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Spectroscopy and dynamics in threshold ion-pair production Hu, Qichi 2005

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SPECTROSCOPY  AND DYNAMICS  IN T H R E S H O L D  ION-PAIR  PRODUCTION  by  QICHI H U B . S c . ( C h e m i s t r y ) P e k i n g U n i v e r s i t y , 1997 M . S c . ( C h e m i s t r y ) U n i v e r s i t y o f W a t e r l o o , 2000  A THESIS SUBMITTED IN PARTIAL F U L F I L L M E N T O F THE REQUIREMENTS FORT H EDEGREE OF  DOCTOR  OF PHILOSOPHY  in  THE FACULTY OF GRADUATE  STUDIES  D e p a r t m e n t of C h e m i s t r y F a c u l t y of Science  T H E UNIVERSITY OF BRITISH COLUMBIA  J u n e 2005 ©  Q i c h i H u , 2005  Abstract T h e h i g h r e s o l u t i o n p h o t o i o n i z a t i o n t e c h n i q u e of T h r e s h o l d I o n - P a i r P r o d u c t i o n  Spectroscopy  ( T I P P S ) i n v o l v e s V U V e x c i t a t i o n of n e u t r a l molecules A B t o t h e h i g h l y v i b r a t i o n a l l y e x c i t e d i o n - p a i r states A {(3 ) +  just below the dissociation l i m i t .  — B~(@~)  +  T h e s e states b e h a v e like  t h e h i g h - n R y d b e r g s t a t e s u s e d for p u l s e d field i o n i z a t i o n zero k i n e t i c e n e r g y  photoelectron  ( P F I - Z E K E ) or m a s s a n a l y z e d t h r e s h o l d i o n i z a t i o n ( M A T I ) s p e c t r o s c o p y , a n d c a n be d e t e c t e d b y p u l s e d f i e l d d i s s o c i a t i o n . D u r i n g t h e p a s t few years T I P P S has b e e n a p p l i e d t o t h e m o l e c u l e s H C 1 / D C 1 , H F / D F , H C N a n d ( H F ) . F o r H C 1 / D C 1 a n d H F / D F , t h e i o n - p a i r t h r e s h o l d s have 2  b e e n p r e c i s e l y m e a s u r e d a n d t h e c l a s s i c a l b o n d d i s s o c i a t i o n energies h a v e b e e n c a l c u l a t e d , a n d therefore o u r r e s u l t s p r o v i d e a n e x p e r i m e n t a l test of t h e B o r n - O p p e n h e i m e r b r e a k d o w n i n t h e t w o p a i r s of i s o t o p o m e r s . T h e i o n - p a i r f o r m a t i o n m e c h a n i s m s i n these m o l e c u l e s were discussed i n l i g h t of these h i g h r e s o l u t i o n r e s u l t s . F o r H C N , we have p r e c i s e l y m e a s u r e d t h e i o n - p a i r t h r e s h o l d E ° p t o b e 122246 ± 4 c m " . O u r r e s u l t also s h o w e d t h a t r o t a t i o n a l l y e x c i t e d i n s t e a d 1  of c o l d C N ~ f r a g m e n t is f a v o r e d as t h e i o n - p a i r d i s s o c i a t i o n p r o d u c t i n t h e t h r e s h o l d r e g i o n . For  ( H F ) 2 , t h e t o t a l i o n y i e l d a n d p u l s e d field i o n i z a t i o n ( P F I ) s p e c t r a of H F H  +  from ( H F ) 2  were r e c o r d e d over t h e e n e r g y r a n g e 14.7-15.9eV. T h e d o m i n a n t p r o c e s s t o p r o d u c e H F H  was  f o u n d t o be ( H F ) 2 +  h i / —> H F H  (HF)2  + F ~ is v i r t u a l l y n o n c o n t r i b u t i n g . P r o d u c t i o n o f v i b r a t i o n a l l y e x c i t e d  HFH  + hu —> H F H +  +  +  + e  +  + F , w h i l e t h e o t h e r e n e r g e t i c a l l y a l l o w e d process  _  f r a g m e n t s w a s o b s e r v e d , a n d a s s i g n m e n t s t o different v i b r a t i o n a l levels w e r e p e r f o r m e d  in comparison w i t h the calculated H F H  +  vibrational spacing i n literature work.  s p e c t r u m we h a v e m e a s u r e d t h e a p p e a r a n c e p o t e n t i a l ( A P ) o f H F H ( r e l a t i v e t o ( H F ) ) , w h i c h gives a v a l u e of 5.07 ± 2  +  F r o m the  t o be 14.50 ±  0.03eV  0 . 0 3 e V for t h e p r o t o n a f f i n i t y of H F . O u r  r e s u l t clarifies t h e d i s c r e p e n c y b e t w e e n p r e v i o u s p h o t o i o n i z a t i o n w o r k a n d t h e r e s u l t s f r o m other methods.  