970. A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n t he Department o f CHEMISTRY We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A u g u s t , 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 t s 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 an t ed 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 unde r s tood that 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 o w e d w i thout my w r i t t e n p e r m i s s i o n . Department o f /CJ^^uJx^yj 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 Date J^LAAQ iCo'L, - i i - ABSTRACT Energy l o s s s p e c t r a o f 2.5 keV e l e c t r o n s , s c a t t e r e d by m o l e c u l a r t a r g e t s t h r o u g h s m a l l a n g l e s , have been s t u d i e d i n t h e r e g i o n s o f t h e r e s p e c t i v e c a r b o n , n i t r o g e n , oxygen and f l u o r i n e K-edges and t h e s u l f u r L J J j j j edges. E l e c t r o n energy l o s s s p e c t r a f o r d i a t o m i c , t r i a t o m i c and p o l y a t o m i c m o l e c u l e s have been s t u d i e d . D i s c r e t e e x c i t a t - i o n s have been i n t e r p r e t e d i n terms o f t h e p r o m o t i o n o f t h e r e s p e c t i v e K - s h e l l e l e c t r o n t o u n f i l l e d v a l e n c e m o l e c u l a r o r b i t a l s and Rydberg o r b i t a l s . Most s p e c t r a show c o n s i d e r a b l e s t r u c t u r e above t h e r e s p e c t i v e K-edge, i n a d d i t i o n t o t h e normal K-continuum. T h i s s t r u c t u r e r e p r e s e n t s t h e s i m u l t a n e o u s t r a n s i t i o n s o f a K - s h e l l and v a l e n c e s h e l l e l e c t r o n s ( i . e . shake-up and s h a k e - o f f e v e n t s f o l l o w i n g t h e c r e a t i o n o f an i n n e r h o l e ) . I n t h e c a s e o f m o l e c u l a r n i t r o g e n and c a r b o n monoxide, a s i m p l e c o r e model was shown t o p r o v i d e an a c c u r a t e d e s c r i p t i o n f o r t h e K - s h e l l e x c i t e d m o l e c u l e . On t h e b a s i s o f t h i s model, e x c i t a t i o n and i o n i z a t i o n e n e r g i e s f o r some e x o t i c c h e m i c a l s p e c i e s have been p r e d i c t e d from t h e r e l a t i v e e n e r g i e s o b s e r v e d i n t h e K - s h e l l e nergy l o s s s p e c t r a o f a number o f m o l e c u l e s . The agreement between the e s t i m a t e d ( c o r e a n a l o g y ) and o b s e r v e d K - s h e l l e x c i t a t i o n e n e r g i e s f o r l a r g e r m o l e c u l e s i s l e s s s a t i s f a c t o r y , p o s s i b l y because o f the l a r g e changes i n m o l e c u l a r geometry w h i c h o c c u r as a r e s u l t o f an e l e c t i o n p r o m o t i o n . F i n a l l y , t h e c a r b o n K - s h e l l e nergy l o s s s p e c t r a o f carbon d i s u l f i d e , c a r b o n y l s u l f i d e and carbon t e t r a f 1 u o r i d e show f e a t u r e s w h i c h a r e p o s s i b l y a s s o c i a t e d w i t h t h e e x i s t e n c e o f an e f f e c t i v e p o t e n t i a l b a r r i e r i n t h e s e m o l e c u l e s . - i i i - TABLE OF CONTENTS Page CHAPTER ONE I n t r o d u c t i o n 1 CHAPTER TWO Theory o f F a s t E l e c t r o n Impact 5 2.1. E l e c t r o n Energy Loss S p e c t r o s c o p y 5 2.2. The V i r t u a l Photon Model 6 2.3. The F i r s t Born A p p r o x i m a t i o n 9 2.4. E l e c t r o n - H y d r o g e n Atom S c a t t e r i n g 10 2.5. G e n e r a l i z a t i o n t o S c a t t e r i n g by Complex Atoms 14 2.6. G e n e r a l i z e d O s c i l l a t o r S t r e n g t h s 17 CHAPTER THREE E x p e r i m e n t a l Methods f o r I n n e r - s h e l l E x c i t a t i o n S t u d i e s 23 CHAPTER FOUR E x p e r i m e n t a l 30 4.1. 180° E l e c t r o s t a t i c A n a l y s e r 30 4.2. The E l e c t r o n Source 34 4.3. The S p e c t r o m e t e r 37 4.3.1. S p e c t r o m e t e r C o n s t r u c t i o n 37 4.3.2. S p e c t r o m e t e r O p e r a t i o n 40 4.3.3. Energy C a l i b r a t i o n 42 4.3.4. Vacuum System 42 4.4. Sample P u r i t y 4 5 - i v- Page CHAPTER FIVE D i a t o m i c M o l e c u l e s 46 5.1. N i t r o g e n and Carbon Monoxide 46 5.1.1. N i t r o g e n 46 a. V a l e n c e S h e l l Spectrum 46 b. N i t r o g e n K - s h e l l E x c i t a t i o n 49 5.1.2. Carbon Monoxide 60 a. V a l e n c e S h e l l Spectrum 60 b. Carbon K - s h e l l E x c i t a t i o n 60 c. Oxygen K - s h e l l E x c i t a t i o n 65 5.2. N i t r i c O xide and Oxygen 69 5.2.1. N i t r i c O x i d e 69 a. N i t r o g e n K - s h e l l E x c i t a t i o n 71 b. Oxygen K - s h e l l E x c i t a t i o n 80 5.2.2. Oxygen 84 a. V a l e n c e S h e l l Spectrum 84 b. Oxygen K - s h e l l Spectrum 86 CHAPTER SIX T r i a t o m i c M o l e c u l e s 90 6.1. Carbon D i o x i d e and N i t r o u s O x i d e 90 6.1.1. Carbon D i o x i d e 90 a. V a l e n c e S h e l l Spectrum 90 b. Carbon K - s h e l l E x c i t a t i o n 92 c. Oxygen K - s h e l l E x c i t a t i o n 102 6.1.2. N i t r o u s Oxide 105 a. V a l e n c e S h e l l Spectrum 105 b. N i t r o g e n K - s h e l l E x c i t a t i o n 106 c. Oxygen K - s h e l l E x c i t a t i o n 112 -V- Page 6.2. Carbon D i s u l f i d e and C a r b o n y l S u l f i d e 116 6.2.1. Carbon D i s u l f i d e 116 a. V a l e n c e S h e l l Spectrum . 116 b. Carbon K - s h e l l E x c i t a t i o n 118 c. S u l f u r L J J ( 2 p ) - s h e l l E x c i t a t i o n 123 6.2.2. C a r b o n y l S u l f i d e 127 a. V a l e n c e S h e l l Spectrum 127 b. Oxygen K - s h e l l E x c i t a t i o n 129 c. Carbon K - s h e l l E x c i t a t i o n 129 d. S u l f u r L n n i ( 2 p ) - s h e l l 133 CHAPTER SEVEN P o l y a t o m i c M o l e c u l e s 136 7.1. I n t r o d u c t i o n 136 7.2. Methane, Ammonia, Water, M e t h a n o l , D i m e t h y l E t h e r and Monomethylamine 137 7.2.1. Methane 137 a. V a l e n c e S h e l l Spectrum 137 b. Carbon K - s h e l l E x c i t a t i o n 139 7.2.2. Ammonia 144 a. V a l e n c e S h e l l Spectrum 145 b. N i t r o g e n K - s h e l l E x c i t a t i o n 145 7.2.3. Water 150 a. V a l e n c e S h e l l Spectrum 150 b. Oxygen K - s h e l l E x c i t a t i o n 150 7.2.4. Methanol 155 a. V a l e n c e S h e l l Spectrum 155 b. Carbon K - s h e l l E x c i t a t i o n 155 c. Oxygen K - s h e l l E x c i t a t i o n 159 - v i - Page 7.2.5. Dimethyl E t h e r 161 a. V a l e n c e S h e l l Spectrum 162 b. Carbon K - s h e l l E x c i t a t i o n 162 c. Oxygen K - s h e l l E x c i t a t i o n 166 7.2.6. Monomethylamine 166 a. V a l e n c e S h e l l Spectrum 166 b. Carbon K - s h e l l E x c i t a t i o n 169 c. N i t r o g e n K - s h e l l E x c i t a t i o n 169 7.2.7. Term V a l u e s 173 7.3. Carbon T e t r a f l u o r i d e 173 a. V a l e n c e S h e l l Spectrum 175 b. Carbon K - s h e l l E x c i t a t i o n 178 c. F l u o r i n e K - s h e l l E x c i t a t i o n 182 7.4. Carbon K - s h e l l Energy L o s s Spectrum o f Acetone 185 7.5. E s t i m a t i o n o f t h e E x c i t a t i o n and I o n i z a t i o n E n e r g i e s o f NH^, H3O and HoF R a d i c a l s u s i n g Core A n a l o g i e s a p p l i e d t o K-shelT E l e c t r o n Energy Loss S p e c t r a 189 CHAPTER EIGHT C o n c l u s i o n 196 REFERENCES 197 - v i i - LIST OF FIGURES F i g u r e Page 1 E l e c t r i c f i e l d , E ( t ) , and c o r r e s p o n d i n g f r e q u e n c y s p e c t r u m , I ( v ) , a s s o c i a t e d w i t h a d i s t a n t c o l l i s i o n o f a f a s t e l e c t r o n and m o l e c u l a r t a r g e t , a. C o l l i s i o n p a r a m e t e r s ; v, e l e c t r o n v e l o c i t y and b, impact p a r a m e t e r s , c. and d, r e a l i s t i c p i c t u r e 7 o 2 R e s o l u t i o n , A X (A), p l o t t e d a g a i n s t energy f o r f i x e d v a l u e s o f r e s o l u t i o n , AE (0.01 t o 0.05) 29 3 S c h e m a t i c diagram o f a h e m i s p h e r i c a l e l e c t r o n e nergy a n a l y s e r 31 4 R e s o l u t i o n , AE (FWHM), V S . e l e c t r o n e nergy f o r t h e 180° e l e c t r o n energy a n a l y s e r : • o b s e r v e d ( c o n v o l u t i o n o f gun and a n a l y s e r s p r e a d s ) , • a n a l y s e r o n l y (gun s p r e a d s u b t r a c t e d ) 35 5 E l e c t r o n gun power s u p p l y 36 6 S c h e m a t i c diagram o f t h e a p p a r a t u s 39 7 Energy c a l i b r a t i o n o f K - s h e l l s p e c t r a ; a. ammonia c a l i b r a t e d u s i n g m o l e c u l a r n i t r o g e n (400.93 eV p e a k ) , b. methane c a l i b r a t e d u s i n g c a r b o n d i o x i d e (290.7 eV p e a k ) . . 43 8 V a l e n c e s h e l l energy l o s s s p e c t r u m o f m o l e c u l a r n i t r o g e n ... 47 9. K - s h e l l energy l o s s s p e c t r u m o f m o l e c u l a r n i t r o g e n 51 10 Comparison o f the r e l a t i v e e n e r g i e s o f v a l e n c e e x c i t e d s t a t e s o f n i t r i c o x i d e and K - s h e l l e x c i t e d s t a t e s o f n i t r o g e n and car b o n monoxide ( c a r b o n K) 53 11 Comparison o f t h e K - s h e l l energy l o s s s p e c t r a o f m o l e c u l a r n i t r o g e n o b t a i n e d u s i n g e l e c t r o n impact and s y n c h r o t r o n r a d i a t i o n 55 12 V a l e n c e s h e l l energy l o s s s p e c t r u m o f car b o n monoxide 61 13 Carbon K - s h e l l energy l o s s s p e c t r u m o f c a r b o n monoxide 62 14 Oxygen K - s h e l l energy l o s s s p e c t r u m o f c a r b o n monoxide. I n s e r t a ( t a k e n from a s e p a r a t e d a t a run) shows t h e t h r e e h i g h e r d i s c r e t e peaks on an expanded s c a l e 67 - v i i i - F i g u r e Page 15 N i t r o g e n K - s h e l l energy l o s s s p e c t r u m o f n i t r i c o x i d e 72 16 Comparison o f t h e r e l a t i v e e n e r g i e s o f : (a) v a l e n c e 0 ? s t a t e s ( e x p e r i m e n t a l ) and N K * s t a t e s ( t h e o r e t i c a l ) ; (b) v a l e n c e NF s t a t e s ( e x p e r i m e n t a l ) and NO^* s t a t e s ( t h e o r e t i c a l ) . N^ +0 and N0^ + s p l i t t i n g s a r e f r o m X-ray PES d a t a 76 17 Oxygen K - s h e l l energy l o s s s p e c t r u m o f n i t r i c o x i d e 81 18 V a l e n c e s h e l l energy l o s s s p e c t r u m o f m o l e c u l a r oxygen 85 19 K - s h e l l energy l o s s s p e c t r u m o f m o l e c u l a r oxygen 87 20 V a l e n c e s h e l l energy l o s s s p e c t r u m o f c a r b o n d i o x i d e 91 21 Q u a l i t a t i v e r e p r e s e n t a t i o n ( n o t t o s c a l e ) o f t h e p o t e n t i a l energy s u r f a c e s , as a f u n c t i o n o f t h e bending c o o r d i n a t e , o f some s t a t e s o f n i t r o g e n d i o x i d e and K - s h e l l e x c i t e d c a r b o n d i o x i d e . Note: These i n d i c a t e t h e n a t u r e o f t h e energy c o r r e c t i o n s w h i c h would have t o be a p p l i e d i n o r d e r t o compare d a t a from t h e two m o l e c u l e s on the b a s i s o f t h e c o r e a n a l o g y model 94 22 The c a r b o n K - s h e l l e nergy l o s s s p e c t r u m o f c a r b o n d i o x i d e .. 95 23 C o r r e l a t i o n o f the o b s e r v e d peaks i n t h e K - s h e l l e n e r g y l o s s s p e c t r a o f c a r b o n d i o x i d e and n i t r o u s o x i d e ( b o t h c a r b o n and oxygen K - s h e l l s ) . The dashed l i n e s r e p r e s e n t the e x p e c t e d p o s i t i o n s o f u n r e s o l v e d peaks (see the t e x t ) . The r e l a t i v e e n e r g i e s ( c o r r e c t e d ) o f a p p r o p r i a t e s t a t e s from t h e v a l e n c e s h e l l s p e c t r u m o f n i t r o g e n d i o x i d e have a l s o been i n c l u d e d f o r c o m p a rison 98 24 The oxygen K - s h e l l energy l o s s spectrum o f c a r b o n d i o x i d e .. 103 25 V a l e n c e s h e l l energy l o s s s p e c t r u m o f n i t r o u s o x i d e 107 26 The n i t r o g e n K - s h e l l e nergy l o s s spectrum o f n i t r o u s o x i d e . 108 27 The oxygen K - s h e l l e n e r g y l o s s s p e c t r u m o f n i t r o u s o x i d e ... 113 28 V a l e n c e s h e l l energy l o s s s p e c t r u m o f c a r b o n d i s u l f i d e 117 29 Carbon K - s h e l l energy l o s s s p e c t r u m o f c a r b o n d i s u l f i d e 119 30 S u l f u r L j j j j j ( 2 p ) energy l o s s s p e c t r u m o f c a r b o n d i s u l f i d e 124 - i x- F i g u r e Page 31 V a l e n c e s h e l l energy l o s s s p e c t r u m o f c a r b o n y l s u l f i d e 128 32 Oxygen K - s h e l l energy l o s s s p e c t r u m o f c a r b o n y l s u l f i d e 130 33 Carbon K - s h e l l e nergy l o s s s p e c t r u m o f c a r b o n y l s u l f i d e 131 34 S u l f u r L J J m ( 2 p ) e n e r 9 v l ° s s s p e c t r u m o f c a r b o n y l s u l f i d e 134 35 S u l f u r L J J m^p) energy l o s s s p e c t r u m o f c a r b o n y l s u l f i d e w i t h an expanded energy s c a l e i n t h e r e g i o n o f t h e L I 1 , 111 e d g e s 1 3 5 36 V a l e n c e s h e l l energy l o s s s p e c t r u m o f methane 138 37 Carbon K - s h e l l energy l o s s s p e c t r u m o f methane 140 38 V a l e n c e s h e l l energy l o s s s pectrum o f ammonia 146 39 N i t r o g e n K - s h e l l e nergy l o s s s p e c t r u m o f ammonia 147 40 V a l e n c e s h e l l e n ergy l o s s s pectrum o f w a t e r 151 41 Oxygen K - s h e l l energy l o s s s p e c t r u m o f w a t e r 152 42 V a l e n c e s h e l l energy l o s s s p e c t r u m o f methanol 156 43 Carbon K - s h e l l energy l o s s s p e c t r u m o f methanol 157 44 Oxygen K - s h e l l energy l o s s s pectrum o f methanol 160 45 V a l e n c e s h e l l energy l o s s s p e c t r u m o f d i m e t h y l e t h e r 163 46 Carbon K - s h e l l energy l o s s s p e c t r u m o f d i m e t h y l e t h e r 164 47 Oxygen K - s h e l l energy l o s s s p e c t r u m o f d i m e t h y l e t h e r 167 48 V a l e n c e s h e l l energy l o s s s p e c t r u m o f monomethylamine 168 49 Carbon K - s h e l l energy l o s s s p e c t r u m o f monomethylamine 170 50 N i t r o g e n K - s h e l l energy l o s s s pectrum o f monomethylamine ... 172 51 V a l e n c e s h e l l e l e c t r o n e nergy l o s s s p e c t r u m o f ca r b o n t e t r a f l u o r i d e 176 52 Carbon K - s h e l l energy l o s s s p e c t r u m o f ca r b o n t e t r a f l u o r i d e 179 53 F l u o r i n e K - s h e l l energy l o s s s p e c t r u m o f carbon t e t r a f l u o r i d e 183 -X- F i g u r e Page 54 Carbon K - s h e l l energy l o s s s p e c t r u m o f a c e t o n e 186 55 The c a r b o n K - s h e l l e l e c t r o n e n e r g y l o s s s p e c t r u m o f methane and c a l c u l a t e d e nergy l e v e l s o f the ammonium r a d i c a l (NH A) . 194 - x i - LIST OF PLATES P l a t e Page 1 The S p e c t r o m e t e r 38 2 Complete E x p e r i m e n t a l Arrangement 44 - x i i - LIST OF TABLES T a b l e Page 1 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and a s s i g n m e n t s o f peaks o b s e r v e d i n Region I o f t h e K - s h e l l s p e c t r a o f m o l e c u l a r n i t r o g e n and c a r b o n monoxide ( c a r b o n K - s h e l l ) 52 2 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n Region I o f t h e oxygen K - s h e l l s p e c t r u m o f c a r b o n monoxide 68 3 E l e c t r o n c o n f i g u r a t i o n s and e l e c t r o n i c s t a t e s o f K - s h e l l e x c i t e d n i t r i c o x i d e and m o l e c u l a r oxygen 70 4 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n t h e n i t r o g e n and oxygen K - s h e l l s p e c t r a o f n i t r i c o x i d e 73 5 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n the K - s h e l l s p e c t r u m o f m o l e c u l a r oxygen 88 6 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n the c a r b o n and oxygen K - s h e l l s p e c t r a o f c a r b o n d i o x i d e 96 7 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n the n i t r o g e n K - s h e l l s p e c t r u m o f n i t r o u s o x i d e 109 8 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n t h e oxygen K - s h e l l s p e c t r u m o f n i t r o u s o x i d e 114 9 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n t h e c a r b o n K - s h e l l s p e c t r u m o f c a r b o n d i s u l f i d e and t h e c a r b o n and oxygen K - s h e l l s p e c t r a o f c a r b o n y l s u l f i d e 120 10 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n the s u l f u r 2p (L J T J T J - s h e l l ) s p e c t r a o f carbon d i s u l f i d e and c a r b o n y l s u l f i d e 125 11 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n the c a r b o n K - s h e l l s pectrum o f methane 141 - x i i i - T a b l e Page 12 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f t h e peaks o b s e r v e d i n t h e n i t r o g e n K - s h e l l s p e c t r u m o f ammonia 148 13 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n t h e oxygen K - s h e l l s p e c t r u m o f w a t e r 153 14 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n t h e c a r b o n and oxygen K - s h e l l s p e c t r a o f methanol 158 15 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n the c a r b o n and oxygen K - s h e l l s p e c t r a o f d i m e t h y l e t h e r 165 16 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n the c a r b o n and n i t r o g e n K - s h e l l s p e c t r a o f monomethylamine 171 17 The 3s and 3p Rydberg term v a l u e s o b s e r v e d f o r K - s h e l l e x c i t a t i o n and v a l e n c e s h e l l e x c i t a t i o n ( o u t e r m o s t e l e c t r o n ) i n methane, ammonia, w a t e r , m e t h a n o l , d i m e t h y l e t h e r and monomethyl amine 174 18 A b s o l u t e e n e r g i e s (eV) o f peaks o b s e r v e d i n t h e v a l e n c e s h e l l s p e c t r u m o f c a r b o n t e t r a f l u o r i d e 177 19 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n the c a r b o n and f l u o r i n e K - s h e l l s p e c t r a o f c a r b o n t e t r a f l u o r i d e 180 20 A b s o l u t e e n e r g i e s ( e V ) , r e l a t i v e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks o b s e r v e d i n t h e c a r b o n K - s h e l l s p e c t r u m o f a c e t o n e 188 21 E s t i m a t e d energy l e v e l s (eV) o f t h e NH., H.,0 and h y p o t h e t i c a l H 9F r a d i c a l s 7...T 193 - x i v- ACKNOWLEDGEMENTS I would l i k e t o thank s i n c e r e l y Dr. C. E. B r i o n f o r h i s i n t e r e s t , encouragement and a s s i s t a n c e and Dr. M. J . van d e r Wi e l f o r t h e s t i m u l u s he i n j e c t e d i n t o t h e work. I a l s o acknowledge many h e l p f u l d i s c u s s i o n s w i t h Dr. A. J . Me r e r and thank him f o r h i s i n t e r e s t . In a d d i t i o n , t h e many d i s c u s s i o n s w i t h Mr. W.-C. Tam and Mr. S. Tong Lee were most h e l p f u l and a r e a p p r e c i a t e d . The c a p a b l e s t a f f o f t h e m e c h a n i c a l and e l e c t r o n i c s workshops were a tremendous a s s e t d u r i n g a l l phases o f t h i s work; i n p a r t i c u l a r , Mr. E. Gomm and Mr. J . Shim. F i n a n c i a l s u p p o r t i n t h e form o f a N a t i o n a l R e s e a r c h C o u n c i l S c i e n c e S c h o l a r s h i p i s a l s o acknowledged. F i n a l l y , from my w i f e , d a u g h t e r and m y s e l f , thank you t o a l l who have c o n t r i b u t e d t o an e n j o y a b l e s t a y i n Van c o u v e r , e s p e c i a l l y ; Ron and Bea Thompson C h r i s and E l i z a b e t h B r i o n B i l l and M a r i l y n Henderson Frank and J o y c e R o b e r t s . -1- CHAPTER ONE INTRODUCTION E l e c t r o n i m p a ct e x c i t a t i o n has been used as a s p e c t r o s c o p i c t e c h n i q u e s i n c e t h e b e g i n n i n g o f t h i s c e n t u r y . I n d eed, t h e f i r s t e x p e r i m e n t a l d e m o n s t r a t i o n o f t h e q u a n t i z a t i o n o f a t o m i c and m o l e c u l a r systems was p r o v i d e d by an energy l o s s measurement o f e l e c t r o n s , i n e l a s t i c a l l y s c a t t e r e d by mercury atoms, i n t h e c l a s s i c F r a n c k - H e r t z 1 2 e x p e r i m e n t . The e l e c t r o n energy l o s s measurements o f Rudberg i n 1930 ar e q u i t e i m p r e s s i v e i n vie w o f t h e l i m i t e d development o f e l e c t r o n o p t i c s , e l e c t r o n energy a n a l y s e r s and s i g n a l p r o c e s s i n g e l e c t r o n i c s a t t h a t t i m e . However, u n t i l t h e e a r l y 1960's t h e r e were few e x p e r i m e n t s i n e l e c t r o n impact s p e c t r o s c o p y . A t t h a t t i m e , t h e r e was a r a p i d growth i n b o t h t h e q u a n t i t y and q u a l i t y o f d a t a , l a r g e l y due t o t h e s t i m u l u s p r o v i d e d by two g r o u p s ; t h a t o f B o e r s c h , G e i g e r e t a l . and t h a t o f L a s s e t t r e e t a l . In f a c t , as e a r l y as 1966, an e l e c t r o n s p e c t r o m e t e r w i t h a r e s o l u t i o n ^ 0.010 eV was i n o p e r a t i o n . T h i s was s u f f i c i e n t t o 3 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 the e l e c t r o n e nergy l o s s s p e c t r u m f o r m o l e c u l a r hydrogen. S i n c e t h e s e measurements, t h e r e s o l u t i o n o f e l e c t r o n energy l o s s s p e c t r o m e t e r s has not been improved. However, i n terms o f q u a n t i t a t i v e measurements, t h e r e has been a c o n t i n u a l improvement i n d e t e r m i n a t i o n s o f g e n e r a l i z e d o s c i l l a t o r s t r e n g t h s and p o r t i o n s o f t h e Bethe s u r f a c e . R e c e n t l y , t h e dependence o f e x c i t a t i o n c r o s s - s e c t i o n s on impact energy ( t h e " e x c i t a t i o n f u n c t i o n " ) and s c a t t e r i n g a n g l e has been -2- used to i d e n t i f y the nature o f atomic and molecu la r t r a n s i t i o n s . In p a r t i c u l a r many e l e c t r i c d i p o l e fo rb idden t r a n s i t i o n s have been i d e n t i f i e d i n t h i s manner us ing low impact ene rg i e s . The developments and cu r ren t 4-9 s ta tus of experiments are d i scussed i n a number o f reviews . In e l e c t r o n energy l o ss spectroscopy a "monoenerget ic" beam of e l e c t - rons i s used to e x c i t e the t a r g e t s p e c i e s . E x c i t a t i o n s are detected as energy losses i n the s ca t t e r ed e l e c t r o n beam. The process may be r e p r e s - ented as f o l l o w s ; X + e -> X* + e * where X i s the t a rge t s p e c i e s , e i s the " c o l l i d i n g " e l e c t r o n and X i s the d i s c r e t e s t a t e of the t a rge t which i s e x c i t e d by the c o l l i s i o n . D i s c r e t e e l e c t r o n energy losses occur f o r every a c c e s s i b l e s t a t e of the t a r g e t . The re fo re , e l e c t r o n energy l o ss spectroscopy i s an a l t e r n a t i v e to the use of photoabsorpt ion f o r i n v e s t i g a t i n g the e x c i t e d s t a t e s of atoms and mo lecu l es . In a d d i t i o n , the s ca t t e r ed i n t e n s i t i e s observed f o r f a s t e l e c t r o n impact and small angle s c a t t e r i n g may be q u a n t i t a t i v e l y r e l a t e d to o p t i c a l o s c i l l a t o r s t r e n g t h s ^ ' ^ . Under these c o n d i t i o n s the impinging e l e c t r o n s imulates a v i r t u a l photon f i e l d and e l e c t r i c d i p o l e t r a n s i t i o n s are dominant. D ipo le s e l e c t i o n r u l e s do not apply f o r low impact e n e r g i e s , p a r t i c u l a r l y a t l a rge s c a t t e r i n g a n g l e s , and magnetic d i p o l e , e l e c t r i c quadrupole and sp in fo rb idden processes may be observed. E l e c t r on impact e x c i t a t i o n i s p a r t i c u l a r l y use fu l at shor t wavelengths (high e x c i t a t i o n energ ies ) where usefu l photon cont inuua are d i f f i c u l t to produce. This t h e s i s desc r ibes the a p p l i c a t i o n of f a s t e l e c t r o n impact to a study of the high energy d i s c r e t e s t a t e s i n the reg ions of inner s h e l l e x c i t a t i o n s of small mo lecu les . These high energy s t a tes r e s u l t from the -3- p r o m o t i o n o f an i n n e r s h e l l e l e c t r o n , f o r example a I s (K) e l e c t r o n (which f o r most m o l e c u l e s i s nonbonding and m a i n l y a t o m i c i n c h a r a c t e r ) t o u n f i l l e d m o l e c u l a r o r b i t a l s and Rydberg o r b i t a l s o f t h e m o l e c u l e . L i t t l e i n f o r m a t i o n i s a v a i l a b l e about t h e s e h i g h energy s t a t e s . To e x c i t e a m o l e c u l e t o a d i s c r e t e s t a t e by p h o t o a b s o r p t i o n , t h e photon energy must be e x a c t l y equal t o the energy r e q u i r e d f o r t h e t r a n s - i t i o n . In t h e e n e r g y r e g i o n f o r K - s h e l l e x c i t a t i o n , two continuum l i g h t s o u r c e s e x i s t : ( i ) B r e m s s t r a h l u n g and ( i i ) e l e c t r o n s y n c h r o t r o n r a d i a t i o n . B r e m s s t r a h l u n g c o n t i n u a a r e d i f f i c u l t s o u r c e s f o r p h o t o a b s o r p t i o n s t u d i e s because t h e y a r e u s u a l l y weak. An e l e c t r o n s y n c h r o t r o n produces a u s e f u l c o n t i n u u m , but i t i s a v e r y e x p e n s i v e f a c i l i t y and o f l i m i t e d a v a i l a b i l i t y . K - s h e l l a b s o r p t i o n s were f i r s t o b s e r v e d f o r m o l e c u l a r n i t r o g e n u s i n g Brems- 12 13 s t r a h l u n g c o n t i n u u a ' . S i n c e t h e n , B r e m s s t r a h l u n g c o n t i n u u a have been used t o o b t a i n a number o f i n n e r s h e l l a b s o r p t i o n s p e c t r a , a l t h o u g h t h e r e s u l t s a r e m a i n l y i n t h e r e g i o n s o f s u l f u r and f l u o r i n e i n n e r s h e l l e d g e s " ^ " ^ . A l t h o u g h s y n c h r o t r o n r a d i a t i o n has been a v a i l a b l e f o r some 27 2 t i m e , o n l y K - s h e l l a b s o r p t i o n s p e c t r a f o r m o l e c u l a r n i t r o g e n and methane have been r e p o r t e d . R e c e n t l y , e l e c t r o n i m p act has been used f o r t h e e x c i t a t i o n o f i n n e r s h e l l e l e c t r o n s . Low r e s o l u t i o n , K - s h e l l e l e c t r o n energy l o s s s p e c t r a o f n i t r o g e n and carbon monoxide have been measured a t an i m p act energy o f 29 10 keV and f u r t h e r measurements have been made i n c o i n c i d e n c e w i t h 30 s p e c i f i c i o n p r o d u c t s . A l s o , the K - s h e l l energy l o s s s p e c t r a o f some n u c l e i c a c i d bases have been o b s e r v e d by p a s s i n g 25 keV e l e c t r o n s t h r o u g h 31 t h i n s o l i d samples . F u r t h e r e v i d e n c e on t h e d i s c r e t e e x c i t a t i o n o f i n n e r s h e l l e l e c t r o n s has been p r o v i d e d by the o c c u r r e n c e o f h i g h energy a u t o i o n i z a t i o n l i n e s i n -4- Auger e l e c t r o n s p e c t r a e x c i t e d by ( i ) e l e c t r o n i m p a ct , and, ( i i ) a c a r e f u l l y s e l e c t e d X-ray l i n e . High energy c o n t r i b u t i o n s t o t h e K a 37-39 e m i s s i o n s p e c t r a o f n i t r o g e n and (as p o i n t e d o u t by Siegbahn ) i n 40 car b o n monoxide , have been a t t r i b u t e d t o reso n a n c e e m i s s i o n from n e u t r a l s t a t e s . The d a t a p r e s e n t l y a v a i l a b l e on t h e d i s c r e t e e x c i t a t i o n o f i n n e r s h e l l 27 e l e c t r o n s i s e x t r e m e l y l i m i t e d and i n c o m p l e t e . M o l e c u l a r n i t r o g e n and 28 methane a r e t h e o n l y common gases f o r w h i c h r e a s o n a b l e K - s h e l l a b s o r p t i o n s p e c t r a have been o b t a i n e d . The o b j e c t o f t h i s r e s e a r c h i s t o o b t a i n and i n t e r p r e t i n n e r s h e l l " a b s o r p t i o n " s p e c t r a f o r a v a r i e t y o f gaseous m o l e c u l e s u s i n g f a s t e l e c t r o n i m p a c t . -5- CHAPTER TWO THEORY OF FAST ELECTRON IMPACT. 2.1. E l e c t r o n Energy Loss S p e c t r o s c o p y . In e l e c t r o n energy l o s s s p e c t r o s c o p y a beam o f "m o n o e n e r g e t i c " e l e c t r o n s i s used t o e x c i t e d i s c r e t e s t a t e s o f an a t o m i c o r m o l e c u l a r t a r g e t . E x c i t a t i o n s a r e d e t e c t e d as e l e c t r o n energy l o s s e s i n t h e s c a t t e r e d e l e c t r o n beam. The p r o c e s s may be r e p r e s e n t e d as f o l l o w s ; X + e ( E Q ) -+ X * ( E n ) + e ( E r e ) where X i s t h e t a r g e t atom o r m o l e c u l e i n i t s ground e l e c t r o n i c s t a t e , E Q * t h i s t h e k i n e t i c e n ergy o f t h e i n c i d e n t e l e c t r o n beam, X i s t h e n e x c i t e d s t a t e o f t h e t a r g e t which has energy E n w i t h r e s p e c t t o t h e ground s t a t e and E-| i s t h e k i n e t i c energy o f an e l e c t r o n w h i c h i s i n e l a s t i c a l l y s c a t t e r e d t h r o u g h a n g l e e ( w i t h r e s p e c t t o t h e i n c i d e n t beam) i n a c o l l i s i o n i n which the t a r g e t i s e x c i t e d t o t h e n^'1 s t a t e . The energy e q u a t i o n i s g i v e n by; E = E, + E + E. o 1 n t where E^ i s t h e k i n e t i c energy o f t h e p r o j e c t i l e e l e c t r o n w h i c h i s t r a n s - f e r r e d t o t r a n s l a t i o n a l e n ergy o f the t a r g e t d u r i n g t h e c o l l i s i o n . U s i n g 41 th e c o n s e r v a t i o n o f energy and momentum, i t has been shown t h a t E t ^ (2mE o/M) |^1 - ( E n / 2 E 0 ) - { l - ( E n / E Q ) } 1 / 2 c o s e j (2.1.1) where m and M a r e t h e masses o f the i n c i d e n t e l e c t r o n and t a r g e t m o l e c u l e r e s p e c t i v e l y . Because o f t h e l a r g e mass d i s p a r i t y between t h e e l e c t r o n and t a r g e t , t h i s term i s v e r y s m a l l and may be n e g l e c t e d . F o r example, _3 Ef °» 10 eV f o r t h e pr o m o t i o n o f a n i t r o g e n K - s h e l l e l e c t r o n ( E n ^ 400 eV) - 6 - w i t h e x p e r i m e n t a l c o n d i t i o n s used f o r t h i s s t u d y (E = 2500 eV and 6 ^ 0 ) . T h e r e f o r e , t h e e l e c t r o n energy l o s s , E - E-|, i s e q u a l t o t h e energy r e q u i r e d t o e x c i t e t h e n e x c i t e d s t a t e o f t h e t a r g e t , E^. A measurement o f t h e d i s c r e t e energy l o s s e s o f t h e s c a t t e r e d e l e c t r o n s produces t h e e l e c t r o n energy l o s s spectrum w h i c h g i v e s t h e energy a b s o r p t i o n s p e c t r u m o f t h e t a r g e t . E x p e r i m e n t a l l y , t h e magnitude o f t h e s c a t t e r e d e l e c t r o n c u r r e n t measured a t a g i v e n e nergy l o s s , E - E-j = E p and s c a t t e r i n g a n g l e e, i s p r o p o r t i o n a l t o t h e d i f f e r e n t i a l c r o s s - s e c t i o n , da °" (E„,e) da v o' f o r t h e e x c i t a t i o n o f t h e n s t a t e o f t h e t a r g e t . I t has a l r e a d y been s t a t e d t h a t f o r h i g h impact e n e r g i e s and s m a l l s c a t t e r i n g a n g l e s , t h e r e i s a q u a n t i t a t i v e r e l a t i o n s h i p between e l e c t r o n impact c r o s s - s e c t i o n s and o p t i c a l d a t a ^ ' ^ . The f o l l o w i n g s e m i c l a s s i c a l t r e a t m e n t shows why a r e l a t i o n s h i p s h o u l d e x i s t . 2.2. The V i r t u a l Photon Model In f a s t charged p a r t i c l e i m p a c t , e x c i t a t i o n s a r e g e n e r a l l y produced by d i s t a n t ( o r g l a n c i n g ) c o l l i s i o n s i n which t h e impact p a r a m e t e r , b, i s l a r g e r than t h e d i m e n s i o n s o f t h e a t o m i c o r m o l e c u l a r t a r g e t [ s e e F i g u r e 1 ( a ) ] . The e l e c t r i c f i e l d e x p e r i e n c e d by t h e t a r g e t i n a d i s t a n t c o l l i s i o n w i t h a f a s t e l e c t r o n ( o r any s t r u c t u r e l e s s c h a r g e d p a r t i c l e ) i s s h a r p l y p u l s e d i n time and u n i f o r m i n space. The f r e q u e n c y components o f t h i s i m p u l s i v e f i e l d may be o b t a i n e d from t h e f o u r i e r t r a n s f o r m r e l a t i o n s h i p : E(t) = / l ( w ) e i a , t du,; I (a,) = / E ( t ) e _ 1 u t dt FIGURE 1. E l e c t r i c f i e l d , E ( t ) , and c o r r e s p o n d i n g f r e q u e n c y s p e c t r u m , I ( v ) , a s s o c i a t e d w i t h a d i s t a n t c o l l i s i o n o f a f a s t e l e c t r o n and m o l e c u l a r t a r g e t . a. C o l l i s i o n p a r a m e t e r s ; v, e l e c t r o n v e l o c i t y and b, impact p a r a m e t e r s . b. I d e a l i z e d case f o r a v e r y f a s t e l e c t r o n . c. and d. r e a l i s t i c p i c t u r e . -8- where E ( t ) i s t h e t i m e dependence o f t h e e l e c t r i c f i e l d and I(co), where co = 2TTV, i s 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 f r e q u e n c y components o f t h e e l e c t r i c f i e l d . The f a s t e r t h e i n c i d e n t p r o j e c t i l e , t h e more E ( t ) r e s e m b l e s a d e l t a f u n c t i o n and i n t h i s h y p o t h e t i c a l l i m i t t h e f o u r i e r t r a n s f o r m o f t h e e l e c t r i c f i e l d , I(co), has eq u a l c o e f f i c i e n t s a t a l l f r e q u e n c i e s [see F i g u r e 1 ( b ) ] . T h e r e f o r e t h e e l e c t r i c f i e l d e x p e r i e n c e d by t h e t a r g e t i n a d i s t a n t c o l l i s i o n w i t h a f a s t e l e c t r o n i s s i m i l a r t o t h e e l e c t r i c f i e l d a s s o c i a t e d w i t h a beam o f w h i t e l i g h t . The i n t e r a c t i o n o f t h e e l e c t r o n and t a r g e t may be viewed as t h e c r e a t i o n o f a v i r t u a l photon f i e l d . In p r a c t i c e , t h e e l e c t r i c f i e l d p u l s e has a f i n i t e w i d t h and t h e r e f o r e I (co) i s not c o n s t a n t o v e r t h e e n t i r e range o f f r e q u e n c i e s . T h i s problem 42 has been c o n s i d e r e d by C h r i s t o p h o r o u . The r e s u l t s a r e i l l u s t r a t e d i n F i g u r e 1 ( c ) and 1(d) where t h e components o f t h e e l e c t r i c f i e l d i n t e n s i t i e s as a f u n c t i o n o f ti m e i n u n i t s o f b/v and t h e f r e q u e n c y s p e c t r a I ( v ) have been p l o t t e d . F i g u r e 1 ( c ) shows t h a t t h e i n t e n s i t y p e r p e n d i c u l a r t o t h e d i r e c t i o n o f i n c i d e n c e , I ^ ( v ) , i s a c o n s t a n t o v e r a l a r g e range o f f r e q u e n c i e s ( o r e n e r g i e s s i n c e energy = hv where h i s P l a n c k ' s c o n s t a n t ) s t a r t i n g a t z e r o f r e q u e n c y and t h e n d e c l i n e s s h a r p l y as v approaches t h e " c u t - o f f " f r e q u e n c y , v/b. F i g u r e 1(d) shows t h a t t h e p a r a l l e l component, I | l (v) i s much l e s s i n t e n s e and has a peaked i n t e n s i t y d i s t r i b u t i o n . T h e r e f o r e , I ( v ) ^ I ^ ( v ) and i s e s s e n t i a l l y c o n s t a n t o v e r the range o f f r e q u e n c i e s n o r m a l l y i n v o l v e d i n t h e e x c i t a t i o n o f atoms and m o l e c u l e s . The w h i t e l i g h t a n a l o g y i s s t i l l m a i n t a i n e d . 42 I t has been shown t h a t t h e number o f v i r t u a l photons (Nco) a t f r e q u e n c y co, i s a p p r o x i m a t e l y i n v e r s e l y p r o p o r t i o n a l t o t h e e n e r g y ; N ^ c o n s t , (l/fico) . CO -9- The number o f e l e c t r o n i c t r a n s i t i o n s , N n , i n d u c e d by t h e s e v i r t u a l photons i s p r o p o r t i o n a l t o t h e number o f photons w i t h e nergy e q u a l t o t h e t r a n s i t i o n e n e r g y , E n , and t h e o p t i c a l o s c i l l a t o r s t r e n g t h , f , o f t h e t r a n s i t i o n . T h e r e f o r e i n t h i s " o p t i c a l a p p r o x i m a t i o n " ; N n * N u - f n * c o n s t . ( f n / E n ) (2.2.1) For f a s t e l e c t r o n impact and l a r g e o s c i l l a t o r s t r e n g t h t r a n s i t i o n s ( i . e . e l e c t r i c d i p o l e a l l o w e d ) , t h e o p t i c a l a p p r o x i m a t i o n g i v e s a r e a s o n a b l e 42 e s t i m a t e f o r t h e number o f p r i m a r y e x c i t a t i o n s . One i m p l i c a t i o n o f e q u a t i o n (2.2.1) i s t h a t v a l e n c e s h e l l e l e c t r o n e x c i t a t i o n p r e d o m i n a t e s o v e r i n n e r s h e l l e l e c t r o n e x c i t a t i o n s i n c e E i s much l a r g e r f o r t h e l a t t e r . n 3 F o r a q u a n t i t a t i v e r e l a t i o n s h i p between o p t i c a l o s c i l l a t o r s t r e n g t h s and f a s t e l e c t r o n impact c r o s s - s e c t i o n d a t a , a quantum m e c h a n i c a l t r e a t m e n t i s r e q u i r e d . In t h e d e r i v a t i o n s w h i c h f o l l o w , i t w i l l be shown t h a t such a r e l a t i o n s h i p does e x i s t and moreover t h a t t h e o p t i c a l a p p r o x i m a t i o n i s i n a p p r o x i m a t e agreement w i t h t h e quantum r e s u l t s . 2.3. The F i r s t Born A p p r o x i m a t i o n . A t h e o r e t i c a l d e s c r i p t i o n o f f a s t e l e c t r o n i m p a ct e x c i t a t i o n was 43 i n i t i a l l y d e r i v e d by Bethe i n t h e 1930's. A r e c e n t r e v i e w w h i c h g i v e s more p h y s i c a l i n s i g h t i n t o t h e Bethe t h e o r y has been w r i t t e n by I n o k u t i ^ . The b a s i s o f t h e Bethe t h e o r y i s t h e f i r s t Born a p p r o x i m a t i o n w h i c h assumes t h a t t h e i n t e r a c t i o n between t h e e l e c t r o n and t h e t a r g e t i s weak and t h e r e f o r e t h e i n c i d e n t wave i s n e g l i g i b l y d i s t o r t e d by t h e i n t e r a c t i o n . The c r i t e r i o n f o r t h e v a l i d i t y o f t h i s a p p r o x i m a t i o n i s somewhat a r b i t r a r y , but g e n e r a l l y t h e f i r s t Born a p p r o x i m a t i o n i s assumed t o be v a l i d i f t h e k i n e t i c e n e r g y o f t h e i n c i d e n t e l e c t r o n i s some 5-7 t i m e s t h e e x c i t a t i o n e nergy o f a p a r t i c u l a r -10- t r a n s i t i o n and.the s c a t t e r i n g a n g l e i s s m a l l . A l t e r n a t i v e l y , t h e momentum whi c h i s t r a n s f e r r e d i n t h e c o l l i s i o n s h o u l d be s m a l l . T h i s c o n d i t i o n i s a l s o s a t i s f i e d by f a s t e l e c t r o n i m p a ct and s m a l l a n g l e s c a t t e r i n g . In o r d e r t o i l l u s t r a t e t he f i r s t Born a p p r o x i m a t i o n , t h e s i m p l e s t e l e c t r o n - a t o m s c a t t e r i n g problem w i l l be c o n s i d e r e d . The r e s u l t s w i l l t h e n be g e n e r a l - i z e d t o more complex atom and m o l e c u l e s c a t t e r i n g . 2.4. E l e c t r o n - H y d r o g e n Atom S c a t t e r i n g . Even t h i s i s a t h r e e body problem and a p p r o x i m a t i o n s a r e n e c e s s a r y . 44 The f o l l o w i n g d e r i v a t i o n i s based on t h a t g i v e n by Massey and Burhop . 45 The t r e a t m e n t by M o i s e i w i t s c h and Smith i s a l s o i n f o r m a t i v e . The S c h r b d i n g e r e q u a t i o n f o r t h e system i s v  2 + v 2 + ^ v l v 2 + ^ 2 2 2 2 E + — + - — r l r 2 r 1 2 * ( r r r 2 ) = 0 (2.4.1) where s u b s c r i p t s 1 and 2 a r e a s s o c i a t e d w i t h t h e i n c i d e n t and a t o m i c e l e c t r o n r e s p e c t i v e l y , E = E Q + E t i s t h e t o t a l energy o f t h e s y s t e m , where E Q i s t h e energy o f t h e ground s t a t e o f t h e hydrogen atom and E^ i s t h e k i n e t i c energy o f t h e i n c i d e n t e l e c t r o n , r-j and a r e r e s p e c t i v e l y t h e p o s i t i o n v e c t o r s o f t h e i n c i d e n t and ato m i c e l e c t r o n s w i t h r e s p e c t t o t h e n u c l e u s ( e s s e n t i a l l y t h e c e n t r e o f m a s s ) , r - ^ i s t h e i n t e r e l e c t r o n d i s t a n c e and ^ ( r - j , r^) i s a w a v e f u n c t i o n d e s c r i b i n g t h e two e l e c t r o n s . The t o t a l w a v e f u n c t i o n 'F(r-j, rv,) may be e x p r e s s e d i n t h e f o r m , H r v r2) - e ^ o ' H . ^ ) + ^ where the f i r s t term on t h e RHS o f e q u a t i o n (2.4.2) r e p r e s e n t s t h e wave- f u n c t i o n i n t h e absence o f any i n t e r a c t i o n ( i . e . an a s y m p t o t i c s o l u t i o n o f -n- e q u a t i o n ( 2 . 4 . 1 ) , r-j -* °°) and c o n s i s t s o f t h e p r o d u c t o f an i n c i d e n t p l a n e wave d e s c r i b i n g t h e i n c i d e n t e l e c t r o n and t h e e l e c t r o n i c w a v e f u n c t i o n o f t h e ground s t a t e o f t h e hydrogen atom, ^ 0 ( r 2 ) . ^he w a v e n u m b e r k Q i s g i v e n by - 2mEt/^2 . The second term on t h e RHS o f e q u a t i o n (2.4.2) r e p r e s e n t s terms i n t r o d u c e d by t h e i n t e r a c t i o n . The w a v e f u n c t i o n <f>(r-|, r 2 ) may be expanded i n terms o f a complete s e t of s t a t e s o f t h e hydrogen atom, (r 2) (2.4.3) where t h e sum and i n t e g r a l a r e o v e r t h e d i s c r e t e and continuum s t a t e s r e s p e c t i v e l y . S u b s t i t u t i o n i n t o ( 2 . 4 . 2 ) , t h e n i n t o (2.4.1) and u s i n g t h e f a c t t h a t v 2 + 2m 2 ^ 2 (•  * 0 * n(r 2) 0 (2.4.4) i m p ! i e s : 2 > / n J " l 2 + kn2> F n < V * „ < ? 