Table of Contents Abstract  ii  Table of C o n t e n t s List of Tables  v  List of F i g u r e s Acknowledgements C h a p t e r 1. 1.1  1.2  iii  Introduction  vi viii 1  B a c k g r o u n d of h i g h - i ; i o n - p a i r states a n d T I P P S  1  1.1.1  R y d b e r g states a n d i o n - p a i r states  1  1.1.2  F i e l d i o n i z a t i o n a n d field dissociation  4  1.1.3  P F I - Z E K E photoelectron spectroscopy  8  1.1.4  T I P P spectroscopy  11  1.1.5  D e t e r m i n a t i o n of b o n d d i s s o c i a t i o n energy  12  Recent work on T I P P S  15  1.2.1  I n v e s t i g a t i o n of B o r n - O p p e n h e i m e r b r e a k d o w n i n d i a t o m i c m o l e c u l e s  16  1.2.2  S t u d y of i o n - p a i r f o r m a t i o n m e c h a n i s m  18  1.2.3  Ion-pair formation i n triatomic and polyatomic molecules  21  Experimental  28  C h a p t e r 2. 2.1  Introduction  28  2.2  Laser system and V U V generation  28  2.2.1  N d : Y A G p u l s e d dye lasers  30  2.2.2  G e n e r a t i o n of V U V p h o t o n s b y f o u r - w a v e m i x i n g  31  2.3  Molecular beam and vacuum system  34  2.4  D e t e c t i o n of t h e i o n - p a i r s i g n a l  35  2.4.1  T i m e - o f - F l i g h t spectrometer  36  2.4.2  L a b V I E W program  40  2.4.3  Ion-counting technique  40  Threshold Ion-Pair Production in H C 1 / D C 1  43  C h a p t e r 3. 3.1  Introduction  43  3.2  Experimental  45  3.3  Results and Discussion  46  3.3.1  T I P P spectra  46  3.3.2  Ion-pair yield spectra  52  Threshold Ion-Pair Production in H F / D F  63  C h a p t e r 4. 4.1  Introduction  63  4.2  Experimental  64  4.3  Results and Discussion  65  4.3.1  T I P P S s p e c t r a a n d B o n d d i s s o c i a t i o n energies of H F / D F  65  4.3.2  T o t a l i o n - p a i r y i e l d s p e c t r a a n d m e c h a n i s m of i o n - p a i r f o r m a t i o n  74  Threshold Ion-Pair Production in H C N  92  C h a p t e r 5. 5.1  Introduction  92  5.2  Experimental  93  5.3  Results and Discussion  94  C h a p t e r 6.  P r o d u c t i o n of H F H + from ( H F )  2  107  6.1  Introduction  6.2  Experimental  110  6.3  Results and Discussion  Ill  C h a p t e r 7.  Concluding Remarks and Future W o r k  107  132  List of Tables 2 . 1 F o u r - w a v e m i x i n g schemes i n t h i s p r o j e c t  33  3 . 1 R e s u l t s f r o m of H C 1 / D C 1 T I P P s p e c t r a  48  3 . 2 D a t a for d e t e r m i n i n g t h e b o n d d i s s o c i a t i o n energies of H C 1  +  and D C 1  +  52  3 . 3 S u m m a r y of c l a s s i c a l b o n d energies £ > ( H - X ) for h y d r i d e s  53  3 . 4 A s s i g n m e n t of R y d b e r g resonances i n H C 1 / D C 1 p h o t o i o n - p a i r y i e l d s p e c t r a  60  4.1Energetic results from H F / D F T I P P spectra  69  4.2Resonances seen i n T I P P s p e c t r a of H F a n d D F  78  4 . 3 L i s t of p r e d i c t e d a n d o b s e r v e d resonances i n D F i o n - p a i r y i e l d s p e c t r u m  80  4 . 4 L i s t of p r e d i c t e d a n d o b s e r v e d resonances i n H F i o n - p a i r y i e l d s p e c t r u m  82  e  4 . 5 A s s i g n m e n t of R y d b e r g sequencess i n H F / D F t o t a l i o n - p a i r y i e l d s p e c t r a  84  4 . 6 F r a n c k - C o n d o n f a c t o r s for t r a n s i t i o n s H F / D F + ( u + )  85  +  6.1Resonances seen i n H F H  +  total yield spectrum  <- H F / D F ( u " = 0 )  128  List of Figures 1.1 C o u l o m b i c a t t r a c t i o n i n H F ( B S ) i o n - p a i r s t a t e  3  1.2 P o t e n t i a l of a R y d b e r g e l e c t r o n i n e l e c t r i c f i e l d  5  1.3 D e t e c t i o n of t h r e s h o l d s i g n a l i n P F I - Z E K E s p e c t r o s c o p y .  10  1.4 T I P P s p e c t r u m of 0  13  2  1.5 B o r n C y c l e for H C 1 / D C 1 B o n d E n e r g i e s 1.6 P o t e n t i a l e n e r g y c u r v e s of H C 1 a n d H C 1  17 20  +  1.7 I o n - p a i r f o r m a t i o n i n H C N  22  2.1 S c h e m a t i c of t h e e x p e r i m e n t a l s e t u p  29  2.2 T i m e - o f - F l i g h t m a s s s p e c t r o m e t e r .  