2 ) 2meJ f i *o < f y  + (2.4.5) where k n (2m/f^) ( E t - E n + E Q ) .th (2.4.6) and E n i s the energy o f t h e n a t o m i c s t a t e . -12- M u l t i p l y i n g b o t h s i d e s o f (2 .4 .5 ) by ^ n * ( ^ 2 ^ ' i n t e g r a t i n g o v e r r 2 and u s i n g t h e o r t h o g a n a l i t y o f t h e a t o m i c f u n c t i o n s , fi>n* ( r 2 ) ̂ - ( f 2 ) d r 2 = 0 , (n f i ) g i v e s ; <*1 2 + k n 2 ) W = + ( ? • + / ) U n m ^ l ) F m ( V (2 .4 .7 ) where U 2meJ nm ^2 E q u a t i o n (2 .4 .7 ) r e p r e s e n t s an i n f i n i t e s e t o f c o u p l e d d i f f e r e n t i a l e q u a t i o n s and a p p r o x i m a t i o n s must be used. I n t h e f i r s t Born a p p r o x i m a t i o n , t h e i n t e r a c t i o n i s assumed t o be weak and t h e r e f o r e t h e s c a t t e r i n g a m p l i t u d e s , F m ( r . j ) , a r e s m a l l . T h e r e f o r e , on t h e RHS o f e q u a t i o n (2 .4 .7 ) we n e g l e c t t h e terms i n U 'F > m f 0 , s i n c e they a r e s m a l l i n co m p a r i s o n w i t h t h e f i r s t term w h i c h i n v o l v e s t h e i n c i d e n t wave. Hence t h e ess e n c e o f t h e f i r s t Born a p p r o x i m a t i o n i s t h a t t h e i n c i d e n t wave i s n e g l i g i b l y d i s t o r t e d by t h e weak i n t e r a c t i o n . We the n must s o l v e ",2 •"„2>'„ff,> - ( 2- 4' 9 ) and we r e q u i r e an a s y m p t o t i c s o l u t i o n o f t h e form Fn (V = r _ 1 V 8 ' * ) (2 .4 .10 ) -13- which i s an o u t g o i n g s p h e r i c a l wave, f (8,<|>) i s t h e s c a t t e r i n g a m p l i t u d e c o r r e s p o n d i n g t o t h e e x c i t a t i o n o f t h e n s t a t e o f t h e hydrogen atom where t h e e l e c t r o n i s s c a t t e r e d a t p o l a r a n g l e s e and $ w i t h r e s p e c t t o th e d i r e c t i o n o f i n c i d e n c e . The d i f f e r e n t i a l c r o s s - s e c t i o n f o r t h e e x c i t a t i o n i s g i v e n by t h e r a t i o o f t h e s c a t t e r e d t o the i n c i d e n t f l u x ; ^ ~ 1 ( e , c O ) = I o n(e,4>) = r I f n ( 8 » * ) I2 (2A.U) o where the f a c t o r k n / k Q i s i n t h e r a t i o o f t h e s c a t t e r e d t o i n c i d e n t v e l o c i t y (V = -hk/m). To d e t e r m i n e f (e,<|>) a s o l u t i o n o f (2 .4 .9 ) i s r e q u i r e d such t h a t t h e a s y m p t o t i c form o f F n ( r - j ) i s g i v e n by ( 2 . 4 . 1 0 ) . T h i s may be done u s i n g t h e 45 46 method o f Green's F u n c t i o n and g i v e s ' f n ( e . + ) = - ( 4 7 T ) - 1 j U Q n e i ( * o " * n ) - r i d + (2 .4 .12) S u b s t i t u t i o n o f (2 .4 .12 ) i n t o (2 .4 .11) g i v e s t h e f o l l o w i n g e x p r e s s i o n f o r t h e d i f f e r e n t i a l c r o s s - s e c t i o n , I o n ( e , * ) = ( ^ ) " Z ^ / U o n ^ l ) e H t ° ' t n ) ' " ] d ^ l 2 (2 .4 .13 ) -> I t i s c o n v e n i e n t t o i n t r o d u c e t h e momentum t r a n s f e r v a r i a b l e - U K where, -tfK = - ^ k n , the momentum t r a n s f e r i n t h e c o l l i s i o n and "fik and f i k a r e t h e momenta o f o n t h e i n c i d e n t and s c a t t e r e d e l e c t r o n r e s p e c t i v e l y . The magnitude o f K i s g i v e n by, K 2 = k ^ 2 + k „ 2 " 2 k k cose (2 .4 .14 ) o n o n x ' where e i s t h e s c a t t e r i n g a n g l e . -14- The d i f f e r e n t i a l c r o s s - s e c t i o n (2.4.13) i s then I o n ( e , 4 . ) = ^ r 2 M U o n ( f l ) e l i ' l d ^ l I 2 ( 2- 4- 1 5^ o J The t o t a l c r o s s - s e c t i o n , Q Q n, may be o b t a i n e d by i n t e g r a t i n g (2.4.15). Q o n = ffl orS*'^ sined0d<i), 2.5 G e n e r a l i z a t i o n t o S c a t t e r i n g by Complex Atoms. I f t h e i n t e r a c t i o n between t h e p r o j e c t i l e and t h e atom i s C o u l o m b i c , then t he i n t e r a c t i o n p o t e n t i a l i s V = - e 2 ̂  ( r - ^ r 1 + ^ (2.5.1) where r g i s the p o s i t i o n v e c t o r o f t h e s*'1 a t o m i c e l e c t r o n , r i s t h e p o s i t i o n v e c t o r o f t h e i n c i d e n t e l e c t r o n w i t h r e s p e c t t o t h e n u c l e u s ( e s s e n t i a l l y t h e c e n t r e o f m a s s ) , i s t h e n u c l e a r c h a r g e and t h e sum extends o v e r a l l N a t o m i c e l e c t r o n s . The d i f f e r e n t i a l c r o s s - s e c t i o n i s then g i v e n by (2.4.15) w i t h U ( r ^ ) g i v e n by "on + t ^ i + h r ^ d f N (2.5.2) The e x p r e s s i o n f o r t h e d i f f e r e n t i a l c r o s s - s e c t i o n may be s i m p l i f i e d by i n t e g r a t i n g o v e r t h e c o o r d i n a t e s o f t h e i n c i d e n t e l e c t r o n ( r ) u s i n g t h e 43 r e l a t i o n ( B e t h e 1 s i n t e g r a l ) , f - r s I"1 e i K ' ? d r = 4rr K" 2 e i K ^ s (2.5.3) -15- I f t h e a t o m i c w a v e f u n c t i o n s a r e o r t h o g o n a l , t h e n u c l e a r term does n o t c o n t r i b u t e t o t h e d i f f e r e n t i a l c r o s s - s e c t i o n . The n u c l e a r term w i l l t h e r e f o r e be o m i t t e d i n t h e f o l l o w i n g d i s c u s s i o n and i f t h e w a v e f u n c t i o n s a r e not o r t h o g o n a l , i t i s a t r i v i a l t a s k t o add t h i s term. The d i f f e r e n t i a l c r o s s - s e c t i o n t h e n becomes i-(9,*> = rvfe1 £/"•»* i, ei""s *°dT",2 <2-5-4) 4TT TI 0 K J 5=1 where d x ^ i n d i c a t e s i n t e g r a t i o n o v e r a l l t h e c o o r d i n a t e s o f t h e N a t o m i c e l e c t r o n s . I t i s c o n v e n i e n t t o e x p r e s s ( 2 . 5 . 4 ) i n t h e f o l l o w i n g f o r m , 'on <e-*> ' W K " 4 K/V I 2 (2-5.5) where t h e m a t r i x e l e m e n t s , e o n ( K ) > a r e g i v e n by 1 . i K - f c /v E - n ( K ) = / * „ * X * S K d x N ( 2 . 5 . 6 ) For most e x c i t a t i o n s t h e d i f f e r e n t i a l c r o s s - s e c t i o n i s o n l y a f u n c t i o n o f e [ i . e . I C©»<t>) = I o n ( e ) l b e c a u s e 1 1 ; e i t h e r t h e i n i t i a l s t a t e tyQ i s s p a t i a l l y s ymmetric o r t h e t a r g e t atoms a r e randomly o r i e n t e d . Under t h e s e c o n d i t i o n s l e o n ( K ) | i s o n l y a f u n c t i o n o f |K|. From an e x p e r i m e n t a l v i e w , i t i s more c o n v e n i e n t t o e x p r e s s t h e d i f f e r e n t i a l c r o s s - s e c t i o n as a f u n c t i o n o f K r a t h e r t h a n e. D i f f e r e n t i a t i n g ( 2 . 4 . 1 4 ) where k Q and k n a r e c o n s t a n t , g i v e s ; d ( K 2 ) = 2k k s i n e d e = k k — ( 2 . 5 . 7 ) ' o n o n TT V ' da 0 S i n c e I (e,<t>) = (e,cf>) we r e p l a c e dn by 2ir..sinede = -rrd(K ) / k Q k n and f i n a l l y o b t a i n from ( 2 . 5 . 5 ) -16- d°onM = k 0 " 2 K - 4 | e o n ( K ) | 2 d ( K 2 ) (2.5.8) I t i s r e l a t i v e l y easy t o g e n e r a l i z e (2.5.8) t o t h e c a s e o f e l e c t r o n - m o l e c u l e s c a t t e r i n g . C o n s i d e r a m o l e c u l e h a v i n g M n u c l e i and N e l e c t r o n s . In t h i s c a s e t h e d i f f e r e n t i a l c r o s s - s e c t i o n f o r t h e e x c i t a t i o n o f t h e n t h s t a t e i s g i v e n by (2.5.8) where e Q n ( K ) i s d e f i n e d by; • o n ™ " \ ^ *0 dV*N < " - 9 > where t h e <y's denote m o l e c u l a r w a v e f u n c t i o n s w h i c h a r e f u n c t i o n s o f e l e c t r o n i c , v i b r a t i o n a l and r o t a t i o n a l quantum numbers, R m and r g a r e r e s p e c t i v e l y the p o s i t i o n v e c t o r s o f t h e m t h n u c l e u s and s t h m o l e c u l a r e l e c t r o n w i t h r e s p e c t t o t h e c e n t r e o f mass, i s t h e n u c l e a r c h a r g e o f t h e mt'1 n u c l e u s and d r ^ i n d i c a t e s i n t e g r a t i o n o v e r a l l n u c l e a r c o o r d i n a t e s . In terms o f t h e Born-Oppenheimer 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 w a v e f u n c t i o n s a r e e x p r e s s e d as t h e p r o d u c t o f an e l e c t r o n i c w a v e f u n c t i o n depending o n l y on the p o s i t i o n s o f t h e e l e c t r o n s ( a t a f i x e d n u c l e a r s e p a r a t i o n ) and a w a v e f u n c t i o n depending on t h e n u c l e a r m o t i o n . T h e r e f o r e * e v r ( r , Q ) = ^ e ( r , Q o ) - ^ v r ( Q ) (2.5.10) where v e v r d e s i g n a t e s the t o t a l w a v e f u n c t i o n d e s c r i b i n g t h e e l e c t r o n i c ( e ) , v i b r a t i o n a l (v). and r o t a t i o n a l ( r ) m o t i o n s , r a r e t h e c o o r d i n a t e s o f t h e e l e c t r o n s , Q t h e c o o r d i n a t e s o f t h e n u c l e i and Q Q a f i x e d n u c l e a r c o n f i g - u r a t i o n . The n u c l e a r terms i n (2.5.9) w i l l v a n i s h upon i n t e g r a t i o n o v e r t h e e l e c t r o n i c c o o r d i n a t e s i f t h e e l e c t r o n i c w a v e f u n c t i o n s a r e o r t h o g o n a l . The i n t e n s i t i e s o f v i b r a t i o n a l e x c i t a t i o n accompanying a g i v e n e l e c t r o n i c t r a n s i t i o n i s s i m p l y g i v e n by the Franck-Condon f a c t o r s which a r e p r o p o r t i o n a l t o t h e o v e r l a p between the i n i t i a l and f i n a l v i b r a t i o n a l w a v e f u n c t i o n s -17- (see R e f e r e n c e s 11 and 4 7 ) . I t s h o u l d be n o t e d t h a t t h e f i r s t Born a p p r o x i m a t i o n has been assumed t o be v a l i d . However, e x p e r i m e n t a l l y , i t 48 49 has been f o u n d ' t h a t t h e Franck-Condon f a c t o r s d e r i v e d f r o m e l e c t r o n impact d a t a a r e i n agreement w i t h o p t i c a l v a l u e s even when t h e e x c i t a t i o n energy i s such t h a t t h e f i r s t Born a p p r o x i m a t i o n no l o n g e r a p p l i e s . I n a d d i t i o n , t h e r e l a t i v e i n t e n s i t i e s o f v i b r a t i o n a l peaks b e l o n g i n g t o t h e 49 same e l e c t r o n i c t r a n s i t i o n a r e a l m o s t i n d e p e n d e n t o f s c a t t e r i n g a n g l e . These f a c t s may be used t o advantage i n e l e c t r o n impact s p e c t r o s c o p y and 5 0 one s p e c i f i c example i s t h e i d e n t i f i c a t i o n o f t h e "C" s t a t e o f ammonia . A c o m p r e h e n s i v e , t h e o r e t i c a l t r e a t m e n t o f t h e e x c i t a t i o n o f v i b r a t i o n a l 51 l e v e l s by e l e c t r o n impact has been g i v e n by Bonham and G e i g e r . 2.6. G e n e r a l i z e d O s c i l l a t o r S t r e n g t h s . In d i s c u s s i n g e l e c t r o n impact e x c i t a t i o n i t i s c o n v e n i e n t t o use t h e g e n e r a l i z e d o s c i l l a t o r s t r e n g t h , f n ( K ) , w h i c h was f i r s t i n t r o d u c e d by B e t h e 4 3 . f n ( K ) = (E n/Q) \eQn(K) | 2 (2.6.1) 2 2 11 where Q = fi K /2m and has t h e u n i t s o f energy. U s i n g t h e Bohr r a d i u s , a Q = t i 2 / m e 2 = 0.52918 x 1 0 " 8 cm and t h e Rydberg e n e r g y , R = m e 4 / 2 t i 2 = 13.606 eV, (2.6.1) becomes; f n ( K ) = ( E n / R ( K a o ) " 2 | c o n ( K ) | 2 (2.6.2) f n ( K ) i s then a g e n e r a l i z a t i o n o f the o p t i c a l o s c i l l a t o r s t r e n g t h d e f i n e d by f n - ( E n / P O M 0 2 n (2.6.3) where -18- on o J " 4̂ 1 2 M i s t h e d i p o l e m a t r i x element s q u a r e d and f n i s p r o p o r t i o n a l t o t h e c r o s s - s e c t i o n f o r t h e e x c i t a t i o n o f t h e n ^ s t a t e by p h o t o a b s o r p t i o n ( d i p o l e a p p r o x i m a t i o n ) . -y C o n s i d e r t h e cas e when K i s d i r e c t e d a l o n g t h e z a x i s and l e t z g be t h -> the z c o o r d i n a t e o f t h e s a t o m i c e l e c t r o n , K«r s = K z $ i n ( 2 . 5 . 6 ) . E q u a t i o n ( 2 . 6 . 2 ) becomes f n ( K ) = ( E n / R ) ( K a Q ) " 2 | e l K Z s *o d r N ^ ( 2 ' 6 ' 5 ) -> F o r s m a l l K, t h e e x p o n e n t i a l i n ( 2 . 6 . 5 ) may be expanded i n a power s e r i e s i n K, e i K z s * 1 + ( T K z s ) + | ( i K z s ) 2 + ... + ^ . ( 1 K z s ) n ( 2 . 6 . 6 ) 4 Assuming t h a t n and <j;Q a r e o r t h o g o n a l we o b t a i n , e o n ( K ) * £ ] ( i K ) + e 2 ( i K ) 2 + E s ( i K ) 3 + . . . ( 2 . 6 . 7 ) 2 = a ~ 2 I L * V * , „ H T ! 2 ( 2 - 6 . 4 ) and f n ( K ) = ( E n / R ) a 0 " 2 + ( e 2 2 - 2 E l e 3 ) K 2 + . . . + 0 ( K 4 ) ( 2 . 6 . 8 ) where e * - i: /vE Z s *o d T N ( 2- 6-9) s In e x p r e s s i n g f p ( K ) as a f u n c t i o n o f even powers o f K i n ( 2 . 6 . 8 ) i t has been assumed t h a t the w a v e f u n c t i o n s i|>n and ^ o a r e r e a l (odd powers o f K i n e * e , ( 2 . 6 . 7 ) s u b s t i t u t e d i n t o ( 2 . 6 . 5 ) , a r e i m a g i n a r y . -19- For v e r y s m a l l momentum t r a n s f e r s , as K ^ 0, t h e r i g h t s i d e o f (2.6.8) i s dominated by t h e f i r s t t erm and f n ( K ) = (E n/R) a o - 2 e ] 2 = f n (2.6.10) l i m K + 0 where f i s the o p t i c a l o s c i l l a t o r s t r e n g t h d e f i n e d i n ( 2 . 6 . 3 ) . L a s s e t t r e 52 e t a l . have shown t h a t (2.6.10) a p p l i e s r e g a r d l e s s o f t h e f i r s t Born a p p r o x i m a t i o n . However, e x t r a p o l a t i o n s o f f" n(K) from r e l a t i v e l y l a r g e 2 2 v a l u e s o f K , t o K = 0 , may be s u b j e c t t o c o n s i d e r a b l e e r r o r . F o r example, minima may o c c u r i n t h e g e n e r a l i z e d o s c i l l a t o r s t r e n g t h f u n c t i o n a t s m a l l v a l u e s o f K 2 as i l l u s t r a t e d by t h e X -> B t r a n s i t i o n o f c a r b o n m o n oxide^ 3. On t h e b a s i s o f (2.6.10) i t i s easy t o d i s t i n g u i s h between e l e c t r i c d i p o l e a l l o w e d and f o r b i d d e n t r a n s i t i o n s : f (K) -v f > 0 -> e l e c t r i c d i p o l e a l l o w e d n n r 1 im K -»• 0 f n ( K ) f - 0 -> e l e c t r i c d i p o l e f o r b i d d e n 1im K 0 In e l e c t r o n impact s p e c t r o s c o p y i t i s c o n v e n t i o n a l t o d e f i n e an a l l o w e d t r a n s i t i o n as one wh i c h i s r i g o r o u s l y a l l o w e d by e l e c t r i c d i p o l e s e l e c t i o n r u l e s even a t low e n e r g i e s and l a r g e s c a t t e r i n g a n g l e s where t h e f i r s t Born a p p r o x i m a t i o n does not h o l d . T r a n s i t i o n s f o r w h i c h = 0 and 0 i n (2.6.8) a r e termed " e l e c t r i c q u a d r u p o l e " t r a n s i t i o n s . However, t h e "qua d r u p o l e moment", £r>, i s not i d e n t i c a l t o t h e e l e c t r i c q u a d r u p o l e moment 5 2 2 2 2 wh i c h o c c u r s i n o p t i c a l s p e c t r o s c o p y . E x p r e s s i n g z as r /3 + ( z - r / 3 ) , (=2 becomes e 2 = 1 / 3 r l *o d T N + < zs " r s / 3 > *o d T N (2.6.11) -20- I n o p t i c a l s p e c t r o s c o p y o n l y t h e second term on t h e RHS o f (2.6.11) 5 c o n t r i b u t e s t o t h e i n t e n s i t y o f e l e c t r i c q u a d r u p o l e t r a n s i t i o n s ; t h e r e i s no analogue t o t h e f i r s t term. The L y m a n - B i r g e - H o p f i e l d bands o f m o l e c u l a r n i t r o g e n p r o v i d e an example where o n l y t h e second term o f (2.6.11) i s nonzero w h i l e f o r t h e l ^ S -»- 2 1 S t r a n s i t i o n i n h e l i u m o n l y t h e 5 54 f i r s t term i s nonzero . R e c e n t l y , some group t h e o r e t i c a l s e l e c t i o n r u l e s have been d e r i v e d w h i c h a r e v a l i d f o r a l l i mpact e n e r g i e s . The r e l a t i o n s h i p between t h e g e n e r a l i z e d o s c i l l a t o r s t r e n g t h and t h e o p t i c a l o s c i l l a t o r s t r e n g t h (2.6.10) has i m p o r t a n t i m p l i c a t i o n s f o r e l e c t r o n impact s p e c t r o s c o p y ; 1. f o r s m a l l momentum t r a n s f e r s , e l e c t r i c d i p o l e s e l e c t i o n r u l e s a p p l y t o t h e e x c i t a t i o n o f atoms and m o l e c u l e s by e l e c t r o n i m p a ct 2. o p t i c a l o s c i l l a t o r s t r e n g t h s may be deduced from e l e c t r o n impact d a t a and 3. o p t i c a l o s c i l l a t o r s t r e n g t h s may be used t o n o r m a l i z e e x p e r i m e n t a l e l e c t r o n impact d a t a . Three c l a s s e s o f e l e c t r o n impact e x p e r i m e n t s have been used t o d e r i v e o p t i c a l o s c i l l a t o r s t r e n g t h s : 1. F i x t h e i n c i d e n t e n e r g y , E Q , v a r y e and e x t r a p o l a t e f n ( K ) t o K -> 0 ( r e c a l l t h a t K 2 = k 2 + k 2 - 2 k k cose from ( 2 . 4 . 1 4 ) . T h i s method has o n o n been used e x t e n s i v e l y by L a s s e t t r e and co-workers ( f o r examples see R e f e r e n c e s 53,55 and 5 6 ) . 2 2. F i x t h e s c a t t e r i n g a n g l e e, v a r y E Q and e x t r a p o l a t e f" n(K) t o K -> 0. T h i s method has been used by H e r t e l and R o s s ^ ' ^ . 3. Use h i g h i n c i d e n t e n e r g i e s (k ^ k n ) and s m a l l s c a t t e r i n g a n g l e s 2 such t h a t K = 0, t h e g e n e r a l i z e d o s c i l l a t o r s t r e n g t h i s e q u a l t o t h e o p t i c a l -21- o s c i l l a t o r s t r e n g t h . T h i s method has been used e x t e n s i v e l y by G e i g e r e t a l . (see R e f e r e n c e s 3, 59-62) and van d e r Wi e l (see R e f e r e n c e 6 3 ) . When t h e f i r s t Born a p p r o x i m a t i o n i s v a l i d , t h e g e n e r a l i z e d o s c i l l a t o r s t r e n g t h can be d i r e c t l y r e l a t e d t o t h e d i f f e r e n t i a l c r o s s - s e c t i o n . U s i n g (2.6.2) and ( 2 . 5 . 5 ) , Jl o da I t i s c o n v e n i e n t t o i n t r o d u c e 1 1 an e f f e c t i v e g e n e r a l i z e d o s c i l l a t o r s t r e n g t h f ^ ( K , E Q ) which can be c a l c u l a t e d e n t i r e l y from e x p e r i m e n t a l measurements r e g a r d l e s s o f t h e v a l i d i t y o f t h e f i r s t Born a p p r o x i m a t i o n . E. A/i o E da When t h e f i r s t Born a p p r o x i m a t i o n i s v a l i d ( l a r g e E ) we can use t h e Born e x p r e s s i o n f o r t he d i f f e r e n t i a l c r o s s - s e c t i o n (2.5.5) t o show, f n ( K , E Q ) + f n ( K ) , l a r g e E Q (2.6.13) where f p ( K ) i s t h e g e n e r a l i z e d o s c i l l a t o r s t r e n g t h d e f i n e d by ( 2 . 6 . 2 ) . A n e c e s s a r y , a l t h o u g h n ot s u f f i c i e n t , c o n d i t i o n 1 1 f o r t h e v a l i d i t y o f t h e f i r s t Born a p p r o x i m a t i o n i s t h a t t h e e f f e c t i v e g e n e r a l i z e d o s c i l l a t o r s t r e n g t h - f ^ ( K , E Q ) s h o u l d have t h e same K dependence a t d i f f e r e n t i n c i d e n t e n e r g i e s , E Q . T h e r e f o r e , i f f n ( K , E Q ) i s a d i f f e r e n t f u n c t i o n o f K a t d i f f e r e n t impact e n e r g i e s , E Q , t h e f i r s t Born a p p r o x i m a t i o n i s c l e a r l y i n v a l i d . On t h i s b a s i s , S k e r b e l e and L a s s e t t r e have f o u n d t r a n s i t i o n s t n 64 53 n i t r o g e n and c a r b o n monoxide where d e v i a t i o n s a r e a p p a r e n t even when the i n c i d e n t e n e r g i e s a r e h i g h enough t o e x p e c t t h e f i r s t Born a p p r o x i m a t i o n t o a p p l y . On t h e b a s i s o f a s u r v e y o f a number o f a t o m i c and m o l e c u l a r 53 t r a n s i t i o n s , i t has been found t h a t d e v i a t i o n s from t h e f i r s t Born -22- a p p r o x i m a t i o n a r e o b s e r v e d when the term symbols o f t h e i n i t i a l and f i n a l s t a t e s a r e i d e n t i c a l . T h e r e f o r e , t h e d e v i a t i o n s a r e dependent on an o p e r a t o r w h i c h i s t o t a l l y s y m m e t r i c . E x p e r i m e n t a l l y , i n an e l e c t r o n energy l o s s measurement, t h e d i f f e r - e n t i a l c r o s s - s e c t i o n f o r t h e t r a n s i t i o n i s measured a t a f i x e d i n c i d e n t e n e r g y , E Q , and s c a t t e r i n g a n g l e 9. U s i n g (2.6.12) and assuming t h a t the f i r s t Born a p p r o x i m a t i o n a p p l i e s such t h a t f ^ ( K , E Q ) = f n ( K ) , o n = 4a 2 dQ 4 a o 1 - V 2 ( K a Q r 2 ( R / E n ) f n ( K ) (2.6.14) A t e = 0° such t h a t Ka„ i s a minimum and E << E , i t has been shown o n o t h a t , 11 da on dQ 1 6 a o 2 r 2 E o E n " 3 f n (2.6.15) where f p i s t h e o p t i c a l o s c i l l a t o r s t r e n g t h and E n i s t h e e x c i t a t i o n energy o r e q u i v a l e n t l y t h e e l e c t r o n e nergy l o s s . -23- CHAPTER THREE EXPERIMENTAL METHODS FOR INNER-SHELL EXCITATION STUDIES For m o l e c u l e s composed o f second row e l e m e n t s , t h e i n n e r s h e l l ( o r c o r e ) e l e c t r o n s a r e t h e I s (K) e l e c t r o n s w h i c h are nonbonding and m a i n l y a t o m i c i n c h a r a c t e r . T r a n s i t i o n s i n v o l v i n g t h e d i s c r e t e e x c i t a t - i o n o f a K - s h e l l e l e c t r o n o c c u r i n t h e a p p r o x i m a t e energy r e g i o n s ; 200 eV (62 A) f o r b o r o n , 300 eV (41 A) f o r c a r b o n , 400 eV (31 A) f o r o o n i t r o g e n , 550 eV (22.5 A) f o r oxygen and 690 eV (18 A) f o r f l u o r i n e . The 2s ( L j ) and 2p ( L j j J J J ) e l e c t r o n s o f t h i r d row e l e m e n t s a r e a l s o c o r e e l e c t r o n s when t h e s e elements a r e i n c o r p o r a t e d i n a m o l e c u l a r e n v i r o n m e n t . F o r s u l f u r - c o n t a i n i n g m o l e c u l e s , i n n e r s h e l l e x c i t a t i o n s o o r e q u i r e a p p r o x i m a t e l y 2475 eV (5 A) f o r s u l f u r K, 220 eV (56 A) f o r o s u l f u r L j and 160 eV (77.5 A) f o r s u l f u r L ^ J J J . A b i b l i o g r a p h y o f i n n e r s h e l l e x c i t a t i o n s t u d i e s has p r e v i o u s l y been g i v e n i n C h a p t e r I . I n f o r m a t i o n on d i s c r e t e e x c i t a t i o n s has been p r o v i d e d by f o u r t y p e s o f e x p e r i m e n t s ; p h o t o a b s o r p t i o n , Auger e l e c t r o n s p e c t r o s c o p y , X-ray e m i s s i o n s p e c t r o s c o p y and e l e c t r o n e nergy l o s s s p e c t r o s c o p y . Each o f t h e s e t e c h n i q u e s has c e r t a i n l i m i t a t i o n s . D i s c r e t e e x c i t a t i o n by p h o t o a b s o r p t i o n r e q u i r e s a photon w i t h an energy e x a c t l y equal t o t h e energy r e q u i r e d f o r t h e t r a n s i t i o n . The d i f f i c u l t y o f p r o d u c i n g a u s e f u l photon continuum i n the e n e r g y r e g i o n r e q u i r e d f o r K - s h e l l e x c i t a t i o n s ( s o f t X -ray) has p r e v i o u s l y been m e n t i o n e d ; B r e m s s t r a h l u n g c o n t i n u u a a r e weak ( p a r t i c u l a r l y below 1000 eV) and e l e c t r o n -24- s y n c h r o t r o n f a c i l i t i e s a r e not r e a d i l y a v a i l a b l e . In a d d i t i o n , an e l e c t r o n s y n c h r o t r o n p r o d u ces a l a r g e i n t e n s i t y o f photons h a v i n g e n e r g i e s h i g h e r than t h a t r e q u i r e d f o r K - s h e l l e x c i t a t i o n and o r d e r o v e r l a p p i n g i n t h e s p e c t r o g r a p h seems t o be a problem. In t h e cas e o f t h e K - s h e l l 27 a b s o r p t i o n spectrum o f n i t r o g e n , an e x c e s s o f oxygen (which has s t r o n g o a b s o r p t i o n below 20 A) had t o be i n c l u d e d i n o r d e r t o s u p p r e s s t h i s e f f e c t . The d e s i g n and c o n s t r u c t i o n o f monochroma.tors f o r the s o f t X - r a y r e g i o n i s a l s o d i f f i c u l t . S u r f a c e r e f l e c t i v i t i e s a r e e x t r e m e l y poor a t such s h o r t w a v e l e n g t h s and g r a z i n g i n c i d e n c e monochromators must be used. A l s o , s i n c e r e s o l u t i o n i s on a w a v e l e n g t h s c a l e , i t becomes p r o g r e s s i v e l y more d i f f i c u l t t o o b t a i n h i g h energy r e s o l u t i o n i n t h e s h o r t w a v e l e n g t h r e g i o n o f t h e energy spectrum. S i n c e r e s o l u t i o n i s g a i n e d a t t h e expense o f i n t e n s i t y , t h e r e i s a p r a c t i c a l l i m i t t o t h e r e s o l u t i o n w h i c h can be o b t a i n e d i n t h e s o f t X-ray r e g i o n . In o r d e r t o d i s c u s s t h e a p p l i c a t i o n o f Auger e l e c t r o n s p e c t r o s c o p y t o i n n e r s h e l l e x c i t a t i o n s t u d i e s i t i s c o n v e n i e n t t o g i v e a b r i e f i n t r o d u c t - i o n t o t h e t e c h n i q u e . The e j e c t i o n o f an i n n e r s h e l l e l e c t r o n by X-ray a b s o r p t i o n , e l e c t r o n impact o r o t h e r methods, r e s u l t s i n t h e p r o d u c t i o n o f a h i g h l y u n s t a b l e s p e c i e s . The dominant r e l a x a t i o n p r o c e s s , f o r m o l e c u l e s 65 composed o f second row e l e m e n t s , i s by Auger e l e c t r o n e j e c t i o n . The p r o c e s s may be r e p r e s e n t e d as f o l l o w s ; i . X n-—> X + e I n i t i a l I o n i z a t i o n o r hv n . X (E ) + X (E ) + e ( E ^ where ( i ) r e p r e s e n t s t h e i n i t i a l i o n i z a t i o n o f a K - s h e l l e l e c t r o n and ( i i ) r e p r e s e n t s t h e Auger r e l a x a t i o n p r o c e s s i n w h i c h the i n n e r s h e l l " h o l e " i s -25- f i l l e d by a v a l e n c e s h e l l e l e c t r o n and t h e e nergy l i b e r a t e d i n t h e p r o c e s s appears as k i n e t i c e n ergy (E-|) o f a s econd v a l e n c e s h e l l e l e c t r o n ( t h e Auger e l e c t r o n ) w h i c h i s e j e c t e d i n t h e p r o c e s s . The k i n e t i c e n ergy o f t h e Auger e l e c t r o n i s g i v e n by; E ] - E K + - E + + where E i s t h e energy o f t h e i n i t i a l K - s h e l l i o n s t a t e and E i s t h e e n ergy o f a d o u b l y i o n i z e d s t a t e o f t h e m o l e c u l e ( b o t h v a c a n c i e s i n t h e v a l e n c e s h e l l ) . I f t h e i n i t i a l K - s h e l l " h o l e " s t a t e i n ( i i ) i s n e u t r a l , t h e n t h e main r e l a x a t i o n p r o c e s s i s a u t o i o n i z a t i o n w h i c h produces a s i n g l y c h arged f i n a l s t a t e : i i i . X K * ( E K * ) - X + ( E + ) + e ( E 2 ) where X + r e p r e s e n t s a s i n g l y c h a r g e d i o n s t a t e w i t h t h e vacancy i n one o f t h e v a l e n c e s h e l l s . The k i n e t i c energy o f t h e e j e c t e d e l e c t r o n , E 2 , i s e q u a l t o t h e d i f f e r e n c e i n energy between th e i n i t i a l K - s h e l l e x c i t e d s t a t e and t h e s i n g l y c h a r g e d i o n s t a t e ; E 2 = E K * - E + R e f e r r i n g t o p r o c e s s e s ( i i ) and ( i i i ) , t h e e n e r g i e s o f t h e i n i t i a l K - s h e l l i o n ( i i ) and d i s c r e t e s t a t e s ( i i i ) u s u a l l y d i f f e r by l e s s t h a n 10 eV w h i l e t h e d o u b l y c h a r g e d i o n s t a t e s i n ( i i ) a r e t y p i c a l l y 20 - 30 eV h i g h e r i n energy t h a n t h e s i n g l y c h a r g e d s t a t e s i n ( i i i ) . T h e r e f o r e , a u t o i o n i z a t i o n p r o c e s s e s can be i d e n t i f i e d s i n c e t h e k i n e t i c e n e r g i e s o f t h e e j e c t e d e l e c t - r o n s a r e h i g h e r t h a n t h e maximum energy w h i c h can be t a k e n up by an Auger e l e c t r o n . I t i s a l s o p o s s i b l e f o r " e x c i t e d " K - s h e l l i o n s t a t e s ( p r o d u c e d by t h e shake-up o f v a l e n c e e l e c t r o n s i n c o n j u n c t i o n w i t h K - s h e l l i o n i z a t i o n ) -26- t o g i v e r i s e t o h i g h energy Auger peaks. However, i t can be e s t a b l i s h e d t h a t the i n i t i a l s t a t e i s n e u t r a l and n o t an " e x c i t e d " K - s h e l l i o n s t a t e by comparing t h e r e s u l t s o f e x c i t a t i o n by e l e c t r o n i m p a ct and X-ray 33- 36 a b s o r p t i o n " . The d i s c r e t e s t a t e s a r e not e x c i t e d by an X-ray l i n e w i t h energy f a r i n ex c e s s o f t h e t r a n s i t i o n e n e r g i e s and t h e c o r r e s p o n d i n g a u t o i o n i z a t i o n l i n e s a r e a b s e n t from t h e Auger spectrum. However, t h e energy o f an a u t o i o n i z a t i o n l i n e i s e q u a l t o t h e energy d i f f e r e n c e between the i n i t i a l n e u t r a l e x c i t e d s t a t e and some f i n a l s t a t e o f t h e s i n g l y charged s p e c i e s . T h e r e f o r e , an a m b i g u i t y i n peak a s s i g n m e n t may a r i s e u n l e s s one o f t h e s t a t e s i n v o l v e d i n the p r o c e s s can be p o s i t i v e l y i d e n t - i f i e d . A c o m p e t i t i v e r e l a x a t i o n p r o c e s s f o r a m o l e c u l e w i t h an i n n e r s h e l l v a c a n c y , i s X-ray e m i s s i o n , i n w h i c h t h e " h o l e " i s f i l l e d by a v a l e n c e s h e l l e l e c t r o n and t h e l i b e r a t e d energy appears as a photon ( X - r a y f l u o r - e s c e n c e ) . I f t h e i n i t i a l s t a t e i s n e u t r a l , then t h e en e r g y o f t h e e m i t t e d photon i s equal t o t h e e x c i t a t i o n energy o f t h e d i s c r e t e s t a t e ( r e s o n a n c e op on cc e m i s s i o n ) . R e c e n t l y , Siegbahn e t a l . ' ' , have c o n s t r u c t e d a h i g h r e s o l u t i o n X - r a y e m i s s i o n s p e c t r o m e t e r (AE (FWHM) - 0.1 eV) wh i c h i s c a p a b l e o f r e s o l v i n g some 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 o f e m i s s i o n bands. The resonance e m i s s i o n from t h e l o w e s t K - s h e l l e x c i t e d s t a t e o f m o l e c u l a r 38 39 n i t r o g e n has been c l e a r l y o b s e r v e d ' . However, e m i s s i o n bands from t h e h i g h e r energy K - s h e l l e x c i t e d s t a t e s ( o b s e r v e d i n R e f e r e n c e 27) have not been r e p o r t e d . In a d d i t i o n , h i g h energy s a t e l l i t e l i n e s i n t h e K - s h e l l X-ray e m i s s i o n s p e c t r u m o f carbon monoxide have v e r y low i n t e n s i t i e s and 66 e n e r g i e s and a s s i g n m e n t s o f t h e s e l i n e s have not been g i v e n . However, f o r low a t o m i c numbers, e m i s s i o n i n t e n s i t i e s a r e e x p e c t e d t o be s m a l l s i n c e -27- t h e c o m p e t i t i o n between Auger e m i s s i o n and X - r a y f l u o r e s c e n c e i s d o m inated by t h e n o n r a d i a t i v e p r o c e s s (see R e f e r e n c e 6 5 ) . In a d d i t i o n , as i n t h e c a s e o f a u t o i o n i z a t i o n , an a m b i g u i t y i n peak a s s i g n m e n t may a r i s e , s i n c e t h e f i n a l s t a t e i n v o l v e d i n t h e e m i s s i o n p r o c e s s may e i t h e r be t h e ground e l e c t r o n i c s t a t e o r any o f t h e n e u t r a l , v a l e n c e s h e l l e x c i t e d s t a t e s o f the m o l e c u l e . The o n l y c o n d i t i o n f o r t h e e x c i t a t i o n o f a n e u t r a l s t a t e by e l e c t r o n i mpact i s t h a t t h e k i n e t i c e n e r g y o f t h e i n c i d e n t e l e c t r o n must be g r e a t e r than t h e t r a n s i t i o n energy. S i n c e t h e k i n e t i c e n e r g y o f t h e i n c i d e n t e l e c t r o n i s d e t e r m i n e d by t h e p o t e n t i a l d i f f e r e n c e between the e l e c t r o n s o u r c e and c o l l i s i o n r e g i o n , t h e problems o f an e nergy s o u r c e f o r K - s h e l l e x c i t a t i o n s e n c o u n t e r e d i n p h o t o a b s o r p t i o n s t u d i e s , do not e x i s t . In a d d i t i o n , t h e l i f e t i m e s o f t h e s e h i g h energy s t a t e s a r e r e l a t i v e l y s h o r t and t h e r e f o r e t h e n a t u r a l l i n e w i d t h s a r e l a r g e . The u n c e r t a i n t y b r o a d e n i n g o f the e x c i t e d s t a t e s i n t h e r e g i o n s o f t h e r e s p e c t i v e c a r b o n , n i t r o g e n and oxygen K-edges 39 i s p r o b a b l y around 0.1 eV. T h i s e s t i m a t e i s s u p p o r t e d by t h e c l e a r r e s o l u t i o n o f v i b r a t i o n a l s t r u c t u r e (0.26 eV s p a c i n g s ) i n the h i g h e nergy 32 a u t o i o n i z a t i o n band o f carbon monoxide . In p r i n c i p l e t h e r e s o l u t i o n o b t a i n a b l e i n e l e c t r o n e nergy l o s s s p e c t r o s c o p y i s an o r d e r o f magnitude l o w e r than t h e n a t u r a l l i n e w i d t h s (see f o r example R e f e r e n c e 3) and t h e r e f o r e , e l e c t r o n impact s p e c t r o s c o p y can p o t e n t i a l l y p r o v i d e as much i n f o r m a t i o n about t h e s e s t a t e s as o p t i c a l a b s o r p t i o n s p e c t r o s c o p y . A l s o , w i t h t h e use o f a s u i t a b l e r e t a r d i n g l e n s , e l e c t r o n s can be e n e r g y a n a l y s e d a t c o n s t a n t e n e r g y , E , and t h e r e s o l u t i o n ( A E / E ) i s c o n s t a n t o v e r t h e e n t i r e energy l o s s spectrum. T h e r e f o r e , even f o r i n c i d e n t e l e c t r o n s w i t h e n e r g i e s i n t h e keV r a n g e , and energy l o s s e s i n t h e 300 - 700 eV e n e r g y -28- r e g i o n , " h i g h " r e s o l u t i o n can be o b t a i n e d by p r e r e t a r d i n g t h e s c a t t e r e d e l e c t r o n s and en e r g y a n a l y s i n g a t a s u f f i c i e n t l y low energy. F i g u r e 2 shows t h e parameters r e l e v a n t t o a c o m p a r i s o n o f r e s o l u t i o n on a w a v e l e n g t h and energy b a s i s and i s h e l p f u l i n comparing and c o n t r a s t i n g p h o t o a b s o r p t i o n and e l e c t r o n impact s p e c t r o s c o p y . In F i g u r e 2 t h e r e s o l u t i o n , AA ( A ) , c o r r e s p o n d i n g t o f i x e d v a l u e s o f r e s o l u t i o n , A E ( e V ) , has been p l o t t e d as a f u n c t i o n o f energy. To i l l u s t r a t e t h e use o f F i g u r e 2, c o n s i d e r t h e f o l l o w i n g examples; f o r c a r b o n K - s h e l l e x c i t a t i o n , ^ 300 eV (41 A ) , a r e s o l u t i o n , A E , o f 0.5 eV c o r r e s p o n d s t o a r e s o l u t i o n AA o f ^ 0.1 A, w h i l e o f o r f l u o r i n e K - s h e l l e x c i t a t i o n , ^ 700 eV (18 A ) , a r e s o l u t i o n o f 0.5 eV o c o r r e s p o n d s t o a r e s o l u t i o n , A A , o f 0.02 A. The K - s h e l l p h o t o a b s o r p t i o n 27 ° spectrum o f n i t r o g e n was o b t a i n e d w i t h a r e s o l u t i o n < 0.03 A w h i c h c o r r e s - o ponds t o a r e s o l u t i o n < 0.4 eV a t 400 eV (31 A ) . In the case o f e l e c t r o n i m p a c t , a r e s o l u t i o n o f ^ 0.1 eV can be o b t a i n e d w i t h o n l y modest "mono- c h r o m a t i o n " o f t h e i n c i d e n t beam. T h i s , and t h e f a c t t h a t t h e r e i s no d i f f i c u l t y i n o b t a i n i n g an energy s o u r c e i n e l e c t r o n i m p a c t s p e c t r o s c o p y , s u g g e s t s t h a t t h e r e a r e advantages t o t h e use o f e l e c t r o n i m p a c t o v e r p h o t o a b s o r p t i o n methods i n t h e s o f t X-ray and X-ray r e g i o n s . o FIGURE 2. R e s o l u t i o n , AX ( A ) , p l o t t e d a g a i n s t energy f o r f i x e d v a l u e s o f r e s o l u t i o n , AE (0.01 t o 0.05 e V ) . -30- CHAPTER FOUR EXPERIMENTAL 4.1. 180° E l e c t r o s t a t i c A n a l y s e r . One o f t h e most i m p o r t a n t components o f an e l e c t r o n s p e c t r o m e t e r i s t h e e l e c t r o n e n e r g y , o r momentum a n a l y s e r . The k i n e t i c e n e r g i e s o f s c a t t e r e d e l e c t r o n s a r e u s u a l l y measured by d e f l e c t i n g t h e e l e c t r o n s i n an e l e c t r i c o r m a g n e t i c f i e l d . A l t e r n a t i v e l y , a c o m b i n a t i o n o f e l e c t r i c and m a g n e t i c f i e l d s may be used. The p r o p e r t i e s and r e l a t i v e m e r i t s o f d i f f e r e n t t y p e s o f e l e c t r o n energy a n a l y s e r s have been d i s c u s s e d i n a o q . c~j CQ number o f r e v i e w a r t i c l e s . ' ' A 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 s e r was s e l e c t e d f o r t h i s work f o r t he f o l l o w i n g r e a s o n s ; ( i ) e l e c t r o s t a t i c d e f l e c t i o n was chosen o v e r magnetic d e f l e c t i o n because u n i f o r m m a g n e t i c f i e l d s a r e more d i f f i c u l t t o produce and c o n t r o l t h a n e l e c t r o s t a t i c f i e l d s . In a d d i t i o n , problems o f f r i n g e f i e l d s a r e more s e v e r e i n t h e case o f a m a g n e t i c a n a l y s e r , ( i i ) t h e two d i m e n s i o n a l f o c u s i n g p r o p e r t i e s o f t h e 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 s e r a r e i d e a l l y s u i t e d f o r c o u p l i n g t o s t r o n g l y d e c e l e r a t i n g l e n s e s o f a x i a l symmetry. The p r o p e r t i e s 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 s e r s have been 70 71 72 73 68 d i s c u s s e d by P u r c e l l , Simpson , Simpson and K u y a t t ' and S a r - E l A s c h e m a t i c diagram i s shown i n F i g u r e 3. E l e c t r o n s a r e d e f l e c t e d by t h e 1/r e l e c t r o s t a t i c f i e l d produced by t h e p o t e n t i a l d i f f e r e n c e , V-^' between 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 w i t h r a d i i > r - j . The magnitude -31- FIGURE 3. S c h e m a t i c diagram o f a h e m i s p h e r i c a l e l e c t r o n energy a n a l y s e r . -32- o f t h e e l e c t r o s t a t i c f i e l d i s g i v e n by S ( r ) = C / r 2 (4.1.1) where C i s a c o n s t a n t . C o n s i d e r an e l e c t r o n w i t h k i n e t i c energy E Q = mu o/2 whi c h e n t e r s t h e a n a l y s e r a t t h e p o i n t x, = 0 and a t an a n g l e a = 0°. In o r d e r f o r t h e e l e c t r o n t o f o l l o w a c i r c u l a r p a t h w i t h r a d i u s r , t h e c e n t r i f u g a l f o r c e must be e q u a l t o the e l e c t r o s t a t i c f o r c e , e , a t r Q ; m u 2 / r Q - e£(r Q) = e C / r 2 (4.1.2) 2 S i n c e the k i n e t i c e n e r g y , mu Q/2 i s equal t o e V Q i n e l e c t r o n v o l t s ; from ( 4 . 1 . 2 ) , C = 2 r 0 V Q = ( r , + r 2 ) V Q (4.1.3) where r Q i s chosen as t h e m i d p o i n t . The p o t e n t i a l a t p o i n t r i s g i v e n by; V ( r ) = £ + B (4.1.4) where B i s a c o n s t a n t . The p o t e n t i a l s o f the i n n e r and o u t e r hemispheres w i t h r e s p e c t t o p o i n t r Q a r e then V ( r ^ - V ( r Q ) = C/r, - C / r Q (4.1.5) V ( r Q ) - V ( r 2 ) = C / r Q - C / r 2 (4.1.6) and t h e r e f o r e t h e p o t e n t i a l d i f f e r e n c e a c r o s s t h e hemispheres i s g i v e n by V ( r 1 ) - V ( r 2 ) = C/r, - C / r 2 = V Q [ ( r z / r } ) - (^/r^J (4.1.7) L e t x, be t h e r a d i a l d i s t a n c e from r Q o f an e l e c t r o n e n t e r i n g t h e 73 a n a l y s e r a t a n g l e a and energy E. I t has been shown t h a t t h e d e v i a t i o n , x 2 , o f t h e t r a n s m i t t e d e l e c t r o n from t h e r a d i a l p a t h , r , i s g i v e n by; -33- ( x 2 / r Q ) = - ( x - , / r o ) + 2 ( A E / E o ) - 2 a 2 (4.1.8) where AE = E - E . S i n c e t h e r e i s no l i n e a r term i n a, t h e a n a l y s e r has f i r s t o r d e r a n g u l a r f o c u s i n g ( t h e a n g u l a r f o c u s i n g i s p e r f e c t a t 360°)^ 4. In a d d i t i o n , because o f t h e s p h e r i c a l symmetry, t h e a n a l y s e r has two d i m e n s i o n a l f o c u s i n g p r o p e r t i e s . The energy r e s o l t u i o n ( i . e . t h e t r a n s m i s s i o n o f e l e c t r o n s as a f u n c t i o n o f energy t a k i n g i n t o a c c o u n t t h e d i s t r i b u t i o n o f i n c i d e n t 73 e l e c t r o n s o v e r space and a n g l e can be a p p r o x i m a t e d by AE(FWHM)/E = S / 2 r Q (4.1.9) where AE(FWHM) i s t h e f u l l w i d t h a t h a l f maximum o f t h e t r a n s m i t t e d beam and S i s t h e s l i t w i d t h o r d i a m e t e r f o r a c i r c u l a r a p e r t u r e . E x p r e s s i o n 2 2 (4.1.9) n e g l e c t s a term i n a and i s o n l y a p p l i c a b l e i f a << S / 2 r Q . The a n a l y s e r used f o r t h i s r e s e a r c h has t h e f o l l o w i n g d i m e n s i o n s ; r-| = 1.5 i n c h e s r Q = 2.0 i n c h e s r ^ = 2.5 i n c h e s S = 0.050 i n c h e s The r a t i o o f t h e p o t e n t i a l d i v i d i n g r e s i s t o r s R2/R-|) i s g i v e n by t h e r a t i o o f t h e p o t e n t i a l s V^/V-j and f o r t h e d i m e n s i o n s o f o u r a n a l y s e r u s i n g (4.1.5) and ( 4 . 1 . 6 ) : R 2 / R 1 = V 2 / V 1 = 3/5 (4.1.10) The t h e o r e t i c a l r e s o l u t i o n ( n e g l e c t i n g t h e a term) i s g i v e n by ( 4 . 1 . 9 ) , AE/E Q = S / 2 r Q = 0.0125 = 1.25% (4.1.11) In o r d e r t o t e s t t h e performance o f t h e a n a l y s e r , t h e r e s o l u t i o n , -34- ( i . e . AE(FWHM), as a f u n c t i o n o f e l e c t r o n e n e r g y , E Q = e V Q ) was measured a t 0° s c a t t e r i n g w i t h h e l i u m as a t a r g e t gas. The r e s u l t s a r e shown i n F i g u r e 4. The AE(obs) c u r v e i s a c o n v o l u t i o n o f t h e energy s p r e a d o f t h e gun and energy t r a n s m i s s i o n f u n c t i o n o f t h e a n a l y s e r . By e x t r a p o l - a t i n g t o E Q = 0, t h e gun s p r e a d was e s t i m a t e d as ^ 0.28 eV. The l o w e r c u r v e r e p r e s e n t s t h e energy t r a n s m i s s i o n w i d t h o f t h e a n a l y s e r and was o b t a i n e d by c o r r e c t e d AE(obs) f o r t h e gun s p r e a d ( t h e energy d i s t r i b u t i o n s 73 were assumed t o be a p p r o x i m a t e l y G a u s s i a n ). The e x p e r i m e n t a l r e s o l u t i o n o f t h e a n a l y s e r i s 1 . 2 3 % , i n good agreement w i t h t h e t h e o r e t i c a l e s t i m a t e o f 1 . 2 5 % ( s e e 4 . 1 . 1 1 ) . An a d d i t i o n a l check o f t h e a n a l y s e r was p e r f o r m e d by p l o t t i n g t h e f o c u s p o t e n t i a l , V-^s o f t h e hemispheres as a f u n c t i o n o f e l e c t r o n energy eV Q. The e x p e r i m e n t a l p l o t was a s t r a i g h t l i n e , V-|2 = 1.07 V , i n e x a c t agreement w i t h t h e t h e o r e t i c a l v a l u e c a l c u l a t e d u s i n g ( 4 . 1 . 7 ) and t h e d i m e n s i o n s o f o u r a n a l y s e r . 4.2. The E l e c t r o n Source. The e l e c t r o n s o u r c e f o r t h e s p e c t r o m e t e r was a P h i l i p s 6AW59 t e l e v i s i o n gun c o n s i s t i n g o f an i n d i r e c t l y h e ated o x i d e cathode ( B a S r O ) , g r i d , anode and f o c u s i n g element ( E i n z e l l e n s ) . A c i r c u i t d iagram o f the e l e c t r o n gun power s u p p l y i s shown i n F i g u r e 5. The advantage o f an o x i d e cathode s o u r c e i s t h a t a r e a s o n a b l y m o n o e n e r g e t i c beam, AE(FWHM) ^ 0.25 eV , can be produced w i t h o u t u s i n g an energy s e l e c t o r . T h i s i s a r e s u l t o f t h e low work f u n c t i o n o f t h e mixed o x i d e c a t h o d e s , w h i c h means t h a t t h e y can be o p e r a t e d a t much l o w e r t e m p e r a t u r e s t h a n o t h e r e m i t t e r s . F o r a t h e r m i o n i c c a t h o d e 6 7 ' 7 2 , AE(FWHM) = 2.54 kT where k i s t h e Boltzmann c o n s t a n t ( 1 / 1 1 6 0 0 eV/°K). T h i s i m p l i e s a AE(FWHM) o f ^ 0.25 eV f o r a normal F I G U R E 4. R e s o l u t i o n , AE ( F W H M ) , VS. e l e c t r o n energy f o r the 180° e l e c t r o n energy a n a l y s e r : • o b s e r v e d ( c o n v o l u t i o n o f gun and a n a l y s e r s p r e a d s ) , • a n a l y s e r o n l y (gun s p r e a d s u b t r a c t e d ) . RE 5. E l e c t r o n gun power s u p p l y ; 1. f i l a m e n t , 2. c a t h o d e , 3. a r i d , 4. anode and 5. f o c u s . R e s i s t o r s a r e i n Kft and c a p a c i t o r s i n yF. -37- o p e r a t i n g t e m p e r a t u r e o f a p p r o x i m a t e l y 1100 °K. The e x p e r i m e n t a l v a l u e o b t a i n e d by e x t r a p o l a t i o n o f t h e o b s e r v e d AE(FWHM) i s % 0.28 eV (see F i g u r e 4 ) . In a d d i t i o n , t h e t e l e v i s i o n gun produces a w e l l f o c u s e d beam. However, a m a j o r d i s a d v a n t a g e i s t h a t t h e o x i d e c a t h o d e i s r e a d i l y p o i s o n e d by most g a s e s , p a r t i c u l a r l y by s t r o n g o x i d i z e r s s u c h as oxygen and n i t r i c o x i d e . In t h e p r e s e n t e x p e r i m e n t , t h i s d i f f i c u l t y was p a r t i a l l y overcome by o p e r a t i n g t h e gun w i t h a h i g h e r f i l a m e n t v o l t a g e w h i c h tended t o reduce i t s u s e f u l l i f e t i m e . A more s a t i s f a c t o r y s o l u t i o n would be t o b u i l d a d i f f e r e n t i a l l y pumped s o u r c e . 4.3. The S p e c t r o m e t e r . P l a t e 1 i s a p h o t o graph o f t h e s p e c t r o m e t e r and F i g u r e 6 shows a s c h e m a t i c diagram. The main components a r e : A, t h e e l e c t r o n gun ( o x i d e c a t h o d e ) and E i n z e l l e n s ; B and F, q u a d r u p o l e e l e c t r i c d e f l e c t i o n p l a t e s ; -4 c, c o l l i s i o n chamber o p e r a t e d a t t y p i c a l gas p r e s s u r e s o f 10 t o r r ; D, gas i n l e t ; E, a n g u l a r s e l e c t i o n p l a t e ; G, d e c e l e r a t i n g l e n s ; H, hemi- s p h e r i c a l a n a l y s e r ; and I , channel e l e c t r o n m u l t i p l i e r . 4.3.1. S p e c t r o m e t e r C o n s t r u c t i o n . The s p e c t r o m e t e r was c o n s t r u c t e d from b r a s s w i t h t h e e x c e p t i o n o f t h e a n g u l a r s e l e c t i o n p l a t e , E, and t h e a p e r t u r e p l a t e s o f t h e a n a l y s e r , w h i c h were machined from molybdenum. The d e f l e c t i o n p l a t e s were 0.4" x 0.6" and t h e a n g u l a r s e l e c t i o n p l a t e and a n a l y s e r " s l i t " p l a t e s have a p e r t u r e s o f 0.050". The l e n s , G, i s an equal d i a m e t e r (D = 1.9") two tube c y l i n d r i c a l l e n s w i t h a h i g h v o l t a g e element ( l e n g t h 1.21 D), a gap o f 0.16 D and a low v o l t a g e e l ement ( l e n g t h 0.84 D ) , ( t h e l e n s p a r a - PLATE 1 The S p e c t r o m e t e r . X-Y PLOT MCA VAR VOLTS DISC RATE METER REC RAMP 2500 VOLTS FIGURE 6. S c h e m a t i c diagram o f t h e a p p a r a t u s . -40- meters a r e e s s e n t i a l l y t h o s e used by van d e r Wiel ). E l e c t r i c a l i n s u l - a t i o n between components o p e r a t i n g a t d i f f e r e n t p o t e n t i a l s was i n i t i a l l y p r o v i d e d by boron n i t r i d e s p a c e r s . However, t h e s e p r o v e d t o be v e r y b r i t t l e , w h i c h was an i n c o n v e n i e n c e when t h e machine was d i s m a n t l e d f o r c l e a n i n g . T h i s problem was overcome by r e b u i l d i n g t h e s p e c t r o m e t e r u s i n g p r e c i s i o n s a p p h i r e b a l l s ( l o c a t e d i n u n d e r s i z e d h o l e s ) as i n s u l a t o r s . An a d d i t i o n a l advantage i s t h a t t h e b a l l s a l s o s e r v e as a c c u r a t e l o c a t e r s . I n i t i a l l y a l l b r a s s s u r f a c e s were g o l d p l a t e d t o p r o v i d e a u n i f o r m s u r f a c e p o t e n t i a l . However, i t was found t h a t t h e performance o f t h e s p e c t r o m e t e r was not degraded by t h e o m i s s i o n o f t h i s s t e p . The s u r f a c e s o f t h e hemi- sph e r e s and a p e r t u r e p l a t e s were c o a t e d w i t h a u n i f o r m l a y e r o f benzene s o o t t o m i n i m i z e t h e number o f s u r f a c e s c a t t e r e d e l e c t r o n s . In a d d i t i o n , a s l o t (0.13" x 2.8") was m i l l e d i n t h e back hemisphere ( b e h i n d t h e e n t r a n c e s l i t ) t o reduce t h e number o f b a c k - s c a t t e r e d and s e c o n d a r y e m i t t e d e l e c t r o n s . The performance o f t h e a n a l y s e r was n o t i m p a i r e d by t h i s m o d i f i c a t i o n . 4.3.2. S p e c t r o m e t e r O p e r a t i o n . The e l e c t r o n beam was a c c e l e r a t e d towards t h e c o l l i s i o n r e g i o n by a 2.5 kV p o t e n t i a l d i f f e r e n c e . The q u a d r u p o l e d e f l e c t i o n p l a t e s B and F were used t o c o n t r o l t h e beam d i r e c t i o n and t h e e l e c t r o n c u r r e n t was m o n i t e r e d by d e f l e c t i n g t h e beam on t o t h e a n g u l a r s e l e c t i o n p l a t e , E, which was f l o a t e d , t h r o u g h a p r e c i s i o n e l e c t r o m e t e r , by t h e h i g h v o l t a g e power s u p p l y . F o r K - s h e l l energy l o s s measurements, i t was not p o s s i b l e t o o b t a i n s p e c t r a a t a 0° s c a t t e r i n g a n g l e because o f t h e l a r g e i n t e n s i t y (even w i t h t h e s l o t ) o f s c a t t e r e d and s e c o n d a r y e m i t t e d e l e c t r o n s produced by t h e f a s t p r i m a r y e l e c t r o n beam c o l l i d i n g w i t h t h e back hemisphere. The -41- p r i m a r y beam was t h e r e f o r e d e f l e c t e d by t h e p l a t e s , B, such t h a t i t was i n t e r c e p t e d by t h e a n g u l a r s e l e c t i o n p l a t e , E. E l e c t r o n s h a v i n g an _2 average s c a t t e r i n g a n g l e o f 2 x 10 r a d i a n s passed t h r o u g h t h e a n g u l a r s e l e c t i o n p l a t e a p e r t u r e i n t o t h e d e c e l e r a t i n g l e n s . Energy l o s s s p e c t r a were o b t a i n e d by s c a n n i n g t h e e l e c t r i c p o t e n t i a l a p p l i e d t o t h e d e c e l e r - a t i n g l e n s ( u s u a l l y from ground t o +40 V) w h i l e t h e cathode p o t e n t i a l was f l o a t e d a t a n e g a t i v e v o l t a g e ( c o r r e s p o n d i n g t o t h e a p p r o x i m a t e K - s h e l l energy l o s s ; e.g. f o r n i t r o g e n K - s h e l l , -425 V ) . The a c c u r a c y o f t h e energy s c a l e o b t a i n e d was ± 0.02 eV. The t r a n s m i s s i o n e nergy o f t h e a n a l y s e r was s e t a t 25 eV and t h e r e s o l u t i o n [ A E ( F W H M ) ] was ^ 0 . 5 eV. Output p u l s e s from t h e m u l t i p l i e r were p r o c e s s e d by s t a n d a r d p u l s e e l e c t r o n - i c s (see F i g u r e 6) and s t o r e d i n a m u l t i s c a l e r , whose channel advance was synchronous w i t h the s c a n n i n g v o l t a g e a p p l i e d t o t h e d e c e l e r a t i n g l e n s . The z e r o o f the energy s c a l e f o r each s p e c t r u m was d e t e r m i n e d by r e c o r d i n g t h e peak from e l a s t i c a l l y s c a t t e r e d e l e c t r o n s (measured as a d.c. c u r r e n t u s i n g t h e e l e c t r o n m u l t i p l i e r as a Faraday cup) and t h e K - s h e l l s p e c t r u m ( p u l s e c o u n t i n g ) under i d e n t i c a l e x p e r i m e n t a l c o n d i t i o n s (beam i n t e n s i t y , d e f l e c t i o n a n g l e and t a r g e t gas p r e s s u r e ) . The e l a s t i c peak was t o o i n t e n s e t o r e c o r d i n t h e p u l s e c o u n t i n g mode. In o r d e r t o measure t h e v a l e n c e s h e l l energy l o s s s p e c t r u m , t h e beam i n t e n s i t y was re d u c e d such t h a t t h e p u l s e c o u n t i n g mode c o u l d be used. K - s h e l l energy l o s s s p e c t r a , were u s u a l l y o b t a i n e d u s i n g p r i m a r y beams i n t e n s i t i e s o f 0.1 yA t o 1 yA. A l t h o u g h some s t r u c t u r e was u s u a l l y a p p a r e n t a f t e r a s i n g l e s c a n , i t was u s u a l l y n e c e s s a r y t o s i g n a l a verage f o r some hours ( t y p i c a l l y o v e r n i g h t ) i n o r d e r t o o b t a i n a spectrum w i t h a good s i g n a l t o n o i s e r a t i o f o r t h e weaker i n t e n s i t y s t r u c t u r e s . -42- 4.3.3. Energy C a l i b r a t i o n . The energy s c a l e was u s u a l l y f i x e d w i t h r e s p e c t t o the e l a s t i c peak as d e s c r i b e d i n S e c t i o n ( 4 . 3 . 2 ) . The v o l t a g e s were measured u s i n g a d i g i t a l v o l t m e t e r w i t h an a c c u r a c y o f ± 0.1 v o l t on t h e 1000 v o l t r a n g e . S i n c e a c a l i b r a t e d v o l t a g e s o u r c e i n t h e 200 - 700 v o l t r e g i o n was n o t c o n t i n u o u s l y a v a i l a b l e , t h e l o w e s t energy d i s c r e t e peaks i n t h e K - s h e l l energy l o s s s p e c t r a o f m o l e c u l a r n i t r o g e n and c a r b o n monoxide ( b o t h c a r b o n and oxygen K - s h e l l s ) were measured w i t h a F l u k e 343 A c a l i b r a t i o n power s u p p l y . T h i s p r o v i d e d t h r e e i n t e r n a l energy s t a n d a r d s (N^ = 400.93 ± 0.05 eV, C K = 287.28 ± 0.05 eV and 0 K = 534.0 ± 0.1 eV) w h i c h were p e r i o d i c a l l y used t o c a l i b r a t e t h e d i g i t a l v o l t m e t e r . A b s o l u t e e n e r g i e s were d e t e r m i n e d f o r t h e ammonia spec t r u m by r e c o r d i n g t h e n i t r o g e n K - s h e l l s pectrum o f s e v e r a l m i x t u r e s c o n t a i n i n g d i f f e r e n t p a r t i a l p r e s s u r e s o f methane and m o l e c u l a r n i t r o g e n and c a l i b r a t i n g t h e ammonia peaks w i t h r e s p e c t t o t h e i n t e n s e 400.93 eV peak o f n i t r o g e n (see F i g u r e 7 ) . The c a r b o n K - s h e l l energy l o s s s p e c t r u m o f methane was s i m i l a r l y c a l i b r a t e d a g a i n s t t h e f i r s t d i s c r e t e peak o b s e r v e d i n t h e K - s h e l l e nergy l o s s s p e c t r u m o f c a r b o n d i o x i d e (290.7 ± 0.2 e V ) , (see F i g u r e 7 ) . The a b s o l u t e e n e r g i e s a r e a c c u r a t e t o ± 0.2 eV f o r a l l K - s h e l l s p e c t r a u n l e s s o t h e r w i s e s t a t e d . 4.3.4. Vacuum System. The c o m p lete e x p e r i m e n t a l arrangement i s shown i n P l a t e 2. The vacuum chamber c o n s i s t s o f a 16" o u t s i d e d i a m e t e r aluminum t u b e , 16" i n h e i g h t w i t h a l / 2 " - t h i c k w a l l . The bottom o f t h e tube r e s t s on a v i t o n 0 - r i n g l o c a t e d i n a machined groove i n t h e b a s e p l a t e (see P l a t e 1 ) . The t o p o f t h e chamber i s s i m i l a r l y c l o s e d w i t h an aluminum l i d ( c o n t a i n i n g an a i r •.v NH < 4l C H , CO i 2.7 —f— 400.93 eV — I — 290.7 eV FIGURE 7. Energy c a l i b r a t i o n o f K-shel l s p e c t r a ; a . ammonia c a l i b r a t e d using molecu lar n i t rogen (400.93 eV peak) , b. methane c a l i b r a t e d using carbon d iox ide (290.7 eV peak). -44- PLATE 2 Complete E x p e r i m e n t a l Arrangement. -45- i n l e t v a l v e and i o n i z a t i o n gauge head) and 0 - r i n g s e a l . A l l e l e c t r i c a l c o n n e c t i o n s a r e made v i a h i g h v o l t a g e c e r a m i c o c t a l - p l u g s o r s i n g l e f e e d - t h r o u g h s . These a r e s o l d e r e d i n t o f l a n g e s w h i c h a r e b o l t e d t o t h e low e r s i d e o f t h e b a s e p l a t e and s e a l e d w i t h v i t o n 0 - r i n g s . The l i d and vacuum chamber may t h e r e f o r e be e a s i l y removed (no b o l t s a r e used on t h e main chamber) t o p r o v i d e r e ady a c c e s s t o t h e s p e c t r o m e t e r . The vacuum chamber i s s c r e e n e d f r o m m a g n e t i c f i e l d s by a mu-metal s h i e l d ( n ot shown i n P l a t e 2 ) . The vacuum i s produced by an NRC 4" d i f f u s i o n pump ( u s i n g C o n v a l e x 10 p o l y p h e n y l e t h e r ) w i t h a w a t e r b a f f l e , l i q u i d n i t r o g e n t r a p , and 5" g a t e v a l v e between t h e pump and vacuum chamber. The t y p i c a l base p r e s s u r e o f - ft t h e system i s ̂  1 x 10~ t o r r . 4.4. Sample P u r i t y . A l l c h e m i c a l samples used i n t h i s s t u d y were c o m m e r c i a l l y p u r c h a s e d and used 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 . F o r l i q u i d samples t h e normal d e g a s s i n g p r o c e d u r e was f o l l o w e d . The s t a t e d minimum p u r i t y o f t h e samples was as f o l l o w s : N 2 99.99% CH 4 99.99% CO 99.5% NH 3 99.99% 0 £ 99% H 20 DISTILLED NO 98.5% 99.9% FISHER SPECTROANALYSED 98% 99.99% 99.7% 98% 99.9% COS 97.5% FISHER SPECTROANALYSED -46- CHAPTER FIVE DIATOMIC MOLECULES 5.1 . N i t r o g e n and Carbon Monoxide 5 . 1 . 1 . N i t r o g e n . The ground s t a t e e l e c t r o n c o n f i g u r a t i o n o f t h e n i t r o g e n m o l e c u l e i s ( l s a g ) 2 ( l s a u ) 2 ( 2 s a g ) 2 ( 2 5 a / (2p^f ( 2 p a g ) 2 , 1 E g + . a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l e l e c t r o n e nergy l o s s s p e c t r u m i s w e l l known and was r e c o r d e d t o t e s t t h e s p e c t r o m e t e r performance and a l s o t o p r o v i d e a p u r i t y check o f t h e sample. In a d d i t i o n , some i n d i c a t i o n o f t h e p o s s i b i l i t y o f f o r b i d d e n t r a n s i t i o n s c o n t r i b u t i n g t o t h e K - s h e l l s p e c t r u m may be o b t a i n e d . The v a l e n c e s h e l l e l e c t r o n energy l o s s s p e c t r u m o f m o l e c u l a r n i t r o g e n i s shown i n F i g u r e 8. M o l e c u l a r n i t r o g e n has been t h o r o u g h l y s t u d i e d i n t h i s energy r e g i o n by b o t h o p t i c a l (see R e f e r e n c e 75) and e l e c t r o n energy l o s s s p e c t r o s c o p y (see R e f e r e n c e 5 ) . An e l e c t r o n e nergy 62 l o s s s p e c t r u m has been o b t a i n e d w i t h 25 keV i n c i d e n t e nergy e l e c t r o n s and a r e s o l u t i o n , AE(FWHM), o f 0.01 eV. The l o c a t i o n s o f t h e peaks o b s e r v e d i n o u r low r e s o l u t i o n spectrum (AE(FWHM) 0.5 eV) a r e c o n s i s t e n t w i t h t h e h i g h e r r e s o l u t i o n r e s u l t s . Peak A w i t h a maximum a t 9.2 eV i n o u r spec t r u m i s a s s o c i a t e d w i t h t h e L y m a n - B i r g e - H o p f i e l d bands, ^ x g + -*- a ^ n g (2pa g -> 2ptr Q ) . T h i s t r a n s i t i o n i s f o r b i d d e n by e l e c t r i c d i p o l e s e l e c t i o n r u l e s (g g ) , a l t h o u g h i t g i v e s r i s e t o weak p h o t o a b s o r p t i o n because o f m a g n e t i c d i p o l e and e l e c t r i c q u a d r u p o l e i n t e r a c t i o n s (see R e f e r e n c e 76) . The t r a n s i t i o n i s Intensity (arbitrary units) -48- a l s o f o r b i d d e n i n our e x p e r i m e n t s i n c e t h e f i r s t Born a p p r o x i m a t i o n s h o u l d be v a l i d ( i . e . E (2500 eV) » E and e ^ 0°). In e l e c t r o n o n a v e r . impact e x c i t a t i o n t h e i n t e r a c t i o n between t h e i n c i d e n t and t a r g e t e l e c t r o n s i s assumed t o be p u r e l y e l e c t r o s t a t i c and t h e r e f o r e , t h e t r a n s i t i o n i s o n l y a s s o c i a t e d w i t h an e l e c t r i c q u a d r u p o l e i n t e r a c t i o n . S i n c e t h e term symbol o f t h e i n i t i a l and f i n a l s t a t e s d i f f e r , o n l y t h e second term i n e q u a t i o n (2.6.11) c o n t r i b u t e s ( i . e . t h e same m a t r i x element a s s o c i a t e d w i t h e l e c t r i c q u a d r u p o l e t r a n s i t i o n s by p h o t o a b s o r p t i o n ) . F o r f o r w a r d s c a t t e r i n g and f a s t e l e c t r o n i m p a c t , t h e r a t i o o f d i p o l e t o q u a d r u p o l e 5 c r o s s - s e c t i o n s has been g i v e n as a d/o q = 2 E E l 2 / E n 2 £ 2 2 where E = E Q - ( E n / 2 ) and e-j and e 2 a r e t h e d i p o l e and q u a d r u p o l e m a t r i x elements r e s p e c t i v e l y [see ( 2 . 6 . 9 ) ] . T h i s i m p l i e s t h a t t h e d i p o l e t o q u a d r u p o l e i n t e n s i t y r a t i o i n c r e a s e s l i n e a r l y w i t h i n c i d e n t energy. A t 48 eV i n c i d e n t energy and 49 6 = 0 ° , t h e r a t i o i s ^ 16. The r a t i o o b s e r v e d i n our s p e c t r u m , 2500 eV i n c i d e n t energy and 6 a v e r ^ 0-02 r a d . i s ^ 18. A l t h o u g h o u r r e s u l t s were not f o r f o r w a r d s c a t t e r i n g , we s t i l l e x p e c t much l e s s i n t e n s i t y t o be a s s o c i a t e d w i t h a q u a d r u p o l e t r a n s i t i o n . The r e a s o n f o r t h i s d i s c r e p a n c y i s not c l e a r . In f a c t , Bonharr/^, u s i n g 10 keV e l e c t r o n i m p a ct has a l s o o b s e r v e d t h e L y m a n - B i r g e - H o p f i e l d bands, w h i l e G e i g e r a t 25 keV and 9 = 0 has not (see R e f e r e n c e 5 ) . The main i n t e n s i t y o f peak B (12.8 eV) i s a s s o c i a t e d w i t h t h e e x c i t a t i o n o f t h e b s t a t e (12.84 eV, v' = 4 R e f e r e n c e 6 2 ) . The h i g h e r energy peaks (C = 14.0 eV, D = 15.8 eV and E = E = 16.9 eV) r e s u l t from t h e e x c i t a t i o n o f a number o f e l e c t r o n i c s t a t e s (see R e f e r e n c e s 60 and 6 2 ) . The l o c a t i o n o f t h e f i r s t i o n i z a t i o n p o t e n t i a l i n o u r spectrum i s based on t h e e x p e r i m e n t a l v a l u e ^ ' ^ o f 15.57 eV. -49- b. N i t r o g e n K - s h e l l E x c i t a t i o n . The l s a g and I s a ^ e l e c t r o n s a r e i n d i s t i n g u i s h a b l e i n X - r a y PES 32 s t u d i e s a l t h o u g h i n t h e o r y t h e r e s h o u l d be a s m a l l e nergy d i f f e r e n c e 32 between th e two o r b i t a l s ' : The e l e c t r o n s f i l l i n g t h e s e o r b i t a l s a r e e s s e n t i a l l y nonbonding and a r e d e s i g n a t e d " K - s h e l l " e l e c t r o n s because o f t h e i r a t o m i c c h a r a c t e r . The removal o f a K - s h e l l e l e c t r o n w h i c h i s l o c a l - 1 2 i z e d on one n i t r o g e n n u c l e u s produces a n i t r i c o x i d e t y p e " c o r e " . (The " c o r e " i n c l u d e s the two n u c l e i and t h e i r K - s h e l l s . ) I t s h o u l d be n o t e d t h a t a l l s t a t e s h a v i n g a K - s h e l l h o l e a r e i n t e r m e d i a t e s t a t e s t h a t o n l y e x i s t f o r a p p r o x i m a t e l y 1 0 ~ 1 4 seconds b e f o r e t h e y decay by Auger e m i s s i o n ( r e l a x a t i o n v i a a r a d i a t i v e t r a n s i t i o n has a v e r y low p r o b a b i l i t y f o r e l e m e n t s o f low a t o m i c number 6^). I f t h e K - s h e l l e l e c t r o n i s promoted t o t h e f i r s t unoccup- i e d m o l e c u l a r o r b i t a l o f n i t r o g e n , t h e a n t i b o n d i n g 2 p i T g , t h e r e s u l t i n g o u t e r e l e c t r o n i c c o n f i g u r a t i o n i s t h e same as t h a t o f t h e ground s t a t e o f n i t r i c o x i d e . S i m i l a r l y , p r o m o t i o n o f t h e K - s h e l l e l e c t r o n i n n i t r o g e n t o h i g h e r energy o r b i t a l s produces s p e c i e s r e s e m b l i n g n i t r i c o x i d e i n e x c i t e d s t a t e s . Only t h o s e e x c i t e d s t a t e s o f n i t r i c o x i d e w h i c h a r e produced by t h e p r o m o t i o n o f t h e 2pTr* e l e c t r o n can be c o r r e l a t e d w i t h t h e s t a t e s o f n i t r o g e n p r o duced by t h e s i n g l e t r a n s i t i o n o f a K - s h e l l e l e c t r o n . These i n c l u d e t h e d i s s o c i a t - 2 + i v e , non-Rydberg, A' z s t a t e and a l l o f t h e Rydberg s t a t e s o f n i t r i c o x i d e which converge t o t h e ground i o n i c s t a t e . Complete e j e c t i o n o f t h e K - e l e c t - ron produces a s t a t e analogous t o t h e ground s t a t e o f N0 +. T h e r e f o r e , t h e energy p o s i t i o n s o f t h e s t r u c t u r e o b s e r v e d i n t h e n i t r o g e n K - s h e l l spectrum r e l a t i v e t o t h e f i r s t d i s c r e t e peak, s h o u l d r e p r o d u c e t h e energy l e v e l s o f t h e f i r s t Rydberg s e r i e s o f t h e n i t r i c o x i d e m o l e c u l e . Nakamura 27 e t a l . have s u c c e s s f u l l y used t h i s a n a l o g y w i t h n i t r i c o x i d e t o i n t e r p r e t -50- the d i s c r e t e s t r u c t u r e o b s e r v e d i n t h e o p t i c a l a b s o r p t i o n s p e c t r u m o f n i t r o g e n , w h i c h was o b t a i n e d u s i n g s y n c h r o t r o n r a d i a t i o n . The K - s h e l l e l e c t r o n e nergy l o s s s p e c t r u m o f N 2 i s shown i n F i g u r e 9 and t h e peak p o s i t i o n s a r e l i s t e d i n T a b l e 1. The r e l a t i v e e n e r g i e s o f 79 th e n i t r i c o x i d e m o l e c u l e have been drawn above t h e s p e c t r u m i n F i g u r e 9 such t h a t t h e f i r s t Rydberg l e v e l o f n i t r i c o x i d e matches up w i t h t h e second d i s c r e t e peak i n o u r spectrum,. A d i f f i c u l t y i n a p p l y i n g t h i s a n a l o g y i s t h a t we a r e deal-jng w i t h e x c i t a t i o n i n t h e Frank-Condon r e g i o n o f t h e n i t r o g e n ground s t a t e t o " N O - l i k e " s t a t e s . T h e r e f o r e v i b r a t i o n a l pppul-r a t i o n s a r e u n c e r t a i n . The second d i s c r e t e peak was chosen as a r e f e r e n c e p o i n t f o r two r e a s o n s : ( i ) t h e f i r s t d i s c r e t e peak i s b r o a d and i t has been s u g g e s t e d t h a t i t r e p r e s e n t s t r a n s i t i o n s t o two f i n a l s t a t e s a n d , ( i i ) t h e second peak, w h i c h r e p r e s e n t s e x c i t a t i o n t o a Rydberg o r b i t a l (3sa) i s e x p e c t e d t o have an i n t e r n u c l e a r s e p a r a t i o n c l o s e t o t h a t o f t h e n i t r o g e n ground s t a t e and t h e r e f o r e s h o u l d have l e s s v i b r a t i o n a l e x c i t a t i o n . The spectrum has been d i v i d e d i n t o t h r e e r e g i o n s on t h e b a s i s o f t h e r e l a t - i v e energy l e v e l s o f n i t r i c o x i d e ( s e e F i g u r e 9 ) . Region I i n c l u d e s a l l o f th e d i s c r e t e s t r u c t u r e and e x t e n d s up t o t h e f i r s t i o n i z a t i o n p o t e n t i a l o f n i t r i c o x i d e (which c o r r e s p o n d s t o t h e K-edge o f n i t r o g e n ) , R e gion I I extends from t h e f i r s t t o t h e second i o n i z a t i o n p o t e n t i a l o f n i t r i c o x i d e and Region I I I i n c l u d e s a l l t h e s t r u c t u r e above t h e second i o n i z a t i o n p o t - e n t i a l . The d i s c r e t e p a r t o f t h e s p e c t r u m ( R e g i o n I ) i s i n good agreement w i t h t h e r e l a t i v e n i t r i c o x i d e l e v e l s and a l s o w i t h t h e p h o t o a b s o r p t i o n 27 spectrum (see F i g u r e 1 0 ) . The ground v i b r a t i o n a l s t a t e o f n i t r i c o x i d e c o r r e s p o n d s t o an energy 0.4 eV below t h e maximum o f t h e f i r s t d i s c r e t e peak i n our s p e c t r u m (see F i g u r e 9 ) , whereas i n t h e p h o t o a b s o r p t i o n s p e c t - I I I 1 1 1 , 1 — 400 4KD 420 430 energy loss(eV) FIGURE 9. K - s h e l l energy l o s s spectrum o f m o l e c u l a r n i t r o g e n . TABLE 1 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND ASSIGNMENTS OF PEAKS OBSERVED IN REGION I OF THE K-SHELL SPECTRA OF N 2 AND CO (CARBON K-SHELL). Peak N i t r o g e n C O ( C K - s h e l l ) T h i s work A s s i g n m e n t 6 N i t r i c oxide T h i s work O p t i c a l 2 7 ( r e f . 79) Energy A E Energy AE Energy AE O r b i t a l 9 S t a t e S t a t e Energy 1 4 0 0 . 6 2 a 0 4 0 0 . 1 1 c 286.86 d 0 2pirg n u 0 2 406.10 5.48 b 405.59 5.48 b 292.34 5 .48 b 3 s a g 1 + z u A V 5.48 3. 407.00 6.38 406.50 6.39 293.31 6 .45 3 p * u n u C 2 n 6.49 406.72 6.61 3 P a u 1 + z u DV 6.61 4. 408.39 7.77 407.66 7.55 294.77 7 .91 4 s a g 1 + z u EV 7.55 407.90 7.79 3aV 9 n u H , 2 n 7.88 3dc I 7.88 K-edge 1 409.9 409.5 296.1 a Peak maximum a t 400.93 ± 0.05 eV. b The second peak was used t o p o s i t i o n t h e r e l a t i v e n i t r i c o x i d e l e v e l s . c C e n t r e o f t r u n c a t e d peak a t 400.84 eV. d Peak maximum a t 287.28 ± 0.05 eV. e Omit t h e g and t h e u f o r c a r b o n monoxide s t a t e s . f V a l u e s f r o m E S C A 3 2 : N 2 , 409.9 eV; CO, carbon-K 295.9 eV. g O n l y t h e o u t e r o r b i t a l s i n v o l v e d i n t h e K - e x c i t a t i o n s have been i n c l u d e d . NO CJO I 10 eV Relative Energy FIGURE 10. Comparison o f t h e r e l a t i v e e n e r g i e s o f v a l e n c e e x c i t e d s t a t e s o f n i t r i c o x i d e and K - s h e l l e x c i t e d s t a t e s o f n i t r o g e n and carbon monoxide (carbon K ) . -54- 27 rum t h e ground v i b r a t i o n a l s t a t e was 0.5 eV below t h e c e n t r e o f t h e t r u n c a t e d f i r s t d i s c r e t e peak. The d i f f e r e n c e i s p a r t i a l l y e x p l a i n e d by t h e asymmetric shape we f i n d f o r t h i s peak. A c o m p a r i s o n o f t h e n i t r o g e n K - s h e l l energy l o s s s p e c t r u m o b t a i n e d w i t h a r e s o l u t i o n [AE(FWHM)] o f 27 0.5 eV ( t h i s work) and the p h o t o a b s o r p t i o n s p e c t r u m [AE(FWHM) < 0.2 eV] i s shown i n F i g u r e 11. The f i r s t d i s c r e t e peak o b s e r v e d i n t h e photo- a b s o r p t i o n spectrum i s " t r u n c a t e d " because o f t o t a l a b s o r p t i o n o f t h e a v a i l 30 a b l e r a d i a t i o n . The d i s c r e t e p a r t o f t h e s p e c t r u m has a l r e a d y been 27 a s s i g n e d by Nakamura e t a l . (see T a b l e 1 ) , w i t h t h e i n t e n s e peak we o b s e r v e a t 400.93 ± 0.05 eV b e i n g a t t r i b u t e d t o t h e p r o m o t i o n o f a l s a u e l e c t r o n t o t h e l o w e s t u n f i l l e d m o l e c u l a r o r b i t a l o f n i t r o g e n , t h e a n t i - bonding 2p7Tg. N 2 ( l s a g ) 2 ( l s a j 2 ( 2 p a g ) 2 , X 1 ^ * - O s a g ) 2 ( l s a ^ 1 ( 2 p a g ) 2 ( 2 p i r g ) \ \ 29 30 From a c o n s i d e r a t i o n o f e l e c t r o n - i o n c o i n c i d e n c e s p e c t r a ' , and t h e 27 36 p h o t o a b s o r p t i o n d a t a , W u i l l e u m i e r and Krause have c o n c l u d e d t h a t two e x c i t e d s t a t e s c o n t r i b u t e t o t h e f i r s t peak, t h e ^ and t h e ''z + s t a t e s . The ẑ..+ s t a t e r e s u l t s f r o m t h e p r o m o t i o n o f a l s o e l e c t r o n t o t h e 2pa,, u r q u 2 + o r b i t a l and i s a n alogous t o t h e d i s s o c i a t i v e A 1 z s t a t e o f t h e n i t r i c o x i d e m o l e c u l e . From t h e s h a r p Auger peaks r e s u l t i n g f r o m t h e decay o f t h e s e s t a t e s , i t was c o n c l u d e d t h a t a maximum o f two v i b r a t i o n a l l e v e l s o f t h e s t a t e were e x c i t e d . T h e r e f o r e , i n t h e energy l o s s s p e c t r u m t h e r e s h o u l d be one o r two v i b r a t i o n a l t r a n s i t i o n s t o t h e 'n s t a t e and a b r o a d e r continuum c o n t r i b u t i o n t o t h e h i g h e r e n e r g y s i d e o f t h e peak f r o m t h e ẑ + s t a t e . Our r e s u l t s show t h a t t h e f i r s t d i s c r e t e peak has a FWHM -55- N 2 K-shell N x10 2.5 kV Electron Impact 31.0 30.2 Synchrotron Tokyo 29.5 o A 400 410 420 eV FIGURE 11. Comparison o f t h e K - s h e l l e nergy l o s s s p e c t r a o f m o l e c u l a r n i t r o g e n o b t a i n e d u s i n g e l e c t r o n impact and s y n c h r o t r o n r a d i a t i o n . -56- o f 0.8 eV, wh i c h i s s i g n i f i c a n t l y more th a n t h e 0.5 eV FWHM o f t h e peak from t h e e l a s t i c a l l y s c a t t e r e d e l e c t r o n s under i d e n t i c a l 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 , t h e peak i s s l i g h t l y a s y m m e t r i c on t h e h i g h energy s i d e . We a r e not a b l e t o make any d e f i n i t e new c o n c l u s i o n from o u r r e s u l t s . However, we o b s e r v e a base w i d t h ( a t 5% o f t h e peak h e i g h t ) o f about 2 eV i n c o n t r a s t t o t h e 3 eV r e p o r t e d by van d e r W i e l and 30 E l - S h e r b i n i a t an impact energy o f 10 keV and a l s o , i f t h e r e a r e two s t a t e s , t h e y must be l e s s t h a n 0.5 eV a p a r t . I t i s p o s s i b l e t h a t t h e r e l a t i v e c r o s s s e c t i o n s f o r t h e two p r o c e s s e s a r e s i g n i f i c a n t l y d i f f e r e n t a t 2.5 and 10 keV. A v a l u e o f 409.9 eV was d e r i v e d f o r t h e K - s h e l l b i n d i n g e nergy from t h e r e l a t i v e n i t r i c o x i d e l e v e l s and t h i s i s i n e x c e l l e n t agreement w i t h 32 t h e X-ray PES v a l u e . We have d e r i v e d an a p p r o x i m a t e c o n t i n u u m shape by 80 + u s i n g s e m i e m p i r i c a l X-ray mass a b s o r p t i o n c o e f f i c i e n t s f o r n i t r o g e n (see the hatched r e g i o n i n F i g u r e 9 ) . S t r u c t u r e i s o b s e r v e d above t h e K-edge i n s t e a d o f a smooth c o n t i n u o u s d e c r e a s e . T h i s s t r u c t u r e r e p r e s e n t s a v a r i e t y o f m u l t i p l e e l e c t r o n t r a n s i t i o n s i n v o l v i n g one K - e l e c t r o n and one o r more v a l e n c e e l e c t r o n s . The f o l l o w i n g t w o - e l e c t r o n t r a n s i t i o n s a r e 1 The r e l a t i o n s h i p between h i g h impact e n e r g y . l o s s s p e c t r a and p h o t o a b s o r p t - i o n d a t a has been d e r i v e d i n C h a p t e r Two. For o u r e x p e r i m e n t a l c o n d i t i o n s t h e c o n v e r s i o n f a c t o r i s a p p r o x i m a t e l y (energy l o s s ) " 3 ( s e e 2.6.15), ( i f a t a momentum t r a n s f e r , K, o f about 1 au, t h e h i g h e r terms i n K i n t h e gener- a l i z e d o s c i l l a t o r s t r e n g t h (2.6.8) can be n e g l e c t e d ) . T h i s f a c t o r was used t o o b t a i n t h e r e l a t i v e b e h a v i o u r o f t h e e x t r a p o l a t e d mass a b s o r p t i o n co- e f f i c i e n t s i n t h e r e g i o n o f o u r spectrum. The a b s o r p t i o n c o e f f i c i e n t s have a c o n t r i b u t i o n from shake-up and s h a k e - o f f p r o c e s s e s , but a t e n e r g i e s f a r above t h e K-edge we e x p e c t t h i s c o n t r i b u t i o n t o be a c o n s t a n t f r a c t i o n o f the K-continuum. The K-continuum was c o n s t r u c t e d by n o r m a l i z i n g t h e d a t a t o the h e i g h t o f our spectrum a t t h e K-edge. -57- e x p e c t e d t o make th e l a r g e s t c o n t r i b u t i o n s : i . d o u b l e e x c i t a t i o n ; i . e . , shake-up o f a v a l e n c e e l e c t r o n i n c o n j u n c t i o n w i t h K - s h e l l e x c i t a t i o n , d e s i g n a t e d by ( N 2 " ) where t h e s u p e r s c r i p t K-1 denotes a h o l e i n t h e K - s h e l l . I t i s i n t e r e s t - i n g t o n ote t h a t t h e s e s t a t e s s h o u l d c o r r e l a t e w i t h t h e Rydberg and non-Rydberg s t a t e s o f n i t r i c o x i d e produced by t h e e x c i t a t i o n o f a 2pOg o r 2 p i r u e l e c t r o n , i i . e x c i t a t i o n and i o n i z a t i o n ; i n v o l v i n g an e l e c t r o n f r o m b o t h t h e K- and v a l e n c e s h e l l , where one o f t h e e l e c t r o n s i s e j e c t e d and t h e o t h e r remains b e h i n d i n a h i g h e r u n f i l l e d o r b i t a l , d e s i g n a t e d by K— 1 +* ( N 2 ~ ) • The s i m u l t a n e o u s i o n i z a t i o n o f a K - e l e c t r o n and a v a l e n c e e l e c t r o n r e q u i r e d an e n e r g y o u t s i d e t h e range o f t h e s p e c t r u m . The broad band o b s e r v e d i n o u r s p e c t r u m i n R e g i o n I I must be a s s o c i a t e d K-1 ** w i t h d i s c r e t e s t r u c t u r e a r i s i n g from d o u b l e e x c i t a t i o n s , i . e . ( N 2 ) s t a t e s , s i n c e f r o m t h e n i t r i c o x i d e a n a l o g y , t h e l o w e s t p o s s i b l e ( N 2 " ) s t a t e s h o u l d c o r r e s p o n d t o t h e f i r s t e x c i t e d i o n s t a t e o f n i t r i c o x i d e (a 3 z + ) w h i c h i s 6.4 e V ^ above t h e ground i o n i c s t a t e . I t i s i n t e r e s t i n g 79 t o n o t e t h a t a number o f a u t o i o n i z i n g s t a t e s o f n i t r i c o x i d e has been o b s e r v e d between t h e f i r s t and t h e second i o n i z a t i o n p o t e n t i a l s w h i c h a r e analogous t o t h e ( N 2 " ) s t a t e s . The i n t e n s i t y o f t h e f i r s t bump above t h e K-edge i s a p p r o x i m a t e l y 5% o f t h a t o f t h e d i s c r e t e peak a t 401 eV w h i c h i s a r e a s o n a b l e r a t i o f o r shake-up e v e n t s ( f o r example see R e f e r e n c e 8 1 ) . T h i s s u g g e s t s t h a t t h e 2p-n^ o r b i t a l i s i n v o l v e d i n t h e s e e x c i t a t i o n s . The second r i s e s t a r t i n g ^ 6 eV above t h e K-edge i s then i d e n t i f i e d w i t h o n s e t s o f i o n i z a t i o n t o a s e r i e s o f ( N 2 ) s t a t e s analogous t o NO s t a t e s whose 79 t h r e s h o l d s (known from PES) a r e too c l o s e t o g e t h e r t o o b s e r v e them s e p a r - -58- a t e l y . However, as shown i n F i g u r e 9, t h e p o s i t i o n o f t h e second bump c o r r e l a t e s w i t h t h e s e s t a t e s ( i t i s p o s s i b l e t h a t d o u b l y e x c i t e d s t a t e s , K-1 ** ( N 2 ) , a l s o c o n t r i b u t e t o t h e i n t e n s i t y i n Region I I I ) . These ( N 2 " ) s t a t e s s h o u l d g i v e r i s e t o a s e r i e s o f s a t e l l i t e peaks i n an X-ray p h o t o e l e c t r o n spectrum a t t h e low energy s i d e o f t h e n i t r o g e n K - s h e l l peak. I t i s t h e r e f o r e i n t e r e s t i n g t o compare t h e s t r u c t u r e we o b s e r v e above 81 416 eV w i t h t h e s a t e l l i t e peaks o b s e r v e d by C a r l s o n e t a l . . The s c a t t e r o f d a t a p o i n t s a t t h e base o f the i n t e n s e K - s h e l l peak o f R e f e r e n c e 81 does not a l l o w a c o n c l u s i o n about t h e p o s s i b l e p r e s e n c e o f s a t e l l i t e s around 6 eV below the K - s h e l l peak. In o u r s p e c t r u m h i g h e r o n s e t s a r e not d i s t i n c t enough t o compare w i t h t h e s a t e l l i t e l i n e s o b s e r v e d i n R e f e r e n c e 81. (However, i n c a r b o n monoxide, o n s e t s a r e c l e a r l y o b s e r v e d and c o r r e l a t e w i t h t h e s a t e l l i t e p e aks.) In o r d e r t o compare t h e i n t e n s i t i e s we o b s e r v e K-1 +* 81 f o r the ( N 2 ~ ) c o n t i n u a w i t h t h e l i n e i n t e n s i t i e s i n t h e ESCA spec t r u m , we note t h e f o l l o w i n g f e a t u r e s o f o u r s p e c t r u m : ( i ) The h e i g h t o f t h e jumps i n Regions I I and I I I a r e o f t h e same magnitude as t h a t o f t h e K-jump. ( i i ) A t t h e h i g h energy l i m i t o f o u r s p e c t r u m t h e s t r u c t u r e has d e c r e a s e d t o a h e i g h t w h i c h i s r o u g h l y 30% h i g h e r than the K-continuum, K-1 + 81 ( N 2 ~ ) . S i n c e C a r l s o n e t a l . o b s e r v e d a t o t a l s a t e l l i t e i n t e n s i t y o f a p p r o x i m a t e l y 15% o f the K - s h e l l peak a t a photon energy o f 1487 eV, o u r d a t a a t t h e h i g h energy s i d e o f t h e s p e c t r u m a r e c o n s i s t e n t w i t h t h i s e a r l i e r work. However, i t has been found i n a wide range o f c a s e s (see f o r i n s t a n c e R e f e r e n c e 82) where e j e c t i o n o f a deep i n n e r e l e c t r o n i s i n v o l v e d , t h a t the i n t e n s i t y r a t i o o f d o u b l e t r a n s i t i o n s (one i n n e r and one o u t e r s h e l l e l e c t r o n ) r e l a t i v e t o s i n g l e t r a n s i t i o n s ( i n n e r e l e c t r o n ) as a -59- f u n c t i o n o f photon energy r i s e s s t e e p l y from t h r e s h o l d and the n becomes c o n s t a n t . On t h i s b a s i s we would e x p e c t t h e s t r u c t u r e s t o have h e i g h t s o f o n l y a few p e r c e n t o f t h e K-jump t h r o u g h o u t Region I I I . The f a c t t h a t much l a r g e r s t r u c t u r e s a r e p r e s e n t s u g g e s t s t h a t t h e r e i s a s t r o n g c o n t r i b u t i o n from a s e r i e s o f (N^ ~ ) s t a t e s c o n v e r g i n g t o each o f t h e i n d i c a t e d t h r e s h o l d s . An a l t e r n a t i v e e x p l a n a t i o n would t a k e a c c o u n t o f t h e i n d i s t - i n g u i s h a b i l i t y o f t h e e l e c t r o n s , due t o w h i c h a s h a k e - o f f i n c o n j u n c t i o n w i t h d i s c r e t e K - e x c i t a t i o n g i v e s r i s e t o t h e same ( N 2 ~ ) s t a t e as a shake-up " f o l l o w i n g " K - i o n i z a t i o n . However, t h e f i r s t p r o c e s s might have more o f t h e c h a r a c t e r i s t i c s o f a resonance t r a n s i t i o n and t h e r e f o r e m ight K - l +* l o c a l l y enhance t h e "n o r m a l " i n t e n s i t y o f t h e ( N 2 ) c o n t i n u a . The o n s e t o f t h e i n c r e a s e i n i n t e n s i t y beyond t h e K-edge i n t h e photo- 27 a b s o r p t i o n s p e c t r u m o f n i t r o g e n a g r e e s w i t h t h e o n s e t o f s t r u c t u r e i n Region I o f o u r sp e c t r u m ( s e e F i g u r e 1 1 ) . A l s o , t h e s t r u c t u r e we o b s e r v e above t h e K-edge i s q u a l i t a t i v e l y s i m i l a r t o t h a t o b s e r v e d i n t h e e l e c t r o n energy l o s s spectrum o f n i t r o g e n measured i n c o i n c i d e n c e w i t h t h e N 2 + + + 30 ( p l u s N ) i o n s produced by Auger decay. (From the s i m i l a r i t y w i t h t h e c o i n c i d e n c e s t u d i e s o f car b o n monoxide where an a m b i g u i t y o f i o n i c s t a t e s does not e x i s t , we can c o n c l u d e t h a t most o f t h e i n t e n s i t y i s due t o N 2 + + i o n s . ) A normal Auger decay o f t h e d o u b l y e x c i t e d s t a t e s would produce a s i n g l y c h a r g e d i o n , o f w h i c h t h e c o i n c i d e n c e s p e c t r u m shows no a p p r e c i a b l e i n t e n s i t y i n t h i s energy range. The i n f e r e n c e t h a t N 2 + + i s t h e predominant p r o d u c t i n d i c a t e s t h a t t h e ( N 2 " ) s t a t e s f i r s t a u t o i o n i z e t o form K-1 • + ( N 2 " ) , wh i c h t h e n undergo a normal Auger decay. T h i s i s s u p p o r t e d by t h e f a c t t h a t t h e decay r a t e o f the f i r s t a u t o i o n i z i n g s t e p w i l l c e r t a i n l y be f a s t e r t h a n t h a t o f an Auger t r a n s i t i o n . -60- 5.1.2. Carbon Monoxide. Carbon monoxide i s i s o e l e c t r o n i c w i t h n i t r o g e n and has a ground s t a t e e l e c t r o n c o n f i g u r a t i o n o f ( l s a 0 ) 2 ( l s a c ) 2 ( 2 s a ) 2 ( 2 s a * ) 2 ( 2 P T T ) 4 ( 2 p a ) 2 , V. We have s t u d i e d both t h e c a r b o n and oxygen K - s h e l l e n e r g y l o s s s p e c t r a . The v a l e n c e s h e l l s p e c t r u m has a l s o been r e c o r d e d . a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l e l e c t r o n energy l o s s s p e c t r u m o f c a r b o n monoxide i s shown i n F i g u r e 12. The l o c a t i o n s o f peaks a r e c o n s i s t e n t w i t h h i g h e r 48 53 55 83 75 r e s o l u t i o n e nergy l o s s s p e c t r a ' ' ' and o p t i c a l d a t a . Peak A w i t h a maximum a t 8.4 eV i s a s s o c i a t e d w i t h t h e " f o u r t h p o s i t i v e g r o u p " o f carbon monoxide, w h i c h i s a n a l o g o u s t o t h e f o r b i d d e n L y m a n - B i r g e - H o p f i e l d bands o f n i t r o g e n . However, i n the c a s e o f c a r b o n monoxide, t h e t r a n s i t i o n i s a l l o w e d , ->- A (2p<? -v TT*) and t h i s i s r e f l e c t e d i n t h e much h i g h e r i n t e n s i t y o f t h i s peak compared w i t h peak A i n t h e n i t r o g e n v a l e n c e s h e l l s p e c t r u m ( F i g u r e 8 ) . Peaks B (10.7 eV) and C (11.3 eV) a r e a s s o c i a t e d w i t h t h e B and C 1E +/E Rydberg s t a t e s r e s p e c t i v e l y (see R e f e r e n c e s 55 and 75 ) . The h i g h e r energy p e a k s , D (13.4 e V ) , E (16.3 eV) and F (17.0 eV) a r e a s s o c i a t e d w i t h a number o f o v e r l a p p i n g e x c i t a t i o n s . The l o c a t i o n o f t h e 75 f i r s t i o n i z a t i o n p o t e n t i a l on t h e s p e c t r u m i s based on t h e o p t i c a l and U V - PES 7 8 v a l u e o f 14.00 eV. b. Carbon K - s h e l l E x c i t a t i o n . The p r o d u c t i o n o f a c a r b o n K - s h e l l " h o l e " s h o u l d produce a n i t r i c o x i d e t y p e c o r e and t h e r e f o r e we w o u l d e x p e c t t h e c a r b o n K - s h e l l s p e c t r u m o f carbon monoxide t o be s i m i l a r t o t h e K - s h e l l s p e c t r u m o f n i t r o g e n . As i s shown i n F i g u r e 13, t h i s i s t h e c a s e , and t h e r e l a t i v e e n e r g i e s o f the ELASTIC 1 s t L P C ' IG CO B- 0 10 20 30 40 Energy Loss (eV) FIGURE 12. V a l e n c e s h e l l energy l o s s spectrum o f carbon monoxide. energy loss (eV) FIGURE 13. Carbon K - s h e l l energy l o s s spectrum o f carbon monoxic -63- peaks a r e i n good agreement w i t h t h o s e o f t h e n i t r i c o x i d e l e v e l s (see F i g u r e 1 0 ) . T h e r e f o r e we have a s s i g n e d t h e d i s c r e t e peaks i n Region I by a n a l o g y w i t h the Rydberg s t a t e s o f n i t r i c o x i d e ( T a b l e 1 ) . The i n t e r p r e t - a t i o n o f both the d i s c r e t e and continuum p a r t o f the s p e c t r u m i s t h e same as t h a t f o r n i t r o g e n and t h e r e f o r e t h e arguments f o r peak a s s i g n m e n t s w i l l n ot be r e p e a t e d . I n s t e a d we w i l l d i s c u s s t h e d i f f e r e n c e s between t h e two s p e c t r a . The r e l a t i v e i n t e n s i t i e s o f t h e peaks i n Region I o f t h e two s p e c t r a a r e s l i g h t l y d i f f e r e n t . A f u r t h e r d i f f e r e n c e i s t h a t t h e f i r s t d i s c r e t e peak a t 287.28 ± 0.05 eV i n the car b o n K - s h e l l s p e c t r u m has a FWHM o f 0.56 eV and i s s y m m e t r i c , i n d i c a t i n g t h a t o n l y one o r two v i b r a t i o n a l l e v e l s o f t h e s t a t e a r e e x c i t e d . T h i s i s i n e x c e l l e n t agreement w i t h t h e Auger 32 + r e s u l t s where t h e v i b r a t i o n a l s p a c i n g s o f t h e f i n a l CO s t a t e s (produced by a u t o i o n i z a t i o n o f the "'n s t a t e ) have been r e s o l v e d . The base w i d t h ( a t 5% o f t h e peak h e i g h t ) o f t h e f i r s t peak i s o n l y 1.5 eV, r a t h e r t h a n t h e 30 3 eV r e p o r t e d i n R e f e r e n c e 30. (The c o i n c i d e n c e s p e c t r a produced t h e same broad asymmetric shape f o r t h e f i r s t peak i n both n i t r o g e n and c a r b o n monoxide.) T h i s m i ght i n d i c a t e t h a t the r e l a t i v e e x c i t a t i o n c r o s s - s e c t i o n s f o r t he s t a t e s r e p r e s e n t e d by t h e f i r s t peak a r e q u i t e d i f f e r e n t a t 2.5 and 10 keV. An energy o f 296.1 eV was d e r i v e d f o r the K-edge, w h i c h i s i n good 32 agreement w i t h t h e X-ray PES v a l u e o f 295.9 eV. The a p p r o x i m a t e continuum shape ( h a t c h e d r e g i o n i n F i g u r e 13) was c o n s t r u c t e d by s m o o t h l y j o i n i n g t h e e x t r a p o l a t e d b e h a v i o u r f o r t h e X-ray a b s o r p t i o n c o e f f i c i e n t s 84 o f methane and m e t h y l a l ( m a i n l y carbon-K) t o the continuum d e c r e a s e near t h e K-edge. The s t r u c t u r e o b s e r v e d i n Region I I i s a s s o c i a t e d w i t h d i s c r e t e -64- s t a t e s , ( C N ~ ' 0 ) ( i . e . c a r b o n K - e x c i t a t i o n and v a l e n c e s h a k e - u p ) . Shaw 85 and Thomas have i n v e s t i g a t e d t h e X-ray PES s p e c t r u m i n t h e energy r e g i o n 5.4 t o 6 eV below t h e main c a r b o n K - s h e l l peak and have put a l i m i t f o r t h e i n t e n s i t y o f any s a t e l l i t e s t r u c t u r e s ( f r o m C O ) s t a t e s ) as 0.4% o f the c a r b o n K - s h e l l peak. T h i s s u p p o r t s o u r a s s i g n m e n t o f t h e s t r u c t u r e i n K-1 +* Region I I . The s t r u c t u r e i n Region I I I r e p r e s e n t s (C 0) s t a t e s and t h e r e i s p r o b a b l y a l a r g e c o n t r i b u t i o n from d o u b l y e x c i t e d s t a t e s (C ~ 0) . +* 79 The e n e r g i e s o f t h e NO s t a t e s as g i v e n by PES c o r r e l a t e w i t h the f i r s t b road bump i n R e g i o n I I I . O n s ets have been r e s o l v e d where t h e i o n i z a t i o n p o t e n t i a l s a r e s u f f i c i e n t l y f a r a p a r t . The e n e r g i e s o f the s a t e l l i t e l i n e s 81 o b t a i n e d by C a r l s o n e t a l . u s i n g an X-ray energy o f 1487 eV a r e i n e x c e l l e n t agreement w i t h t h e o n s e t s o b s e r v e d i n our spectrum (see F i g u r e 13, X-ray PES l i n e s ) . The l o w e s t s a t e l l i t e l i n e o b s e r v e d by C a r l s o n e t a l . i s a t 8.5 eV below t h e K - s h e l l peak, but t h e s c a t t e r o f d a t a p o i n t s would p r o b a b l y mask a broad band o f s a t e l l i t e l i n e s c l o s e t o t h e i n t e n s e K - s h e l l peak. There a r e o b v i o u s d i f f e r e n c e s between t h e s t r u c t u r e s o b s e r v e d above the K-edge i n the n i t r o g e n spectrum and t h e car b o n K - s h e l l spectrum o f ca r b o n monoxide. I n R e g i o n I I t h e components making up t h e d i s c r e t e s t r u c t u r e s do not have t h e same r e l a t i v e i n t e n s i t i e s as i n n i t r o g e n . In carbon monoxide the s t r u c t u r e i s g e n e r a l l y more i n t e n s e r e l a t i v e t o t h e K-jump and the h i g h e r o n s e t s i n Region I I I a r e c l e a r l y r e s o l v e d . I t i s r e a s o n a b l e t o e x p e c t d i f f e r e n t shake-up and s h a k e - o f f p r o b a b i l i t i e s i n t h e two m o l e c u l e s , n i t r o g e n and c a r b o n monoxide. Carbon monoxide has two d i f f e r e n t n u c l e i , and t h e r e f o r e , each m o l e c u l a r o r b i t a l w i l l g e n e r a l l y have unequal "carbon and oxygen" e l e c t r o n d e n s i t i e s . A change i n the s c r e e n i n g o f t h e carbon n u c l e u s by t h e p r o d u c t i o n o f a c a r b o n K - s h e l l h o l e , s h o u l d -65- p r e f e r e n t i a l l y produce shake-up and s h a k e - o f f o f e l e c t r o n s from m o l e c u l a r o r b i t a l s w i t h the h i g h e r " c a r b o n " e l e c t r o n d e n s i t i e s . A l s o i f a d i r e c t i n t e r a c t i o n between t h e i n n e r and t h e v a l e n c e e l e c t r o n s i s i n v o l v e d t o a s i g n i f i c a n t e x t e n t , we would e x p e c t t h i s e f f e c t t o c o n t r i b u t e more t o t h e carbon K - s h e l l s p e c t r u m t h a n t h e n i t r o g e n s p e c t r u m s i n c e t h e c a r b o n K-orb- i t a l i s c l o s e r i n energy t o t h e v a l e n c e s h e l l . The continuum s t r u c t u r e i s q u a l i t a t i v e l y t h e same as t h a t o b s e r v e d i n t h e carbon K - s h e l l energy l o s s s p e c t r u m o f c a r b o n monoxide measured i n c o i n c i d e n c e w i t h C 0 + + i o n s produced by Auger decay"^. As i n t h e case o f n i t r o g e n , t h e d o u b l y e x c i t e d s t a t e s o b s e r v e d i n Region I I do not c o n t r i b u t e t o t h e CO c o i n c i d e n c e s p e c t r u m i m p l y i n g t h a t t h e s e (C " 0) s t a t e s f i r s t a u t o i o n i z e t o form ( C ^ - 1 0 ) + w h i c h t h e n undergo a normal Auger decay t o C 0 + + . c. Oxygen K - s h e l l E x c i t a t i o n . From the c l o s e agreement o b s e r v e d i n both the n i t r o g e n K - s h e l l spectrum and t h e c a r b o n K - s h e l l s p e c t r u m o f c a r b o n monoxide w i t h t h e r e l a t i v e n i t r i c o x i d e l e v e l s , we e x p e c t t h e oxygen K - s h e l l s p e c t r u m t o r e p r o d u c e t h e r e l a t i v e e n ergy s p a c i n g s o f t h e s t a t e s o f t h e CF r a d i c a l ( t h e p r o d u c t i o n o f an oxygen K - s h e l l h o l e s h o u l d produce a CF t y p e c o r e ) . Three s t a t e s o f the c a r b o n m o n o f l u o r i d e r a d i c a l a r e w e l l known from s p e c t - 86 89 2 2 + 2 r o s c o p i c s t u d i e s ' ; t h e ground X n , t h e A E and the B A s t a t e s . 2 2 + R e c e n t l y a D n and p o s s i b l y a C E s t a t e have been o b s e r v e d (see R e f e r e n c e 90 f o r d e t a i l s ) . The i o n i z a t i o n p o t e n t i a l o f CF has been d e r i v e d i n a wide 89 91 range o f e x p e r i m e n t s w i t h e s t i m a t e s from s p e c t r o s c o p i c d a t a ' and 90 92 c a l c u l a t e d v a l u e s ' , i n f a i r agreement (see R e f e r e n c e 92 f o r a complete r e v i e w ) . The oxygen K - s h e l l energy l o s s s p e c t r u m o f c a r b o n monoxide i s shown -66- i n F i g u r e 14. The r e l a t i v e e n e r g i e s o f t h o s e s t a t e s o f CF w h i c h have o u t e r e l e c t r o n i c c o n f i g u r a t i o n s i d e n t i c a l t o t h o s e produced by a s i n g l e t r a n s i t i o n o f a K - e l e c t r o n i n CO, a r e a l s o shown ( t h e B A s t a t e has t h e r e f o r e not been i n c l u d e d i n t h e c o r r e l a t i o n ) . The r e s u l t s a r e l i s t e d i n T a b l e 2 and t e n t a t i v e a s s i g n m e n t s o f t h e d i s c r e t e s t r u c t u r e s have been made on t h e b a s i s o f t h e c a r b o n m o n o f l u o r i d e s t a t e s . The second d i s c r e t e peak has been used t o n o r m a l i z e t h e r e l a t i v e CF energy l e v e l s . A l s o i n c l u d e d i n F i g u r e 14 a r e t h e t h r e e h i g h e r d i s c r e t e peaks i n t h e s p e c t r u m on an expanded s c a l e ( i n s e r t a ) . (The f u l l s p e c t r u m and t h e i n s e r t a r e from d i f f e r e n t d a t a r u n s . ) As shown by F i g u r e 14, t h e r e l a t i v e e n e r g i e s o f t h e p r e s e n t l y known s t a t e s o f CF agree w i t h t h e peaks i n o u r spectrum. The i n t e n s e peak o b s e r v e d a t 534.0 ± 0 . 1 eV ( a n a l o g o u s t o t h e ground 2 X n s t a t e o f CF) has a FWHM o f 1.3 eV and t h e r e f o r e we c o n c l u d e t h a t a number o f v i b r a t i o n a l l e v e l s a r e p o p u l a t e d (5 t o 7 ) . T h i s i s s u p p o r t e d by t h e f a c t t h a t t h e r e l a t i v e CF energy s c a l e i m p l i e s t h a t t h e ground v i b r a t i o n a l s t a t e i s ^ 0.5 eV below t h e c e n t r e o f t h e f i r s t d i s c r e t e peak (see F i g u r e 1 4 ) . The i o n i z a t i o n p o t e n t i a l o f CF, e s t i m a t e d as 8.9 ± 0 . 1 eV from 89 91 s p e c t r o s c o p i c d a t a ' and 9.2 ± 0.5 eV from a H a r t r e e - F o c k SCF c a l c u l - 92 a t i o n l e a d s t o v a l u e s o f 542.4 eV and 542.7 eV r e s p e c t i v e l y f o r t h e oxygen K-edge o f c a r b o n monoxide. These v a l u e s compare f a v o u r a b l y w i t h 32 the e x p e r i m e n t a l X-ray PES energy o f 542.1 eV. An a p p r o x i m a t e K - c o n t m - uum as i n d i c a t e d by the h a t c h e d r e g i o n i n F i g u r e 14 was c o n s t r u c t e d by 80 e x t r a p o l a t i n g X-ray mass a b s o r p t i o n c o e f f i c i e n t s f o r oxygen ( c o r r e c t e d by an (energy l o s s ) f a c t o r ) t o t h e K-edge. Broad s t r u c t u r e i s o b s e r v e d above t h e K-edge wh i c h r e p r e s e n t s shake-up and s h a k e - o f f e v e n t s a s s o c i a t e d I.O-. —I ' 1 ' 1 1 •—I— 5 3 0 5 4 0 5 5 0 5 6 0 energy loss (eV) FIGURE 14. Oxygen K - s h e l l e n e r g y l o s s s p e c t r u m o f carbon monoxide. I n s e r t a (taken from a s e p a r a t e d a t a r u n ) shows the t h r e e h i g h e r d i s c r e t e peaks on an expanded s c a l e . TABLE 2 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND POSSIBLE ASSIGNMENTS OF PEAKS OBSERVED IN REGION I OF THE OXYGEN K-SHELL SPECTRUM OF CARBON MONOXIDE. Peak C O ( 0 K - s h e l l ) t h i s work P o s s i b l e a s signments C F 8 6 - 9 0 Energy AE O r b i t a l d S t a t e S t a t e Energy 1 5 3 3 . 5 a 0 2 p i T * \ X 2 n 0 2 538.8 5 . 3 b 3sa V AV 5.32 3 539.8 6.3 3piT \ D 2n 6.40 4 3pa V C 2 z + ? 6.65 4 540.9 7.4 K-edge c 542.4 a. Peak maximum a t 534.0 ± 0.1 eV. b. The second peak was used t o p o s i t i o n t h e CF l e v e l s . c. E S C A 3 2 v a l u e , 542.1 eV. d. Only t h e o u t e r o r b i t a l s i n v o l v e d i n t h e K - e x c i t a t i o n s have been i n c l u d e d . -69- w i t h K - e x c i t a t i o n and i o n i z a t i o n . R e l a t i v e t o t h e K-jump, t h i s s t r u c t u r e i s more i n t e n s e than i n e i t h e r o f t h e p r e v i o u s s p e c t r a . 5.2. N i t r i c Oxide and Oxygen. 5.2.1. N i t r i c O x i d e . The ground e l e c t r o n i c s t a t e o f t h e n i t r i c o x i d e m o l e c u l e has t h e e l e c t r o n c o n f i g u r a t i o n ( 1 s a Q ) 2 ( l s a N ) 2 ( 2 s a ) 2 ( 2 s a * ) 2 ( 2 p a ) 2 ( 2 P T T ) 4 ( 2 P T T * ) 1 , 2 n. A v a l e n c e s h e l l s p e c t r u m was n o t r e c o r d e d . The l s o g and I s o ^ m o l e c u l a r o r b i t a l s , formed from t h e oxygen K - s h e l l and n i t r o g e n K - s h e l l a t o m i c o r b i t a l s r e s p e c t i v e l y , a r e nonbonding and m a i n l y a t o m i c i n c h a r a c t e r . The e x c i t a t i o n o r i o n i z a t i o n o f an i n n e r s h e l l e l e c t r o n r e s u l t s i n a number o f e l e c t r o n i c s t a t e s f o r each o r b i t a l c o n f i g u r a t i o n because o f t h e c o u p l i n g o f u n p a i r e d e l e c t r o n s p i n s between t h e c o r e and v a l e n c e s h e l l ( s e e T a b l e 3 ) . Thus t h e i o n i z a t i o n o f a n i t r o g e n I s e l e c t r o n o r oxygen I s 3 1 e l e c t r o n r e s u l t s i n n and n m o l e c u l a r i o n s t a t e s . U s i n g X-ray PES, 3 1 n - n e n e r g y s p l i t t i n g s (exchange s p l i t t i n g s ) o f 1.42 eV and 0.55 eV 93 94 have been o b s e r v e d ' f o r n i t r o g e n I s i o n i z a t i o n and oxygen I s i o n i z a t i o n r e s p e c t i v e l y . On t h e b a s i s o f t h e c o r e a n a l o g y model we e x p e c t t h e prom- o t i o n o f a n i t r o g e n I s e l e c t r o n i n n i t r i c o x i d e t o d i s c r e t e l e v e l s below the I s i o n i z a t i o n l i m i t , t o produce a n i t r i c o x i d e s p e c i e s w i t h r e l a t i v e energy l e v e l s s i m i l a r t o t h o s e o f m o l e c u l a r oxygen i n i t s ground and v a l e n c e s h e l l e x c i t e d s t a t e s (produced by t h e e x c i t a t i o n o f an O2, 2pi\^ e l e c t r o n ) . I o n i z a t i o n o f t h e n i t r o g e n I s e l e c t r o n s h o u l d produce a n i t r i c o x i d e s p e c i e s s i m i l a r t o oxygen i n i t s ground i o n i c s t a t e . S i m i l a r l y t h e p r o m o t i o n o f an oxygen I s e l e c t r o n i n n i t r i c o x i d e s h o u l d produce an T A B L E 3 E L E C T R O N C O N F I G U R A T I O N S A N D E L E C T R O N I C S T A T E S O F K - S H E L L E X C I T E D N I T R I C O X I D E AND M O L E C U L A R O X Y G E N . E L E C T R O N C O N F I G U R A T I O N RYDBERG O R B I T A L S M O L E C U L A R S T A T E S 0 N I T R I C 3 O X I D E 1 s a 0 1 S A N 2sa 2 s a * 2pa 2DTT 2 p 7 T * na n?r CO NO 2 2 2 2 2 4 1 X 2 n N K * 0 2 1 2 2 2 4 2 V , V , 2 A , V N K * 0 2 1 2 2 2 4 1 1 4 n , 2 n , 2 n N K * 0 2 1 2 2 2 4 1 1 V , W . V ( 2 ) . 2 A ( 2 ) , 2 E + ( 2 ] N K + 0 2 1 2 2 2 4 1 1 n , n O X Y G E N 0 l S a g l s a u 2 s a g 2 s ° u 2P°g 20, u 2 P * g °2 K * 0 2 2 2 1 2 2 2 2 2 2 4 4 2 3 X 3 E " a }A , b V g g g 3 n , ! n 0 K * U 2 2 1 2 2 2 4 2 1 V , V ( 2 ) , 3 A , V , V , ] A , V o f 2 1 2 2 2 2 2 1 5 n , 3 n ( 4 ) , 3 o , \(3), V N K + °2 2 1 2 2 2 2 2 1 4 - 2 - 2 2 + E , E , A-, D E a The same m o l e c u l a r s t a t e s a r e o b t a i n e d by oxygen I s e x c i t a t i o n i n NO b The numbers i n b r a c k e t s r e f e r t o t h e number o f s t a t e s o f t h a t symmetry. c g, u d e s i g n a t i o n s do n o t a p p l y t o an oxygen m o l e c u l e w i t h a l o c a l i z e d I s h o l e . -71- " N F - l i k e " s p e c i e s . a. N i t r o g e n K - s h e l l E x c i t a t i o n . The n i t r o g e n K - s h e l l e l e c t r o n energy l o s s s p e c t r u m o f n i t r i c o x i d e i s shown i n F i g u r e 15 and t h e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 4. The g e n e r a l appearance o f t h e s p e c t r u m i s s i m i l a r t o t h a t o b s e r v e d f o r t h e d i a t o m i c m o l e c u l e s n i t r o g e n and c a r b o n monoxide i n t h a t t h e s p e c t r u m i s dominated by t h e f i r s t d i s c r e t e peak. T h i s i n t e n s e peak, l o c a t e d a t 399.1 eV i s i n t e r p r e t e d as a r i s i n g from t h e p r o m o t i o n o f a n i t r o g e n I s e l e c t r o n t o t h e l o w e s t a v a i l a b l e m o l e c u l a r o r b i t a l , t h e 2p-rr*. The r e s u l t i n g e l e c t r o n c o n f i g u r a t i o n can g i v e r i s e t o 4 - 2 - 2 2 + E , E , A and z e l e c t r o n i c s t a t e s (see T a b l e 3 ) . E l e c t r i c d i p o l e s e l e c t i o n r u l e s a p p l y t o e l e c t r o n i m p a ct e x c i t a t i o n f o r h i g h i n c i d e n t e n e r g i e s and s m a l l s c a t t e r i n g a n g l e s ( i . e . when t h e f i r s t Born approxim- a t i o n i s v a l i d ) . A l t h o u g h t h e f i r s t Born a p p r o x i m a t i o n may n o t always a p p l y t o o u r e x p e r i m e n t ( t h e i n c i d e n t e nergy i s s i x t i m e s t h e e x c i t a t i o n energy f o r n i t r o g e n K - s h e l l p r o m o t i o n w h i l e t h e r a t i o d e c r e a s e s t o 4.5 f o r oxygen K - s h e l l p r o m o t i o n ) t h e e x p e r i m e n t a l c o n d i t i o n s a r e such t h a t 4 - s p i n f o r b i d d e n t r a n s i t i o n s s h o u l d n o t be o b s e r v e d . T h e r e f o r e , t h e E 2 - 2 2 + s t a t e i s not e x p e c t e d t o c o n t r i b u t e t o o u r spectrum. The E , A and E ? s t a t e s a r e a l l d i p o l e c o n n e c t e d t o t h e ground n s t a t e and peaks a s s o c i a t e d w i t h t h e e x c i t a t i o n o f t h e s e s t a t e s s h o u l d be o b s e r v e d i n o u r s p e c t r u m . The f i r s t d i s c r e t e peak has a AE(FWHM) o f 1.0 eV compared w i t h a A E ( FWHM) o f 0.4 eV o b s e r v e d f o r the peak a s s o c i a t e d w i t h e l a s t i c s c a t t e r e d e l e c t r o n s and i s symmetric ( t h e s l i g h t asymmetry on the t a i l o f t h e peak i s i n s t r u - m e n t a l ) . T h i s s u g g e s t s t h a t the energy s p a c i n g s o f t h e t h r e e d o u b l e t s t a t e s i s l e s s t h a n 1 eV. S i n c e t h i s f i r s t peak i s by f a r t h e most i n t e n s e Energy Loss (eV) FIGURE 15. N i t r o g e n K - s h e l l energy l o s s spectrum o f n i t r i c o x i d e . TABLE 4 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND POSSIBLE ASSIGNMENTS OF PEAKS OBSERVED I N THE K-SHELL ENERGY LOSS SPECTRA OF NO (NITROGEN AND OXYGEN K-SHELLS). N K - S H E L L O R B I T A L 9 A S S I G N M E N T S T A T E S 0 K - S H E L L P E A K ENERGY A E P E A K ENERGY A E 1 399.7 0 2 p T T * 2 - 2 2 + E , A , E 1 532.7 0 2 404.7 5.0 2pa* 2 n , 2 n 3 406.6 6.9 3sa 2 n 2 540.2 7.5 4 407.6 7.9 3 s a , 3 p i T 2 n , 7 5 409.0 ^ 409.8 9.3 ^ 10.1 3p K - E D G E B 410.3 10.6 CO 3 n K - E D G E B 543.3 10.6 6 ^ 410.4 10.7 K - E D G E B 411.8 12.1 CO K - E D G E B 544.0 11.3 413.1 414.5 13.4 14.8 ( S H A K E - U P < AND ( S H A K E - O F F 546.3 13.6 a Only t h e o u t e r o r b i t a l s i n v o l v e d i n t h e K - e x c i t a t i o n s have been i n c l u d e d , b These v a l u e s a r e from X - r a y P E S . 3 2 -74- s t r u c t u r e i n the s p e c t r u m , we e x p e c t t h e s e d i s c r e t e l e v e l s t o g i v e t h e l a r g e s t c o n t r i b u t i o n t o t h e h i g h e n e r g y a u t o i o n i z a t i o n l i n e s o b s e r v e d i n oc t h e Auger s p e c t r u m o f n i t r i c o x i d e e x c i t e d by e l e c t r o n i m p a ct . A t h e o r e t i c a l e s t i m a t e o f t h e energy d i f f e r e n c e s between t h e 4 £ ~ , 2 - 2 2 + £ , A and E s t a t e s ( a r i s i n g f r o m t h e t r a n s i t i o n l s o ^ -> 2p^*) can be made on t h e b a s i s t h a t t h e o r b i t a l w a v e f u n c t i o n s o f t h e f o u r s t a t e s a r e i d e n t i c a l ( 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 ) . O m i t t i n g t h e f i l l e d o r b i t a l s , 4 - 2 the E and A s t a t e s can be r e p r e s e n t e d by t h e s i n g l e d e t e r m i n a n t s , *( Z ) = IlS TT TT | M 2 A ) = | l S TT+*TT+*| where t h e o r b i t a l s have been w r i t t e n i n complex form and t h e b a r r e p r e s e n t s B s p i n . The E s t a - s i n g l e d e t e r m i n a n t s 2 B s p i n . The E s t a t e s a r e a s s o c i a t e d w i t h l i n e a r c o m b i n a t i o n s o f t h e t h r e e * A = | l s i r 4 " T T " * | , <j> = | i s T7+*TT"*| and < f r = | l s TT+*TT"*| ,2 L i n e a r c o m b i n a t i o n s w h i c h a r e e i g e n f u n c t i o n s o f S , t h e t o t a l s p i n a n g u l a r momentum o p e r a t o r , can be found by the Nesbet m e t h o d 9 5 . C o m b i n a t i o n s which s a t i s f y t h e above c o n d i t i o n and have t h e c o r r e c t r e f l e c t i o n symmetry a r e M>(2i') = 1 (2<|>c - $ A - * ). . *(V) = 1 - • ) /2 A B From t h e t o t a l energy e x p r e s s i o n s f o r t h e f o u r s t a t e s and t h e f a c t t h a t K-|S7T+* = K - ] S T r * t h e energy d i f f e r e n c e s a r e E ( 2 Z " ) = E ( 4 E " ) + 3 K l s 7 r + * .... (5.2.1) E ( 2 A ) = E ( V ) + K l s i i + * + V V * .... (5.2.2) - 7 5 - E(V) = (E 4z~) + K, +* + 2K +* _* . . . . ( 5 . 2 . 3 ) ISTT IT TT where i s t h e exchange i n t e g r a l between o r b i t a l s i and j . On t h e b a s i s o f t h e c o r e a n a l o g y m o d e l , n i t r o g e n I s e x c i t a t i o n i n n i t r i c o x i d e ( i . e . K* p r o d u c t i o n o f N 0 s t a t e s ) i s e x p e c t e d t o produce m o l e c u l a r oxygen l i k e s t a t e s . T h e r e f o r e we assume t h a t t h e charg e d i s t r i b u t i o n s o f v a l e n c e K* m o l e c u l a r o r b i t a l s f o r N 0 s t a t e s have an eq u a l c o n t r i b u t i o n from b o t h K* a t o m i c c e n t r e s . The 2 p i T * o r b i t a l i n N 0 i s then a p p r o x i m a t e d by the w a v e f u n c t i o n 4>(2pir*) = 0.707PTTNK* - 0 .707pTr 0 where p̂ K* and p ^ r e p r e s e n t t h e a t o m i c p-rr o r b i t a l s a s s o c i a t e d w i t h t h e n i t r o g e n n u c l e u s w i t h a I s K* h o l e and t h e oxygen n u c l e u s r e s p e c t i v e l y i n N 0 . S i n c e p-rr^K* ̂  p ^ and ISpjK* % lSg, o n e - c e n t r e exchange i n t e g r a l s f o r a t o m i c o x y g e n ^ 6 may be used t o c a l c u l a t e t h e exchange i n t e g r a l s i n e q u a t i o n s ( 5 . 2.1) t o ( 5 . 2 . 3 ) . The t w o - c e n t r e exchange i n t e g r a l s a r e t y p i c a l l y an o r d e r o f 94 magnitude s m a l l e r and have been n e g l e c t e d (see R e f e r e n c e 94 where an an- al o g o u s p r o c e d u r e has been used t o e s t i m a t e t h e exchange s p l i t t i n g f o r I s i o n i z a t i o n i n open s h e l l s y s t e m s ) . The r e s u l t s a r e shown i n F i g u r e 16(a) 75 where t h e e x p e r i m e n t a l energy l e v e l s o f t h e t h r e e s t a t e s o f m o l e c u l a r oxygen (X z~, a Ag and b z g) a r i s i n g from t h e same v a l e n c e e l e c t r o n 4 2 c o n f i g u r a t i o n ( 2pTr u 2p7r g ) have been i n c l u d e d . On t h e b a s i s o f t h e 3 _ c o r e a n a l o g y model t h e z s t a t e o f m o l e c u l a r oxygen g i v e s r i s e t o 4 - 2 - K* c o r r e s p o n d i n g z and z s t a t e s i n N 0 w h i l e t h e energy d i f f e r e n c e between th e and ̂ z * s t a t e s i s e x p e c t e d t o be s i m i l a r t o t h e e n e r g y d i f f e r e n c e 2 2 + K* between t h e A and z s t a t e s o f N 0 . The c a l c u l a t i o n s u g g e s t s t h a t t h e 2 - 2 z and A s t a t e s s h o u l d be c l o s e i n energy and a p p r o x i m a t e l y 1 eV below 2 + t h e z s t a t e . T h i s r e s u l t i s i n q u a l i t a t i v e agreement w i t h 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 oxygen energy l e v e l s . The e x p e r i m e n t a l FWHM o f 1 . 0 eV x 2 n 9 b1Z* a'A eV •n 1.42 3n I 9 ? — 1 0,65 9 ~ ~ 0i98 x % " _ L _ T 1.1 1 1.3 1.6 2 s - 1.4 o 2 Exptl. (a) IMK*0 Theory x zn a1A 0.9 1.4 3 v - N F Exptl. eV 2S + 1.2 f 1.8 0.9 J _ 4 2 - (b) Theory FIGURE 16. Comparison o f the r e l a t i v e e n e r g i e s o f : (a) v a l e n c e 0 2 s t a t e s ( e x p e r i m e n t a l ) and N K * s t a t e s ( t h e o r e t i c a l ) ; (b) v a l e n c e NF s t a t e s ( e x p e r i m e n t a l ) and NOK* s t a t e s ( t h e o r e t i c a l ) . N^O and N 0 K + s p l i t t i n g s a r e from X-rav PES d a t a . i C D -77- f o r t h e f i r s t d i s c r e t e peak and t h e symme t r i c peak shape i n d i c a t e s t h a t i f a l l t h r e e d o u b l e t s t a t e s a r e e x c i t e d w i t h a p p r o x i m a t e l y e q u a l i n t e n s i t y , t h e i r energy s p a c i n g s must be s m a l l e r t h a n t h o s e c a l c u l a t e d . Our a s s i g n m e n t o f the f i r s t d i s c r e t e peak ( l s a N -* 2 p 7 r * ) i s s u p p o r t e d by t h e c l o s e agreement between t h e o b s e r v e d peak e n e r g y , 399.7 ± 0.2 eV, and t h e v a l u e o f 399.8 eV e s t i m a t e d u s i n g t he c o n c e p t o f e q u i v a l e n t c o r e s 97-99 and t h e thermochemical method . The f o l l o w i n g r e a c t i o n scheme was used: 1. N0 ( X 2n) ->- N K + 0 ( 3 n ) + e A E 1 A E 2 = 410.3 e V 3 2 2. N K + 0 ( 3 n ) -> N K +0(W) = 0.4 3. N K +0(W) + 0 6 + ->• 0 2 + ( W ) + N 6 + A E 3 = 6 4. o2+(w) -> o2+(x) A E 4 = -0.01 5. 0 2 + ( X ) + e -+ 0 2 ( X 3 E " ) A E 5 = - 1 2 . 0 7 1 0 0 6. o2(xV) o2(w) A E 6 = 0.6 7. 0 2(W) + N 6 + -> N K*0(W) + 0 6 + A E 7 = -6' 8. N 0(W) -> N K*0(w) AEg ^ 0.6 N0 ( X 2 n ) -> N K*0(w) AE = 399.8 + 6 - &' % 399.8 eV K+ + where (X) i n d i c a t e s t h e ground s t a t e s p e c i e s ; N 0(W), 0 2 (W), 0 2(W) and N K 0(W) r e s p e c t i v e l y r e p r e s e n t the w e i g h t e d a v e r a g e s o f t h e 3 n and N K + 0 2 2 3 1 s t a t e s , t h e ground i o n i c s t a t e s ( no. , n i , ) o f 0 o , t h e X E , a A and 7 2 12 2' g' g 1 + d - 9 - ? 9 + K* K* b z g s t a t e s o f 0 2 and t h e E , E , A and cn s t a t e s o f N 0. N 0(w) i s the w e i g h t e d average o f t h e d o u b l e t N 0 s t a t e s i n N 0(W), c f . F i g u r e 1 6 ( a ) . The w e i g h t e d a v erages have been c a l c u l a t e d from t h e r e l a t i v e e n e r g i e s shown 2 2 i n F i g u r e 1 6 ( a ) , e x c e p t A E^ w h i c h i s based on t h e n 3 ^ , n ^ s p l i t t i n g o f t h e ground s t a t e 0 2 + s p e c i e s 1 ^ . In the e q u i v a l e n t c o r e s a p p r o x i m a t i o n , -78- 6 ^ 6'(s e e R e f e r e n c e 9 8 ) . The broad band o f s t r u c t u r e w i t h a maximum a t ̂  404.7 eV i s t o o h i g h i n energy t o be a s s o c i a t e d w i t h p r o m o t i o n t o t h e 2p-rr* o r b i t a l and t o o low i n energy t o be a s s o c i a t e d w i t h t h e l o w e s t Rydberg e x c i t a t i o n . A p o s s i b l e e x p l a n a t i o n o f t h i s band i s t h a t i t r e p r e s e n t s t h e e x c i t a t i o n o f a n i t r o g e n I s e l e c t r o n t o t h e a n t i b o n d i n g 2pa* v a l e n c e o r b i t a l . The b r o a d n a t u r e o f t h e peak c o u l d be a s s o c i a t e d w i t h e x c i t a t i o n o f t h e two n s t a t e s ( s e e . T a b l e 3) r e s u l t i n g from t h i s e l e c t r o n c o n f i g u r a t i o n ( t h e 4 n s t a t e i s f o r - b i d d e n ) . The r e s u l t i n g s t a t e s a r e e x p e c t e d t o have some d i s s o c i a t i v e c h a r a c t e r s i n c e t h e c o r r e s p o n d i n g v a l e n c e s h e l l e x c i t a t i o n i n n i t r i c o x i d e r e s u l t s i n a d i s s o c i a t i v e A' 2 E + s t a t e ^ . T h i s d i s s o c i a t i v e c h a r a c t e r would c o n t r i b u t e t o t h e b r o a d e n i n g o f t h e s t r u c t u r e . S i n c e a l l t h e v a l e n c e o r b i t a l s have been a c c o u n t e d f o r , the h i g h e r d i s c r e t e peaks i n t h e s p e c t r u m a r e p r o b a b l y a s s o c i a t e d w i t h t h e p r o m o t i o n o f a n i t r o g e n I s e l e c t r o n t o Rydberg o r b i t a l s . T h i s a s s i g n m e n t i s s u p p o r t e d by t h e magnitudes o f t h e d e r i v e d quantum d e f e c t s . The p r o m o t i o n o f a I s a e l e c t r o n t o e i t h e r n s a o r npa Rydberg o r b i t a l s r e s u l t s i n t h r e e s e p a r a t e Rydberg s e r i e s ( n, n and 2 4 2 n). Two o f t h e s e s e r i e s , t he n and one o f t h e n s e r i e s w i l l c o n v e r g e 3 2 t o the n K - s h e l l i o n s t a t e w h i l e the r e m a i n i n g n s e r i e s c o n v e r g e s t o t h e 1 2 n K - s h e l l i o n s t a t e . Only t h e n s t a t e s a r e e x p e c t e d t o c o n t r i b u t e t o our s p ectrum. The f i r s t Rydberg t r a n s i t i o n , l s a ^ -»- 3s o , s h o u l d r e s u l t i n 2 3 two n s t a t e s , one c o n v e r g i n g a t t h e n i o n i c l i m i t and t h e o t h e r c o n v e r g i n g t o t h e ^JI i o n i c l i m i t . The t h i r d peak l o c a t e d a t 406.6 eV i s a s s i g n e d t o 2 3 t h e n s t a t e w h i c h i s a s s o c i a t e d w i t h t h e n l i m i t . U s i n g t h e o b s e r v e d 3 32 peak energy and t h e X-ray PES v a l u e o f 410.3 eV f o r t h e n l i m i t , we deduce a quantum d e f e c t o f 1.08, w h i c h compares f a v o u r a b l y w i t h t h e quantum -79- d e f e c t s o b s e r v e d f o r 3s Rydberg e x c i t a t i o n i n t h e v a l e n c e s h e l l s p e c t r u m o f n i t r i c o x i d e 7 9 ' 1 0 1 , 2p-rr* 3 s a , 6 = 0.97 and t h e v a l e n c e s h e l l s p e c t r u m 102 o f m o l e c u l a r oxygen , 2pTr g ->- 3 s a g , 6 = 1.1. The f o u r t h peak o b s e r v e d a t an energy o f 407.6 eV has a quantum d e f e c t o f 1.2 w i t h r e s p e c t t o t h e i o n s t a t e . T h i s peak c o u l d t h e n have a c o n t r i b u t i o n from t h e r e m a i n i n g n s t a t e produced by t h e e x c i t a t i o n o f a l s a ^ e l e c t r o n t o t h e 3sa Rydberg o r b i t a l . The much l a r g e r i n t e n s i t y o f peak f o u r compared t o peak t h r e e c o u l d r e s u l t from a c o n t r i b u t i o n from t h e t r a n s i t i o n l s o ^ -> 3PTT where t h e 3 i o n i z a t i o n l i m i t i s t h e n i o n s t a t e . The quantum d e f e c t o f peak f o u r 3 w i t h r e s p e c t t o t h e n l i m i t i s 0.75 and i s c o n s i s t e n t w i t h t h e quantum 79 d e f e c t , 6 = 0.76 , o b s e r v e d f o r t h e c o r r e s p o n d i n g v a l e n c e s h e l l t r a n s i t i o n i n n i t r i c o x i d e (2pTr* -> 3pir). S i m i l a r l y peak f i v e o b s e r v e d a t 409.0 eV has a quantum d e f e c t o f 0.80 w i t h r e s p e c t t o t h e l i m i t w h i c h i s c o n s i s t - ent w i t h what we would e x p e c t f o r 3p e x c i t a t i o n where t h e Rydberg s t a t e c onverges t o t h e l i m i t . However, the p r o m o t i o n o f a I s a ^ e l e c t r o n t o t h e 3pTr and 3pa Rydberg l e v e l s r e s u l t s i n s i x and two d i p o l e a l l o w e d f i n a l s t a t e s r e s p e c t i v e l y (see T a b l e 3) and t h e a s s i g n m e n t s o f t h e s e h i g h e r Rydberg peaks a r e c l e a r l y u n c e r t a i n . Peak s i x o b s e r v e d a t ^ 410.4 eV i s p r o b a b l y a s s o c i a t e d w i t h Rydberg s t a t e s w h i c h c o n v e r g e t o t h e "'n i o n s t a t e . 3 1 The p o s i t i o n s o f t h e n and n K-edges i n o u r s p e c t r u m a r e based on 32 the e x p e r i m e n t a l X-ray PES v a l u e s o f 410.3 and 411.8 eV r e s p e c t i v e l y . The broad band o f s t r u c t u r e w i t h maxima a t a p p r o x i m a t e l y 413.1 eV and 414.5 eV i s a s s o c i a t e d w i t h the shake-up and s h a k e - o f f o f v a l e n c e e l e c t r o n s i n c o n j u n c t i o n w i t h K - s h e l l e x c i t a t i o n . On the b a s i s o f the c o r e a n a l o g y model we e x p e c t t h e energy s p a c i n g 4 - 2 - between t h e average energy o f the z and z s t a t e s r e s u l t i n g from l s a w - 8 0 - promotion t o th e 2pTr* o r b i t a l and th e average energy o f t h e n and n N 0 s t a t e s i n n i t r i c o x i d e t o r e p r o d u c e t h e energy s p a c i n g between t h e ground s t a t e and f i r s t i o n s t a t e o f m o l e c u l a r oxygen, 12.07 e V 1 ^ [s e e F i g u r e 1 6 ( a ) ] . U s i n g t h e f i r s t peak maximum as an i n d i c a t i o n o f t h e z" 4 - 2 - energy l e v e l and o u r e s t i m a t e o f t h e z - z s p l i t t i n g o f 1.4 eV, y i e l d s an energy s p a c i n g o f 12.0 ± 0.4 eV i n q u a l i t a t i v e agreement w i t h t h e + 3 1 p r e d i c t e d v a l u e . The s p l i t t i n g between t h e n and n i o n s t a t e s produced by i o n i z a t i o n o f a n i t r o g e n I s e l e c t r o n i n n i t r i c o x i d e i s 1.42 eV ' . Us i n g t h e same a p p r o x i m a t i o n assumed i n d e r i v i n g e q u a t i o n s ( 5 . 2 . 1 ) t o 3 1 3 2 9 4 ( 5 . 2 . 3 ) , t h e n, n energy s p a c i n g i s equal t o 2 K ^ S ^ + * ' . U s i n g t h e v a l u e o f K-| S i t +* c a l c u l a t e d w i t h o n e - c e n t r e a t o m i c oxygen exchange i n t e g r a l s g i v e s a v a l u e o f 0.96 eV f o r t h e ESCA s p l i t t i n g . b. Oxygen K - s h e l l E x c i t a t i o n . The oxygen K - s h e l l e l e c t r o n energy l o s s s p e c t r u m o f n i t r i c o x i d e i s shown i n F i g u r e 1 7 and the e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f s t r u c t u r e a re l i s t e d i n T a b l e 4. The poor s i g n a l t o n o i s e r a t i o compared w i t h t h a t of t h e n i t r o g e n K - s h e l l spectrum o f n i t r i c o x i d e ( F i g u r e 1 5 ) r e f l e c t s t h e f a c t t h a t t h e i n e l a s t i c s c a t t e r i n g i n t e n s i t y o f f a s t e l e c t r o n s f o r f o r w a r d 11 3 s c a t t e r i n g d e c r e a s e s by a f a c t o r , « (energy l o s s ) . The i n t e r p r e t a t i o n o f t h e f i r s t d i s c r e t e peak o b s e r v e d a t 532. 7 eV i s analogous t o t h a t o f t h e f i r s t peak i n t h e n i t r o g e n K - s h e l l s p e c t r u m o f n i t r i c o x i d e ( s e e T a b l e 4 ) . The peak i s v e r y b r o a d and has a FWHM o f 2.1 eV compared w i t h an e l a s t i c FWHM o f 0.4 eV. T h i s r e s u l t s u g g e s t s t h a t t h e t h r e e d o u b l e t N 0 K * s t a t e s a s s o c i a t e d w i t h t h i s peak have a w i d e r energy s p a c i n g t h a n t h e c o r r e s p o n d i n g 4- A b s o l u t e b i n d i n g e n e r g i e s i n R e f e r e n c e s 9 3 and 9 4 a r e o n l y ± 0.5 eV. T h e r e f o r e i n T a b l e 4 th e N^+O e n e r g i e s a r e from R e f e r e n c e 3 2 . Intensity (arbitrary units) G"5 CZ 73 o X << fD 7< I ro -5 << o to (/> c/> •a ro o r+ -s -S o o Q-fD P Oi 01 CO o m 3 (0 - i (Q o a) o i o fl> o i > Oi ^ o Oi O) o o ro — Oi .»» * .r'r CD a -L8- -82- K * N 0 s t a t e s . The i n t e n s i t y o f t h e f i r s t peak r e l a t i v e t o t h e o t h e r s i n d i c a t e s t h a t t h e s e d i s c r e t e l e v e l s a r e e x p e c t e d t o g i v e t h e l a r g e s t c o n t r i b u t i o n t o t h e h i g h energy a u t o i o n i z a t i o n l i n e s o b s e r v e d i n t h e 35 oxygen K - s h e l l Auger sp e c t r u m o f n i t r i c o x i d e e x c i t e d by e l e c t r o n impact . Oxygen I s e x c i t a t i o n i n n i t r i c o x i d e s h o u l d produce an " N F - l i k e " s p e c i e s . In a n a l o g y t o t h e case o f n i t r o g e n I s e l e c t r o n p r o m o t i o n , we have t h e o r e t i c a l l y e s t i m a t e d t h e r e l a t i v e energy s p a c i n g s o f t h e ^E~, 2 - 2 2 + E , A and E s t a t e s as a r e s u l t o f oxygen I s pr o m o t i o n t o t h e 2p-rr* K* m o l e c u l a r o r b i t a l . A wave f u n c t i o n f o r t h e 2pn* o r b i t a l i n NO was 1Q3 c a l c u l a t e d u s i n g an INDO c a l c u l a t i o n ( u n r e s t r i c t e d H a r t r e e - F o c k w i t h one- c e n t r e exchange) f o r NF w i t h an i n t e r n u c l e a r s e p a r a t i o n e q u a l t o t h a t o f n i t r i c o x i d e . U s i n g t h e r e s u l t i n g w a v e f u n c t i o n , <j>(2pir*) ^ 0 . 8 5 9 p T r N - 0.512PTTQ|<*, P T Q K * % P^pj ^ S Q K * ^ lSp and t h e o n e - c e n t r e a t o m i c f l u o r i n e 96 exchange i n t e g r a l s , t h e c a l c u l a t e d energy s p a c i n g s shown i n F i g u r e 16(b) were o b t a i n e d . + The e x p e r i m e n t a l energy s p a c i n g s 1 ^ ' 1 ^ o f t h e X 3E~, a  and b ^E + s t a t e s o f NF ( l a r g e r t h a n t h e c o r r e s p o n d i n g s p a c i n g s i n oxygen) a r e i n q u a l i t a t i v e agreement w i t h t h e e s t i m a t e d v a l u e s f o r NO [ s e e F i g u r e 1 6 ( b ) ] . The c a l c u l a t i o n s u g g e s t s t h a t t h e t h r e e d o u b l e t s t a t e s produced by oxygen I s p r o m o t i o n t o t h e 2p-n* m o l e c u l a r o r b i t a l i n n i t r i c o x i d e , have an energy s p a c i n g o f ^ 1.8 eV i n good agreement w i t h t h e e x p e r i m e n t a l FWHM o f the f i r s t d i s c r e t e peak ( F i g u r e 17). The en e r g y r e q u i r e d f o r t h e t r a n s i t i o n I s a ^ -> 2pir* can be e s t i m a t e d u s i n g t h e e q u i v a l e n t c o r e s a p p r o x i m a t i o n and t h e thermochemical method i n e x a c t a n a l o g y t o t h e + The energy s p a c i n g s wege a l s o c a l c u l a t e d u s i n g a w a v e f u n c t i o n c a l c u l a t e d f o r NF w i t h r e = 1.3173 A, the e x p e r i m e n t a l i n t e r n u c l e a r s e p a r a t i o n ^ o f NF, X^E". The l a r g e s t d e v i a t i o n from t h e energy d i f f e r e n c e s shown i n F i g u r e 16(b) was < 0.1 eV. -83- method used f o r t h e n i t r o g e n K - s h e l l c a s e , I s o ^ -> 2 p 7 r * . The e s t i m a t e d v a l u e o f ̂  531.9 eV i s i n good agreement w i t h t h e o b s e r v e d peak e n e r g y , 532.7 eV, c o n s i d e r i n g t h a t t h e o b s e r v e d FWHM o f t h e peak i s 2.1 eV. There appears t o be a broad band o f s t r u c t u r e w i t h a maximum a t 3 1 a p p r o x i m a t e l y 540.2 eV. The quantum d e f e c t s w i t h r e s p e c t t o t h e n and n i o n i z a t i o n l i m i t s a r e c o n s i s t e n t w i t h t h o s e e x p e c t e d f o r e x c i t a t i o n o f an oxygen I s e l e c t r o n t o Rydberg s t a t e s w i t h quantum number t h r e e . 3 1 The p o s i t i o n s o f the n and n K-edges i n o u r spect r u m a r e based on 32 the e x p e r i m e n t a l X-ray PES v a l u e s o f 543.3 eV and 544.0 eV r e s p e c t i v e l y . The b r o a d band o f s t r u c t u r e w i t h a maximum a t a p p r o x i m a t e l y 546 eV r e p r e s - e n t s t h e shake-up and s h a k e - o f f o f v a l e n c e e l e c t r o n s i n c o n j u n c t i o n w i t h I S O Q e x c i t a t i o n . The c o r e a n a l o g y model s u g g e s t s t h a t the energy d i f f e r e n c e between t h e 3 1 K+ 4 - 2 - K* average e n e r g i e s o f t h e n and n NO s t a t e s and the E and E NO s t a t e s [ s e e F i g u r e 1 6 ( b ) ] s h o u l d have a magnitude s i m i l a r t o t h e f i r s t i o n i z a t i o n p o t e n t i a l o f NF. Assuming t h a t t h e o n s e t o f t h e f i r s t d i s c r e t e K* 2 - peak i n t h e NO spectrum c o r r e s p o n d s t o e x c i t a t i o n o f t h e E s t a t e , we d e r i v e a v a l u e o f ^ 13 eV f o r t h e i o n i z a t i o n p o t e n t i a l o f NF. The v a l u e o f 13.1 ± 0.2 eV d e r i v e d 1 ^ from e x p e r i m e n t a l appearance p o t e n t i a l s ' ' 1 ^ and t h e t h e o r e t i c a l v a l u e 1 1 ^ o f 13.2 ± 0.3 eV a r e i n r e a s o n a b l e agreement w i t h t h i s p r e d i c t i o n . The magnitude o f t h e m o l e c u l a r exchange i n t e g r a l , K^ s * d e r i v e d K* E from t h e INDO 2p?r* w a v e f u n c t i o n f o r NO , i m p l i e s t h a t t h e exchange s p l i t t i n g ( t o the same degree o f a p p r o x i m a t i o n as i n t h e c a s e o f t h e 3 1 K+ d i s c r e t e m u l t i p l e t s p l i t t i n g s ) o f the n and n NO s t a t e s i s 0.6 eV. 93 94 T h i s compares f a v o u r a b l y w i t h t h e e x p e r i m e n t a l v a l u e ' o f 0.55 eV. - 8 4 - 5.2.2. Oxygen. The ground e l e c t r o n i c s t a t e o f t h e oxygen m o l e c u l e has t h e e l e c t r o n c o n f i g u r a t i o n : ( l s a g ) 2 ( l s a j 2 ( 2 s a g ) 2 ( 2 s a u ) 2 ( 2 p a g ) 2 (2p«/ ( 2 P l T g ) 2 , 3 E G a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l energy l o s s spectrum o f m o l e c u l a r oxygen i s shown i n F i g u r e 18. A h i g h r e s o l u t i o n s p e c t r u m , AE(FWHM) = 0.01 eV, i n t h e 6.8 t o 21 eV energy r e g i o n , has been o b t a i n e d 6 1 u s i n g 25 keV i n c i d e n t e l e c t r o n s . In a d d i t i o n , a s s i g n m e n t s o f o p t i c a l l y f o r b i d d e n t r a n s i t i o n s and some Rydberg s t a t e s have been made on t h e b a s i s o f a n g u l a r dependence s t u d i e s u s i n g l o w e r impact e n e r g i e s 1 0 2 ' 1 1 6 . The l o c a t i o n s o f peaks i n our s p e c t r u m a r e c o n s i s t e n t w i t h t h e s e h i g h e r r e s o l u t i o n measurements. The f i r s t e x c i t a t i o n i n oxygen i s 2 P T T U -* 2pTrg r e s u l t i n g i n s i x p o s s i b l e 1 3 + 1 3 1 3 -f i n a l e l e c t r o n i c s t a t e s ; ' E U , ' A U and ' Y.^. The o n l y o p t i c a l l y 3 - 3 - a l l o w e d t r a n s i t i o n i s X E G -> B E^ ( t h e Schuman-Runge c o n t i n u u m ) . Peak B i n our s p e c t r u m (maximum 8.3 eV) i s a s s o c i a t e d w i t h t h i s 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 energy from o t h e r w o r k e r s i s 8.6 eV; see R e f e r e n c e 1 1 6 ) . The broad band A w i t h a maximum a t a p p r o x i m a t e l y 6.0 eV i s i n t h e 3 + 3 1 -energy r e g i o n where t h e f o r b i d d e n A E , C A U and c E^ s t a t e s o c c u r (see R e f e r e n c e 1 6 ) . The +«-|->- s e l e c t i o n r u l e i s not r i g o r o u s f o r n o n a x i a l 54 s c a t t e r i n g i n e l e c t r o n impact and t h e i n t e n s i t y o f t h e A band i s p r o b a b l y 3 + 3 1 -a s s o c i a t e d w i t h t h e A E^ s t a t e . The A U and E^ e x c i t a t i o n s a r e l e s s p r o b a b l e a t impact energy o f 2.5 keV s i n c e t h e f i r s t i n v o l v e s AA = 2 and t h e second i n v o l v e s a s p i n f o r b i d d e n t r a n s i t i o n . The ^ l * and ^ s t a t e s have not been o b s e r v e d i n e i t h e r e l e c t r o n impact o r o p t i c a l s t u d i e s . Peak C (10.0 eV) and s h o u l d e r D (10.9 eV) a r e a s s o c i a t e d w i t h t h e " l o n g e s t " V a l e n c e s h e l l energy l o s s spectrum o f m o l e c u l a r oxygen. -86- (9.97 e V D I ) and "second" (10.29 e V b l ) band r e s p e c t i v e l y . The h i g h e r energy p e a k s , E (13.0 e V ) , F (15.3 eV) G (16.9 e V ) , H (19.0 e V ) , I (20.1 e V ) , J (21.8 e V ) , K (23.5 eV) and L (24.5 eV) a r e a s s o c i a t e d w i t h a l a r g e number o f o v e r l a p p i n g t r a n s i t i o n s (see R e f e r e n c e 6 1 ) . The l o c a t i o n o f t h e f i r s t i o n i z a t i o n p o t e n t i a l shown i n our spect r u m i s based on t h e e x p e r i - m e n t a l 1 0 0 UV PES v a l u e o f 12.07 eV. b. Oxygen K - s h e l l E x c i t a t i o n . In g e n e r a l , t h e p r o m o t i o n o f a c o r e e l e c t r o n ( l s a = oxygen K) t o d i s c r e t e l e v e l s r e s u l t s i n a number o f p o s s i b l e f i n a l s t a t e s f o r a g i v e n e l e c t r o n c o n f i g u r a t i o n (see T a b l e 3 ) . + The exchange s p l i t t i n g between the 109 m u l t i p l e t components can be q u i t e l a r g e as shown by t h e e x p e r i m e n t a l v a l u e 4 - 2 - o f 1.11 eV measured by X-ray PES f o r t h e s p l i t t i n g between E and £ i o n s t a t e s produced by I s i o n i z a t i o n i n m o l e c u l a r oxygen. The K - s h e l l e l e c t r o n e n e r g y l o s s s p e c t r u m o f oxygen i s shown i n F i g u r e 19 and t h e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 5. The f i r s t d i s c r e t e peak o b s e r v e d a t 530.8 eV i s a t t r i b u t e d t o t h e promotion o f an oxygen l s a e l e c t r o n t o t h e l o w e s t u n f i l l e d m o l e c u l a r o r b i t a l , t h e 2piTg. The r e s u l t i n g c o n f i g u r a t i o n : ( l s a ) 1 (2pu) 4 ( 2PTT*) 3 3 1 g i v e s r i s e t o a n and a n s t a t e and o n l y t he t r i p l e t s t a t e i s e x p e c t e d t o c o n t r i b u t e t o o u r spectrum.. The o b s e r v e d peak has a FWHM o f 0.5 eV, i n d i c a t i n g t he e x c i t a t i o n o f a number o f v i b r a t i o n a l l e v e l s . S i n c e t h e i n t e n s i t y o f t h i s peak i s much l a r g e r t h a n t h e h i g h e r energy d i s c r e t e peaks 3 o b s e r v e d i n t h e s p e c t r u m , e x c i t a t i o n t o t h e n s t a t e i s e x p e c t e d t o g i v e f F o r m a l l y , g and u symmetry does not a p p l y t o an oxygen m o l e c u l e w i t h a l o c a l i z e d I s h o l e ( f o r example see R e f e r e n c e 117). Intensity (arbitrary units) -o P 01 b CD cr 70 7^ I CO ZT fD -S C Q O CO cn co T 3 fD o r+ -s o -t) 3 o ro o cr _ J o> -s o X i.Q rc> m CD C D O • • • CO o o 01 o o ro M . 1 M I 7 CD a (Q CD -Z8- TABLE 5 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND POSSIBLE ASSIGNMENTS OF PEAKS OBSERVED IN THE K-SHELL ENERGY LOSS SPECTRUM OF 0 2 . PEAK ENERGY AE ASSIGNMENT 9 ORBITAL STATES 1 530.8 0 2% 2 539.2 8.4 3 s 0 g V ( 2 ) , 3 541.9 11.1 3p, 3d, e t c K-EDGE 0 543.1 12.3 CO V K-EDGE 0 544.2 13.4 CO V a O n l y t h e f i n a l o r b i t a l i n v o l v e d i n the e x c i t a t i o n and m o l e c u l a r s t a t e s d i p o l e c o n n e c t e d t o t h e ground 3£g s t a t e have been i n c l u d e d . However, t h e 3 E s t a t e has been i n c l u d e d s i n c e the -+-/->+ r u l e does n o t a p p l y t o e l e c t r o n i m p a ct f o r n o n - a x i a l s c a t t e r i n g . 5 1 * b As d e t e r m i n e d by X-ray P E S . 3 2 1 -89- t h e l a r g e s t c o n t r i b u t i o n t o the h i g h e n e r g y a u t o i o n i z a t i o n l i n e s o b s e r v e d 32 35 i n t h e oxygen I s Auger sp e c t r u m e x c i t e d by e l e c t r o n impact ' . The second and t h i r d peaks w i t h maxima a t 539.2 and a p p r o x i m a t e l y 541.9 eV r e s p e c t i v e l y a r e p r o b a b l y a s s o c i a t e d w i t h t h e e x c i t a t i o n o f a I s a e l e c t r o n t o t h e 3s, 3p and h i g h e r energy Rydberg l e v e l s . A quantum d e f e c t o f 1.25 i s d e r i v e d from t h e o b s e r v e d energy p o s i t i o n o f t h e second peak and t h e 32 e x p e r i m e n t a l X-ray PES v a l u e s f o r t h e K-edges. The magnitude o f t h e quantum d e f e c t i s s i m i l a r t o t h a t deduced from t h e r e p o r t e d e x c i t a t i o n 102 energy f o r t h e c o r r e s p o n d i n g v a l e n c e s h e l l t r a n s i t i o n i n oxygen , 2pir + 3sag, f o r w h i c h 6 = 1 .1 . 4 - 2 - The p o s i t i o n s o f t h e z and z K-edges i n o u r sp e c t r u m a r e based on t h e X-ray PES v a l u e s 3 2 o f 543.1 eV and 544.2 eV r e s p e c t i v e l y . On t h e b a s i s o f t h e c o r e a n a l o g y model we e x p e c t K - s h e l l e x c i t a t i o n s i n m o l e c u l a r oxygen t o produce an " O F - l i k e " s p e c i e s . The e x i s t e n c e o f the oxygen m o n o f l u o r i d e r a d i c a l has been f i r m l y e s t a b l i s h e d by m a t r i x t e c h n i q u e s 1 1 0 ' 1 1 2 , and r e c e n t l y , gas phase d e t e c t i o n has been c l a i m e d 1 1 3 . 114 An i o n i z a t i o n p o t e n t i a l o f 13.1 ± 0.5 eV has been c a l c u l a t e d , w h i c h 114 agrees w i t h t h e v a l u e o f 13.1 ± 0.3 eV e s t i m a t e d from t h e appearance 115 + p o t e n t i a l o f OF from 0 2F 2. , From t h e energy d i f f e r e n c e between t h e 4 - 2 - K+ average energy o f t h e z~ and z~ 0 2 s t a t e s and t h e e s t i m a t e d energy o f 3 1 K* the n and n 0 2 s t a t e s , we deduce a v a l u e o f 12.7 ± 0.4 eV f o r t h e i o n i z a t i o n p o t e n t i a l o f OF. Our e s t i m a t e d v a l u e i s i n r e a s o n a b l e agreement w i t h t h e t h e o r e t i c a l and " e x p e r i m e n t a l " v a l u e s . -90- CHAPTER SIX TRIATOMIC MOLECULES 6.1. Carbon D i o x i d e and N i t r o u s O x i d e . 6.1.1. Carbon D i o x i d e . The c a r b o n d i o x i d e m o l e c u l e i s l i n e a r i n i t s ground e l e c t r o n i c s t a t e and has t h e e l e c t r o n c o n f i g u r a t i o n <V2 ( l a u ) 2 ( V 2 ( V 2 { 2 a f ( % > 2 < 3 o u ) 2 ( 1 - u ) 4 'v The la and l a u o r b i t a l s a r e l i n e a r c o m b i n a t i o n s o f oxygen Is a t o m i c o r b i t a l s , w h i l e t h e 2ag o r b i t a l i s formed from t h e c a r b o n Is a t o m i c o r b i t a l . The l a g , l a u and 2ag o r b i t a l s a r e e s s e n t i a l l y l o c a l i z e d + on t h e i r n u c l e i and a r e t h e r e f o r e nonbonding. To i n d i c a t e t h e i r " a t o m i c " c h a r a c t e r , t h e e l e c t r o n s f i l l i n g t h e s e o r b i t a l s a r e d e s i g n a t e d oxygen K - s h e l l and c a r b o n K - s h e l l e l e c t r o n s . We have s t u d i e d b o t h t h e c a r b o n and oxygen K - s h e l l energy l o s s s p e c t r a . A v a l e n c e s h e l l s p e c t r u m has a l s o been r e c o r d e d . a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l energy l o s s spectrum o f car b o n d i o x i d e i s shown i n F i g u r e 20. The o b s e r v e d l o c a t i o n s o f peaks a r e c o n s i s t e n t w i t h 5 49 118 h i g h r e s o l u t i o n e l e c t r o n impact s p e c t r a 5 * and a " h i g h " r e s o l u t i o n 119 o p t i c a l s p e c t r u m . The broad peak A w i t h a maximum a t a p p r o x i m a t e l y 9 eV i s a s s o c i a t e d w i t h lTT g 2-IT̂  (TT*) t r a n s i t i o n s . I n t h e p h o t o a b s o r p t i o n + Recent quantum m e c h a n i c a l c a l c u l a t i o n s on t h e I s -hole s t a t e s o f t h e O2 m o l e c u l a r ion"' 17 have been i n t e r p r e t e d as an i n d i c a t i o n t h a t t h e c o r e h o l e s are l o c a l i z e d . ELASTIC T • 1 " 1 1 1 1 p 0 10 20 30 40 Energy Loss (eV) FIGURE 20. V a l e n c e s h e l l energy l o s s spectrum o f carbon d i o x i d e . -92- s p e c t r u m 1 1 3 , two o v e r l a p p i n g bands have been o b s e r v e d i n t h i s e n ergy r e g i o n ; one (peak maximum a t 8.41 eV) a s s i g n e d t o t h e ^ i * -* 1 A u (^Bg) t r a n s i t i o n and t h e second (peak maximum a t 9.31 eV) a s s i g n e d t o t h e ^->- ^iig t r a n s i t i o n . Both t r a n s i t i o n s a r e f o r b i d d e n i n symmetry, but i n C 2 V symmetry they each have an a l l o w e d ^ component. The h i g h e r energy peaks i n our s p e c t r u m ; B (10.9 eV, s h o u l d e r ^ 11.2 e V ) , C (12.3 e V ) , D (13.3 eV) and E (16.0 eV) a r e p r o b a b l y a s s o c i a t e d w i t h 112 Rydberg t r a n s i t i o n s . The h i g h e r r e s o l u t i o n e l e c t r o n energy l o s s s p e c t r u m has been i n t e r p r e t e d on t h e b a s i s o f a Rydberg a s s i g n m e n t . The l o c a t i o n o f t h e f i r s t i o n i z a t i o n p o t e n t i a l shown i n F i g u r e 20 i s based on t h e e x p e r - i m e n t a l v a l u e 7 8 ' 1 2 0 o f 13.77 eV. b. Carbon K - s h e l l E x c i t a t i o n . The K - s h e l l s p e c t r a o f t h e d i a t o m i c m o l e c u l e s ( C h a p t e r F i v e ) were i n t e r p r e t e d on t h e b a s i s o f a s i m p l e " e q u i v a l e n t c o r e " m o d e l , i n which a h o l e i n the K - s h e l l i s c o n s i d e r e d t o have t h e same e f f e c t on t h e p o t e n t i a l e x p e r i e n c e d by t h e o u t e r v a l e n c e e l e c t r o n s as one more p o s i t i v e c harge on t h e n u c l e u s . I f t h e c o r e a n a l o g y model i s v a l i d f o r c a r b o n d i o x i d e , we would e x p e c t t h e r e l a t i v e e n e r g i e s o f the peaks o b s e r v e d i n t h e c a r b o n K - s h e l l s p e c t r u m ( w i t h r e s p e c t t o the l o w e s t energy d i s c r e t e peak) t o r e p r o d u c e t h o s e o b s e r v e d i n the e x c i t a t i o n o f t h e 6a, e l e c t r o n t o Rydberg s t a t e s i n n i t r o g e n d i o x i d e . However, b e f o r e comparing d a t a from t h e two m o l e c u l e s , s e v e r a l f a c t o r s s h o u l d be c o n s i d e r e d . i . The ground e l e c t r o n i c s t a t e o f t h e n i t r o g e n d i o x i d e m o l e c u l e , 2 X A,, i s b e n t ( t h e e q u i l i b r i u m bond a n g l e i s 134°) and t h e e x t e n t o f v i b r a t i o n a l e x c i t a t i o n accompanying e l e c t r o n p r o m o t i o n i s d e t e r m i n e d by -93- the o v e r l a p o f t h e f i n a l and i n i t i a l s t a t e v i b r a t i o n a l w a v e f u n c t i o n s ( i . e . t he Franck-Condon f a c t o r s ) . T h e r e f o r e , t h e p r o m o t i o n o f t h e 6a, e l e c t r o n t o t h e l i n e a r Rydberg s t a t e s and t h e i o n s t a t e i s e x p e c t e d t o e x c i t e many v i b r a t i o n a l q u anta ( p a r t i c u l a r l y o f t h e b e n d i n g mode, V 2 ) . T h i s i s i l l u s t r a t e d i n F i g u r e 2 1 , where a q u a l i t a t i v e r e p r e s e n t a t i o n o f some o f t h e s t a t e s o f n i t r o g e n d i o x i d e has been drawn i n t h e bending c o o r d i n a t e . I t s h o u l d be noted t h a t t h e i n d e p e n d e n t s t r e t c h i n g c o o r d i n a t e a l s o c o n t r i b u t e s t o t h e v i b r a t i o n a l s t r u c t u r e o f e x c i t e d s t a t e s . i i . In ca r b o n K - s h e l l e x c i t a t i o n s i n c a r b o n d i o x i d e , v i b r a t i o n a l p o p u l a t i o n s a r e d e t e r m i n e d by t h e Franck-Condon r e g i o n o f t h e l i n e a r ground s t a t e . T h i s i s shown i n F i g u r e 2 1 , where i t has been assumed t h a t t h e K - s h e l l e x c i t e d s t a t e s o f carbon d i o x i d e have t h e same r e l a t i v e e n e r g i e s as t h o s e s t a t e s o f n i t r o g e n d i o x i d e r e s u l t i n g from t h e p r o m o t i o n o f a 6a, v a l e n c e e l e c t r o n . In o r d e r t o compare t h e two s e t s o f d a t a , t h e n i t r o g e n d i o x i d e e n e r g i e s must be c o r r e c t e d by s u b t r a c t i n g b o t h t h e b a r r i e r t o l i n e a r i t y 121 o f n i t r o g e n d i o x i d e (1.83 eV) and the e x c e s s v i b r a t i o n a l p o p u l a t i o n o f t h e upper s t a t e s . The l a t t e r q u a n t i t y can be e s t i m a t e d as b e i n g a p prox- i m a t e l y 1.6 eV on the b a s i s o f t h e d i f f e r e n c e between t h e v e r t i c a l (11.25 e V ) 1 2 2 and the a d i a b a t i c U 9.62 e V ) 1 2 3 v a l u e s f o r t h e f i r s t i o n i z - a t i o n p o t e n t i a l o f n i t r o g e n d i o x i d e . We e x p e c t a s i m i l a r c o r r e c t i o n t o a p p l y t o t h e Rydberg s t a t e s . The c a r b o n K - s h e l l energy l o s s s p e c t r u m o f c a r b o n d i o x i d e i s shown i n F i g u r e 22 and t h e e n e r g i e s and t e n t a t i v e a s s i g n m e n t s o f s t r u c t u r e a r e l i s t e d i n T a b l e 6. A l t h o u g h c o mplete d a t a on t h e v a l e n c e s h e l l e x c i t a t i o n o f n i t r o g e n d i o x i d e i s not a v a i l a b l e , t h e c o r r e c t e d r e l a t i v e e n e r g i e s o f -94- e l35°l8Cfl35° C02 Carbon K-Excitation FIGURE 21. Q u a l i t a t i v e r e p r e s e n t a t i o n ( n ot t o s c a l e ) o f the p o t e n t i a l energy s u r f a c e s , as a f u n c t i o n o f t h e bending c o o r d i n a t e , o f some s t a t e s o f n i t r o g e n d i o x i d e and K - s h e l l e x c i t e d c a r b o n d i o x i d e . Mote: These i n d i c a t e t h e n a t u r e o f t h e energy c o r r e c t i o n s which would have t o be a p p l i e d i n o r d e r t o compare d a t a from t h e two m o l e c u l e s on t h e b a s i s o f t h e c o r e a n a l o g y model. 1.0 H 4-1 mmm c 4-1 5 0.5 0) c k-edge 1 2 34 : i ; I I I (a) 3 T 1 1 1 — 293 295 ( c ^ o f * x8 co2 C k-shell 290 T T 300 310 Energy Loss 320 CeV) 330 tn I FIGURE 22. The c a r b o n K - s h e l l energy l o s s spectrum o f carbon d i o x i d e . TABLE 6 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND POSSIBLE ASSIGNMENTS OF PEAKS OBSERVED IN THE CARBON AND OXYGEN K-SHELL SPECTRA OF CARBON DIOXIDE. PEAK CARBON K- SHELL OXYGEN K-•SHELL POSSIBLE . ENERGY AE CALCULATED V A L U E 3 ENERGY AE CALCULATED ENERGY 3 ASSIGNMENT 1 290.7 0 - (535.4 0 _ 1TTu (6a, + lb,) 3sag 3 D C T U 3DTT v u 4sa 2 3 292.7 i(294.5)d I 294.9 2.0 (3.8) 4.2 294.1 294.9 295.2 (MASKED 0 538.7 ? 3.3 537.7 538.5 538.8 296.0 539.6 4 296.3 5.6 296.2 296.4 539.9 4.5 539.8 539.9 9 v u 00 K-EDGE E 297.5 6.8 - 541.1 5.4 - 5 3̂01 'vlO - lir + SHAKE-UP ITT + SHAKE-UP 6 <314 ^13 - C a l c u l a t e d u s i n g t h e Rydberg f o r m u l a ; E n=A-R/(n-<5) 2 , where E n i s t h e e x c i t a t i o n e n e r g y ; A, t h e K - s h e l l i o n i z a t i o n e n ergy o f C 0 2 ; R, the Rydberg c o n s t a n t ; n, t h e quantum number; and 6 , t h e quantum d e f e c t . The quantum d e f e c t s used were t h o s e from the v a l e n c e s h e l l Rydberg s e r i e s o f ca r b o n d i o x i d e 1 2 4 w i t h ; 5 ( n s a ) = 1 . 0 , fi(npa)=0.71, and 6(np7r)=0.56. O n l y t h e f i n a l o r b i t a l / s i n v o l v e d i n t h e K - e x c i t a t i o n s have been i n c l u d e d . I f the h o l e s t a t e s a r e l o c a l i z e d (see t e x t ) t h e g and u d e s i g n a t i o n s s h o u l d be o m i t t e d f o r oxygen K - s h e l l e x c i t a t i o n s s i n c e t h e m o l e c u l e would have C symmetry. The i n t e n s e f i r s t d i s c r e t e peak p r o b a b l y i n c l u d e s t h e f i r s t Rydberg t r a n s i t i o n (see t h e t e x t ) . T h i s e x t r a peak i s from a h i g h e r r e s o l u t i o n s c a n . F i g u r e 2 ( i n s e r t a ) . These v a l u e s a r e from X-ray PES m e a s u r e m e n t s . 1 2 5 c. d. e. 1 cn 1 -97- the i d e n t i f i e d Rydberg s t a t e s 1 ^ w h i c h converge t o t h e f i r s t i o n s t a t e do not match t h e d a t a from t h e car b o n K - s h e l l s p e c t r u m (see t h e c o r r e l a t i o n d i a g r a m , F i g u r e 23). Q u a l i t a t i v e l y , 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 o s e p r e v i o u s l y o b s e r v e d f o r n i t r o g e n and carb o n monoxide, w i t h t h e d i s c r e t e s t r u c t u r e dominated by t h e l o w e s t energy peak. T h i s i n t e n s e peak o b s e r v e d a t 290.7 ± 0.2 eV i s i n t e r p r e t e d as a r i s i n g from t h e p r o m o t i o n o f a ca r b o n K - s h e l l e l e c t r o n (2a g ) t o t h e l o w e s t u n f i l l e d m o l e c u l a r o r b i t a l o f carbon d i o x i d e , t h e 2TTU. C0 o ( l a g ) 2 ( l a u ) 2 ( 2 a g ) 2 . . . . ( l r r g ) 4 , X ] Z G + < l C T g ) 2 ( 1 ° u ) 2 ( 2 a g ) ] ' ' ' ' ( V 4 ( 2 l T u } 1 ' V For l i n e a r s t a t e s o f ca r b o n d i o x i d e , t h e 2TTu o r b i t a l i s d o u b l y d e g e n e r a t e . T h i s degeneracy i s removed i n bent s t a t e s w i t h t h e p r o d u c t i o n o f t h e 6a-, and t h e l b , R e n n e r - T e l l e r components (see F i g u r e 21). A r e a s o n a b l e e s t i m - a t i o n f o r t h e e q u i l i b r i u m bond a n g l e o f a s t a t e produced by e x c i t i n g a carbon K - s h e l l e l e c t r o n (2a g becomes 2a, i n C2y symmetry) t o the 6a, o r b i t a l i s a p p r o x i m a t e l y 135° . T h i s e s t i m a t i o n i s based on two f a c t s ; a bond a n g l e o f 134° f o r t h e ground s t a t e o f n i t r o g e n d i o x i d e , and a bond a n g l e o f 120 122° f o r t h e f i r s t v a l e n c e e x c i t e d s t a t e o f c a r b o n d i o x i d e w h i c h r e s u l t s from t h e t r a n s i t i o n 1 TT^(4b^) l n ^ ( 6 a , ) . The analogous carbon K - s h e l l e x c i t e d s t a t e r e s u l t i n g from t h e t r a n s i t i o n 2a, -> 6a, i s e x p e c t e d t o have a bond a n g l e l a r g e r than 122° s i n c e t h e 4 b 2 o r b i t a l i s now f i l l e d and on t h e 1 ?o b a s i s o f a Walsh diagram t h i s o r b i t a l f a v o u r s l a r g e r bond a n g l e s . The peak which we have a s s o c i a t e d w i t h t h e t r a n s i t i o n s 2a, -> 6a, and 2a g -> 2TTu (6a, + l b , ) has a FWHM o f 0.9 eV ( e l a s t i c peak 0.5 eV) i n d i c a t i n g t h a t a number o f v i b r a t i o n s a r e e x c i t e d . In t h e bending c o o r d i n a t e , - 9 8 - Excited Orbital 3po 4pa n 3sa 3pn 4pn k + c o 2~C>k N 2 0 - O k COo- Ci • • • » • • • • • • • NNO-N k NNO-N k NO2 corrected 1 1 1 0 5 10 eV FIGURE 23. C o r r e l a t i o n o f the o b s e r v e d peaks i n t h e K - s h e l l e nergy l o s s s p e c t r a o f c a r b o n d i o x i d e and n i t r o u s o x i d e (bot h carbon and oxygen K - s h e l l s ) The dashed l i n e s r e p r e s e n t t h e e x p e c t e d p o s i t i o n s o f u n r e s o l v e d peaks (see the t e x t ) . The r e l a t i v e e n e r g i e s ( c o r r e c t e d ) o f a p p r o p r i a t e s t a t e s from t h e v a l e n c e s h e l l s p e c t r u m o f n i t r o g e n d i o x i d e have a l s o been i n c l u d e d f o r c o m p a r i s o n . -99- maximum Franck-Condon o v e r l a p i s e x p e c t e d f o r t h e 0 ->- 0 t r a n s i t i o n t o t h e l i n e a r l b , component. T h e r e f o r e , most o f t h e o b s e r v e d i n t e n s i t y i s p r o b a b l y a s s o c i a t e d w i t h v i b r a t i o n a l e x c i t a t i o n o f t h e l o w e r members o f t h e l i n e a r component w h i l e some o f t h e i n t e n s i t y on t h e low energy s i d e o f t h e peak, c o u l d be e x c i t a t i o n o f 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 o f t h e be n t 6a-, component below t h e b a r r i e r t o l i n e a r i t y . These " h o l e " s t a t e s decay by Auger e m i s s i o n i n a much s h o r t e r t i m e t h a n t h a t r e q u i r e d f o r a v i b r a t i o n and t h e r e f o r e t h e e x c i t e d m o l e c u l e does not r e a c h t h e bent e q u i l i b r i u m c o n f o r m a t i o n . However, such t r a n s i t i o n s may s t i l l o c c u r s i n c e t h e wave- f u n c t i o n s a r e f i n i t e , a l t h o u g h s m a l l , a t t h e l i n e a r p o s i t i o n ( i . e . between th e two w e l l s ) . In Auger e m i s s i o n s t u d i e s t h e K - s h e l l e x c i t e d s t a t e s a r e o b s e r v e d when th e y decay by a u t o i o n i z i n g t o s i n g l y c h a r g e d i o n s t a t e s . The e n e r g i e s o f t h e e j e c t e d e l e c t r o n s a r e h i g h e r t h a n t h e maximum en e r g y w h i c h can be t a k e n up by a "normal" Auger e l e c t r o n . In t h e c a r b o n K - s h e l l Auger 35 spectrum o f c a r b o n d i o x i d e , two h i g h e nergy peaks a t 272.6 ± 0.5 eV and 268.2 ± 0.5 eV have been o b s e r v e d . Assuming t h a t t h e f i r s t d i s c r e t e s t a t e a t 290.7 ± 0.2 eV i s t h e i n i t i a l n e u t r a l e x c i t e d s t a t e i m p l i e s t h a t s i n g l y c h a r g e d i o n s t a t e s o f c a r b o n d i o x i d e o c c u r a t 18.1 eV and 22.5 eV. The 78 f i r s t energy agrees w i t h t h a t n e c e s s a r y t o remove a 3a e l e c t r o n , w h i l e t h e second p r q b a b l y r e p r e s e n t s shake-up i n c o n j u n c t i o n w i t h t h e i o n i z a t i o n o f a v a l e n c e e l e c t r o n . The h i g h e r e nergy d i s c r e t e peaks i n t h e s p e c t r u m a r e a s s o c i a t e d w i t h s t a t e s produced by p r o m o t i n g a c a r b o n K - s h e l l e l e c t r o n t o Rydberg o r b i t a l s , p r o d u c i n g s t a t e s w h i c h converge t o t h e c a r b o n K - s h e l l i o n i z a t i o n l i m i t . The much l o w e r i n t e n s i t y , w i t h r e s p e c t t o t h a t o f t h e f i r s t d i s c r e t e peak, i s e x p e c t e d , s i n c e t h e f i r s t d i s c r e t e peak i s a s s o c i a t e d -100- w i t h t h e p r o m o t i o n o f a K - s h e l l e l e c t r o n t o a v a l e n c e m o l e c u l a r o r b i t a l w i t h a p r i n c i p a l quantum number o f two, w h i l e t h e h i g h e r energy d i s c r e t e peaks a r e a s s o c i a t e d w i t h h i g h e r quantum number (n = 3,4) Rydberg o r b i t a l s . Two o r b i t a l s w h i c h a r e sometimes i n c l u d e d i n t h e v a l e n c e s h e l l , t h e 5ag 119 and t h e 4au, a r e e x p e c t e d t o c o r r e s p o n d w i t h o u t e r Rydberg o r b i t a l s , Energy v a l u e s c a l c u l a t e d u s i n g quantum d e f e c t s from t h e v a l e n c e s h e l l 124 Rydberg s e r i e s o f c a r b o n d i o x i d e and a s e r i e s l i m i t o f 297.5 eV as 125 d e t e r m i n e d by X-ray PES , and i n good agreement w i t h peaks o b s e r v e d i n t h e s p e c t r u m (see T a b l e 6 ) . The l a r g e s t d e v i a t i o n i s f o u n d f o r t h e peak a t 292.7 eV, whi c h i s a s s i g n e d t o t h e f i r s t Rydberg t r a n s i t i o n , 2ag -> 3sag. However, t h i s i s e x p e c t e d s i n c e t h e 3sag o r b i t a l i s v e r y c l o s e t o t h e v a l e n c e s h e l l and i s p r o b a b l y not a " t r u e " Rydberg o r b i t a l . T h i s t r a n s i t i o n i s o p t i c a l l y f o r b i d d e n by t h e s e l e c t i o n r u l e g g and i s a l s o f o r b i d d e n i n o u r e x p e r i m e n t i f t h e f i r s t Born a p p r o x i m a t i o n i s v a l i d . However, i n e l e c t r o n impact s p e c t r o s c o p y symmetry f o r b i d d e n t r a n s i t i o n s have been o b s e r v e d even a t h i g h e r e n e r g i e s where t h e f i r s t Born a p p r o x i m a t i o n i s 53 n o r m a l l y e x p e c t e d t o be v a l i d . In f a c t , S k e r b e l e and L a s s e t t r e have proposed t h e s e l e c t i o n r u l e t h a t d e v i a t i o n s from t h e f i r s t Born approx- i m a t i o n a r e l a r g e s t when t h e e x c i t e d s t a t e and t h e ground s t a t e b e l o n g t o t h e same symmetry s p e c i e s , i . e . t h e d e v i a t i o n depends upon a t o t a l l y symmetric o p e r a t o r (see R e f e r e n c e 126). In t h i s c a s e , b o t h s t a t e s have a 1 t e r m m a n i f o l d and t h e r e f o r e d e v i a t i o n s from t h e Born t h e o r y a r e t o be e x p e c t e d . The t h i r d peak a t 294.9 eV i s a s s i g n e d t o the Rydberg t r a n s i t i o n s 20^ -> 3pau, 3p-rru. A h i g h e r r e s o l u t i o n scan i s shown i n F i g u r e 22 ( i n s e r t a) and i n d i c a t e s t h a t t h i s band i s composed o f a number o f peaks. The quantum d e f e c t c a l c u l a t i o n s a r e i n good agreement w i t h t h e s e a s s i g n m e n t s , g i v i n g -101- v a l u e s o f 294.9 eV f o r t h e 3pau peak and 295.2 eV f o r t h e 3pTr u peak. I t i s p o s s i b l e t h a t t h e h i g h energy s h o u l d e r has a c o n t r i b u t i o n from t h e t r a n s i t i o n 2a ̂  -> 4sag ( c a l c u l a t e d v a l u e 296.0 e V ) . The f o u r t h d i s c r e t e band w i t h a maximum a t 296.4 eV i s p r o b a b l y a s s o c i a t e d w i t h 2ag -> 4p Rydberg t r a n s i t i o n s . A v a l u e o f 297.5 eV has been o b t a i n e d f o r t h e c a r b o n K - s h e l l b i n d i n g 125 energy by X-ray PES . An i o n i z a t i o n p o t e n t i a l o f 298.0 eV i s o b t a i n e d u s i n g t h e c o r e a n a l o g y model (energy o f t h e o n s e t o f t h e f i r s t d i s c r e t e peak i n t h e carb o n K - s h e l l s p e c t r u m , p l u s a c o r r e c t e d v a l u e f o r t h e i o n - i z a t i o n p o t e n t i a l o f n i t r o g e n d i o x i d e ) w h i c h i s o n l y i n f a i r agreement w i t h the e x p e r i m e n t a l v a l u e (see F i g u r e 23). S t r u c t u r e i s o b s e r v e d above t h e K-edge r e p r e s e n t i n g a v a r i e t y o f m u l t i p l e e l e c t r o n t r a n s i t i o n s i n v o l v i n g one K - s h e l l e l e c t r o n and one o r more v a l e n c e s h e l l e l e c t r o n s . The f o l l o w i n g two e l e c t r o n t r a n s i t i o n s a r e e x p e c t e d t o make t h e l a r g e s t c o n t r i b u t i o n s ; i . d o u b l e e x c i t a t i o n ; i . e . shake-up o f a v a l e n c e e l e c t r o n i n con- j u n c t i o n w i t h K - s h e l l e x c i t a t i o n , d e s i g n a t e d by (C ~ 02) where t h e s u p e r s c r i p t K - l denotes a h o l e i n t h e carb o n K - s h e l l . i i . e x c i t a t i o n and i o n i z a t i o n ; i n v o l v i n g an e l e c t r o n from both t h e carbo n K- and v a l e n c e s h e l l s , where one o f t h e e l e c t r o n s i s e j e c t e d and the o t h e r remains b e h i n d i n a h i g h e r u n f i l l e d o r b i t a l , d e s i g n a t e d by ( c K - ' o 2r. The broad s t r u c t u r e o b s e r v e d i n Region I I o f t h e carb o n K - s h e l l spectrum i s a s s o c i a t e d w i t h d i s c r e t e s t r u c t u r e a r i s i n g from d o u b l e e x c i t a t i o n s , K 1 i . e . (C " 02) s t a t e s . The i n t e n s i t y o f t h e s e bands i s r o u g h l y t h e same as t h a t o f t h e K-jump and a few p e r c e n t o f t h e i n t e n s i t y o f t h e f i r s t d i s c r e t e peak a t 290.7 eV. T h i s s u g g e s t s t h a t most o f t h e i n t e n s i t y a r i s e s -102- from t h e shake-up o f v a l e n c e e l e c t r o n s i n c o n j u n c t i o n w i t h c a r b o n K - s h e l l p r o m o t i o n t o the 2TT m o l e c u l a r o r b i t a l . D i s c r e t e s t a t e s r e s u l t i n g f r o m t h e p r o m o t i o n o f a c a r b o n K - s h e l l e l e c t r o n t o t h e Rydberg o r b i t a l s a r e much l e s s i n t e n s e and t h e r e f o r e we e x p e c t shake-up a s s o c i a t e d w i t h t h e s e t r a n s i t i o n s t o c o n t r i b u t e l i t t l e i n t e n s i t y t o t h e o b s e r v e d shake-up s t r u c t u r e . C o n t r i b - K 1 +* u t i o n s t o t h e i n t e n s i t y o f t h i s s t r u c t u r e from (C ~ 0 2 ) s t a t e s a r e i m p r o b a b l e s i n c e t h e l o w e s t shake-up s t a t e a s s o c i a t e d w i t h K - s h e l l i o n - 125 i z a t i o n , as d e t e r m i n e d by X-ray PES , s h o u l d s t a r t a t 10.8 eV above t h e K-edge. The s t r u c t u r e o b s e r v e d i n Region I I I o f t h e carbon K - s h e l l s p e c t r u m K—1 i s i d e n t i f i e d w i t h t h e o n s e t s o f i o n i z a t i o n t o (C " 0 2 ) s t a t e s . These K— 1 (C " 0 2 ) s t a t e s s h o u l d g i v e r i s e t o a s e r i e s o f shake-up peaks i n t h e X-ray PES s p e c t r u m on t h e low k i n e t i c e n ergy s i d e o f t h e c a r b o n K - s h e l l peak. In F i g u r e 22 we have drawn t h e shake-up peaks a s s o c i a t e d w i t h c a r b o n 125 K - i o n i z a t i o n o f c a r b o n d i o x i d e o b s e r v e d by Siegbahn e t a l . and C a r l s o n 81 e t a l . . I t can be seen t h a t t h e energy r e g i o n o f t h e band o b s e r v e d i n o u r spectrum c o r r e l a t e s w i t h t h e shake-up peaks. The t o t a l shake-up i n t e n s i t y o b s e r v e d i n t h e X-ray PES e x p e r i m e n t s i s r o u g h l y 20% o f t h a t o f t h e K - s h e l l peak, w h i l e t h e i n t e n s i t y o f s t r u c t u r e i n Region I I I o f o u r s p e c t r u m i s a t l e a s t t w i c e t h a t o f t h e K-jump. The l a r g e shake-up i n t e n s i t y o b s e r v e d i n Region I I I o f o u r spectrum p r o b a b l y has a s i g n i f i c a n t c o n t r i b u t i o n from a s e r i e s o f (C ~ 0 2 ) s t a t e s , which converge t o each o f t h e i n d i c a t e d t h r e s h o l d s [ c f . t h e n i t r o g e n K - s h e l l spectrum o f m o l e c u l a r n i t r o g e n ( 5 . 1 . 1 ) . c. Oxygen K - s h e l l E x c i t a t i o n . The oxygen K - s h e l l energy l o s s s p e c t r u m o f c a r b o n d i o x i d e i s shown i n F i g u r e 24 and t e n t a t i v e a s s i g n m e n t s o f o b s e r v e d s t r u c t u r e s a r e L O A J2 c 3 >» I 5 0.5 (0 co c a> HI c o 2 O k-shell co^" 1)^ O co * ^ * v w ^ , x 2 540 550 560 E n e r g y Loss (eV) r 570 FIGURE 24. The oxygen K-shell energy loss spectrum of carbon dioxide. -1 CH- l i s t e d i n T a b l e 6. The i n t e r p r e t a t i o n o f t h e sp e c t r u m i s analogous t o t h a t o f t h e c a r b o n K - s h e l l and t h e r e f o r e o n l y d i f f e r e n c e s w i l l be d i s c u s s e d i n d e t a i l . The f i r s t d i s c r e t e peak a t 535.4 ± 0.2 eV has a FWHM o f 1.4 eV compared w i t h t h e e l a s t i c peak FWHM o f 0.55 eV. In a d d i t i o n t o t h e promot- i o n o f an oxygen K - s h e l l e l e c t r o n t o t h e two components o f t h e 2TT o r b i t a l , t h e 6a, and t h e l b , , i t i s p o s s i b l e t h a t some o f t h e l i n e b r o a d e n i n g and i n t e n s i t y on t h e h i g h energy s i d e o f t h e peak i s a s s o c i a t e d w i t h t h e l o w e s t energy Rydberg t r a n s i t i o n , l a -> 3sa . T h i s t r a n s i t i o n i s o p t i c a l l y a l l o w e d ( i n c o n t r a s t t o 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 v o l v i n g a c a r b o n K - s h e l l e l e c t r o n ) s i n c e f o r m a l l y one oxygen K - s h e l l o r b i t a l has aQ symmetry and t h e o t h e r has a u symmetry. The r e s u l t s o f t h e c o r r e l a t i o n diagram shown i n F i g u r e 23 a r e c o n s i s t e n t w i t h t h e p o s s i b i l i t y t h a t t h e f i r s t d i s c r e t e peak i n c l u d e s t h e f i r s t Rydberg t r a n s i t i o n . The a s s i g n m e n t s o f the r e m a i n i n g d i s c r e t e peaks f o l l o w t h o s e o f t h e c a r b o n K - s h e l l as shown by T a b l e 6 and F i g u r e 23. The energy p o s i t i o n s o f t h e s e d i s c r e t e peaks a r e i n good agreement w i t h t h o s e e x p e c t e d on t h e b a s i s o f c a l c u l a t i o n s i n v o l v i n g 124 quantum d e f e c t s from t h e v a l e n c e s h e l l s p e c t r a o f carb o n d i o x i d e and a s e r i e s l i m i t o f 541.1 e V 1 2 5 (see T a b l e 6 ) . 35 In t h e oxygen K - s h e l l Auger sp e c t r u m o f c a r b o n d i o x i d e , a peak has been o b s e r v e d a t 511.3 ± 0.3 eV whi c h i s t o o h i g h i n energy t o a t t r i b u t e t o a "normal" Auger p r o c e s s . The a s s u m p t i o n t h a t t h e f i r s t d i s c r e t e s t a t e o b s e r v e d a t 535.4 eV i n o u r s p e c t r u m , i s t h e i n i t i a l n e u t r a l e x c i t e d s t a t e , i m p l i e s t h e e x i s t e n c e o f a s i n g l y c h a r g e d s t a t e o f carb o n d i o x i d e a t an 78 energy o f 24.1 eV. The c l o s e s t known i o n s t a t e o f carb o n d i o x i d e i s a t 19.4 eV, a r i s i n g from t h e e j e c t i o n o f a 4a g e l e c t r o n . T h e r e f o r e , t h e f i n a l s t a t e p r o b a b l y a r i s e s from t h e shake-up o f a v a l e n c e e l e c t r o n i n c o n j u n c t i o n -105- w i t h t h e i o n i z a t i o n o f a second v a l e n c e e l e c t r o n . A v a l u e o f 541.1 eV has been o b t a i n e d f o r t h e oxygen K - s h e l l b i n d i n g 125 energy by X-ray PES . Above t h i s edge t h e s p e c t r u m has been d i v i d e d i n t o two r e g i o n s ; Region I I e x t e n d s from t h e K-edge up t o t h e l o w e s t energy K-1 +* 81125 where (CO^ " ) s t a t e s have been o b s e r v e d by X-ray PES ' , and Region I I I e x t e n d s f r o m t h i s p o i n t t o t h e h i g h energy l i m i t o f t h e spectrum. Thus i n K— 1 * * Region I I we would e x p e c t t o o b s e r v e d i s c r e t e e x c i t a t i o n s , (CO2 " ) s t a t e s , w h i l e i n Region I I I t h e s t r u c t u r e s p r o b a b l y a r i s e from b o t h d i s c r e t e s t a t e s , (CO2 " ) , and continuum s t a t e s , (CO2 ~ ) ( c f . t h e c a r b o n K - s h e l l s p e c t r u m ) . The i n t e n s i t i e s o f shake-up s t r u c t u r e s o b s e r v e d i n Region I I , r e l a t i v e t o t h e i n t e n s i t i e s o f Region I I I and t h e K-jump, appear t o be s m a l l . I n F i g u r e 24 we have i n d i c a t e d t h e energy p o s i t i o n s where s t a t e s a r i s i n g from shake-up a s s o c i a t e d w i t h oxygen K - s h e l l i o n i z a t i o n o f c a r b o n Q"l "I pc d i o x i d e have been o b s e r v e d by X-ray PES ' . The broad r e g i o n o f s t r u c t u r e o b s e r v e d i n Region I I I o f o u r spectrum c o r r e l a t e s w i t h t h e shake-up peaks. 6„ 1.2. N i t r o u s Oxide ( M ) . The n i t r o u s o x i d e m o l e c u l e i s i s o e l e c t r o n i c w i t h c a r b o n d i o x i d e and t h e l i n e a r ground e l e c t r o n i c s t a t e has t h e e l e c t r o n i c c o n f i g u r a t i o n ( l a ) 2 (2a ) 2 (3a ) 2 (4a ) 2 (5a ) 2 (6a ) 2 (7a ) 2 ( I T T ) 4 (2T T) 4, V. The l a o r b i t a l i s e s s e n t i a l l y t h e oxygen K - s h e l l o r b i t a l and t h e 2a and 3a o r b i t a l s r e p r e s e n t n i t r o g e n K - s h e l l o r b i t a l s . We have s t u d i e d b o t h t h e n i t r o g e n and t h e oxygen K - s h e l l energy l o s s s p e c t r a . A v a l e n c e s h e l l s pectrum has a l s o been r e c o r d e d . a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l energy l o s s s p e c t r u m o f n i t r o u s o x i d e i s shown -106- i n F i g u r e 25. The l o c a t i o n s o f peaks a r e c o n s i s t e n t w i t h h i g h e r r e s o l u t i o n s p e c t r a 4 9 ' 1 ^ ' 1 1 9 . The weak band w i t h a maximum a t ^ 7.0 eV (peak A) i s p r o b a b l y a s s o c i a t e d w i t h a TT -* TT* t r a n s i t i o n , a n a l o g o u s t o t h e f i r s t band i n t h e c a r b o n d i o x i d e v a l e n c e s h e l l s p e c t r u m ( s e e F i g u r e 20). I n a h i g h e r 119 r e s o l u t i o n p h o t o a b s o r p t i o n s p e c t r u m , a b r o a d , weak band w i t h a maximum a t 6.81 eV has been a s s i g n e d t o t h e f o r b i d d e n , -> ^A t r a n s i t i o n . In C s symmetry, both R e n n e r - T e l l e r components, A' and A", a r e a l l o w e d , w h i c h e x p l a i n s why t h e t r a n s i t i o n i s o b s e r v e d i n o u r spectrum. The h i g h e r energy peaks o b s e r v e d i n o u r s p e c t r u m , B (8.5 e V ) , C (9.6 e V ) , D(11.2 e V ) , E (12.3 e V ) , F( 14.1 e V ) , G (14.8 e V ) , H (16.0 eV) and I (18.5 eV) a r e p r o b a b l y a s s o c i a t e d w i t h Rydberg t r a n s i t i o n s . A Rydberg i n t e r p r e t a t i o n o f 118 the h i g h e r r e s o l u t i o n e l e c t r o n impact spectrum has been s u g g e s t e d , 119 a l t h o u g h i n t h e p h o t o a b s o r p t i o n s p e c t r u m , t h e peak c o r r e s p o n d i n g t o B i n our spectrum has been a s s i g n e d t o t h e t r a n s i t i o n -> ^JI (a -»- T T * ) . The l o c a t i o n o f the f i r s t i o n i z a t i o n p o t e n t i a l shown on o u r s p e c t r u m i s 78 120 based on t h e e x p e r i m e n t a l v a l u e ' o f 12.89 eV. b. N i t r o g e n K - s h e l l E x c i t a t i o n . I f we assume t h a t t h e c o r e e l e c t r o n s a r e l o c a l i z e d on t h e i r n u c l e i , t h e 2a and 3a m o l e c u l a r o r b i t a l s r e p r e s e n t c e n t r a l n i t r o g e n and t e r m i n a l n i t r o g e n K - s h e l l o r b i t a l s r e s p e c t i v e l y . On t h e b a s i s o f t h e c o r e a n a l o g y m odel, e x c i t a t i o n o f a t e r m i n a l n i t r o g e n K - s h e l l e l e c t r o n i n n i t r o u s o x i d e s h o u l d produce s t a t e s analogous t o a p p r o p r i a t e s t a t e s o f n i t r o g e n d i o x i d e . The n i t r o g e n K - s h e l l energy l o s s spectrum i s shown i n F i g u r e 26 and t e n t a t i v e peak a s s i g n m e n t s a r e l i s t e d i n T a b l e 7. The a s s i g n m e n t s a r e analogous t o t h o s e p r e v i o u s l y g i v e n f o r the c o r r e s p o n d i n g peaks i n t h e 20 Energy Loss (eV) 30 40 FIGURE 25. V a l e n c e s h e l l e n ergy l o s s s pectrum o f n i t r o u s o x i de. k + Terminal N k+ Central N i ^ J K i i i i i l i f y. /I w I I H I I 1 2 345 67 1 r - 410 400 " T - 430 T 420 Energy Loss CeV) FIGURE 26. The n i t r o g e n K - s h e l l energy l o s s spectrum o f n i t r o u s o x i d e 440 -109- TABLE 7 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND POSSIBLE ASSIGNMENTS OF PEAKS OBSERVED IN THE NITROGEN K-SHELL SPECTRUM OF NITROUS OXIDE. PEAK ENERGY AE ASSIGNMENT 9 ^ V A L U E ^ 1 401.1 0 N T 3Tr(a' + a") 2 404.7 3 .6 C ( N c - 3 T r(a' + a") ( N T 3so ? 405.1 3 406.2 5.1 N T + 3pa 406.0 1 3pff 406.2 N T + 3da 406.7 1 3dTT 406.9 N T 4sa 407.0 4 407.6 6.5 N T -> 4pa, 4pTr 407.3 1 4da 407.5 4dTr 407.6 5 408.0 6.9 N c + 3s a 409.1 TERMINAL^ K-EDGE 408.5 7.4 Hj -> °° 6 410.0 8.9 N - 3pa 410.0 3PT: 410.2 N 3d a 410.7 3d7T 410.9 N c 4sa 411.0 7 411.2 10.1 N -> 4pa, 4 PTT 411.3 4da 411.5 4diT 411.6 CENTRAL d K-EDGE 4 1 2 - 5 1 1 ' 4 N c - a Only the f i n a l o r b i t a l / s i n v o l v e d i n t h e K - e x c i t a t i o n s have been i n c l u d e d . b C a l c u l a t e d u s i n g t h e Rydberg f o r m u l a . The quantum d e f e c t s used were t h o s e from t h e v a l e n c e s h e l l Rydberg s e r i e s o f n i t r o u s o x i d e 1 2 7 ( a v e r a g e d v a l u e s ) w i t h 6(nso)=1.0, 6(npa)=0.68, 6(npir)=0.57, 6(nda)=0.29 and 6 (ndTr)=0.07 c A more a c c u r a t e d e t e r m i n a t i o n i s 3.62 + 0.05 eV (from a d i f f e r e n t d a t a r u n ) . d These v a l u e s a r e from X-ray PES e x p e r i m e n t s 3 2 . -110- c a r b o n d i o x i d e s p e c t r a (see F i g u r e 2 3 ) , a l t h o u g h t h e n i t r o u s o x i d e s p e c t r u m i s more complex s i n c e two s e p a r a t e s p e c t r a a r e o v e r l a p p e d . T h e r e f o r e , we w i l l d i s c u s s t h e s e p a r a t i o n o f t h e s p e c t r u m i n t o i t s two component p a r t s and any f e a t u r e s w h i c h a r e u n i q u e t o the n i t r o u s o x i d e s p ectrum. The spectrum i s dominated by t h e f i r s t two d i s c r e t e peaks w h i c h have approx- i m a t e l y e q u a l i n t e n s i t i e s . The l o w e s t energy peak i s a t t r i b u t e d t o t h e pr o m o t i o n o f a t e r m i n a l n i t r o g e n K - s h e l l e l e c t r o n t o t h e l o w e s t u n f i l l e d m o l e c u l a r o r b i t a l , t h e 3TT. The h i g h e r e nergy peak i s t h e n 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 t r a n s i t i o n i n v o l v i n g a c e n t r a l n i t r o g e n K - s h e l l e l e c t r o n . The o b s e r v e d energy d i f f e r e n c e between t h e two d i s c r e t e s t a t e s produced by t h e s e t r a n s i t i o n s i s 3.62 ± 0.05 eV. The two d i s c r e t e peaks have d i f f e r e n t w i d t h s , w i t h t h e peak a s s o c i a t e d w i t h t h e t e r m i n a l n i t r o g e n h a v i n g a FWHM o f 1.1 eV w h i l e t h e c o r r e s p o n d i n g peak a s s o c i a t e d w i t h t h e c e n t r a l n i t r o g e n has a FWHM o f 1.3 eV. These a r e much l a r g e r t h a n t h e w i d t h o f t h e peak from e l a s t i c a l l y s c a t t e r e d e l e c t r o n s (FWHM o f 0.5 eV) and i n d i c a t e K - s h e l l e x c i t a t i o n t o a number o f v i b r a t i o n a l l e v e l s o f bo t h components, a 1 and a", o f the 3TT l e v e l (TT d e g e n e r a c i e s i n n i t r o u s o x i d e a r e l i f t e d i n C g symmetry w i t h t h e f o r m a t i o n o f a' and a" components). I t i s p o s s i b l e t h a t t h e f i r s t Rydberg t r a n s i t i o n a s s o c i a t e d w i t h t h e p r o m o t i o n o f a t e r m i n a l n i t r o g e n K - s h e l l e l e c t r o n i s masked by t h e i n t e n s e s econd d i s c r e t e peak and t h e r e f o r e , may c o n t r i b u t e t o i t s w i d t h . I t i s i n t e r e s t i n g t h a t t h e w i d t h s o f t h e peaks 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 i o n s t a t e s ( s e p a r a t e d by 32 4.0 e V ) , o b s e r v e d by X-ray PES , f o l l o w t h e r e v e r s e o r d e r i n t h a t i o n i z a t i o n o f a t e r m i n a l n i t r o g e n K - s h e l l e l e c t r o n g i v e s r i s e t o a peak w i t h a FWHM o f 1.05 eV, w h i l e i o n i z a t i o n o f a c e n t r a l n i t r o g e n K - s h e l l e l e c t r o n produces a peak h a v i n g a FWHM o f 0.95 eV. The a b s o l u t e m agnitudes o f t h e FWHM's - I l l - measured i n t h e two d i f f e r e n t e x p e r i m e n t s a r e not d i r e c t l y c o m p a r a b l e , s i n c e t h e l a r g e n a t u r a l l i n e w i d t h s o f t h e i n c i d e n t X - r a y s a r e the main c o n t r i b u t o r t o the FWHM's o f t h e peaks a s s o c i a t e d w i t h t h e i o n s t a t e s . The s e p a r a t i o n o f t h e h i g h e r energy d i s c r e t e peaks i n t h e spec t r u m i n t o peaks a s s o c i a t e d w i t h each o f t h e n i t r o g e n i n n e r s h e l l s has been made on t h e b a s i s t h a t t h e energy s p l i t t i n g o b s e r v e d between c o r r e s p o n d i n g Rydberg s t a t e s ( i . e . one a s s o c i a t e d w i t h t h e p r o m o t i o n o f a t e r m i n a l n i t r o g e n K - s h e l l e l e c t r o n and t h e o t h e r a s s o c i a t e d w i t h t h e p r o m o t i o n o f a c e n t r a l n i t r o g e n K - s h e l l e l e c t r o n t o t h e same f i n a l o r b i t a l ) s h o u l d be from 3.6 t o 4.0 eV. In f a c t , f o r a " t r u e " Rydberg t y p e o r b i t a l , we would e x p e c t an energy s p l i t - t i n g c l o s e r t o t h e 4.0 eV s e p a r a t i o n o f t h e i o n s t a t e s . Peaks 6 and 7 l i e above t h e t e r m i n a l n i t r o g e n K-edge and a r e a s s i g n e d t o Rydberg t r a n s i t i o n s i n v o l v i n g t h e c e n t r a l n i t r o g e n ( s e e T a b l e 7 ) . However, i t i s p o s s i b l e t h a t i n t h i s energy r e g i o n t h e r e c o u l d be a c o n t r i b u t i o n from d o u b l y e x c i t e d K-1 \** (N NO) s t a t e s . On t h e b a s i s o f t h e f i r s t a s s i g n m e n t i t i s p o s s i b l e t o t e n t a t i v e l y a s s i g n t h e o t h e r d i s c r e t e peaks. The energy p o s i t i o n s o f t h e a s s i g n e d peaks a r e i n good agreement w i t h t h o s e e x p e c t e d on t h e b a s i s o f c a l c u l a t e d v a l u e s u s i n g quantum d e f e c t s f r o m t h e v a l e n c e s h e l l s p e c t r a o f 127 32 n i t r o u s o x i d e and s e r i e s l i m i t s as p r o v i d e d by X-ray PES (see T a b l e 7 ) . The energy d i f f e r e n c e between a l l o f the c o r r e s p o n d i n g s t a t e s i s w i t h i n t h e range 3.6 t o 4.0 eV and t h e s p e c t r u m a s s o c i a t e d w i t h each s h e l l c o r r e l a t e s w i t h t h o s e o f car b o n d i o x i d e ( s e e F i g u r e 23). The a s s o c i a t i o n o f t h e peak a t 408.0 eV w i t h t h e f i r s t Rydberg t r a n s i t i o n i n v o l v i n g a c e n t r a l n i t r o g e n K - s h e l l e l e c t r o n (2a ->- 3 s a ) , i n d i c a t e s t h a t t h e peak from 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 v o l v i n g a t e r m i n a l n i t r o g e n K - s h e l l e l e c t r o n s h o u l d be i n t h e energy r e g i o n o f t h e i n t e n s e second d i s c r e t e peak. In some r e g i o n s i t i s -112- i m p o s s i b l e t o s p e c i f y w h i c h Rydberg t r a n s i t i o n s a r e r e s p o n s i b l e f o r t h e main i n t e n s i t y ( f o r example 3p o r 3d). F o r i n s t a n c e , i n t h e v a l e n c e s h e l l 127 s p e c t r a , a -»- 3dTr Rydberg t r a n s i t i o n s a r e u s u a l l y i n t e n s e . The r e l a t i v e e n e r g i e s o f t h e peaks a s s i g n e d t o t h e p r o m o t i o n o f a t e r m i n a l n i t r o g e n K - s h e l l e l e c t r o n do not match t h o s e o b s e r v e d i n the c a r b o n K - s h e l l s p e c t r u m o f c a r b o n d i o x i d e o r t h e c o r r e c t e d r e l a t i v e e n e r g i e s o f t h e n i t r o g e n d i o x i d e m o l e c u l e (see F i g u r e 2 3 ) . T h e r e f o r e , t h e d e s c r i p t i o n o f t h e K - s h e l l e x c i t e d s t a t e s o f t h e s e t r i a t o m i c m o l e c u l e s i n terms o f t h e c o r e a n a l o g y model i s not as a c c u r a t e as t h a t f o r t h e d i a t o m i c s . T h i s r e s u l t i s n o t s u r p r i s i n g i n view o f t h e a d d i t i o n a l m o l e c u l a r c o m p l e x i t i e s o f the t r i a t o m i c m o l e c u l e s . The energy p o s i t i o n s o f t h e t e r m i n a l and c e n t r a l n i t r o g e n K-edges, 32 shown i n F i g u r e 26, a r e t h o s e o b t a i n e d by X-ray PES . S t r u c t u r e s o b s e r v e d between t h e two edges cannot be a s s i g n e d w i t h c e r t a i n t y , a l t h o u g h t h e c o r r e l a t i o n d i a g r a m , F i g u r e 23, i s c o n s i s t e n t w i t h t h e d i s c r e t e a s s i g n m e n t . Above t h e c e n t r a l n i t r o g e n K-edge t h e o b s e r v e d s t r u c t u r e s c o r r e s p o n d t o t h e shake-up and s h a k e - o f f o f v a l e n c e e l e c t r o n s i n c o n j u n c t i o n w i t h K - s h e l l e x c i t a t i o n o r i o n i z a t i o n o f e i t h e r a t e r m i n a l o r c e n t r a l n i t r o g e n K - s h e l l e l e c t r o n . The f i r s t band o f s t r u c t u r e o b s e r v e d a t ^ 414 eV i n our s p e c t r u m K - l \** i s p r o b a b l y a s s o c i a t e d w i t h (N NO) s t a t e s , w h i l e t h e band c e n t r e d around 418 eV c o u l d have a c o n t r i b u t i o n from (NN ~ 0) s t a t e s . The p o s i t i o n o f ( N K _ 1 N 0 ) + * and ( N N K _ 1 0 ) + * s t a t e s as d e t e r m i n e d by X-ray P E S 1 2 8 have been i n c l u d e d i n F i g u r e 26. c. Oxygen K - s h e l l E x c i t a t i o n . The oxygen K - s h e l l energy l o s s spectrum o f n i t r o u s o x i d e i s shown i n F i g u r e 27 and t e n t a t i v e peak a s s i g n m e n t s a r e l i s t e d i n T a b l e 8. The i n t e r p r e t a t i o n o f t h e s p e c t r u m i s a nalogous t o t h a t o f each n i t r o g e n -113- -114- TABLE 8 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND POSSIBLE ASSIGNMENTS OF PEAKS OBSERVED IN THE OXYGEN K-SHELL SPECTRUM OF NITROUS OXIDE. PEAK ENERGY AE ASSIGNMENT 9 CAbi?Fh(5ED 1 534.6 0 3 i r(a' + a") - 2 536.5 1.9 3sa 537.8 3 538.8 4.2 3pa 538.7 3piT 538.9 3da 539.3 3diT 539.6 4sa 539.7 4 540.0 5.4 4pa, 4piT 540.0 4da 540.2 4dTT 540.3 K-EDGE 0 541.2 6.6 a. Only t h e f i n a l o r b i t a l / s i n v o l v e d i n t h e K - e x c i t a t i o n s have been i n c l u d e d . b. C a l c u l a t e d u s i n g t h e Rydberg f o r m u l a . The quantum d e f e c t s used were t h o s e from t h e v a l e n c e s h e l l Rydberg s e r i e s o f n i t r o u s o x i d e 1 2 7 w i t h 6 (nso) = 1.0, 6(npa) = 0.68, 6(npiT) = 0.57, 6(nda) = 0.29 and 6(nda) = 0.07. c. T h i s v a l u e i s from X-ray P E S 3 2 . -115- K - s h e l l s p e c t r u m and t h e r e f o r e o n l y d i f f e r e n c e s and i n t e r e s t i n g f e a t u r e s w i l l be d i s c u s s e d . The f i r s t d i s c r e t e peak has a FWHM o f 1.2 eV, w h i c h i s s i m i l a r t o t h o s e o b s e r v e d f o r t h e c o r r e s p o n d i n g peaks i n t h e n i t r o g e n s p e c t r u m o f n i t r o u s o x i d e . ( I n c o n t r a s t , f o r c a r b o n d i o x i d e , t h e FWHM's were 0.9 eV f o r t h e c a r b o n K - s h e l l and 1.4 eV f o r t h e oxygen K - s h e l l . T h i s p r o v i d e s some r e i n f o r c e m e n t o f t h e s u g g e s t i o n t h a t t h e f i r s t peak i n t h e oxygen K - s h e l l s p e c t r u m o f carb o n d i o x i d e has a c o n t r i b u t i o n from a t r a n s i t i o n t o the 3sa Rydberg o r b i t a l . ) The i n t e n s i t i e s o f t h e h i g h e r energy d i s c r e t e peaks i n the s p e c t r u m , r e l a t i v e t o t h a t o f t h e f i r s t d i s c r e t e peak, a r e l a r g e r t h a n t h o s e o b s e r v e d i n e i t h e r o f t h e p r e v i o u s s p e c t r a . P a r t o f t h i s i n t e n s i t y may be a s s o c i a t e d w i t h t r a n s i t i o n s t o d-type Rydberg o r b i t a l s s i n c e t h e v a l e n c e s h e l l s p e c t r u m has a s t r o n g c o n t r i b u t i o n from 127 sigma t o nd Rydberg o r b i t a l s •.. However, we would a l s o e x p e c t t h i s t o o c c u r i n t h e case o f t h e n i t r o g e n K - s h e l l s p e c t r u m o f n i t r o u s o x i d e , where a "normal" i n t e n s i t y was o b s e r v e d . A s h a r p d e c r e a s e i n i n t e n s i t y i s ob s e r v e d a f t e r t he K-edge. In t h e n i t r o g e n K - s h e l l s p e c t r u m i t i s d i f f i c u l t t o c o n c l u d e whether a s i m i l a r s i t u a t i o n o c c u r s because both K - r e g i o n s a r e o b s c u r e d , one by d i s c r e t e s t r u c t u r e and t h e o t h e r by a continuum. The p o s i t i o n o f t h e oxygen K-edge i n F i g u r e 2 7 , i s based on t h e 32 e x p e r i m e n t a l v a l u e p r o v i d e d by X-ray PES . A s u r p r i s i n g f e a t u r e above t h e K-edge i s t h e e x t r e m e l y s m a l l i n t e n s i t y o f s t r u c t u r e s a s s o c i a t e d w i t h (N^O ~ ) and (N^O." ) s t a t e s . The energy p o s i t i o n s where shake-up i n c o n j u n c t i o n w i t h K - s h e l l i o n i z a t i o n ( i . e . ( f ^ O " ) s t a t e s ) have been 128 o b s e r v e d by X-ray PES a r e i n c l u d e d i n F i g u r e 27. -116- 6.2. Carbon D i s u l f i d e and C a r b o n y l S u l f i d e . 6.2.1. Carbon D i s u l f i d e . The c a r b o n d i s u l f i d e m o l e c u l e i s l i n e a r i n i t s ground e l e c t r o n i c s t a t e and has t h e e l e c t r o n c o n f i g u r a t i o n : c 4 r2 c 4 c 1 2 ,c x 2 s2 ic. N2 , c ,2 ,0 N4 ^4 1 + b l s C 1 s S 2 s S 2 p (5cV < 4 ou> {6agy ( 5 a u ) ( 2 7 T u } {2lTg) ' E g ' We have s t u d i e d t h e carbon I s (K) and s u l f u r 2p ( L J J j T T ) energy l o s s s p e c t r a . C r o s s - s e c t i o n s f o r d i s c r e t e t r a n s i t i o n s i n t h e r e g i o n o f t h e s u l f u r 2s ( L j ) edge appear t o be s m a l l and a sp e c t r u m was n o t r e c o r d e d . The v a l e n c e s h e l l o f carbon d i s u l f i d e i s i s o e l e c t r o n i c w i t h t h o s e o f carbon d i o x i d e and n i t r o u s o x i d e . a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l e n ergy l o s s s p e c t r u m o f c a r b o n d i s u l f i d e i s shown i n F i g u r e 28. The o b s e r v e d l o c a t i o n s o f peaks a r e c o n s i s t e n t w i t h 118 a h i g h e r r e s o l u t i o n s p e c t r u m . Peak A, w i t h a maximum a t 4.1 eV, i s a s s o c i a t e d w i t h t h e ̂ R e n n e r - T e l l e r component o f t h e ''A s t a t e w h i c h r e s u l t s from t h e t r a n s i t i o n , 2TT -> 3ir ( T T * ) . T h i s t r a n s i t i o n has been g u p o s i t i v e l y i d e n t i f i e d i n t h e c o r r e s p o n d i n g energy r e g i o n o f a h i g h 129 r e s o l u t i o n o p t i c a l s pectrum . The weak i n t e n s i t y o f t h i s band i s a s s o c i a t e d w i t h t h e f o r b i d d e n n a t u r e o f t h e t r a n s i t i o n i n Dro^ symmetry. The i n t e n s e peak, B, w i t h a maximum a t 6.2 eV i s a s s o c i a t e d w i t h t h e 2n ->- 3TTU ( T T*), ^E * (^2) t r a n s i t i o n (see t h e i n t e r p r e t a t i o n o f t h e o p t i c a l 119 129 s p e c t r a ' ). T h i s t r a n s i t i o n i s e l e c t r i c d i p o l e a l l o w e d i n both D ^ and C 2 V symmetry, w h i c h a c c o u n t s f o r i t s s t r o n g i n t e n s i t y r e l a t i v e t o peak A. The l o c a t i o n s o f h i g h e r energy peaks i n o u r s p e c t r u m a r e ; C (8.5 e V ) , D (9.3 e V ) , E (11.1 e V ) , F (11.9 e V ) , G (13.4 eV) and H (15.1 e V ) . In t h e p h o t o a b s o r p t i o n s p e c t r u m 1 1 9 and a h i g h e r r e s o l u t i o n . ELASTIC B 1 s t I. P T » 1 i 1 - i 1 1 r 0 10 20 30 40 Energy Loss (eV) GURE 28. V a l e n c e s h e l l e n e r g y l o s s spectrum o f carbon d i s u l f i d e -118- e l e c t r o n i m p a ct s p e c t r u m 1 l o , c o r r e s p o n d i n g peaks have been a s s i g n e d t o Rydberg e x c i t a t i o n s . The l o c a t i o n o f th e f i r s t i o n i z a t i o n p o t e n t i a l 78 shown i n o u r s p e c t r u m i s based on t h e UV-PES v a l u e o f 10.06 eV. b. Carbon K - s h e l l E x c i t a t i o n . The c a r b o n K - s h e l l energy l o s s s p e c t r u m o f c a r b o n d i s u l f i d e i s shown i n F i g u r e 29 and the e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 9. The i n t e n s e peak o b s e r v e d a t 286.1 eV i s i n t e r p r e t e d as a r i s i n g from t h e p r o m o t i o n o f a carb o n K - s h e l l e l e c t r o n t o t h e l o w e s t u n f i l l e d m o l e c u l a r o r b i t a l , t h e 3TTU ( T T * ) . The peak has a FWHM o f 0.56 eV compared w i t h a FWHM o f 0.38 eV f o r t h e peak a s s o c i a t e d w i t h e l a s t i c a l l y s c a t t e r e d e l e c t r o n s . T h i s i n d i c a t e s t h e e x c i t a t i o n o f a number o f v i b r a t - i o n a l l e v e l s . T h i s peak i s analogous t o th e f i r s t d i s c r e t e peak o b s e r v e d i n the c a r b o n K - s h e l l spectrum o f ca r b o n d i o x i d e . The second and t h i r d peaks l o c a t e d a t ̂  289.6 eV and 290.6 eV r e s p e c t i v e l y , a r e p r o b a b l y a s s o c - i a t e d w i t h t h e p r o m o t i o n o f a ca r b o n K - s h e l l e l e c t r o n t o Rydberg o r b i t a l s . S i n c e c a r b o n and s u l f u r b e l o n g t o d i f f e r e n t rows o f t h e p e r i o d i c t a b l e , a c h o i c e o f p r i n c i p a l quantum numbers e x i s t s . The l o w e s t Rydberg o r b i t a l may be d e s i g n a t e d 3s a p p r o p r i a t e f o r carbon o r 4s a p p r o p r i a t e f o r s u l f u r . The quantum d e f e c t s may be a p p r e c i a b l y d i f f e r e n t from t h o s e d e r i v e d f o r m o l e c u l e s c o n t a i n i n g o n l y second row atoms (see R e f e r e n c e 130). The quantum d e f e c t s (assuming n = 3) d e r i v e d from t h e e x p e r i m e n t a l e n e r g i e s o f peaks two and t h r e e and t h e X-ray PES v a l u e f o r t h e c a r b o n K-edge i n c a r b o n 125 d i s u l f i d e a r e 1.03 and 0.67 r e s p e c t i v e l y . I f a Rydberg a s s i g n m e n t i s c o r r e c t , peak two i s p r o b a b l y a s s o c i a t e d w i t h 3s e x c i t a t i o n and peak t h r e e w i t h 3p e x c i t a t i o n . The broad s h o u l d e r on t h e low energy s i d e o f peak f o u r may a l s o be a s s o c i a t e d w i t h Rydberg t r a n s i t i o n s . 1XH CO 4ml c 1} 1 . CD CO c CD 4-1 c 2 8 0 K-edge to I 2 9 0 3 0 0 310 Energy Loss (eV) 3 2 0 FIGURE 29. Carbon K - s h e l l energy l o s s spectrum o f c a r b o n d i s u l f i d e . TABLE 9 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND POSSIBLE ASSIGNMENTS OF PEAKS OBSERVED IN THE CARBON K-SHELL ENERGY LOSS SPECTRUM OF CARBON DISULFIDE AND THE CARBON AND OXYGEN K-SHELL ENERGY LOSS SPECTRA OF CARBONYL SULFIDE. C A R B O N D I S U L F I D E P O S S I B L E 9 CARBONYL S U L F I D E CARBON K - S H E L L A S S I G N M E N T CARBON K - S H E L L OXYGEN K - S H E L L P E A K E N E R G Y A E P E A K ENERGY A E ENERGY AE 1 286.1 0 TT* 1 288.2 0 533.7 0 2 289.6 3.5 ns? 2 291.0 2.8 3 290.6 4.5 np? 3 291.5 3.3 4 ^ 293.7 5.5 5 294.4 6.2 EDGE 0 293.1 7.0 00 K-EDGE 0 295.2 7.0 540.3 6.6 4 293.4 7.3 6 297.5 9.3 5 295.8 9.7 7 298.3 10.1 6 a, 299.4 ^ 13.3 a Only t h e o u t e r o r b i t a l s i n v o l v e d i n t h e K - e x c i t a t i o n s have been i n c l u d e d , b These v a l u e s a r e from X-ray P E S 1 2 5 . - 1 2 1 - The p o s i t i o n o f t h e carbon K-edge i n d i c a t e d on o u r s p e c t r u m i s based 1?5 on t h e e x p e r i m e n t a l X-ray PES v a l u e o f 293.1 ± 0.1 eV. Peak f o u r w i t h a maximum a t 2 9 3 . 4 ± 0 . 2 eV has two p o s s i b l e e x p l a n a t i o n s : i . the peak may be a s s o c i a t e d w i t h t h e shake-up and/or s h a k e - o f f o f v a l e n c e e l e c t r o n s i n c o n j u n c t i o n w i t h t h e e x c i t a t i o n o f a carbon K - s h e l l e l e c t r o n t o t h e 3TT m o l e c u l a r o r b i t a l and, i i . t h e carbon d i s u l f i d e m o l e c u l e may have an e f f e c t i v e p o t e n t i a l b a r r i e r i n t h e r e g i o n s o f t h e s u l f u r atoms (see R e f e r e n c e s 1 3 1 - 1 3 4 ) . D i s c r e t e l e v e l s can o c c u r up t o t h e t o p o f t h e b a r r i e r w h i c h may be w e l l above t h e i o n i z a t i o n l i m i t . The f i r s t i n t e r p r e t - a t i o n i s s u p p o r t e d by t h e shake-up l i n e s o b s e r v e d i n c o n j u n c t i o n w i t h 125 carbon K - s h e l l i o n i z a t i o n i n c a r b o n d i s u l f i d e as d e t e r m i n e d by X - r a y PES. The two l o w e s t shake-up s t a t e s , (C ~ S 2 ) s t a t e s , o c c u r a t 6 . 5 1 25 and 9.1 eV above t h e K - s h e l l i o n s t a t e (see F i g u r e 29) w i t h r e l a t i v e i n t e n s i t i e s ( w i t h r e s p e c t t o t h e main K - s h e l l i o n peak) o f 7 and 1 6 . 4 % r e s p e c t i v e l y . The shake-up s t a t e s a s s o c i a t e d w i t h t h e d i s c r e t e e x c i t a t i o n o f a c a r b o n K - s h e l l e l e c t r o n are e x p e c t e d t o have s i m i l a r r e l a t i v e e n e r g i e s ( w i t h r e s p e c t t o t h e main d i s c r e t e peak) and i n t e n s i t i e s r o u g h l y 5 t o 20% o f t h a t o f t h e main peak. T h e r e f o r e , peaks f o u r and f i v e , w h i c h a r e o b s e r v e d a t 7 .3 eV and 9 . 7 eV r e s p e c t i v e l y above t h e i n t e n s e 3 i r u peak a r e c o n s i s t e n t w i t h a shake-up i n t e r p r e t a t i o n . S i m i l a r l y t h e broad s t r u c t - ure (peak 6) l o c a t e d a t a p p r o x i m a t e l y 2 9 9 . 4 eV c o u l d be a s s o c i a t e d w i t h t h e shake-up o f v a l e n c e e l e c t r o n s i n c o n j u n c t i o n w i t h K - s h e l l e x c i t a t i o n t o t h e 3TTu o r b i t a l and/or K - s h e l l i o n i z a t i o n . S i m i l a r shake-up s t r u c t u r e s were o b s e r v e d i n the K - s h e l l s p e c t r a ( c a r b o n , n i t r o g e n and oxygen) o f t h e d i a t o m i c m o l e c u l e s , n i t r o g e n , S e c t i o n ( 5 . 1 . 1 ) , and c a r b o n monoxide, S e c t i o n ( 5 . 1 . 2 ) , and t h e t r i a t o m i c m o l e c u l e s , c a r b o n d i o x i d e , S e c t i o n ( 6 . 1 . 1 ) , and n i t r o u s -122- o x i d e , S e c t i o n ( 6 . 1 . 2 ) . However, i n t h e s e c a s e s t h e shake-up bands a r e r a t h e r b r o a d i n c o n t r a s t t o t h e r e l a t i v e l y s h a r p n a t u r e o f peak f o u r . The second i n t e r p r e t a t i o n , t h a t t h e r e i s an e f f e c t i v e p o t e n t i a l b a r r i e r , i s based on t h e o b s e r v a t i o n t h a t t h e c a r b o n K - s h e l l s p e c t r u m o f c a r b o n d i s u l f i d e has p r o p e r t i e s s i m i l a r t o t h o s e o b s e r v e d i n t h e i n n e r s h e l l a b s o r p t i o n s p e c t r a o f S F g 1 6 " 1 9 , B F 3 2 0 " 2 2 and o t h e r m o l e c u l e s 1 5 , 2 1 ' 2 3 - 2 5 c o n s i s t i n g o f a c e n t r a l atom " s u r r o u n d e d " by e l e c t r o n e g a t i v e atoms. F o r t h e s e m o l e c u l e s t h e i n n e r s h e l l a b s o r p t i o n s p e c t r a o f t h e c e n t r a l atom (and i n some c a s e s t h e s u r r o u n d i n g atoms) a r e g e n e r a l l y c h a r a c t e r i z e d by s t r o n g d i s c r e t e peaks b o t h above and below t h e i o n i z a t i o n l i m i t , as w e l l as weak Rydberg s e r i e s and s m a l l K-jumps. These e f f e c t s have been a t t r i b - 131-134 u t e d t o t h e e x i s t e n c e o f an e f f e c t i v e p o t e n t i a l b a r r i e r on t h e o u t e r r i m o f t h e s e m o l e c u l e s . T h i s b a r r i e r s e p a r a t e s an i n n e r p o t e n t i a l 132 133 w e l l from an o u t e r , s h a l l o w w e l l o f l a r g e r a d i u s . C a l c u l a t i o n s f o r t h e e x c i t e d s t a t e s o f BFg s u p p o r t a p o t e n t i a l b a r r i e r i n t h i s m o l e c u l e . T h i s phenomenon i s n o t l i m i t e d t o m o l e c u l e s w h i c h c o n s i s t o f a c e n t r a l atom c o m p l e t e l y s u r r o u n d e d by e l e c t r o n e g a t i v e atoms. In f a c t t h e 115 s u l f u r L j j j j j a b s o r p t i o n s p e c t r u m o f SO^ has some o f t h e c h a r a c t e r i s t i c s 131 131 w h i c h a r e u s u a l l y a s s o c i a t e d w i t h a p o t e n t i a l b a r r i e r . Dehmer has 15 21 p o i n t e d o u t t h a t t h e s u l f u r L J J J J J p h o t o a b s o r p t i o n s p e c t r u m ' o f c a r b o n d i s u l f i d e i s n o t c o n s i s t e n t w i t h t h e e x i s t e n c e o f a p o t e n t i a l b a r r i e r . However, t h i s r e s u l t does n o t n e g a t e t h e p o s s i b i l i t y o f a p o t e n t i a l b a r r i e r t o t h e p r o m o t i o n o f a c a r b o n K - s h e l l e l e c t r o n ( i . e . , f r o m t h e c e n t r a l atom) o f c a r b o n d i s u l f i d e . The f a c t t h a t peaks f o u r and f i v e i n t h e c a r b o n K - s h e l l s p e c t r u m a r e l o c a t e d above t h e K-edge and a r e r e l a t i v e l y narrow s t r u c t u r e s i n d i c a t e s t h e p o s s i b l e e x i s t e n c e o f a p o t e n t i a l b a r r i e r . -123- The p r e s e n c e o f a b a r r i e r i s n o t e x p e c t e d t o s i g n i f i c a n t l y reduce t h e o v e r l a p between Rydberg o r b i t a l s ( o u t e r - w e l l ) and t h e i n n e r - w e l l c a r b o n K - s h e l l o r b i t a l s i n c e the b a r r i e r would not c o m p l e t e l y s u r r o u n d t h e m o l e c u l e . T h e r e f o r e , Rydberg e x c i t a t i o n s a r e a l s o e x p e c t e d t o be o b s e r v e d (see R e f e r e n c e 131). W i t h t h e e x c e p t i o n o f t h e NF^, BF^ and BClg m o l e c u l e s , c h a r a c t e r i s t i c s a t t r i b u t e d t o a p o t e n t i a l b a r r i e r have o n l y been o b s e r v e d (see R e f e r e n c e 131) i n m o l e c u l e s c o n t a i n i n g s u l f u r o r s i l i c o n In t h e s e m o l e c u l e s , t h e p a r t i c i p a t i o n o f d r o r b i t a l s i n t h e bonding may be an i m p o r t - a n t f a c t o r . The c a r b o n K - s h e l l e nergy l o s s spectrum o f c a r b o n d i o x i d e , S e c t i o n ( 6 . 1 . 1 ) , does not s u p p o r t such an i n t e r p r e t a t i o n f o r t h i s m o l e c u l e . These r e s u l t s s u g g e s t t h a t t h e e l e c t r o n e g a t i v i t y o f t h e p e r i p h e r a l atoms i s not t h e o n l y c o n s i d e r a t i o n , s i n c e oxygen i s more e l e c t r o n e g a t i v e than s u l f u r . c. S u l f u r L J J J J J (2p) S h e l l E x c i t a t i o n . The s u l f u r L J J j j j - s h e l l energy l o s s s p e c t r u m o f c a r b o n d i s u l f i d e i s shown i n F i g u r e 30 and t h e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 10. The o p t i c a l a b s o r p t i o n s p e c t r u m o f c a r b o n d i s u l f i d e 15 i n t h i s energy r e g i o n has p r e v i o u s l y been o b t a i n e d u s i n g a B r e m s t r a h l u n g continuum. The i n s t r u m e n t a l r e s o l u t i o n was % 0.4 eV ( i . e . t h e same as i n our spectrum) and t h e a b s o l u t e c a l i b r a t i o n i s r e p o r t e d t o be ± 0.1 eV. The o p t i c a l r e s u l t s a r e l i s t e d i n T a b l e 10. Below t h e L J J j j j - e d g e , the o p t i c a l 15 spectrum and o u r energy l o s s s p e c t r u m show i d e n t i c a l s t r u c t u r e , a l t h o u g h t h e a b s o l u t e c a l i b r a t i o n s d i f f e r by 0.4 eV ( t h i s i s 0.1 eV l a r g e r t h a n t h e sum o f t h e e x p e r i m e n t a l u n c e r t a i n t i e s ) . The s p e c t r a do not show c h a r a c t e r - i s t i c s w h i c h a r e u s u a l l y a s s o c i a t e d w i t h a p o t e n t i a l b a r r i e r . I t has been 131 s u g g e s t e d t h a t t h e d i s c r e t e s t r u c t u r e o b s e r v e d i n t h e o p t i c a l s p e c t r u m c/> 4-1 a mmm c Z3 >> 2 • •••• n 1.0 H £ 0.54 c 0 O J I F 5S M l * * * * * * (CS£- 1) +* X-ray PES v ! I I IMI I I 1 2 3456 7 8 T 160 T 170 180 Energy Loss (eV) 190 FIGURE 30. S u l f u r L n I H ( 2 p ) energy loss spectrum of carbon d i s u l f i d e . ro i T A B L E 10 A B S O L U T E E N E R G I E S ( e V ) , R E L A T I V E E N E R G I E S A N D P O S S I B L E A S S I G N M E N T S OF P E A K S O B S E R V E D IN T H E S U L - FUR 2p ( L T T T J T - S H E L L ) ENERGY L O S S S P E C T R A O F CARBON D I S U L F I D E AND C A R B O N Y L . C A R B O N D I S U L F I D E . CARBONYL S U L F I D E P E A K T H I S WORK O P T I C A L a P O S S I B L E " O R B I T A L A S S I G N M E N T T H I S WORK E N E R G Y AE E N E R G Y AE PEAK ENERGY A E 1 163.1 0 163.5 0 IT* 1 164.2 0 2 164.2 1.1 164.6 1.1 TT* 2 165.6 1.4 3 165.9 2.8 166.4 2.9 IT* 3 166.9 2.7 4 166.5 3.4 167.0 3.5 4 168.1 3.9 5 167.4 4.3 167.7 4.2 5 168.6 4.4 6 168.2 5.1 168.6 5.1 6 170.0 5.8 7 169.5 6.4 169.9 6.4 L - E D G E C ( 2 n 3 / 2 ) 169.8 6.7 0 0 L - E D G E C < 2"3 / 2 ) 170.6 6.4 8 170.8 7.4 171.1 7.6 o/171.0 6.8 L - E D G E 0 ( 2n,,.) 171.0 / 2 7.9 0 0 L - E D G E 0 ) 171.8 7.6 ^177.1 a,14.0 S H A K E - U P M 9 1 a.26.8 a. R e f e r e n c e 15. b. O n l y t h e o u t e r o r b i t a l i n v o l v e d i n t h e t r a n s i t i o n i s g i v e n . c. R e f e r e n c e 125. -126- p r o b a b l y d e r i v e s f r o m s u p e r i m p o s e d Rydberg l i n e s , b u t an i n d i v i d u a l a s s i g n - ment o f t h e peaks has not been a t t e m p t e d . S p i n - o r b i t c o u p l i n g i n t h e s u l f u r 2p s h e l l o f c a r b o n d i s u l f i d e i s l a r g e , as shown by t h e 1.2 eV 1 p r p p s p l i t t i n g o b s e r v e d by X-ray PES between t h e n 3 / and n 1 ; s u l f u r 2p 12 12 i o n s t a t e s o f ca r b o n d i s u l f i d e . T h e r e f o r e R u s s e l l - S a u n d e r s c o u p l i n g does not a p p l y and c o u p l i n g g i v e s a more a p p r o p r i a t e d e s c r i p t i o n ( s e e R e f e r e n c e 75). The l o w e s t energy d i s c r e t e peaks o b s e r v e d i n t h e s u l f u r 2p spect r u m a r e e x p e c t e d t o be a s s o c i a t e d w i t h t h e p r o m o t i o n o f a s u l f u r 2p e l e c t r o n t o t h e v a l e n c e m o l e c u l a r o r b i t a l ( c f . t h e c a r b o n K - s h e l l s pectrum o f carbon d i s u l f i d e ) . S i x groups o f m o l e c u l a r s t a t e s a r e 75 e x p e c t e d as a r e s u l t o f t h i s e x c i t a t i o n , s i n c e t h e l o n e s u l f u r 2p e l e c t r o n may be a a\. , T T 3 o r T T I , . Peaks one and two a r e p r o b a b l y a s s o c i a t e d 12 12 12 w i t h 3T T u e x c i t a t i o n . Peak one i s a p p r o x i m a t e l y t h e same en e r g y below t h e 2 n31 edge, as t h e f i r s t d i s c r e t e peak i n t h e c a r b o n K - s h e l l s p e c t r u m o f c a r b o n d i s u l f i d e i s below t h e c a r b o n K-edge. Peaks t h r e e t o seven a r e p r o b a b l y a s s o c i a t e d w i t h Rydberg e x c i t a t i o n s , a l t h o u g h some o f t h e i n t e n s i t y ( p a r t i c u l a r l y i n the low energy r e g i o n o f t h i s group o f peaks) may be a s s o c i a t e d w i t h e x c i t a t i o n . 2 2 The p o s i t i o n s o f t h e n 3 ^ and L-edges i n d i c a t e d i n o u r s p e c t r u m 1 2 5 a r e based on t h e X-ray PES v a l u e s o f 169.8 ± 0.1 and 171.0 ± 0.1 eV r e s p e c t i v e l y . The band o f s t r u c t u r e w i t h an o n s e t a t ^ 177 eV i s p r o b a b l y a s s o c i a t e d w i t h t h e shake-up o f v a l e n c e e l e c t r o n s i n c o n j u n c t i o n w i t h s u l f u r 2p i o n i z a t i o n . The o n s e t i s 7.3 eV above t h e n 3 ^ edge i n e x a c t agreement w i t h t h e energy o f t h e l o w e s t shake-up s t a t e o b s e r v e d by X-ray PES f o r t h e s u l f u r 2p s h e l l o f . c a r b o n d i s u l f i d e . The p o s i t i o n s o f t h e shake-up s t a t e s , ( C S ? L ~ 1 ) + * , o b s e r v e d 1 2 5 by X-ray PES a r e i n d i c a t e d i n -127- F i g u r e 30. 6.2.2. C a r b o n y l S u l f i d e . The c a r b o n y l s u l f i d e m o l e c u l e i s l i n e a r i n i t s g round e l e c t r o n i c s t a t e and has t h e e l e c t r o n c o n f i g u r a t i o n : S l s °ls C l s S 2 s S 2 p ( 6 a ) 2 ( 7 a ) 2 { 8 a ) 2 ( 9 a ) 2 ( 2 i t ) 4 ( 3 ^ ) 4 ' 1 e + - We have s t u d i e d t h e oxygen I s ( K ) , c a r b o n I s ( K ) and s u l f u r 2p ( L J J J J J ) s h e l l e nergy l o s s s p e c t r a . C r o s s - s e c t i o n s f o r d i s c r e t e t r a n s i t i o n s i n t h e r e g i o n o f t h e s u l f u r 2s ( L j ) edge appear t o be s m a l l and a s p e c t r u m was not r e c o r d e d . The v a l e n c e s h e l l o f c a r b o n y l s u l f i d e i s i s o e l e c t r o n i c w i t h t h o s e o f c a r b o n d i o x i d e , n i t r o u s o x i d e and c a r b o n d i s u l f i d e . A v a l e n c e s h e l l s p e c t r u m was r e c o r d e d . a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l s p e c t r u m o f c a r b o n y l s u l f i d e i s shown i n F i g u r e 31. The l o c a t i o n s o f peaks a r e c o n s i s t e n t w i t h h i g h e r r e s o l u t i o n e l e c t r o n i m p a c t 1 1 8 and o p t i c a l s p e c t r a 1 1 9 . The weak broad band, A , w i t h a maximum a t a p p r o x i m a t e l y 5.7 eV i s p r o b a b l y a s s o c i a t e d w i t h t h e t r a n s i t i o n 2TT •> 3TT (TT*) [ ^ + ->- 1 A ( 1 A ' ) ] , see R e f e r e n c e 119. The h i g h e r energy peaks o b s e r v e d i n o u r s p e c t r u m a r e ; B (7.4 e V ) , C (8.1 e V ) , D (9.5 e V ) , E (12.1 e V ) , F (13.2 eV) and G (13.8 e V ) . In t h e h i g h e r r e s o l u t i o n e l e c t r o n 118 impact spectrum t h e c o r r e s p o n d i n g peaks have been a s s i g n e d t o Rydberg 119 t r a n s i t i o n s . However, i n t h e o p t i c a l s p e c t r u m , peaks c o r r e s p o n d i n g t o B and C i n o u r spectrum have been a s s i g n e d t o non-Rydberg, 1 E + •+ \ and -> 1 £ + t r a n s i t i o n s r e s p e c t i v e l y . The l o c a t i o n o f t h e f i r s t i o n i z a t i o n 120 p o t e n t i a l shown i n F i g u r e 31 i s based on the o p t i c a l and e x p e r i m e n t a l I 1 I • I • s I I 0 10 20 30 40 Energy Loss (eV) FIGURE 31. V a l e n c e s h e l l e n ergy l o s s spectrum o f c a r b o n y l s u l f i d e . -129- UV-PES v a l u e ' " o f 11.2 eV. b. Oxygen K - s h e l l E x c i t a t i o n . The oxygen K - s h e l l energy l o s s s p e c t r u m o f c a r b o n y l s u l f i d e i s shown i n F i g u r e 32 and t h e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f s t r u c t u r e s a r e l i s t e d i n T a b l e 9. The c a l i b r a t i o n a c c u r a c y o f the s p e c t r u m i s ± 0 . 3 eV. The poor s i g n a l t o n o i s e and s i g n a l t o background r a t i o s a r e p a r t i a l l y due t o t h e f a c t t h a t f o r f a s t e l e c t r o n impact and f o r w a r d s c a t t e r i n g , t h e i n e l a s t i c s c a t t e r i n g i n t e n s i t y d e c r e a s e s 1 1 by a f a c t o r , oc (energy l o s s ) - . The broad peak w i t h a maximum a t 533.7 ± 0.3 eV i s i n t e r p r e t e d as a r i s i n g from t h e p r o m o t i o n o f an oxygen K - s h e l l e l e c t r o n t o the l o w e s t u n f i l l e d m o l e c u l a r o r b i t a l , t h e 4u ( I T * ) . The peak has a FWHM o f 1.2 eV ( e l a s t i c FWHM 0.5 eV) i n d i c a t i n g the e x c i t a t i o n o f a number o f v i b r a t i o n a l l e v e l s . The p o s i t i o n o f t h e oxygen K-edge i n d i c a t e d on o u r sp e c t r u m i s based on t h e X-ray PES v a l u e 1 2 5 o f 540.3 ± 0.1 eV. c. Carbon K - s h e l l E x c i t a t i o n . The c a r b o n K - s h e l l energy l o s s spectrum o f c a r b o n y l s u l f i d e i s shown i n F i g u r e 33 and th e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 9. The g e n e r a l appearance o f t h e s p e c t r u m i s s i m i l a r t o the carbon K - s h e l l s p e c t r u m o f ca r b o n d i s u l f i d e , a l t h o u g h t h e r e l a t i v e e n e r g i e s o f s t r u c t u r e s a r e d i f f e r e n t . The i n t e n s e d i s c r e t e peak o b s e r v e d a t 288.2 eV i s i n t e r p r e t e d as a r i s i n g from t h e p r o m o t i o n o f a c a r b o n K - s h e l l e l e c t r o n t o t h e l o w e s t u n f i l l e d m o l e c u l a r o r b i t a l o f c a r b o n y l s u l f i d e , the 4TT (IT * ) . T h i s i n t e r p r e t a t i o n i s analogous t o t h a t o f t h e f i r s t d i s c r e t e peak o b s e r v e d i n t h e carbon K - s h e l l spectrum o f ca r b o n d i s u l f i d e , S e c t i o n ( 6 . 2 . 1 ) , and carbon d i o x i d e , S e c t i o n ( 6 . 1 . 1 ) . In each c a s e t h e Intensity (arbitrary units) -0£L- 1.0 I 5 0.5 03 K-edge • l li II :1 . 2 3 4 5 II 6 7 290 T (C K- 1OS) +* X-ray PES i CO I 300 310 Energy Loss (eV) 320 FIGURE 33. Carbon K - s h e l l energy l o s s spectrum o f c a r b o n y l s u l f i d e . -132- peak i s a p p r o x i m a t e l y t h e same energy below t h e r e s p e c t i v e K-edge. The peak i n t h e c a r b o n y l s u l f i d e s p e c t r u m has a FWHM o f 0.85 eV ( e l a s t i c FWHM 0.56 eV) i n d i c a t i n g t h e e x c i t a t i o n o f a number o f v i b r a t i o n a l l e v e l s . H i g h e r energy d i s c r e t e s t r u c t u r e s below t h e K-edge a r e p r o b a b l y a s s o c i a t e d w i t h t h e p r o m o t i o n o f a ca r b o n K - s h e l l e l e c t r o n t o Rydberg o r b i t a l s . The d e r i v e d quantum d e f e c t s o f peaks two and t h r e e (assuming n = 3) a r e 1.2 and 1.08 r e s p e c t i v e l y . T h e r e f o r e peaks two and t h r e e c o u l d r e p r e s e n t p r o m o t i o n t o the 3s and 3p Rydberg o r b i t a l s r e s p e c t i v e l y . The quantum d e f e c t o f 1.08 i s somewhat l a r g e f o r a 3p Rydberg e x c i t a t i o n and s u g g e s t s t h a t t h i s Rydberg o r b i t a l has more p e n e t r a t i o n i n t o t h e s u l f u r c o r e t h a n does t h e 3s. In a d d i t i o n t o Rydberg e x c i t a t i o n s , peak f i v e may have a c o n t r i b u t i o n from t h e shake-up o f v a l e n c e e l e c t r o n s i n c o n j u n c t i o n w i t h t h e prom o t i o n o f a carbon K - s h e l l e l e c t r o n t o the 4TT m o l e c u l a r o r b i t a l . I t i s a l s o p o s s i b l e t h a t t h e c a r b o n y l s u l f i d e m o l e c u l e has an e f f e c t i v e p o t e n t i a l b a r r i e r ( c f . t h e p o s s i b l e i n t e r p r e t a t i o n o f the c a r b o n K - s h e l l s pectrum o f c a r b o n d i s u l f i d e ) . I f t h i s i s t h e c a s e , t h e e x c i t e d s t a t e s a r e e x p e c t e d t o be a m i x t u r e o f Rydberg ( o u t e r - w e l l s t a t e s ) and i n n e r - w e l l s t a t e s . The p o s i t i o n o f t h e K-edge i n d i c a t e d i n o u r spect r u m i s based on t h e 125 X-ray PES v a l u e o f 295.2 ± 0.1 eV. The broad s t r u c t u r e o b s e r v e d above t h e K-edge (peaks s i x and seven) may be a s s o c i a t e d w i t h shake-up s t a t e s where t h e p r o m o t i o n o f a carb o n K - s h e l l e l e c t r o n t o the 4TT m o l e c u l a r o r b i t a l i s i n v o l v e d . The r e l a t i v e e n e r g i e s o f t h e s e s t r u c t u r e s w i t h r e s p e c t t o the f i r s t d i s c r e t e peak i s ^ 9 eV w h i c h i s c o n s i s t e n t w i t h t h e 125 lov/est shake-up s t a t e o b s e r v e d i n c o n j u n c t i o n w i t h c a r b o n K - s h e l l 125 i o n i z a t i o n ( i . e . 8.3 eV above t h e K-edge). The X-ray PES shake-up -133- l i n e s c o r r e s p o n d i n g t o C N _ l 0 S ) + s t a t e s have been i n c l u d e d i n F i g u r e 33. A l t e r n a t i v e l y , peaks s i x and seven may r e p r e s e n t d i s c r e t e s t a t e s r a i s e d above t h e K-edge by an e f f e c t i v e p o t e n t i a l b a r r i e r . d. S u l f u r L J J j j j ( 2 p ) - s h e l l E x c i t a t i o n . The s u l f u r L J J j j j - s h e l l energy l o s s s p e c t r u m o f c a r b o n y l s u l f i d e i s shown i n F i g u r e 34 and t h e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 10. F i g u r e 35 shows t h e d i s c r e t e s t r u c t u r e below t h e edge on an expanded s c a l e . The i n t e r p r e t a t i o n o f t h e s p e c t r u m i s s i m i l a r t o t h a t o f t h e s u l f u r s pectrum o f carbon d i s u l f i d e . The f i r s t t h r e e peaks a r e r e l a t i v e l y i n t e n s e and a r e p r o b a b l y a s s o c i a t e d w i t h t h e p r o m o t i o n o f a s u l f u r 2p e l e c t r o n t o t h e 4TT ( T T * ) m o l e c u l a r o r b i t a l . I n e x a c t a n a l o g y t o c a r b o n d i s u l f i d e , t h e p r o m o t i o n o f a s u l f u r 2p e l e c t r o n t o t h e 4TT m o l e c u l a r o r b i t a l r e s u l t s i n s i x groups o f m o l e c u l a r s t a t e i n 120 c o u p l i n g . H i g h e r energy d i s c r e t e peaks ( f o u r - s i x ) a r e p r o b a b l y a s s o c i a t e d w i t h Rydberg t r a n s i t i o n s . S h o u l d an e f f e c t i v e p o t e n t i a l b a r r i e r e x i s t i n t h e c a r b o n y l s u l f i d e m o l e c u l e , i t i s u n l i k e l y (as i n t h e c a s e o f carbon d i s u l f i d e ) t h a t i t would have a s i g n i f i c a n t e f f e c t on t h e e x c i t a t i o n o f a s u l f u r 2p e l e c t r o n . 2 2 The p o s i t i o n s o f t h e n 3 ^ and L-edges i n d i c a t e d i n F i g u r e s 34 125 and 35 a r e based on t h e e x p e r i m e n t a l X-ray PES v a l u e s o f 170.6 ± 0.1 eV 125 and 171.8 ± 0.1 eV r e s p e c t i v e l y . The shake-up l i n e s o b s e r v e d i n c o n j u n c t i o n w i t h s u l f u r 2p i o n i z a t i o n c o r r e s p o n d i n g t o (COS " ) s t a t e s , have been i n c l u d e d i n F i g u r e 34. The broad band o f s t r u c t u r e w i t h an o n s e t a t a p p r o x i m a t e l y 191 eV i s p r o b a b l y a s s o c i a t e d w i t h t h e e x c i t a t i o n o f 125 shake-up/shake-off s t a t e s . The X-ray PES spe c t r u m , showing shake-up s t r u c t u r e s , i s not r e p o r t e d above 190 eV. 'n MIN I 1 2 3 4 5 6 T (COS L" 1) +* X-ray PES T 160 170 180 Energy Loss (eV) H r 190 200 FIGURE 34. S u l f u r L I I j n ( 2 p ) energy l o s s spectrum o f c a r b o n y l s u l f i d e . c/> c 3 S5 CD CO c 1.0- 0.5- l i t l i p • * V v 2 3 45 6 7 160 ~ i — r 164 172 Energy Loss (eV) C O cn 176 FIGURE 35. S u l f u r L n j n ( 2 p ) energy l o s s spectrum o f c a r b o n y l s u l f i d e w i t h an expanded energy s c a l e i n t h e r e g i o n o f the ^ edges. -136- CHAPTER SEVEN POLYATOMIC MOLECULES. 7.1. I n t r o d u c t i o n . The prominent f e a t u r e s o b s e r v e d i n t h e a b s o r p t i o n s p e c t r a ( v a l e n c e s h e l l r e g i o n s ) o f s a t u r a t e d p o l y a t o m i c m o l e c u l e s a r e u s u a l l y a s s o c i a t e d w i t h Rydberg t r a n s i t i o n s ( f o r examples see R e f e r e n c e 120). On t h i s b a s i s , Rydberg t r a n s i t i o n s a r e a l s o e x p e c t e d t o dominate t h e K - s h e l l s p e c t r a o f t h e s e m o l e c u l e s . In comparing t h e Rydberg s t a t e s o b s e r v e d as a r e s u l t o f K - s h e l l p r o m o t i o n w i t h t h o s e o b s e r v e d i n t h e p r o m o t i o n o f a v a l e n c e s h e l l e l e c t r o n , i t i s c o n v e n i e n t t o use t h e term v a l u e s ( d i f f e r e n c e between t h e e x c i t a t i o n energy and t h e c o r r e s p o n d i n g i o n i z a t i o n p o t e n t i a l ) s i n c e t h e y have been e x t e n s i v e l y used i n i n t e r p r e t i n g t h e v a l e n c e s h e l l s p e c t r a . F o r t h e same p r i n c i p a l quantum number, n, a d i s c u s s i o n o f t h e quantum d e f e c t , 6, o r the term v a l u e i s e q u i v a l e n t s i n c e t h e term v a l u e i s e q u a l t o R/(n - 6) where R i s t h e Rydberg c o n s t a n t . F o r v a l e n c e s h e l l e x c i t a t i o n t o 3p and 3d Rydberg l e v e l s , i t has been found t h a t t h e term v a l u e s a r e a p p r o x i m a t e l y c o n s t a n t i n a wide range o f compounds, w h i l e t h e 3s term 135 v a l u e s v a r y c o n s i d e r a b l y . The o b s e r v e d 3s d e v i a t i o n s c o r r e l a t e w i t h t h e n a t u r e o f t h e s u b s t i t u e n t groups o f t h e m o l e c u l e and o c c u r because t h e p e n e t r a t i o n o f t h e 3s o r b i t a l e i t h e r i n c r e a s e s ( t h e b i n d i n g energy i n c r e a s e s and t h e r e f o r e t h e term v a l u e i n c r e a s e s ) o r d e c r e a s e s ( l o w e r t e r m v a l u e ) f o r the a d d i t i o n o f an e l e c t r o n e g a t i v e o r e l e c t r o p o s i t i v e s u b s t i t u e n t r e s p e c t - i v e l y . The 3p and 3d Rydberg o r b i t a l s a r e much l e s s s e n s i t i v e t o the n a t u r e -137- o f t h e s u b s t i t u e n t s s i n c e t h e y have much l e s s p e n e t r a t i o n t h a n t h e 3 s . In p r i n c i p l e , f o r K - s h e l l e x c i t a t i o n t o Rydberg o r b i t a l s , we e x p e c t t o o b s e r v e l a r g e r term v a l u e s t h a n t h o s e o b s e r v e d f o r v a l e n c e s h e l l e x c i t - a t i o n i n t h e same m o l e c u l e . T h i s r e s u l t i s e x p e c t e d s i n c e , as a r e s u l t o f a K - s h e l l e x c i t a t i o n , one o f t h e c o r e s ( n u c l e u s + K - s h e l l ) o f t h e m o l e c u l e has e f f e c t i v e l y one more p o s i t i v e c h a r g e . A p e n e t r a t i n g o r b i t a l such as a 3s Rydberg i s t h e r e f o r e e x p e c t e d t o be more t i g h t l y bound ( h i g h e r term v a l u e ) f o r a m o l e c u l e w i t h a K - s h e l l v a c a n c y t h a n i t i s when t h e m o l e c u l e has a v a l e n c e vacancy. 7.2. Methane, Ammonia, Water, M e t h a n o l , D i m e t h y l E t h e r and Monomethylamine. 7.2.1. Methane. The ground e l e c t r o n i c s t a t e o f t h e methane m o l e c u l e has t e t r a h e d r a l symmetry and t h e e l e c t r o n c o n f i g u r a t i o n : ( l a , ) 2 ( 2 a , ) 2 ( l t 2 ) 6 , ]A,. The l a , m o l e c u l a r o r b i t a l i s formed from t h e c a r b o n I s a t o m i c o r b i t a l and i s e s s e n t i a l l y l o c a l i z e d on t h e c a r b o n n u c l e u s . In r e c o g n i t i o n o f t h i s " a t o m i c " c h a r a c t e r t h e e l e c t r o n s f i l l i n g t h i s o r b i t a l a r e d e s i g n a t e d c a r b o n K - s h e l l e l e c t r o n s . a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l e l e c t r o n energy l o s s s p e c t r u m o f methane i s shown i n F i g u r e 36. The l o c a t i o n s o f peaks a r e c o n s i s t e n t w i t h a h i g h e r r e s o l u t i o n s p e c t r u m , where a Rydberg assignment has been p r o p o s e d . The peak p o s i t i o n s i n our spectrum a r e : A (10.0 e V ) , B (11.6 eV) and C (13.4 e V ) . The l o c a t i o n o f t h e f i r s t i o n i z a t i o n p o t e n t i a l shown i n F i g u r e 36 i s based .• ELASTIC Energy Loss (eV) FIGURE 36. V a l e n c e s h e l l energy l o s s spectrum o f methane. -139- on t h e a d i a b a t i c v a l u e 1 ^ o f 13.0 eV. b. Carbon K - s h e l l E x c i t a t i o n . A b s o r p t i o n i n t h e r e g i o n o f t h e carbon K-edge i n methane has 137-140 been i n v e s t i g a t e d u s i n g B r e m s s t r a h l u n g c o n t i n u u a and more r e c e n t l y pQ w i t h t h e c o n t i n u o u s r a d i a t i o n produced by an e l e c t r o n s y n c h r o t r o n . The s p e c t r a o b t a i n e d w i t h B r e m s s t r a h l u n g r a d i a t i o n a r e c h a r a c t e r i z e d by weak a b s o r p t i o n s superimposed by t h e second o r d e r s p e c t r u m o f t h e l o w e r wave- l e n g t h r e g i o n , making i t d i f f i c u l t t o i d e n t i f y carbon-K a b s o r p t i o n bands. pQ However, t h e much " c l e a n e r " s y n c h r o t r o n spectrum shows two d i s c r e t e a b s o r p t i o n s . Energy l e v e l s f o r some o f t h e c o r e e x c i t e d s t a t e s o f methane 141 142 have a l s o been c a l c u l a t e d . The c a r b o n K - s h e l l energy l o s s s p e c t r u m o f methane i s shown i n F i g u r e 37 and t h e e n e r g i e s and t e n t a t i v e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 11. T a b l e 11 a l s o i n c l u d e s e x c i t a t i o n e n e r g i e s o b s e r v e d u s i n g 28 e l e c t r o n s y n c h r o t r o n r a d i a t i o n and c a l c u l a t e d v a l u e s u s i n g SCF wave- 142 f u n c t i o n s . Our spectrum shows more d i s c r e t e s t r u c t u r e t h a n t h e o p t i c a l 28 spectrum and ex t e n d s f u r t h e r i n t o t h e continuum r e g i o n . The f i r s t d i s c r e t e peak o b s e r v e d a t 287.0 eV i s i n t e r p r e t e d as a r i s i n g from the p r o m o t i o n o f a carbon K - s h e l l e l e c t r o n (la-,) t o the 3 s a , Rydberg l e v e l . T h i s e x p e r i m e n t a l 32 v a l u e f o r t h e e x c i t a t i o n energy and t h e X-ray PES v a l u e o f 290.7 eV f o r the K - s h e l l i o n i z a t i o n p o t e n t i a l i m p l i e s a quantum d e f e c t o f 1.08 f o r t h e 3s Rydberg s t a t e . The magnitude o f t h e quantum d e f e c t i s c o n s i s t e n t w i t h t h o s e 130 o b s e r v e d f o r e x c i t a t i o n s t o an ns Rydberg l e v e l . The t r a n s i t i o n 1 a-j -»• 3sa-| i s o p t i c a l l y f o r b i d d e n and i s f o r b i d d e n i n o u r e x p e r i m e n t i f t h e f i r s t Born a p p r o x i m a t i o n i s v a l i d ( t h e impact energy i s 8 t i m e s t h e e x c i t a t i o n energy and e ^ 0°). However, b o t h t h e i n i t i a l and f i n a l s t a t e s  TABLE 11 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND TENTATIVE ASSIGNMENTS OF PEAKS OBSERVED IN THE CARBON K-SHELL SPECTRUM OF METHANE. PEAK ENERGY AE TERM VALUE 9 ASSIGNMENT 5 CALCULATED ENERGYC OPTICAL DATA d SCF e 1 287.0 0 3.7 3sa-| - 287.2 287.3 2 288.0 1.0 2.7 3 p t 2 4 s a 1 3d 289.1 289.2 288.3 288.4 3 289.4 2.4 1.3 4 p t 2 289.4 - - 4 289.8 2.8 0.9 5 p t 2 289.9 - - K-EDGE f 290.7 3.7 0 0 0 5 6 o,303 0,311 o,16 o,24 (SHAKE-UP < AND (SHAKE-OFF a. D e f i n e d as t h e d i f f e r e n c e between t h e i o n i z a t i o n p o t e n t i a l and t h e e x c i t a t i o n energy. b. Only t h e f i n a l o r b i t a l i s l i s t e d ( i n i t i a l o r b i t a l i s la-, = carbon K ) . c. C a l c u l a t e d u s i n g t h e Rydberg f o r m u l a E n = A-R/(n- < 5 ) 2 where E n i s the e x c i t a t i o n energy f o r t h e Rydberg l e v e l h a v i n g quantum number n and quantum d e f e c t 6 , A i s t h e ca r b o n K - s h e l l i o n i z a t i o n p o t e n t i a l and R i s t h e Rydberg c o n s t a n t . The quantum d e f e c t s used f o r ns and np were d e r i v e d from t h e e n e r g y p o s i t i o n s o f t h e f i r s t two peaks. <s(nd) was assumed t o be 0. d. From R e f e r e n c e 28. e. From R e f e r e n c e 142. f . X-ray PES v a l u e ( R e f e r e n c e 3 2 ) . -142- b e l o n g t o t h e same symmetry s p e c i e s and t h e r e f o r e d e v i a t i o n s from t h e Born 53 t h e o r y a r e e x p e c t e d . The f i r s t a b s o r p t i o n i n t h e e l e c t r o n s y n c h r o t r o n 28 142 spectrum was o b s e r v e d a t 287.2 eV. Bagus e t a l . have c a l c u l a t e d a v a l u e o f 287.3 eV f o r t h e Rydberg t r a n s i t i o n , l a , -> 3sa , . From i n t e n s i t y c o n s i d e r a t i o n s and t h e c a l c u l a t e d r e s u l t s , t h e y s u g g e s t t h a t t h e f i r s t a b s o r p t i o n peak i n t h e s y n c h r o t r o n s p e c t r u m s h o u l d be a s s i g n e d t o t h e l a , ->- 3s a , Rydberg t r a n s i t i o n , t h e t r a n s i t i o n b e i n g o b s e r v e d because o f v i b r o n i c c o u p l i n g between t h e ground s t a t e and t h e 3sa , Rydberg s t a t e due t o the t w o , T 2 , v i b r a t i o n a l modes, vg and v^. F i n a l l y , t h e term v a l u e f o r t h i s t r a n s i t i o n i s 3.7 eV i n com p a r i s o n w i t h a term v a l u e o f 3.95 eV 136 o b s e r v e d f o r 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 the v a l e n c e s h e l l s p e c t r u m o f methane, l t 2 3 s a , . The second peak i n o u r s p e c t r u m o b s e r v e d a t 288.0 eV i s a s s i g n e d t o the p r o m o t i o n o f a l a , e l e c t r o n t o t h e f i r s t p-Rydberg o r b i t a l , l a , -v 3 p t 2 , ^ 2 . T h i s t r a n s i t i o n i s e l e c t r i c d i p o l e a l l o w e d and t h e l a r g e i n t e n s i t y r e l a t i v e t o t h a t o f the f i r s t peak i s t h e r e f o r e e x p e c t e d . The e x c i t a t i o n e n ergy i m p l i e s a quantum d e f e c t o f 0.75 w h i c h i s r e a s o n a b l e f o r a 3p Rydberg l e v e l (see R e f e r e n c e 130). The peak has a FWHM o f 1.0 eV ( i n c o n t r a s t t o a FWHM o f 0.5 eV f o r t h e peak a s s o c i a t e d w i t h e l a s t i c a l l y s c a t t e r e d e l e c t r o n s ) and i s a s y m m e t r i c on t h e h i g h energy s i d e (see t h e i n s e r t i n F i g u r e 3 7 ) . I n a d d i t i o n t o v i b r a t i o n a l e x c i t a t i o n , some o f t h e b r o a d e n i n g and asymmetry c o u l d be a s s o c i a t e d w i t h a J a h n - T e l l e r s p l i t t i n g o f t h e d e g e n e r a t e V 2 e l e c t r o n i c s t a t e . The f i r s t two peaks o b s e r v e d i n the e l e c t r o n i m p a ct spectrum o f t h e v a l e n c e s h e l l 1 o r -I A *5 energy r e g i o n o f methane ' ( c o r r e s p o n d i n g t o peak A i n F i g u r e 36) have an energy d i f f e r e n c e o f 0.68 eV and have been i n t e r p r e t e d as J a h n - T e l l e r components o f the ^T ? s t a t e a r i s i n g from t h e t r a n s i t i o n l t 0 3sa-,. A -143- J a h n - T e l l e r s p l i t t i n g o f 0.8 eV has been o b s e r v e d f o r t h e f i r s t i o n s t a t e 144 o f methane by PES . The o b s e r v e d e n e r g i e s f o r peak 2 i n o u r s p e c t r u m 28 and t h e second peak i n t h e s y n c h r o t r o n s p e c t r u m a r e i n good agreement. 142 Bagus e t a l . have c a l c u l a t e d a v a l u e o f 288.4 eV f o r t h e la-j 3 p t 2 t r a n s i t i o n and have s u g g e s t e d t h a t t h i s i s t h e c o r r e c t i n t e r p r e t a t i o n o f 28 t h e p h o t o a b s o r p t i o n peak o b s e r v e d by Chun a t 288.3 eV. A o n e - c e n t r e 141 H a r t r e e - F o c k c a l c u l a t i o n o f t h e l a ^ -> 3 p t 2 t r a n s i t i o n e nergy gave a v a l u e o f 284.7 ± 0.3 eV, w h i c h i s a p p r e c i a b l y l o w e r t h a n o u r e x p e r i m e n t a l r e s u l t . F i n a l l y , a term v a l u e o f 2.7 eV i s o b t a i n e d f r o m o u r d a t a f o r t h i s t r a n s i t i o n . The magnitude o f t h i s term v a l u e i s s i m i l a r t o t h o s e I oc o b s e r v e d i n t h e v a l e n c e s h e l l s p e c t r a o f t h e f l u o r o m e t h a n e m o l e c u l e s f o r t h e p r o m o t i o n o f an o u t e r m o s t e l e c t r o n t o a 3p Rydberg o r b i t a l ( e.g. CF^: It-j 3p, term v a l u e 2.61 e V ) . We have c a l c u l a t e d t h e e x p e c t e d e x c i t a t i o n e n e r g i e s f o r h i g h e r nsa-j and n p t 2 Rydberg l e v e l s u s i n g t h e quantum d e f e c t s d e r i v e d from o u r e x p e r i m e n t a l v a l u e s f o r t h e n = 3 l e v e l s 32 and a v a l u e o f 290.7 eV f o r t h e c a r b o n K-edge o f methane . The r e s u l t s a r e l i s t e d i n T a b l e 11 and have been used as an a i d i n i n t e r p r e t i n g t h e h i g h e r e n e r g y d i s c r e t e s t r u c t u r e i n t h e s p e c t r u m . T h i s s t r u c t u r e c o n s i s t s o f a peak (number 4) w i t h a maximum a t 289.8 eV and a l o w e r e n e r g y s h o u l d e r (peak number 3) a t 289.4 eV. The o b s e r v e d e n e r g i e s a r e i n e x c e l l e n t a g r e e - ment w i t h t h e c a l c u l a t e d v a l u e s f o r t h e 4 p t 2 and 5 p t 2 Rydberg l e v e l s , s u g g e s t i n g t h a t t h e s e peaks c o u l d have c o n t r i b u t i o n s from t r a n s i t i o n s t o t h e s e o r b i t a l s . The r e l a t i v e i n t e n s i t y o f t h e 3p t r a n s i t i o n t o t h a t o f the 3s t r a n s i t i o n i n d i c a t e s t h a t t r a n s i t i o n s t o h i g h e r quantum number ns s t a t e s would be v e r y weak. F i n a l l y , la-j t o 3d t r a n s i t i o n s c o u l d c o n t r i b u t e t o s t r u c t u r e i n t h i s r e g i o n , as i n d i c a t e d by t h e c a l c u l a t e d t r a n s i t i o n e n e r g y -144- ( t h e quantum d e f e c t was assumed t o be z e r o ) . The p o s i t i o n o f t h e K-edge i n d i c a t e d i n o u r spec t r u m i s based on t h e v a l u e o f 290.7 eV f o r t h e K - s h e l l i o n i z a t i o n p o t e n t i a l d e t e r m i n e d by 32 X-ray PES . The v e r y b r o a d s t r u c t u r e s l o c a t e d a t a p p r o x i m a t e l y 303 and 311 eV a r e a s s o c i a t e d w i t h t h e s i m u l t a n e o u s t r a n s i t i o n s o f a K - s h e l l and v a l e n c e s h e l l e l e c t r o n s ( i . e . the shake-up and s h a k e - o f f o f v a l e n c e e l e c t r o n s i n c o n j u n c t i o n w i t h K - s h e l l e x c i t a t i o n o r i o n i z a t i o n . S i m i l a r s t r u c t u r e s have been o b s e r v e d i n the c a s e o f t h e d i a t o m i c and t r i a t o m i c m o l e c u l e s (see S e c t i o n (5.1.1) f o r d e t a i l s ) . 7.2.2. Ammonia. The ground e l e c t r o n i c s t a t e o f the ammonia m o l e c u l e has p y r a m i d a l geometry, but i s more a p p r o p r i a t e l y d e s c r i b e d by D^h symmetry because o f i n v e r s i o n . However, t h e s m a l l i n v e r s i o n s p l i t t i n g o f t h e = 0 v i b r a t i o n a l l e v e l i n t o a symmetric and a n t i s y m m e t r i c l e v e l r e s u l t s i n 145 s e l e c t i o n r u l e s w h i c h a r e e f f e c t i v e l y t h e same as t h o s e f o r C^ v symmetry The e l e c t r o n c o n f i g u r a t i o n o f t h e ground e l e c t r o n i c s t a t e o f ammonia i n C g v symmetry i s ( l a , ) 2 ( 2 a , ) 2 ( l e ) 4 ( 3 a , ) 2 , 1A,. The l a , m o l e c u l a r o r b i t a l i s formed from t h e n i t r o g e n I s a t o m i c o r b i t a l . P r o m o t i o n o f a l a , e l e c t r o n t o n s a , , npe and npa, Rydberg o r b i t a l s i s e l e c t r i c d i p o l e a l l o w e d . One i n t e r e s t i n g f e a t u r e o f t h e v a l e n c e s h e l l s pectrum o f ammonia i s t h a t a l l o f t h e s t a t e s ( b o t h Rydberg and i o n ) , r e s u l t i n g from t h e p r o m o t i o n o f a 3a, e l e c t r o n , a r e e i t h e r p l a n a r o r v e r y 146 n e a r l y p l a n a r . T h i s produces l o n g p r o g r e s s i o n s i n v 9 and i s a r e s u l t o f -145- the l o s s o f an e l e c t r o n from an o r b i t a l w h i c h s t r o n g l y s t a b i l i z e s p y r a m i d a l geometry. Such l a r g e changes i n geometry a r e not e x p e c t e d f o r t h e p r o m o t i o n o f a 1 a-j e l e c t r o n ( e s s e n t i a l l y a t o m i c and nonbonding) t o Rydberg o r b i t a l s and, t h e r e f o r e , t h e s e t r a n s i t i o n s a r e e x p e c t e d t o r e s u l t . i n l e s s v i b r a t - i o n a l e x c i t a t i o n . a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l e l e c t r o n energy l o s s spectrum o f ammonia i s shown i n F i g u r e 38. The l o c a t i o n s o f peaks a r e c o n s i s t e n t w i t h h i g h e r 120 145 49 50 56 r e s o l u t i o n p h o t o a b s o r p t i o n ' and e l e c t r o n impact r e s u l t s ' ' where t h e peaks have been a s s o c i a t e d w i t h Rydberg t r a n s i t i o n s . In o u r sp e c t r u m , c o r r e s p o n d i n g peaks a r e o b s e r v e d a t : A (6.3 e V ) , B (^ 8.0 e V ) , C (9.2 e V ) , D (11.3 eV) and E (15.2 e V ) . The l o c a t i o n o f t h e f i r s t 120 147 i o n i z a t i o n p o t e n t i a l i n F i g u r e 38 i s based on t h e e x p e r i m e n t a l v a l u e ' o f 10.2 eV. b. N i t r o g e n K - s h e l l E x c i t a t i o n . The n i t r o g e n K - s h e l l e nergy l o s s s p e c t r u m o f ammonia i s shown i n F i g u r e 39 and th e e n e r g i e s and t e n t a t i v e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 12. The g e n e r a l appearance o f t h e s p e c t r u m resembles t h a t o f t h e carbon K - s h e l l s p e c t r u m o f methane and th e s p e c t r u m has been i n t e r p r e t e d i n terms o f th e e x c i t a t i o n o f a n i t r o g e n " K - s h e l l " e l e c t r o n ( l a , ) t o Rydberg o r b i t a l s . T r a n s i t i o n e n e r g i e s e s t i m a t e d u s i n g quantum d e f e c t s 120 145 d e r i v e d from the v a l e n c e s h e l l s p e c t r u m ' o f ammonia have been i n c l u d e d i n T a b l e 12. The f i r s t d i s c r e t e peak o b s e r v e d a t 400.6 eV has been a s s i g n e d t o th e t r a n s i t i o n , l a , -> 3sa-,. The peak has a FWHM o f 0.8 eV (compared w i t h an e l a s t i c peak FWHM o f 0.5 eV) i n d i c a t i n g t h a t a number o f v i b r a t i o n a l l e v e l s a r e e x c i t e d . The 5.0 eV term v a l u e i m p l i e s a quantum E L A S T I C 1st IP NH 3 : B T 0 10 20 Energy Loss (eV) FIGURE 38. V a l e n c e s h e l l energy l o s s spectrum o f ammonia. Intensity (arbitrary units) -LH- TABLE 12 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND TENTATIVE ASSIGNMENTS OF THE PEAKS OBSERVED IN THE NITROGEN K-SHELL SPECTRUM OF AMMONIA ESTIMATED PEAK ENERGY AE TERM VALUE ASSIGNMENT 3 ENERGY 0 1 400.6 0 5.0 3sa-| 401.2 2 402.2 1.6 3.4 3pe 402.8 3 403.5 2.9 2.1 3pa, 403.4 4 404.1 3.5 1.5 4 s a 1 / 3 d 4pe 404.1 404.3 ^ 404.6 4.0 1.0 5pe 404.8 K-EDGE C 405.6 5.0 0 CO 5 <\, 4 1 4 d * 13.5 (SHAKE-UP 6 ^ 4 2 8 d ^ 27.5 < AND (SHAKE-OFF a. Only t h e f i n a l o r b i t a l i n v o l v e d i n t h e e x c i t a t i o n i s g i v e n ( t h e i n i t i a l o r b i t a l i s l a , = N i t r o g e n K ) . b. E s t i m a t e d u s i n g quantum d e f e c t s d e r i v e d from t h e v a l e n c e s h e l l s p e c t r u m 5 0 ' 1 4 3 ' 1 4 5 ; 5(nsa,) = 1.25 n = 3; 1.02 n > 3, 6(npe) = 0.8, 6(npa,) = 0.54, and 6(nd) was assumed t o be 0. c. From X-ray P E S 3 2 . d. Onset. -149- d e f e c t o f 1.35 w h i c h i s comparable w i t h t h e 1.25 quantum d e f e c t o b s e r v e d i n t h e v a l e n c e s h e l l s p e c t r u m f o r t h e t r a n s i t i o n 3a-j -> 3sa-j. I t i s normal f o r t h e 3s quantum d e f e c t t o be a p p r e c i a b l y h i g h e r t h a n t h a t d e t e r m i n e d f o r t h e h i g h e r members o f the s e r i e s i n p o l y a t o m i c m o l e c u l e s . In f a c t , f o r v a l e n c e s h e l l e x c i t a t i o n i n ammonia, t h e 3sa-j o r b i t a l i s n ot a pure cc Rydberg o r b i t a l and shows a p p r e c i a b l e a n t i b o n d i n g c h a r a c t e r . Peak number 2, o b s e r v e d a t 404.2 eV, i s a s s i g n e d t o t h e p r o m o t i o n o f a la-j e l e c t r o n t o th e l o w e s t energy 3p Rydberg o r b i t a l ( 3 p e ) . T h i s peak has a FWHM o f 0.7 eV and a quantum d e f e c t (peak maximum) o f 1.0 i n c o n t r a s t t o the quantum d e f e c t o f 0.8 ( c a l c u l a t e d from the a d i a b a t i c t r a n s i t i o n energy r e p o r t e d i n R e f e r e n c e 50) f o r t h e p r o m o t i o n o f a 3a-j e l e c t r o n t o th e 3pe Rydberg o r b i t a l . The energy d i f f e r e n c e o b s e r v e d f o r t h e two v a l e n c e s h e l l t r a n s i t i o n s 3 a 1 + 3pe and 3a-| + 3pa-| (6 = 0.54.) i s < 0.6 eV . The o b s e r v e d FWHM o f the second peak i n o u r spect r u m does not s u p p o r t a c o n t r i b u t i o n from the t r a n s i t i o n 1 a-j 3pa-| u n l e s s f o r K - s h e l l e x c i t a t i o n t he 3pe and 3pa-j energy d i f f e r e n c e i s s m a l l o r t h e i n t e n s i t y o f one t r a n s i t i o n i s weak. We sug g e s t t h a t t h e t h i r d peak a t 403.5 eV c o u l d r e p r e s e n t t h e t r a n s i t i o n la-| •> 3pa-|. T h i s i m p l i e s a 3p s p l i t t i n g o f 1.3 eV f o r K - s h e l l e x c i t a t i o n . The o b s e r v e d energy o f the f o u r t h peak, 401.1 eV, i s c o n s i s t e n t w i t h t h e energy c a l c u l a t e d f o r t h e e x c i t a t i o n o f a la-j e l e c t r o n t o 3d, 4s and 4p Rydberg o r b i t a l s . The h i g h energy s h o u l d e r p r o b a b l y has c o n t r i b u t i o n s from t h e e x c i t a t i o n o f n - 5 and h i g h e r Rydberg o r b i t a l s . The p o s i t i o n o f t h e K-edge i n o ur spect r u m i s based on the e x p e r i m e n t a l 32 X-ray PES v a l u e o f 405.6 eV f o r t h e la-j b i n d i n g energy i n ammonia. The broad s t r u c t u r e s w i t h o n s e t s a t ̂  414 eV and o, 428 eV a r e i d e n t i f i e d w i t h t h e s i m u l t a n e o u s t r a n s i t i o n s o f a K - s h e l l and v a l e n c e s h e l l e l e c t r o n s . -150- 7.2.3. H a t e r . The ground e l e c t r o n i c s t a t e o f t h e w a t e r m o l e c u l e has C 2 v symmetry and t h e e l e c t r o n c o n f i g u r a t i o n : ( l a , ) 2 ( 2 a , ) 2 ( l b 2 ) 2 ( 3 a , ) 2 ( l b , ) 2 , 1A,. The l a , o r b i t a l i s formed from t h e oxygen I s o r b i t a l and i s l o c a l i z e d on th e oxygen n u c l e u s . The t h r e e p - o r b i t a l s a r e nondegenerate i n C^ v symmetry and have a,, b, and b 2 symmetries. T r a n s i t i o n s i n v o l v i n g t h e p r o m o t i o n o f a l a , e l e c t r o n t o ns and np Rydberg o r b i t a l s a r e e l e c t r i c d i p o l e a l l o w e d . a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l e n ergy l o s s spectrum o f w a t e r i s shown i n F i g u r e 40. The l o c a t i o n s o f p e a k s ; A (7.5 e V ) , B (9.7 e V ) , C (10.1 e V ) , D (11.1 e V ) , E (13.6 eV) and F (17.2 eV) a r e c o n s i s t e n t w i t h h i g h e r 120 49 148 149 r e s o l u t i o n o p t i c a l and e l e c t r o n impact r e s u l t s ' ' . These s p e c t r a have been i n t e r p r e t e d i n terms o f Rydberg t r a n s i t i o n s 1 2 0 ' 1 5 0 , 1 5 1 . The l o c a t i o n o f the f i r s t i o n i z a t i o n p o t e n t i a l i n F i g u r e 40 i s based on t h e 120 e x p e r i m e n t a l v a l u e o f 12.61 eV. b. Oxygen K - s h e l l E x c i t a t i o n . The K - s h e l l energy l o s s s p e c t r u m o f w a t e r i s shown i n F i g u r e 41 and th e e n e r g i e s and t e n t a t i v e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 13. The g e n e r a l appearance o f t h e s p e c t r u m i s s i m i l a r t o t h a t o b s e r v e d f o r t h e K - s h e l l s p e c t r a o f methane and ammonia. The s p e c t r u m i s i n t e r p r e t e d i n terms o f Rydberg e x c i t a t i o n s and e x c i t a t i o n e n e r g i e s e s t i m a t e d u s i n g t h e quantum d e f e c t method a r e i n c l u d e d i n T a b l e 13. The f i r s t peak o b s e r v e d a t 534.0 eV i s a s s i g n e d t o t h e p r o m o t i o n o f an oxygen K - s h e l l e l e c t r o n ( l a , ) t o t h e 3 s a , Rydberg o r b i t a l . The peak has a FWHM o f 1.0 eV, 1st I.P 0 1 10 1 20 ' 30 40 Energy Loss (eV) FIGURE 40. V a l e n c e s h e l l energy l o s s spectrum o f w a t e r . 1.0-1 o i K- edge I I I I 1 234 H 530 T T T 540 550 560 Energy Loss (eV ) FIGURE 41 . Oxygen K - s h e l l energy l o s s spectrum o f wa t e r TABLE 13 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND POSSIBLE ASSIGNMENTS OF PEAKS OBSERVED IN THE OXYGEN K-SHELL SPECTRUM OF WATER. PEAK ENERGY AE TERM VALUE P O S S I B L E , ASSIGNMENT ESTIMATED ENERGY 0 1 534.0 • 0 5.7 3sa-| 534.5 2 535.9 1.9 3.8 3pb^ - 3 537.1 3.1 2.6 |3pa-, /3pb 1 3d 537.1 537.5 4 538.5 4.5 1.2 j 4 s J4p 538.1 538.5 K-EDGE C 539.7 ^ 555 5.7 CO (SHAKE-UP < AND (SHAKE-OFF a. b. O n l y t h e f i n a l o r b i t a l s i n v o l v e d i n t h e K - e x c i t a t i o n s have been i n c l u d e d . E s t i m a t e d u s i n g t h e quantum d e f e c t method w i t h quantum d e f e c t s from t h e v a l e n c e s h e l l s p e c t r u m o f w a t e r 1 4 6 . ( 3 s ) = 1.38 (from the term v a l u e o f 5.2 eV r e p o r t e d by R e f e r e n c e 150, 6 ( n s ) = 1.05 n > 3, 6 ( n p a ] / b 1 ) = 0.7, fi(nd) = 0.05. From X-ray P E S 3 2 . -154- i n d i c a t i n g t h a t a number o f v i b r a t i o n a l l e v e l s a r e e x c i t e d . The term v a l u e i s 5.7 eV w h i c h i m p l i e s a quantum d e f e c t o f 1.45 f o r t h e 3s Rydberg s t a t e . T h i s term v a l u e i s comparable w i t h t h e term v a l u e o f 5.2 eV o b s e r v - 150 ed i n t h e v a l e n c e s h e l l s p e c t r u m o f w a t e r f o r t h e p r o m o t i o n o f an e l e c t r o n from t h e o u t e r m o s t o r b i t a l t o t h e 3s Rydberg l e v e l , i . e . lb-j •> 3sa-j. The second peak, o b s e r v e d a t 535.9 eV, has a FWHM o f 0.9 eV ( d i f f e r e n t d a t a r u n ) which i n d i c a t e s t h e e x c i t a t i o n o f a number o f v i b r a t - i o n a l l e v e l s . T h i s peak i s a s s i g n e d t o t h e t r a n s i t i o n 1 a^ ->• 3 p b 2 and has a term v a l u e o f 3.8 eV. The 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 t h e v a l e n c e s h e l l s p ectrum l b ^ -»• 3 p b 2 i s e l e c t r i c d i p o l e f o r b i d d e n and has n o t been o b s e r v e d . However, t h e lb-j -»- 3 p b 2 e x c i t a t i o n e n e r g y has been c a l c u l a t e d by t h e 152 153 INDO and t h e IVO methods. Both c a l c u l a t i o n s i n d i c a t e t h a t t h e l o w e s t energy 3p Rydberg e x c i t a t i o n s h o u l d r e s u l t from p r o m o t i o n t o t h e b 2 component. The t h i r d peak i n o u r spectrum a t 537.1 eV i s t h e n a s s o c i a t e d w i t h p r o m o t i o n o f a la-j e l e c t r o n t o the 3pa-| and 3pb-j Rydberg o r b i t a l s and has a term v a l u e o f 2.6 eV. The energy d i f f e r e n c e between t h e s e o r b i t a l s 1 50 i n the v a l e n c e s h e l l s pectrum (lb-j p r o m o t i o n ) i s 0.16 eV (term v a l u e s 2.62 eV and 2.46). The term v a l u e f o r t h e K - s h e l l t r a n s i t i o n i s i n good agreement w i t h the term v a l u e s f o r t h e c o r r e s p o n d i n g v a l e n c e t r a n s i t i o n s . The f o u r t h peak o b s e r v e d a t 538.5 eV (term v a l u e 1.2 eV) i s p r o b a b l y a s s o c i a t e d w i t h 4s and 4p Rydberg t r a n s i t i o n s ( c f . t h e e s t i m a t e d v a l u e s i n T a b l e 1 3 ) . The p o s i t i o n o f the K-edge shown on o u r s p e c t r u m i s based on t h e 32 X - r a y PES v a l u e o f 539.7 eV f o r t h e K - s h e l l b i n d i n g e n e r g y o f w a t e r . The broad s t r u c t u r e o b s e r v e d i n t h e continuum r e g i o n ^ 555 eV i s a s s o c i a t e d w i t h t h e s i m u l t a n e o u s t r a n s i t i o n s o f a K - s h e l l and v a l e n c e s h e l l e l e c t r o n s . -155- 7.2.4. M e t h a n o l . The ground e l e c t r o n i c s t a t e o f t h e methanol m o l e c u l e has C g symmetry 150 and t h e e l e c t r o n i c c o n f i g u r a t i o n ; ( l a 1 ) 2 ( 2 a 1 ) 2 ( 3 a ' ) 2 ( 4 a 1 ) 2 ( 5 a 1 ) 2 ( l a " ) 2 ( 6 a 1 ) 2 ( 7 a ' ) 2 ( 2 a 1 1 ) 2 , V . The l a ' and 2a' m o l e c u l a r o r b i t a l s r e p r e s e n t t h e oxygen I s and c a r b o n I s a t o m i c o r b i t a l s r e s p e c t i v e l y . The 3s and 3p Rydberg o r b i t a l s y m m e t r i e s i n t h e C s p o i n t group a r e 3 s a ' , 3pa' ( t w i c e ) and 3pa". The p r o m o t i o n o f an e l e c t r o n from any o f t h e 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 o f methanol t o each o f t h e s e Rydberg o r b i t a l s i s e l e c t r i c d i p o l e a l l o w e d . We have i n v e s t - i g a t e d b o t h t h e c a r b o n and oxygen K - s h e l l r e g i o n s o f m e t h a n o l . A v a l e n c e s h e l l s p e c t r u m was r e c o r d e d . a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l energy l o s s spectrum o f methanol i s shown i n F i g u r e 42. The l o c a t i o n s o f peaks, A (6.8 e V ) , B(7.9 e V ) , C (8.3 e V ) , D (9.8 e V ) , E (12.0 e V ) , F (13.8 eV) and G (15.8 e V ) , a r e c o n s i s t e n t w i t h h i g h e r r e s o l u t i o n e l e c t r o n i m p a c t r e s u l t s 1 ^ ' 1 ^ , where t h e peaks have been a s s o c i a t e d w i t h Rydberg t r a n s i t i o n s . The l o c a t i o n o f t h e f i r s t 120 i o n i z a t i o n p o t e n t i a l i n F i g u r e 42 i s based on t h e a d i a b a t i c v a l u e o f 10.85 eV. b. Carbon K - s h e l l E x c i t a t i o n . The carbon K - s h e l l energy l o s s spectrum o f methanol i s shown i n F i g u r e 43 and t h e e n e r g i e s and t e n t a t i v e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 14. We have i n t e r p r e t e d t h e s p e c t r u m i n terms o f Rydberg t r a n s i t - i o n s . T r a n s i t i o n e n e r g i e s e s t i m a t e d u s i n g term v a l u e s o b s e r v e d i n t h e 150 v a l e n c e s h e l l s pectrum f o r 2a" p r o m o t i o n ( m a i n l y an oxygen l o n e p a i r o r b i t a l ) a r e a l s o l i s t e d i n T a b l e 14. The f i r s t d i s c r e t e peak o b s e r v e d : ELASTIC 1st I. P Energy Loss (eV) IGURE 42. V a l e n c e s h e l l e n e r g y l o s s spectrum o f methanol. K-edge I III 1234 C H 3 0 H CK-shell 290 300 Energy Loss —1 310 (eV) 320 FIGURE 43. Carbon K - s h e l l energy l o s s spectrum o f methanol TABLE 14 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND TENTATIVE ASSIGNMENTS OF PEAKS OBSERVED IN THE CARBON AND OXYGEN K-SHELL SPECTRUM OF METHANOL • CARBON K-SHELL OX YGEN K- SHELL ASSIGNMENT 5 PEAK ENERGY AE TERM VALUE ESTIMATED VALUE 9 PEAK ENERGY AE TERM VALUE ESTIMATED VALUE 3 1 288.1 0 4.2 288.1 1 534.1 0 4.8 534.6 3sa' 2 289.4 1.3 2.9 289.1 3p 3 290.3 2.2 2.0 289.7 290.6 2 537.1 3,0 1.8 3p 3d 4 K-EDGE C 291.3 292.3 3.2 4.2 1.0 291.O d K-EDGE 0 538.9 4.8 4s/4p CO a. E s t i m a t e d u s i n g the term v a l u e s o b s e r v e d i n the v a l e n c e s h e l l s p e c t r u m . 1 5 0 b. Only t h e f i n a l o r b i t a l i n v o l v e d i n t h e e x c i t a t i o n i s g i v e n . c. X-ray PES v a l u e . 3 2 -159- a t 288.1 eV has a term v a l u e o f 4.2 eV and i s a s s i g n e d t o t h e p r o m o t i o n o f a carbon I s e l e c t r o n ( 2 a 1 ) t o t h e 3sa' Rydberg o r b i t a l , 2a' -> 3sa' (6 = 1.2). The term v a l u e f o r t h i s t r a n s i t i o n i s v e r y c l o s e t o t h e term 150 v a l u e o f 4.22 eV o b s e r v e d f o r t h e c o r r e s p o n d i n g v a l e n c e s h e l l t r a n s i t i o n , 2a" -> 3 s a ' . The second and t h i r d d i s c r e t e peaks i n our s p e c t r u m a r e a s s i g n e d t o t h e p r o m o t i o n o f a l a ' e l e c t r o n t o 3p Rydberg o r b i t a l s . The o b s e r v e d energy d i f f e r e n c e between t h e two 3p l e v e l s i s 0.8 eV. I n t h e 150 v a l e n c e s h e l l e l e c t r o n impact s p e c t r u m o f methanol ( o b t a i n e d w i t h much h i g h e r r e s o l u t i o n ) two peaks w i t h an energy d i f f e r e n c e o f 0.4 eV ( c o r r e s p o n d - i n g t o peaks B and C i n F i g u r e 42) have been a s s i g n e d t o 3p Rydberg e x c i t - a t i o n s , 2a" -> 3p. The o b s e r v e d term v a l u e s were 3.24 eV and 2.64 eV w h i c h a r e somewhat h i g h e r than t h o s e o b s e r v e d i n o u r K - s h e l l s p e c t r u m (2.9 and 2.0 e V ) . The f o u r t h band o f s t r u c t u r e w i t h a maximum a t 291.3 eV p r o b a b l y has c o n t r i b u t i o n s from 3d, 4s and 4p Rydberg t r a n s i t i o n s . The p o s i t i o n o f t h e c a r b o n K-edge i n o u r s p e c t r u m i s based on t h e 32 X-ray PES v a l u e o f 292.3 eV f o r t h e K - s h e l l i o n i z a t i o n e nergy. S t r u c t u r e s a r i s i n g from the s i m u l t a n e o u s p r o m o t i o n o f a K - s h e l l and v a l e n c e s h e l l e l e c t r o n s appear t o be weak. c. Oxygen K - s h e l l E x c i t a t i o n . The oxygen K - s h e l l e nergy l o s s s p e c t r u m o f methanol i s shown i n F i g u r e 44 and t h e e n e r g i e s and t e n t a t i v e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 14. The s p e c t r u m has a s l o p i n g b a s e l i n e which i s i n s t r u m e n t a l , a r i s i n g from the l a r g e c o n t i n u o u s background o f s e c o n d a r y e m i t t e d and a p p a r a t u s s c a t t e r e d e l e c t r o n s . T h i s background i s m o n o t o n i c a l l y d e c r e a s i n g as a f u n c t i o n o f energy l o s s and was checked by r e c o r d i n g t h e s i g n a l from background s c a t t e r e d e l e c t r o n s w i t h o u t any t a r g e t gas. The background i s CD CZ TO 4> O X •< CQ m zn I t/> fD ro -s C Q O IO </) T 3 fD O r+ -S cz 3 fD =3 O Intensity ( arbitrary units ) CD 0 < © cn © 3 p o -v7 ro — •.v 7 CD a CD CD :K.. eft- .:y.v- o I 0) u CD n CO O -09L- -161- more prominent i n t h e oxygen K - s h e l l e nergy r e g i o n because o f t h e r a p i d d e c r e a s e i n s c a t t e r i n g i n t e n s i t y w i t h e n e r g y l o s s 1 1 , i . e . a t l e a s t as f a s t as ( e nergy l o s s ) . The appearance o f t h e sp e c t r u m i s a p p r e c i a b l y d i f f e r e n t from t h a t f o r t h e c a r b o n K - s h e l l ( F i g u r e 4 3 ) . The f i r s t peak a t 534.1 eV has a term v a l u e o f 4.6 eV and i s i n t e r p r e t e d as a r i s i n g from t h e p r o m o t i o n o f an oxygen K - s h e l l e l e c t r o n t o t h e 3sa' Rydberg o r b i t a l , l a 1 -> 3 s a ' . The peak has a FWHM o f 1.2 eV i n d i c a t i n g t h e e x c i t a t i o n o f many v i b r a t i o n a l l e v e l s . H i g h e r e n e r g y s t r u c t u r e c o n s i s t s o f a b r o a d peak w i t h a maximum a t 537.1 eV. On t h e b a s i s o f t h e a s s i g n m e n t s o f the p r e v i o u s s p e c t r a , we e x p e c t 3p Rydberg e x c i t a t i o n s t o c o n t r i b u t e t h e most i n t e n s i t y t o t h i s broad s t r u c t u r e . I f t h i s i s t h e c a s 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 3p components must be s i g n i f i c a n t l y d i f f e r e n t from t h o s e o b s e r v e d i n t h e ca r b o n K - s h e l l s p e c t r u m o f m e t h a n o l . T r a n s i t i o n s t o 3d, 4s and 4p Rydberg o r b i t a l s a r e a l s o e x p e c t e d t o c o n t r i b u t e i n t e n s i t y i n t h e r e g i o n o f t h e band maximum. The p o s i t i o n o f t h e K-edge i n d i c a t e d on o u r spectrum i s based on the 32 X-ray PES v a l u e o f 538.9 eV f o r t h e oxygen K - s h e l l b i n d i n g e n e r g y o f metha n o l . 7.2.5. D i m e t h y l E t h e r . The ground e l e c t r o n i c s t a t e o f t h e d i m e t h y l e t h e r m o l e c u l e has C^ v 154 symmetry and t h e e l e c t r o n c o n f i g u r a t i o n : ( l a ^ 2 ( 2 a / ( l b 0 ) 2 ( C H b o n d i n g ) 1 2 ( 2 b 2 ) 2 ( 3 a ] ) 2 ( l b 2 ) 2 , 1 A 1 . The la-j and 2 a - j / l b 2 m o l e c u l a r o r b i t a l s r e p r e s e n t oxygen I s and c a r b o n I s o r b i t a l s r e s p e c t i v e l y . In t h e e x c i t a t i o n o f an oxygen I s e l e c t r o n , t h e -162- f i n a l Rydberg s t a t e s have t h e same symmetries as t h e c o r r e s p o n d i n g s t a t e s i n w a t e r [see S e c t i o n ( 7 . 2 . 3 . ) ] . The p r o m o t i o n o f a c a r b o n I s e l e c t r o n s h o u l d r e s u l t i n a l o w e r i n g o f t h e m o l e c u l a r symmetry t o C $. In e i t h e r p o i n t g r o u p , e x c i t a t i o n o f a K - s h e l l e l e c t r o n ( c a r b o n o r oxygen) t o a l l 3s and 3p Rydberg l e v e l s i s e l e c t r i c d i p o l e a l l o w e d . a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l energy l o s s s p e c t r u m o f d i m e t h y l e t h e r i s shown i n F i g u r e 45. The l o c a t i o n s o f peaks, A (6.7 e V ) , B (7.6 e V ) , C (8.5 e V ) , D (9.2 e V ) , E (11.0 eV) and F (12.9 e V ) , a r e c o n s i s t e n t w i t h 148 a h i g h e r r e s o l u t i o n energy l o s s s p e c t r u m , where s t r u c t u r e s have been a s s i g n e d t o Rydberg t r a n s i t i o n s . Peaks G (14.0 eV) and H (15.5 eV) a r e a s s o c i a t e d w i t h n i t r o g e n i m p u r i t y ( v e r i f i e d by UV-PES) and peaks D and F a l s o have a c o n t r i b u t i o n f r o m t h i s s o u r c e . The l o c a t i o n o f t h e f i r s t i o n i z a t i o n p o t e n t i a l shown i n F i g u r e 45 i s based on the e x p e r i m e n t a l , a d i a b a t i c v a l u e 1 2 0 o f 9.96 eV. b. Carbon K - s h e l l E x c i t a t i o n . The c a r b o n K - s h e l l energy l o s s s p e c t r u m o f d i m e t h y l e t h e r i s shown i n F i g u r e 46 and t h e e n e r g i e s and t e n t a t i v e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 15. The f i r s t e n ergy l o s s s t r u c t u r e appears as a s h o u l d e r a t a p p r o x i m a t e l y 288.5 eV (term v a l u e 3.75 eV) on t h e more i n t e n s e second peak and i s a s s i g n e d t o t h e p r o m o t i o n o f a c a r b o n K - s h e l l e l e c t r o n ( 2 a 1 i n C $ p o i n t group) t o t h e 3sa' Rydberg o r b i t a l . The second peak o b s e r v e d a t 289.4 eV (term v a l u e 2.85 eV) i s a s s i g n e d t o t h e p r o m o t i o n o f a c a r b o n K - s h e l l e l e c t r o n ( 2 a 1 ) t o a 3p Rydberg o r b i t a l . The t h i r d peak i n the s pectrum o b s e r v e d a t 291.1 eV (term v a l u e 1.15 eV) p r o b a b l y has c o n t - r i b u t i o n s from 4s and 4p Rydberg t r a n s i t i o n s . : ELASTIC a •••• c E 4-1 • •••• (U (0 c 0 CH30CH3 CO I 0 10 20 Energy Loss (eV) 1 30 FIGURE 4 5 . V a l e n c e s h e l l e n e r g y l o s s spectrum o f d i m e t h y l e t h e r . 40 1.(H co mmmm c 3 v. CD b 0.5 J- CU CO c CD K-edge \ II I /12 3 CH3OCH3 CK-shell 1 290 300 310 320 Energy Loss (eV) FIGURE 46 Carbon K - s h e l l energy l o s s spectrum o f d i m e t h y l e t h e r . TABLE 15 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND TENTATIVE ASSIGNMENTS OF PEAKS OBSERVED IN T H E C A R B O N AND O X Y G E N K - S H E L L S P E C T R A OF D I M E T H Y L E T H E R ( C H 30CH 3>- CARBON K - S H E L L OXYGEN K-- S H E L L A S S I G N M E N T 5 P E A K E N E R G Y AE TERM V A L U E E S T I M A T E D V A L U E 9 P E A K ENERGY A E TERM V A L U E E S T I M A T E D V A L U E 3 1 288.5 0 3.75 288.9 1 535.5 0 3.1 535.2 3sa-j 2 289.4 0.9 2.85 (289.6 (289.8 290.7 j /535.9 (536.2 537,0 3p 3d 3 291.1 2.6 1.15 2 1538.6 3.1 4s/4p K - E D G E C 292.25 3.75 K - E D G E 0 538.59 3.1 CO a. E s t i m a t e d u s i n g t h e term v a l u e s o b s e r v e d i n t h e v a l e n c e s h e l l spectrum o f d i m e t h y l e t h e r 1 ^ b. O n l y the f i n a l o r b i t a l i n v o l v e d i n t h e e x c i t a t i o n i s l i s t e d . c. X - r a y PES v a l u e s 1 5 5 . - 1 6 6 - The p o s i t i o n o f t h e c a r b o n K-edge i n d i c a t e d i n o u r s p e c t r u m i s based on t h e X-ray PES v a l u e 1 5 5 o f 292.25 0.05 eV. c. Oxygen K - s h e l l E x c i t a t i o n . The oxygen K - s h e l l e nergy l o s s s p e c t r u m o f d i m e t h y l e t h e r i s shown i n F i g u r e 47 and t h e e n e r g i e s and t e n t a t i v e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 15. The s p e c t r u m i s v e r y d i f f e r e n t f r o m t h e c a r b o n K - s h e l l s p e c t r u m o f d i m e t h y l e t h e r ( F i g u r e 46) and r e s e m b l e s t h e oxygen K - s h e l l s pectrum o f methanol ( F i g u r e 4 4 ) . The f i r s t peak a t 535.5 eV ( t e r m v a l u e 3.1 eV) i s a s s i g n e d t o t h e p r o m o t i o n o f an oxygen K - s h e l l e l e c t r o n t o t h e 3sa-| Rydberg o r b i t a l . The broad band o f s t r u c t u r e w i t h a maximum a t 538.6 eV presumably has c o n t r i b u t - i o n s from 3p and h i g h e r quantum number Rydberg t r a n s i t i o n . The p o s i t i o n o f the oxygen K-edge i n d i c a t e d on o u r s p e c t r u m i s based on the X-ray PES v a l u e 1 5 5 o f 538.6 ± 0.05 eV. 7.2.6. Monomethylamine. The ground e l e c t r o n i c s t a t e o f t h e monomethylamine m o l e c u l e has C $ 156 symmetry ( s t a g g e r e d c o n f o r m a t i o n ) and t h e e l e c t r o n c o n f i g u r a t i o n ; ( l a 1 ) 2 ( 2 a 1 ) 2 ( 3 a ' ) 2 ( 4 a 1 ) 2 ( l a " ) 2 ( 5 a ' ) 2 ( 6 a 1 ) 2 ( 2 a " ) 2 ( 7 a 1 ) 2 ( 3 a " ) 2 , V. The l a ' and 2a' o r b i t a l s r e p r e s e n t n i t r o g e n I s and c a r b o n I s o r b i t a l s r e s p e c t i v e l y . a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l e l e c t r o n energy l o s s s p e c t r u m o f monomethylamine i s shown i n F i g u r e 48. E l e c t r o n i m p a c t d a t a f o r monomethylamine has n o t been r e p o r t e d i n t h e l i t e r a t u r e and, o p t i c a l l y , o n l y t h e X A t r a n s i t i o n , 120 w i t h an o n s e t a t 5.2 eV, has been o b s e r v e d . T h i s t r a n s i t i o n has been 120 a s s i g n e d t o t h e e x c i t a t i o n o f a n i t r o g e n l o n e p a i r e l e c t r o n ( 3 a M ) t o t h e 1.0 H co 4-1 c CO k. 4-i !5 03 CO c 0 0.5-̂ 530 J K-edge I I I 2 C H 3 0 C H 3 0K-shell 1 T T 540 550 560 Energy Loss (eV ) CM I 570 FIGURE 47. Oxygen K - s h e l l energy l o s s spectrum o f d i m e t h y l e t h e r . Intensity (arbitrary units) -89 L- -169- 3s Rydberg o r b i t a l , a n a l o g o u s t o t h e f i r s t band i n t h e ammonia v a l e n c e s h e l l s p e c t r u m (peak A i n F i g u r e 3 8 ) . Peak A, w i t h a maximum a t approx- i m a t e l y 5.7 eV i n o u r s p e c t r u m , i s t h e r e f o r e a s s o c i a t e d w i t h t h e X •> A t r a n s i t i o n . Peak B 7.0 eV) p r o b a b l y r e p r e s e n t s t h e e x c i t a t i o n o f a h i g h e r energy Rydberg s t a t e . The l o c a t i o n o f t h e f i r s t i o n i z a t i o n p o t e n t i a l 120 shown i n F i g u r e 48 i s based on t h e a d i a b a t i c v a l u e o f 8.97 eV. b. Carbon K - s h e l l E x c i t a t i o n . The c a r b o n K - s h e l l e nergy l o s s s p e c t r u m o f monomethylamine i s shown i n F i g u r e 49 and the e n e r g i e s and t e n t a t i v e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 16. The f i r s t peak o b s e r v e d a t 287.5 eV (term v a l u e 4.1 eV) i s i n t e r p r e t e d as r e p r e s e n t i n g t h e p r o m o t i o n o f a c a r b o n I s e l e c t r o n t o t h e 3sa' Rydberg o r b i t a l , w h i l e t h e second peak o b s e r v e d a t 288.5 eV ( t e r m v a l u e 3.1 eV) i s a s s o c i a t e d w i t h c a r b o n I s e x c i t a t i o n t o a 3p Rydberg l e v e l . F i n a l l y , t h e broad band w i t h a peak maximum a t 291.5 eV and s h o u l d e r ^ 290.4 eV i s p r o b a b l y a s s o c i a t e d w i t h 3d and h i g h e r quantum number ns and np Rydberg t r a n s i t i o n s . The p o s i t i o n o f t h e c a r b o n K-edge i n d i c a t e d i n o u r sp e c t r u m i s based on 1 ̂ 5 the X-ray PES v a l u e 1 0 o f 291.6 ± 0.05 eV. c. N i t r o g e n K - s h e l l E x c i t a t i o n . The n i t r o g e n K - s h e l l energy l o s s s p e c t r u m o f monomethylamine i s shown i n F i g u r e 50 and t h e e n e r g i e s and t e n t a t i v e a s s i g n m e n t s o f peaks a r e l i s t e d i n T a b l e 16. The f i r s t peak a t 400.6 eV has a term v a l u e o f 4.5 eV (6 = 1.3) and i s a s s i g n e d t o t h e p r o m o t i o n o f a n i t r o g e n I s e l e c t r o n t o the 3sa' Rydberg o r b i t a l . The second peak a t 401.9 eV has a term v a l u e o f 3.2 eV (5 = 0.9) and i s a s s o c i a t e d w i t h t h e p r o m o t i o n o f a n i t r o g e n I s e l e c t r o n t o a 3p Rydberg o r b i t a l . The br o a d s t r u c t u r e w i t h a peak maximum a t 404.6 eV 0) c 3 03 s- 5 03 to H 0.5 H K-edge IIP CH 3 NH 2 CK-shell co c 0 :l I l l /12 34 o I 280 290 300 310 320 Energy Loss (eV ) FIGURE 49. Carbon K-shell energy loss spectrum of monomethylamine. TABLE 16 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND TENTATIVE ASSIGNMENTS OF PEAKS OBSERVED IN THE CARBON AND NITROGEN K-SHELL SPECTRA OF MONOMETHYLAMINE (CH 3NH 2). CARBON K-SHELL PEAK ENERGY 1 287.5 2 0 288.5 1.0 s h o u l d e r M .8? 290.4 2.9 291.5 4.0 4.1 3.1 2.3 1.2 0.1 289.2 290.2 NITROGEN K-SHELL ASSIGNMENT TERM ESTIMATED VALUE VALUE 3 PEAK ENERGY TERM VALUE ESTIMATED VALUE 3 1 400.6 0 4.5 2 401.9 1.3 3.2 3 o403.4 o,2.8 1.7 4 404.6 4.0 0.5 403.3 403.6 3s 3p 4s 3d/4p K-EDGE 291.6 4.1 K-EDGE 405.1 4.5 a. E s t i m a t e d from t h e quantum d e f e c t s d e r i v e d from t h e energy p o s i t i o n s o f t h e f i r s t two peaks. 6 ( n d ) was assumed t o be 0. b. Only t h e f i n a l o r b i t a l i n v o l v e d i n the e x c i t a t i o n i s g i v e n . c. X - r a y PES v a l u e 1 5 5 . d. X-ray PES v a l u e 1 5 5 , 1 5 7 . -172- ( s i i u n A j B j j j q j e ) A j j s u e i u i -173- (term v a l u e 0.5 eV) and a s h o u l d e r a t ^ 403.4 (term v a l u e ^ 1.7 eV) i s p r o b a b l y a s s o c i a t e d w i t h 3d, and h i g h e r quantum number ns and np Rydberg t r a n s i t i o n s . T a b l e 16 i n c l u d e s e s t i m a t e s o f t h e s e e x c i t a t i o n e n e r g i e s on t h e b a s i s o f t h e quantum d e f e c t s d e r i v e d from t h e o b s e r v e d e n e r g i e s o f t h e 3s and 3p peaks i n o u r s p e c t r u m . The p o s i t i o n o f the n i t r o g e n K-edge i n d i c a t e d i n o u r s p e c t r u m i s based on t h e X-ray PES v a l u e o f 405.1 eV f o r t h e n i t r o g e n I s b i n d i n g e nergy 155 157 i n monomethylamine ' 7.2.7. Term V a l u e s . The 3s term v a l u e s i n t h e K - s h e l l s p e c t r a o f methane, ammonia and w a t e r f o l l o w t h e same o r d e r as t h e 3s term v a l u e s d e r i v e d from t h e v a l e n c e s h e l l s p e c t r a o f t h e same m o l e c u l e s 1 2 0 ' 1 3 6 ' 1 4 5 ' 1 4 8 ' 1 5 0 ( f o r p r o m o t i o n o f t h e l e a s t t i g h t l y bound e l e c t r o n ) w i t h CH^ < NH 3 < HgO (see T a b l e 1 7 ) . T h i s r e s u l t i s e x p e c t e d , s i n c e term v a l u e s i n c r e a s e w i t h i n c r e a s i n g e f f e c t i v e n u c l e a r c h a r g e . I n t h e s e r i e s , w a t e r , methanol and d i m e t h y l e t h e r , t h e 3s t e r m v a l u e s d e r i v e d f r o m b o t h t h e v a l e n c e s h e l l s p e c t r a and t h e K - s h e l l s p e c t r a f o l l o w t h e same o r d e r w i t h CH 30CH 3 < CH 30H < H,,0. The t r e n d i n t h e v a l e n c e 135 s h e l l s p e c t r a has been e x p l a i n e d on t h e b a s i s t h a t t h e 3s Rydberg o r b i t a l i n c r e a s e s i t s c a r b o n c h a r a c t e r w i t h i n c r e a s i n g a l k y l a t i o n , w h i c h r e s u l t s i n l e s s p e n e t r a t i o n i n t o t h e c o r e and, t h e r e f o r e , a l o w e r term v a l u e . A s i m i l a r t r e n d i s o b s e r v e d i n the K - s h e l l s p e c t r a o f ammonia and mono- methyl amine w i t h t h e term v a l u e s i n the e x p e c t e d o r d e r , CH QNH 0 < NH,,. 7.3. Carbon T e t r a f 1 u o r i d e . The c a r b o n t e t r a f 1 u o r i d e m o l e c u l e i s t e t r a h e d r a l i n i t s ground TABLE 17. 3s AND 3p RYDBERG TERM VALUES OBSERVED FOR K-SHELL EXCITATION AND VALENCE SHELL EXCITATION (OUTERMOST ELECTRON) IN CH 4, NH3, H 2 0 , CH 30H, CH 30CH, AND C H ^ . Ne K - S h e l l (a) CH, NH. H 20 I t , 3a (c) l b (d) F i n a l O r b i t a l 3.04 3.7 2.7 3.95 it: 29 61 5.0 4.43 2.81 2.24 5.7 (3.8 1 2 . 6 5.2 (2.62 [2.46 3s CH 3NH 2 CH 30H h C K °K C K 2a ..(d) 4.5 4.1 4.8 4.2 4.22 3.2 (3.1 12.3 (2.9 12.1 (3.24 12.64 3s 3p a. R e f e r e n c e 158; b. R e f e r e n c e 136; c. R e f e r e n c e s 120,145 d. R e f e r e n c e 150; e. R e f e r e n c e 148. C H 3 0 C H 3 'K 3.1 3.75 2.85 lb- (e) 3.37 {i: 70 41 3s 3p -175- 144 e l e c t r o n i c s t a t e and has t h e e l e c t r o n c o n f i g u r a t i o n : ( l t 2 ) 6 ( l a , ) 2 ( 2 a , ) 2 ( 3 a , ) 2 ( 2 t 2 ) 6 ( 4 a , ) 2 ( 3 t 2 ) 6 ( l e ) 4 ( 4 t 2 ) 6 ( I t , ) 6 , ]A,. The l t 2 and l a , m o l e c u l a r o r b i t a l s a r e formed from l i n e a r c o m b i n a t i o n s o f f l u o r i n e I s (K) a t o m i c o r b i t a l s . The c a l c u l a t e d e nergy d i f f e r e n c e " ^ 2 ' 1 4 4 between t h e l a , and l t 2 o r b i t a l s i s n e g l i g i b l e (^ 0.001 e V ) . The two o r b i t a l s w i l l be d e s i g n a t e d f l u o r i n e - K because o f t h e i r a t o m i c c h a r a c t e r and assumed t o be d e g e n e r a t e . S i m i l a r l y t h e 2a, m o l e c u l a r o r b i t a l i s formed from t h e carbon I s (K) a t o m i c o r b i t a l and the e l e c t r o n s f i l l i n g t h i s o r b - i t a l w i l l be d e s i g n a t e d c a r b o n K - s h e l l e l e c t r o n s . I t has a l r e a d y been p o i n t e d out i n S e c t i o n 6.2. t h a t the i n n e r s h e l l a b s o r p t i o n s p e c t r a f o r m o l e c u l e s composed o f a c e n t r a l atom " s u r r o u n d e d " by e l e c t r o n e g a t i v e atoms, show analomous f e a t u r e s w h i c h have been a t t r i b u t e d t o an e f f e c t i v e p o t e n t i a l b a r r i e r on t h e o u t e r r i m o f t h e s e m o l e c u l e s (see R e f e r e n c e 131). T h e r e f o r e , a p o t e n t i a l b a r r i e r may e x i s t i n t h e c a r b o n t e t r a f l u o r i d e m o l e c u l e . The f l u o r i n e K - s h e l l a b s o r p t i o n s p e c t r a o f t h e 26 f l u o r o m e t h a n e s , i n c l u d i n g carbon t e t r a f l u o r i d e , have been o b t a i n e d u s i n g B r e m s s t r a h l u n g r a d i a t i o n . However, a b s o r p t i o n s p e c t r a i n t h e r e g i o n o f t h e carbon K-edges, which are the most i n t e r e s t i n g from t h e p o i n t o f view o f a p o s s i b l e p o t e n t i a l b a r r i e r , were not r e p o r t e d , a. V a l e n c e S h e l l Spectrum. The v a l e n c e s h e l l e n ergy l o s s spectrum o f c a r b o n t e t r a f l u o r i d e i s shown i n F i g u r e 51 and t h e e n e r g i e s o f peaks a r e g i v e n i n T a b l e 18. The v a l e n c e s h e l l s pectrum o f carbon t e t r a f l u o r i d e has p r e v i o u s l y been 1 ozr o b t a i n e d w i t h 400 eV i n c i d e n t e l e c t r o n s , z e r o degree s c a t t e r i n g a n g l e 1 3 6 and a r e s o l u t i o n o f ^ 0.045 eV. The o b s e r v e d peaks have been a s s i g n e d 1.0 ELASTIC 1st |p T 1 » 1 1 1 1 r 0 10 20 30 40 Energy Loss (eV) FIGURE 51. V a l e n c e s h e l l e l e c t r o n energy l o s s spectrum o f carbon t e t r a f l u o r i d e . -177- TABLE 18 ABSOLUTE ENERGIES (eV) OF PEAKS OBSERVED IN THE VALENCE SHELL ENERGY LOSS SPECTRUM OF CARBON TETRAFLUORIDE. THIS WORK5 REFERENCE 136° 'EAK ENERGY ENERGY 1 12.5 2 13.7 12.51 113.59 (13.89 3 15.9 1 5 . 8 1 4 16.8 1 6 . 8 6 5 17.2 6 17.8) 7 18.4) 18.01 8 19.3 19.42 9 20.6 20.53 a. 2.5 keV i n c i d e n t e n e r g y , 0.5 eV FWHM e l a s t i c peak and average s c a t t e r i n g a n g l e 2 x 1 0 " 2 r a d . b. 400 eV i n c i d e n t e n e r g y , 0.045 FWHM e l a s t i c peak and z e r o degree s c a t t e r i n g a n g l e . The spectrum has been a s s i g n e d i n Ref e r e n c e 136. -178- t o Rydberg t r a n s i t i o n s u s i n g t h e term v a l u e scheme and t h i s i n t e r p r e t a t i o n 1 oc i s c o n s i s t e n t w i t h t h a t o f t h e o t h e r f l u o r o m e t h a n e m o l e c u l e s . Our spectrum compares f a v o u r a b l y w i t h t h e h i g h e r r e s o l u t i o n s p e c t r u m (see T a b l e 1 8 ) . b. Carbon K - s h e l l E x c i t a t i o n . The c a r b o n K - s h e l l e nergy l o s s s p e c t r u m o f c a r b o n t e t r a f l u o r i d e i s shown i n F i g u r e 52 and t h e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks are l i s t e d i n T a b l e 19. The s p e c t r u m i s dominated by a broad band o f s t r u c t u r e l o c a t e d j u s t below t h e c a r b o n K-edge. T h i s band o f s t r u c t u r e has a number o f components w h i c h a r e c l e a r l y v i s i b l e i n t h e expanded spectrum shown i n t h e i n s e r t i n F i g u r e 52. On t h e b a s i s o f t h e Rydberg i n t e r p r e t a t i o n s o f t h e c a r b o n K - s h e l l spectrum o f methane (see S e c t i o n 7.2.1) and t h e v a l e n c e s h e l l s p e c t r a o f methane and t h e f l u o r o - 12g methanes , we e x p e c t t h e l o w e s t energy t r a n s i t i o n i n t h e c a r b o n K - s h e l l spectrum o f c a r b o n t e t r a f l u o r i d e t o be, 2a, ( carbon-K) -> 3sa-,. T h i s t r a n s i t i o n i s o p t i c a l l y f o r b i d d e n and i s f o r b i d d e n i n o u r e x p e r i m e n t i f the f i r s t Born a p p r o x i m a t i o n i s v a l i d ( t h e i n c i d e n t energy i s e i g h t t i m e s t h e e x c i t a t i o n e n e r g y ) . However, both i n i t i a l and f i n a l s t a t e s b e l o n g t o t h e 53 same symmetry s p e c i e s and d e v i a t i o n s from the Born t h e o r y a r e e x p e c t e d . The spectrum i s e x p e c t e d t o resemble t h e c a r b o n K - s h e l l s p e c t r u m o f methane (see F i g u r e 37) where t h e 3s peak has much l e s s i n t e n s i t y t h a n t h e 3p. As shown by F i g u r e 52, t h i s i s not o b s e r v e d . Moreover, t h e f i r s t f o u r s t r u c t - u r e s on t h e band have quantum d e f e c t s o f 1.2, 1.1, 1.0 and 0.9 (term v a l u e s , 4.0, 3.7, 3.4 and 3.1 eV r e s p e c t i v e l y ) , a l l c o n s i s t e n t w i t h 3s e x c i t a t i o n . The term v a l u e o b s e r v e d f o r t h e f i r s t c a r b o n K - s h e l l a b s o r p t i o n i n t h e methane spectrum ( F i g u r e 37) was 3.7 eV. In the v a l e n c e s h e l l -6ZL- -180- TABLE 19 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND POSSIBLE ASSIGNMENTS OF PEAKS OBSERVED IN THE CARBON AND FLUORINE K-SHELL ENERGY LOSS SPECTRA OF CARBON TETRAFLUORIDE. CARBON K-SHELL FLUORINE K-SHELL n 2 o ? ™ ^ — , — _ (JKDIIAL PEAK ENERGY AE TERM VALUE PEAK ENERGY AE TERM VALUE ASSIGNMENT 1 297.8 0 4.0 s h o u l d e r 2 298.1 0.3 3.7 3 298.4 0.6 3.4 4. 298.7 0.9 3.1 5 299.3 1.5 2.5 1 6 300.2 2.7 1.6 2 K-EDGE d 301.8 4.0 K-EDGE d ^4 3s l 692.9 0 o,694 1.1 695.2 2.3 2.3 1.2 3p 3d a. D e f i n e d as t h e d i f f e r e n c e between the e x c i t a t i o n energy and t h e i o n i z a t i o n p o t e n t i a l ( i . e . t h e b i n d i n g e n e r g y o f t h e e l e c t r o n i n t h e e x c i t e d o r b i t a l ) . b. Only t h e f i n a l o r b i t a l i s g i v e n . c. T h i s a s s i g n m e n t does not a p p l y t o t h e c a r b o n K - s h e l l s p e c t r u m ( s ee t e x t ) . d. X-ray PES v a l u e s 3 2 . -181- spectrum o f c a r b o n t e t r a f l u o r i d e 1 0 0 , 3s term v a l u e s were i n t h e range 4.1 - 3.5 eV. However, t h e e n e r g y s p a c i n g s between t h e f i r s t f o u r components o f t h e band, ^ 0.3 eV, a r e t o o l a r g e t o be a s s o c i a t e d w i t h v i b r a t i o n a l s t r u c t u r e . T h e r e f o r e , t h e appearance o f t h e s e m u l t i p l e t f e a t u r e s i n t h e r e g i o n where a s i n g l e peak i s e x p e c t e d i s h i g h l y u n u s u a l . F e a t u r e s f i v e and s i x have quantum d e f e c t s o f 0.67 and 0.08 r e s p e c t i v e l y (term v a l u e s 2.5 and 1.6 eV. These f e a t u r e s may be a s s o c i a t e d w i t h t h e p r o m o t i o n o f a carbon K - s h e l l e l e c t r o n t o 3p and 3d Rydberg o r b i t a l s r e s p e c t i v e l y . The term v a l u e s a r e c o n s i s t e n t w i t h t h o s e o b s e r v e d f o r c o r r e s p o n d i n g Rydberg 1 3fi e x c i t a t i o n s i n the v a l e n c e s h e l l s p e c t r a o f t h e f l u o r o m e t h a n e m o l e c u l e s (e. g . CF^, I t , •+ 3p and l e -> 3d have term v a l u e s o f 2.6 and 1.6 eV r e s p e c t - i v e l y ) . The l o c a t i o n o f t h e carbon K-edge i n d i c a t e d on o u r s p e c t r u m i s based 32 on t h e e x p e r i m e n t a l X-ray PES v a l u e o f 301.8 eV. The e x t r e m e l y broad s t r u c t u r e o b s e r v e d above t h e K-edge i s p o s s i b l y a s s o c i a t e d w i t h shake-up and s h a k e - o f f p r o c e s s e s i n c o n j u n c t i o n w i t h K - s h e l l e x c i t a t i o n and/or i o n i z - a t i o n . With r e g a r d t o t h e p o s s i b l e e x i s t e n c e o f a p o t e n t i a l b a r r i e r , t h e carbon K - s h e l l energy l o s s s p e c t r u m o f carbon t e t r a f l u o r i d e has two f e a t u r e s n o t o b s e r v e d i n t h e K - s h e l l s p e c t r a o f m o l e c u l e s s u c h as methane, ammonia, and w a t e r , where a p o t e n t i a l b a r r i e r i s not e x p e c t e d (see F i g u r e s 37, 39 and 4 1 ) : i . an unusual number o f components a r e o b s e r v e d i n the energy r e g i o n where a s i n g l e peak a s s o c i a t e d w i t h 3s Rydberg e x c i t a t i o n i s e x p e c t e d . T h i s + The l a r g e s t v i b r a t i o n a l s p a c i n g f o r CF- i n i t s ground e l e c t r o n i c s t a t e i s v 0 = 0.16 e V 1 5 9 . 4 -182- i s a p p a r e n t l y not v i b r a t i o n a l s t r u c t u r e and i t i s a l s o u n l i k e l y t h a t any o f t h e h i g h e r e nergy components ( i . e . peaks 3 and 4) a r e a s s o c i a t e d w i t h a J a h n - T e l l e r s p l i t t i n g o f t h e ^ s t a t e a r i s i n g from t h e t r a n s i t i o n , 2a-, •> 3pt£» wh i c h has been a s s o c i a t e d w i t h peak f i v e . J a h n - T e l l e r i n s t a b - i l i t y i s l a r g e r i n methane t h a n i n c a r b o n t e t r a f l u o r i d e . T h i s i s i l l u s t - r a t e d by the v a l e n c e s h e l l s p e c t r a - where a ' d i s t i n c t s p l i t t i n g has been o b s e r v e d i n methane ( l t 2 -> 3 s , AE = 0.68 eV) w h i l e a s p l i t t i n g i s not a p p a r e n t i n t h e c a r b o n t e t r a f l u o r i d e s p ectrum. S i n c e a J a h n - T e l l e r s p l i t t i n g i s not o b v i o u s i n t h e c a r b o n K - s h e l l s p e c t r u m o f methane, (see F i g u r e 37) we do not e x p e c t v t o o b s e r v e a s p l i t t i n g i n t h e c a r b o n K - s h e l l spectrum o f carbon t e t r a f l u o r i d e ; However, even i f t h e r e i s a p p r e c i a b l e J a h n - T e l l e r s p l i t t i n g , t h e r e . c a n o n l y b e a maximum o f t h r e e f e a t u r e s a s s o c - i a t e d w i t h the Tr, s t a t e . .• • i i . t h e r a t i o o f t h e . i n t e n s i t y 1 i n the continuum r e g i o n o f t h e K-edge to t h a t o f d i s c r e t e s t r u c t u r e s i s s m a l l i n c o m p arison w i t h s i m i l a r r a t i o s o b s e r v e d i n t h e K - s h e l l s p e c t r a o f methane, ammonia and w a t e r (see F i g u r e s 37, 39 and 4 1 ) . T h j s f e a t u r e i s common t o a l l i n n e r s h e l l s p e c t r a i n m o l e c u l e s where the e x i s t e n c e o f a p o t e n t i a l b a r r i e r has been p r o p o s e d . I t a r i s e s because t h e i n t e n s i t y a s s o c i a t e d w i t h d i r e c t i o n i z a t i o n i s s u p p r e s s e d u n t i l t h e e j e c t e d e l e c t r o n has enough energy t o overcome t h e b a r r i e r (see R e f e r e n c e 1.31). U s i n g t h i s model, the i n c r e a s e i n i n t e n s i t y on t h e K-continuum a t a p p r o x i m a t e l y 308 eV may be a s s o c i a t e d w i t h t h e o n s e t o f " d i r e c t " i o n i z a t i o n . c. F l u o r i n e K - s h e l l E x c i t a t i o n . The f l u o r i n e .K-shel1 e l e c t r o n e nergy l o s s s p e c t r u m o f c a r b o n t e t r a f l u o r i d e i s shown i n F i g u r e 53 and t h e e n e r g i e s and p o s s i b l e 1.0 H K-edge CF4 l= -shell 4 - 0.6-I 1 2 T —. • • t* ... .: . CO CO I • i . »^/.V ,*/-A ..;iv 690 700 710 Energy Loss (eV) 720 FIGURE 53. F l u o r i n e K - s h e l l energy l o s s spectrum o f carbon t e t r a f l u o r i d e . -184- a s s i g n m e n t s o f s t r u c t u r e s a r e g i v e n i n T a b l e 19. The o p t i c a l a b s o r p t i o n s p e c t r u m has p r e v i o u s l y been o b t a i n e d u s i n g B r e m s s t r a h l u n g r a d i a t i o n . I t c o n s i s t s o f one broad a b s o r p t i o n band l o c a t e d j u s t below t h e K-edge and s e v e r a l b r o a d bands i n t h e continuum r e g i o n . Our s p e c t r u m shows a b r o a d band below t h e K-edge w i t h a maximum l o c a t e d a t 692.9 eV and a h i g h energy s h o u l d e r l o c a t e d a t a p p r o x i m a t e l y 694 eV. The low energy s i d e o f t h e peak i s asymmetric and appears t o have a c o n t r i b u t i o n from u n r e s o l v e d s t r u c t u r e . The d i s c r e t e s t r u c t u r e o b s e r v e d i n t h e K - s h e l l p h o t o a b s o r p t i o n s p e c t r u m was a t t r i b u t e d t o the p r o m o t i o n o f a f l u o r i n e K - s h e l l e l e c t r o n t o a n t i - bonding v a l e n c e o r b i t a l s . We s u g g e s t t h a t a Rydberg i n t e r p r e t a t i o n i s more l i k e l y . The quantum d e f e c t s d e r i v e d from t h e l o c a t i o n s o f t h e peak maximum and h i g h energy s h o u l d e r a r e 0.57 and 0 r e s p e c t i v e l y (term v a l u e s 2.3 and 1.2 e V ) , c o n s i s t e n t w i t h t h o s e e x p e c t e d f o r 3p and 3d Rydberg e x c i t a t i o n . The s t r u c t u r e on t h e low energy s i d e o f t h e peak may be a s s o c i a t e d w i t h 3s Rydberg e x c i t a t i o n . T h i s i n t e r p r e t a t i o n i s c o n s i s t e n t w i t h t h a t o f t h e carbon K - s h e l l s p e c t r u m o f methane (see T a b l e 11) and t h e v a l e n c e s h e l l s p e c t r a o f t h e f l u o r o m e t h a n e s ( i n c l u d i n g C F ^ ) . The l o c a t i o n o f t h e f l u o r i n e K-edge i n d i c a t e d on o u r sp e c t r u m i s 32 based on the X-ray PES v a l u e o f 695.2 eV. The i n t e n s i t y o f s t r u c t u r e j u s t beyond t h e K-edge i s a p p r o x i m a t e l y o n e - h a l f t h a t o f t h e main d i s c r e t e 26 peak (see a l s o t h e o p t i c a l a b s o r p t i o n s p e c t r u m ). T h i s i s i n s h a r p c o n t r a s t t o t h e low r a t i o o f continuum t o d i s c r e t e s t r u c t u r e o b s e r v e d i n t h e carbon K - s h e l l s p e c t r u m o f c a r b o n t e t r a f 1 u o r i d e and t h e f l u o r i n e K - s h e l l s p e c t r u m 1 6 of SFg. T h i s s u g g e s t s t h a t i f a p o t e n t i a l b a r r i e r e x i s t s i n the c a r b o n t e t r a f l u o r i d e m o l e c u l e (see t h e c a r b o n K - s h e l l d i s c u s s i o n ) i t p r o b a b l y has l i t t l e e f f e c t on t h e e x c i t a t i o n o f a f l u o r i n e K - s h e l l e l e c t r o n . -185- 7.4. Carbon K - s h e l l Energy Loss Spectrum o f A c e t o n e . The ground e l e c t r o n i c s t a t e o f t h e a c e t o n e m o l e c u l e has C 2 v symmetry and t h e e l e c t r o n c o n f i g u r a t i o n : ( l a / ( 2 a / ( 3 a / ( l b / ( v a l e n c e s h e l l ) 2 4 , 1 A ] The la-j and 2a-j m o l e c u l a r o r b i t a l s a r e formed from t h e I s (K) a t o m i c o r b i t a l s o f oxygen and t h e c a r b o n y l c a r b o n r e s p e c t i v e l y . S i m i l a r l y , t h e 3a-j and l b 2 m o l e c u l a r o r b i t a l s r e p r e s e n t l i n e a r c o m b i n a t i o n s o f t h e I s (K) a t o m i c o r b i t a l s o f t h e two methyl c a r b o n s . The e l e c t r o n s f i l l i n g t h e s e o r b i t a l s a r e d e s i g n a t e d K - s h e l l e l e c t r o n s because t h e y a r e l o c a l i z e d on t h e i r r e s p e c t i v e n u c l e i (nonbonding) and a r e m a i n l y a t o m i c i n c h a r a c t e r . 32 The X-ray PES s p e c t r u m o f a c e t o n e c o n s i s t s o f two peaks i n t h e r e g i o n o f t h e carbon K-edge s e p a r a t e d by 2.6 eV ( i n t e n s i t y r a t i o 2:1). These peaks r e p r e s e n t t h e i o n i z a t i o n o f 3 a - j / l b 2 and 2a-j e l e c t r o n s r e s p e c t i v e l y , w i t h t h e methyl carbon a s s o c i a t e d w i t h t h e l o w e r K - s h e l l b i n d i n g energy and t h e l a r g e r i n t e n s i t y peak. On t h e b a s i s o f t h e X-ray PES s p e c t r u m , t h e 3a-j and l b 2 m o l e c u l a r o r b i t a l s a r e c o n s i d e r e d t o be e f f e c t i v e l y d e g e n e r a t e + a t our e x p e r i m e n t a l r e s o l u t i o n 0.5 e V ) . The v a l e n c e s h e l l e l e c t r o n e nergy l o s s s p e c t r u m o f a c e t o n e has r e c e n t l y been r e p o r t e d 1 6 0 ' 1 6 1 and prominent f e a t u r e s have been a s s i g n e d t o Rydberg t r a n s i t i o n s . On t h i s b a s i s we e x p e c t Rydberg t r a n s i t i o n s t o dominate t h e K - s h e l l spectrum. The c a r b o n K - s h e l l energy l o s s s p e c t r u m o f a c e t o n e i s shown i n F i g u r e 54 and t h e e n e r g i e s and p o s s i b l e a s s i g n m e n t s o f peaks a r e g i v e n i n + T h e o r e t i c a l l y a s m a l l energy d i f f e r e n c e i s e x p e c t e d . A s i m i l a r s i t u a t i o n o c c u r s 3 2 i n t h e CF^ m o l e c u l e f o r t h e f l u o r i n e I s (K) e l e c t r o n s . In t h i s c a s e , t h e c a l c u l a t e d 3 2 ' 1 ^ energy s p l i t t i n g i s v e r y s m a l l 0.001 e V ) . K-edge (METHYL) K-edge (CARBONYL) CH3COCH3 C K - shell 280 T 290 300 310 Energy Loss (eV) 320 FIGURE 54. Carbon K - s h e l l energy l o s s spectrum o f ac e t o n e . -187- T a b l e 20. The f i r s t d i s c r e t e peak w i t h a maximum a t 286.8 eV i s i n t e r - p r e t e d as a r i s i n g from t h e p r o m o t i o n o f a carb o n K - s h e l l e l e c t r o n ( m e t h y l ) , 3a-j/lb2> t o t h e 3sa-j Rydberg o r b i t a l . The o b s e r v e d e x c i t a t i o n e nergy and 32 th e X-ray PES v a l u e f o r t h e s e r i e s l i m i t i m p l i e s a quantum d e f e c t o f 1.2. 130 The magnitude o f t h i s quantum d e f e c t i s c o n s i s t e n t w i t h t h a t e x p e c t e d f o r a 3s Rydberg s t a t e and s i m i l a r t o t h e quantum d e f e c t d e r i v e d f o r t h e ns Rydberg s e r i e s i n the v a l e n c e s h e l l s p e c t r u m o f a c e t o n e ( 1 . 0 3 , R e f e r e n c e 160 and 1.09, R e f e r e n c e 161). The f i r s t peak o b s e r v e d i n o u r spect r u m has a FWHM o f 1.0 eV compared w i t h a FWHM o f 0.6 eV f o r t h e peak a s s o c i a t e d w i t h e l a s t i c a l l y s c a t t e r e d e l e c t r o n s . T h i s i n d i c a t e s t h e e x c i t a t i o n o f a number o f v i b r a t i o n a l l e v e l s . I t i s u n l i k e l y t h a t any o f t h i s b r o a d e n i n g i s a s s o c i a t e d w i t h an energy d i f f e r e n c e between the 3a-j and l b , , o r b i t a l s . Peak two, w i t h a maximum a t 288.4 eV, may be a s s o c i a t e d w i t h the e x c i t a t i o n o f a carbon K - s h e l l e l e c t r o n ( m e t h y l ) t o one o r more components o f the 3p Rydberg o r b i t a l ( a - j , b-j and b^). The d e r i v e d quantum d e f e c t i s 0.8, c o n s i s t e n t w i t h t h e quantum d e f e c t o b s e r v e d f o r t h e np Rydberg s e r i e s i n the v a l e n c e s h e l l s p e c t r u m o f acet o n e ( 0 . 8 1 , R e f e r e n c e 160 and 0.76, Ref e r e n c e 161). In a d d i t i o n , we e x p e c t t h e p r o m o t i o n o f a 2a-j e l e c t r o n ( c a r b o n y l c a r b o n K - s h e l l ) t o the 3sa-j Rydberg o r b i t a l t o c o n t r i b u t e t o t h e i n t e n s i t y o b s e r v e d i n t h i s r e g i o n o f t h e sp e c t r u m . The d e r i v e d quantum d e f e c t o f peak two w i t h r e s p e c t t o t h e c a r b o n y l c a r b o n K-edge i s 1.4. The magnitude o f t h i s quantum d e f e c t i s p o s s i b l e f o r a 3s Rydberg s t a t e . A ssignments o f s t r u c t u r e l o c a t e d above t h e f i r s t peak i n o u r spect r u m are c l e a r l y s p e c u l a t i v e . However, peaks a s s o c i a t e d w i t h t h e pro m o t i o n o f a methyl c a r b o n K - s h e l l e l e c t r o n a r e e x p e c t e d t o be a p p r o x i m a t e l y t w i c e as i n t e n s e as t h o s e a s s o c i a t e d w i t h t h e e x c i t a t i o n o f a c a r b o n y l K - s h e l l TABLE 20 ABSOLUTE ENERGIES ( e V ) , RELATIVE ENERGIES AND POSSIBLE ASSIGNMENTS OF PEAKS OBSERVED IN THE CARBON K-SHELL ENERGY LOSS SPECTRUM OF ACETONE. POSSIBLE 9 DERIVED 5 PEAK ENERGY A E ASSIGNMENT QUANTUM DEFECT 1 286.8 0 C 1 •+ 3 s a ] 1.2 2 288.4 1.6 C 1 -* 3p 0.8 C 2 -y 3sa-j 1.4 3 290.0 3.2 K-EDGE ( C / 291.2 4.4 C ] + » 291.9 5.5 ( C 2 + 4s 1.3 ,C 2 -> 3d 0.32 K-EDGE ( C 2 ) C 293.8 7.0 C ? - » 5 * 296.4 a, 9.6) SHAKE-UP > AND 6 -v- 301 ^ 14.2) SHAKE-OFF a. C| = Carbon K ( m e t h y l ) , C 2 = Carbon K ( c a r b o n y l ) b. D e r i v e d from t h e Rydberg f o r m u l a , E = A-R/(n - 6 ) ^ where E i s the o b s e r v e d e x c i t a t i o n e n e r g y ; A, t h e i o n i z a t i o n p o t e n t i a l ; R, t h e Rydberg c o n s t a n t ; n, t h e p r i n c i p a l quantum number and th e quantum d e f e c t . c. X-ray PES v a l u e s 3 2 . -189- e l e c t r o n ( c f . t h e X-ray PES s p e c t r u m o f a c e t o n e ^ ) . The l o c a t i o n s o f t h e two c a r b o n K-edges shown on our s p e c t r u m a r e 32 based on t h e e x p e r i m e n t a l c a r b o n I s b i n d i n g e n e r g i e s d e t e r m i n e d by X-ray PES. Peak 4, l o c a t e d between t h e two edges, may be a s s o c i a t e d w i t h t h e t r a n s i t i o n c a r b o n K ( c a r b o n y l ) •> 4s (6 = 1.3) and/or carbon-K ( c a r b o n y l ) -> 3d (6 = 0.32). I n t h e v a l e n c e s h e l l s p e c t r u m o f "I C O a c e t o n e , t h e 3d Rydberg s e r i e s has a quantum d e f e c t o f 0.32 (0.28, R e f e r e n c e 161). The broad s t r u c t u r e s o b s e r v e d above the K-edges (peaks 5 and 6) a r e a t t r i b u t e d t o t h e s i m u l t a n e o u s t r a n s i t i o n s o f a c a r b o n I s and v a l e n c e s h e l l e l e c t r o n s ( i . e . shake-up and s h a k e - o f f p r o c e s s e s ) . 7.5. E s t i m a t i o n o f t h e E x c i t a t i o n and I o n i z a t i o n E n e r g i e s o f NH^, H^O and H^F R a d i c a l s U s i n g Core A n a l o g i e s A p p l i e d t o K - s h e l l E l e c t r o n Energy Loss S p e c t r a . 27 Nakamura e t a l . have i n t e r p r e t e d t h e K - s h e l l p h o t o a b s o r p t i o n s p e c t r u m o f m o l e c u l a r n i t r o g e n ( o b t a i n e d u s i n g s y n c h r o t r o n r a d i a t i o n ) u s i n g a c o r e a n a l o g y model. The K - s h e l l e x c i t e d n i t r o g e n m o l e c u l e i s e x p e c t e d t o resemble n i t r i c o x i d e i n two r e s p e c t s ; i . t h e o u t e r e l e c t r o n i c c o n f i g - u r a t i o n s o f K - s h e l l e x c i t e d n i t r o g e n s t a t e s and v a l e n c e n i t r i c o x i d e s t a t e s a r e i d e n t i c a l , and, i i . t h e c o r e p o t e n t i a l ( n u c l e i p l u s K - s h e l l s ) i n b o t h m o l e c u l e s i s e x p e c t e d t o be s i m i l a r , s i n c e a h o l e i n one o f t h e K - s h e l l s o f n i t r o g e n i n c r e a s e s t h e e f f e c t i v e c o r e c h a r g e by one p o s i t i v e u n i t . Thus t h e energy s p a c i n g s o f v a l e n c e s h e l l e x c i t e d s t a t e s o f n i t r i c o x i d e and K - s h e l l e x c i t e d s t a t e s o f n i t r o g e n a r e found t o be v e r y s i m i l a r . The r e s u l t s p r e s e n t e d i n t h i s t h e s i s d e m o n s t r a t e t h a t i n n e r s h e l l a b s o r p t i o n s p e c t r a can be more e a s i l y o b t a i n e d u s i n g t e c h n i q u e s o f energy l o s s , e l e c t r o n -190- impact s p e c t r o s c o p y a t h i g h (2.5 keV) impact e n e r g i e s . The K - s h e l l e n e r g y l o s s s p e c t r a o f n i t r o g e n and c a r b o n monoxide (carbon-K) a r e a l m o s t i d e n t - i c a l (see F i g u r e s 9 and 13) as e x p e c t e d on t h e b a s i s o f t h e c o r e a n a l o g y model ( b o t h K - s h e l l e x c i t e d m o l e c u l e s s h o u l d " r e s e m b l e " n i t r i c o x i d e ) . F u r t h e r m o r e , t h e oxygen K - s h e l l s p e c t r u m o f c a r b o n monoxide ( F i g u r e 14) i s c o n s i s t e n t w i t h a "CF d e s c r i p t i o n " o f t h e oxygen K - s h e l l e x c i t e d m o l e c u l e . Thus, s a t i s f a c t o r y e s t i m a t e s o f the e x c i t e d s t a t e e n e r g i e s and i o n i z a t i o n p o t e n t i a l o f t h e c a r b o n m o n o f l u o r i d e r a d i c a l were o b t a i n e d . However, i n the c a s e o f the l i n e a r t r i a t o m i c m o l e c u l e s , n i t r o u s o x i d e and c a r b o n d i o x i d e (see S e c t i o n 6.1.) t h e r e i s poor agreement between th e e n e r g y s p a c i n g s o f the K - s h e l l e x c i t e d s t a t e s and t h o s e e x p e c t e d on t h e b a s i s o f the c o r e a n a l o g y model ( i . e . e x c i t a t i o n o f t h e t e r m i n a l n i t r o g e n K - s h e l l e l e c t r o n i n ^0 and t h e carbon K - s h e l l e l e c t r o n i n C0^ s h o u l d produce "NO2-1ike" s p e c i e s ) . The breakdown o f the model i n t h e s e c a s e s may be p a r t i a l l y a s s o c i a t e d w i t h the l a r g e changes i n m o l e c u l a r geometry w h i c h o c c u r as a r e s u l t o f e l e c t r o n i c e x c i t a t i o n i n t h e s e m o l e c u l e s . In f a v o u r - a b l e c a s e s i t s h o u l d be p o s s i b l e t o p r e d i c t t h e e x c i t e d s t a t e and i o n i z a t i o n e n e r g i e s o f r a d i c a l s p e c i e s u s i n g c o r e a n a l o g i e s a p p l i e d t o i n n e r s h e l l a b s o r p t i o n s p e c t r a . The c o r e a n a l o g y model i s e x p e c t e d t o a p p l y t o t h e K - s h e l l " h o l e " s t a t e s o f t h e methane, ammonia and w a t e r m o l e c u l e s because t h e y have one " c e n t r a l " heavy n u c l e u s w h i c h i s e x p e c t e d t o dominate t h e p o t e n t i a l f i e l d . T h i s " a t o m i c - l i k e " s t r u c t u r e i s e v i d e n t from t h e w e l l - b e h a v e d Rydberg l e v e l s o b s e r v e d i n the K - s h e l l s p e c t r u m o f methane (see T a b l e 11 ) . T h e r e f o r e , the r e l a t i v e e n e r g i e s o f the K - s h e l l e x c i t e d s t a t e s o f methane ( w i t h r e s p e c t t o the l o w e s t energy K - s h e l l e x c i t e d s t a t e ) ; -191- ( l a / ( 2 a i ) 2 ( 2 t 2 ) 6 ( 3 s a / , \ ar e e x p e c t e d t o be s i m i l a r t o t h e r e l a t i v e e n e r g i e s o f t h e e x c i t e d s t a t e s o f t h e ammonium r a d i c a l , NH^, ( w i t h r e s p e c t t o t h e ground s t a t e : ( l a / ( 2 a / ( 2 t 2 ) 6 ( 3 s a / , \ , assuming t e t r a h e d r a l symmetry) p r o d u c e d by t h e e x c i t a t i o n o f a 3s e l e c t r o n , S i m i l a r l y , the r e l a t i v e e n e r g i e s o f t h e K - s h e l l e x c i t e d s t a t e s o f ammonia and w a t e r a r e e x p e c t e d t o be s i m i l a r t o t h o s e o b s e r v e d f o r s t a t e s r e s u l t - i n g from 3s e l e c t r o n p r o m o t i o n i n the hydrogen o x i d e r a d i c a l , H 30, and t h e h y p o t h e t i c a l f l u o r o n i u m r a d i c a l , H 2F, r e s p e c t i v e l y . The ammonium and hydrogen o x i d e r a d i c a l s have been i n v e s t i g a t e d b o t h e x p e r i m e n t a l l y and t h e o r e t i c a l l y . Mass s p e c t r o m e t r y has p r o v i d e d e x p e r i - "ICO 1 C O 1 C *3 mental e v i d e n c e s u g g e s t i n g t h a t NH 4 and H^O ' c o u l d e x i s t i n t h e gas phase. The f o r m a t i o n o f NH^ on s o l i d s u r f a c e s has a l s o been 164 165 c l a i m e d ' and H^O has been p o s t u l a t e d as an i n t e r m e d i a t e i n w a t e r r a d i o l y s i s e x p e r i m e n t s 1 6 6 ' 1 6 ^ . A r e c e n t r e p o r t 1 6 8 o f t h e ESR s p e c t r a o f m a t r i x - s t a b i l i z e d H 30 and D 30 r a d i c a l s has been c h a l l e n g e d on both e x p e r i m - e n t a l 1 6 ^ and t h e o r e t i c a l grounds 1'' 0. T h e o r e t i c a l l y , t he s t a b i l i t y o f gas phase NH 4 and H 30 w i t h r e s p e c t t o d i s s o c i a t i o n i n t o NH 3 + H- and H^O + H« r e s p e c t i v e l y , has n o t been c l e a r l y e s t a b l i s h e d . The c a l c u l a t i o n s o f Gangi and B a d e r 1 ^ 1 f o r H 30 i n d i c a t e a b a r r i e r o f 6.6 K c a l s / m o l e a l o n g the d i s - s o c i a t i o n p a t h , s u g g e s t i n g t h e p o s s i b i l i t y o f low t e m p e r a t u r e i s o l a t i o n . 172 However, t h e c a l c u l a t i o n s by Lathan e t a l . i n d i c a t e t h a t NH^ and H 30 a r e u n s t a b l e w i t h r e s p e c t t o d i s s o c i a t i o n , s i n c e p o t e n t i a l minima were not f o u n d . E x p e r i m e n t a l e v i d e n c e t o s u p p o r t the e x i s t e n c e o f t h e H 2F 172 r a d i c a l has not been r e p o r t e d and t h e o r e t i c a l c a l c u l a t i o n s i n d i c a t e t h a t t h e r a d i c a l does n o t have a t i g h t l y bound s t r u c t u r e . The f l u o r o n i u m -192- + 173 i o n , H^F , has r e c e n t l y been o b s e r v e d i n a s o l i d m i x t u r e and i n s o l u t i o n by (low t e m p e r a t u r e ) i n f r a r e d s p e c t r o s c o p y . T a b l e 21 l i s t s the p r e d i c t e d e x c i t a t i o n and i o n i z a t i o n e n e r g i e s o f t h e NH^, H^O and h^F r a d i c a l s u s i n g t h e c o r e a n a l o g y model. T h e o r e t i c a l 174 v a l u e s f o r the ammonium r a d i c a l have a l s o been i n c l u d e d . The t h e o r e t i c a l and c o r e a n a l o g y v a l u e s f o r t h e e x c i t e d s t a t e s o f NH^ agree 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 i s i s i l l u s t r a t e d i n F i g u r e 55, where we have i n d i c a t e d the r e l a t i v e e n e r g i e s o f t h e c a l c u l a t e d NH^ s t a t e s on t h e c a r b o n K - s h e l l e l e c t r o n energy l o s s s p e c t r u m o f methane. The z e r o o f t h e NH^ energy s c a l e c o r r e s p o n d s t o i t s ground e l e c t r o n i c s t a t e . The p o s i t i o n o f t h e methane K-edge i n d i c a t e d on t h e s p e c t r u m i s based on t h e e x p e r i m e n t a l v a l u e d e t e r - 32 mined by X-ray 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 . O t h e r c a l c u l a t i o n s o f t h e i o n i z a t i o n p o t e n t i a l o f NH 4 g i v e v a l u e s o f 3 . 9 2 1 7 2 , 3 . 9 4 1 7 5 ' 1 7 6 , 3 . 9 7 1 7 5 and 3.8 e V ^ 2 ' ^ 7 7 , i n good agreement w i t h o u r p r e d i c t e d v a l u e o f 3.7 ± 0.3 eV. The e x p e r i m e n t a l v a l u e o f 5.9 eV e s t i m a t e d by s u r f a c e i o n - 162 i z a t i o n t e c h n i q u e s i s much l a r g e r than t h e p r e d i c t e d c o r e a n a l o g y and t h e o r e t i c a l v a l u e s . E x c i t e d s t a t e c a l c u l a t i o n s f o r t h e H^O r a d i c a l ^ g i v e v a l u e s o f 4.4 and 5.2 eV f o r t h e two l o w e s t e x c i t e d s t a t e s , w h i c h do not agree w i t h our p r e d i c t e d v a l u e s . The c a l c u l a t e d i o n i z a t i o n p o t e n t i a l ^ ( u s i n g a l a r g e r b a s i s s e t ) i s 4.6 eV and i s i n b e t t e r agreement w i t h our p r e d i c t e d v a l u e o f 5.0 ± 0.3 eV. O t h e r c a l c u l a t e d v a l u e s f o r t h e i o n - i z a t i o n p o t e n t i a l o f H 30 g i v e v a l u e s o f 5 . 8 1 7 2 , 4 . 6 1 7 1 , 4 . 4 1 7 5 , 4.2 1 7 5' 1 7 8 1 cp 1 cp and 3.9 eV . The e x p e r i m e n t a l v a l u e o f ^ 10.9 eV e s t i m a t e d from t h e appearance p o t e n t i a l o f H^0 + i s a p p r e c i a b l y l a r g e r than our p r e d i c t e d v a l u e . C a l c u l a t i o n s f o r e x c i t e d s t a t e s o f the f l u o r o n i u m r a d i c a l , H^F, have not TABLE 21 ESTIMATED ENERGY LEVELS (eV) OF THE NH^, H 30 AND HYPOTHETICAL H 2F RADICALS. E X C I T E D O R B I T A L N H 4 H 3 0 H 2 F T H I S WORK A E (eV) T H E O R Y 9 A E (eV) T H I S WORK A E - ( e V ) T H I S WORK AE (eV) GROUND S T A T E 0 0 0 0 3p 1.0 1.3 H.6 12.9 H.9 13.1 3d/4s 2.2 4p 2.4 2.6 3.5 4.5 5p 2.8 4.0 - ( I P ) 3.7 3.8 b 5.0 C 5.7 d a. R e f e r e n c e 174, t h e c a l c u l a t e d i o n i z a t i o n p o t e n t i a l i n c r e a s e d t o 4.0 eV w i t h a l a r g e r b a s i s s e t . b. O t h e r c a l c u l a t i o n s o f t h e i o n i z a t i o n p o t e n t i a l o f NH. g i v e v a l u e s o f 3 . 9 2 1 7 2 , 3 . 9 4 1 7 5 , 1 7 6 , 3 . 9 7 1 7 5 , and 3.8 e V 1 6 2 , 1 7 \ 4 c. C a l c u l a t i o n s o f t h e i o n i z a t i o n p o t e n t i a l o f H,0 a i v e v a l u e s o f 5 . 8 1 7 2 , 4 . 6 1 7 1 , 4 . 4 1 7 5 , 4 . 2 1 7 5 , 1 7 8 and 3.9 e V 1 6 2 . 3 d. A v a l u e o f 8.57 eV has been c a l c u l a t e d 1 7 2 -194- 3 s 3 p 4 p 5p oo C H 4 ( ) • I I 1 T 1 T" 286 288 290 292 Energy L o s s (eV) FIGURE 5 5 . The ca r b o n K - s h e l l e l e c t r o n energy l o s s s p e c t r u m o f methane and c a l c u l a t e d energy l e v e l s o f the ammonium r a d i c a l ( N H J . -195- been r e p o r t e d . The c a l c u l a t e d i o n i z a t i o n p o t e n t i a l , 8.57 eV, i s l a r g e r t h a n o u r p r e d i c t e d v a l u e , 5.7 ± 0.3 eV. -196- CHAPTER EIGHT CONCLUSION The K - s h e l l energy l o s s s p e c t r a f o r a v a r i e t y o f s m a l l m o l e c u l e s have been s t u d i e d u s i n g f a s t e l e c t r o n i m p a c t . The r e s u l t s d e m o n s t r a t e t h a t h i g h impact e n e r g y , e l e c t r o n e nergy l o s s s p e c t r o s c o p y i s a v i a b l e a l t e r n a t i v e t o t h e use o f p h o t o a b s o r p t i o n t e c h n i q u e s f o r s t u d y i n g e x c i t a t i o n p r o c e s s e s i n t h e s o f t X-ray and X-ray r e g i o n s . In f a c t , t h e r e a r e some p r a c t i c a l advantages t o t h e use o f e l e c t r o n impact s p e c t r o s c o p y , and w i t h o n l y modest energy s e l e c t i o n o f t h e i n c i d e n t beam, t h e r e s o l u t i o n would be the same magnitude as the n a t u r a l l i n e w i d t h s o f t h e s e h i g h l y e x c i t e d s t a t e s . I t has been shown t h a t t h e K - s h e l l s p e c t r a o f t h e d i a t o m i c m o l e c u l e s , n i t r o g e n and car b o n monoxide, a r e c o n s i s t e n t w i t h t he r e s u l t s e x p e c t e d on the b a s i s o f a s i m p l e c o r e a n a l o g y model. T h i s model has been used t o e s t i m a t e e x c i t a t i o n and i o n i z a t i o n e n e r g i e s f o r some e x o t i c c h e m i c a l s p e c i e s from t h e r e l a t i v e e n e r g i e s o b s e r v e d i n t h e K - s h e l l energy l o s s s p e c t r a o f a number o f m o l e c u l e s . 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