37  2.3 D e t e c t i o n s c h e m e of t h e t h r e h o l d i o n - p a i r s i g n a l  39  3.1 T I P P s p e c t r a of D C 1 a n d H C 1  47  3.2 D e t e r m i n a t i o n of i o n - p a i r t h r e s h o l d s for H C 1 a n d D C 1  49  3.3 T o t a l p h o t o i o n - p a i r y i e l d s p e c t r a for D C 1 a n d H C 1  54  3.4 S i m u l a t i o n of H C 1 t o t a l p h o t o i o n - p a i r y i e l d s p e c t r u m  57  3.5 S i m u l a t i o n of lowest e n e r g y r e s o n a n c e i n t h e D C 1 p h o t o i o n - p a i r y i e l d s p e c t r u m  59  4.1 T I P P s p e c t r u m of H F  66  4.2 T I P P s p e c t r u m of D F  67  4.3 H F T I P P s p e c t r a for J " = 0 ,  1 a n d 3 w i t h d i s c r i m i n a t i o n fields of 2 ( r e d ) , 4 (green) a n d  6 V / c m (blue)  70  4.4 E x t r a p o l a t i o n t o d e t e r m i n e t h e field-free i o n - p a i r t h r e s h o l d for H F  71  4.5 D F T I P P s p e c t r a for J " = 0 a n d 2 w i t h d i s c r i m i n a t i o n fields of 2 ( r e d ) , 4 (green) a n d 6 V / c m (blue)  72  4.6 E x t r a p o l a t i o n t o d e t e r m i n e t h e field-free i o n - p a i r t h r e s h o l d for D F  73  4.7 T o t a l i o n - p a i r y i e l d s p e c t r u m of H F  75  4.8 T o t a l i o n - p a i r y i e l d s p e c t r u m of D F  76  4.9 C o m p a r i s o n of H  +  and H F  +  i n the threshold region  86  4 . 1 0 H F P o t e n t i a l curves  88  5.1 T o t a l i o n - p a i r y i e l d s p e c t r u m of H C N  96  5.2 T I P P s p e c t r u m of H C N  97  5.3 T I P P s p e c t r a of H C N w i t h t w o different V U V i n t e n s i t i e s  98  5.4 S i m u l a t i o n I of t h e H C N T I P P s p e c t r u m  100  5.5 S i m u l a t i o n I I of H C N T I P P s p e c t r u m  101  5.6 R e l a t i v e b r a n c h i n g r a t i o i n t o C N  104  _  f r a g m e n t at different r o t a t i o n a l levels  6.1 ( H F ) 2 e n e r g e t i c s a n d m o l e c u l a r s t r u c t u r e s of ( H F )  2  and H F H  108  +  6.2 I o n species p r o d u c e d f r o m p h o t o i o n i z a t i o n of H F m o l e c u l a r b e a m at 1 5 . 9 0 e V  112  6.3 R e l a t i v e i o n i n t e n s i t i e s of H F  113  +  and H F H  +  at different V U V energies  6.4 T O F s p e c t r u m f r o m H F b e a m m i x e d w i t h A r  115  6.5 T o t a l i o n y i e l d s p e c t r u m of H F H +  116  6.6 P F I s i g n a l of H F H  +  w i t h different d i s c r i m i n a t i o n  fields  117  6.7 C o m p a r i s o n of y i e l d s p e c t r a of H F H 6.8 T O F s p e c t r u m of H F H  +  and ( H F ) H  6.9 C o m p a r i s o n of t h e y i e l d s p e c t r a for H F 6 . 1 0 C o m p a r i s o n of t h e H F H  2  119  +  i o n w i t h different d e l a y t i m e o f t h e e x t r a c t i o n p u l s e  +  +  +  and H F H  120 121  +  a n d e~ s i g n a l s  123  6 . 1 1 P F I s p e c t r u m of H F H +  124  6 . 1 2 M A T I s p e c t r a of A r w i t h different d i s c r i m i n a t i o n fields  126  6 . 1 3 C o m p a r i s o n of t o t a l y i e l d a n d P F I s p e c t r a for H F H  127  +  Acknowledgements I b a s i c a l l y w o r k e d a l o n e o n m y P h D p r o j e c t i n t h e p a s t four y e a r s i n V a n c o u v e r , a n d D r . J o h n W H e p b u r n is b a s i c a l l y t h e o n l y resource for h e l p . A t t h e b e g i n n i n g I was k i n d of s h y t o expose m y w e a k p o i n t s d i r e c t l y t o t h e b o s s , a n d t h e n I r e a l i z e d t h a t I was l u c k y t o have frequent c o m m u n i c a t i o n w i t h h i m . I w a n t t o t h a n k J o h n for his p a t i e n c e a n d e n c o u r a g e m e n t a l l t h e w a y a l o n g . I also w a n t t o t h a n k h i m for s e n d i n g m e t o a d o z e n conferences, a n d c o r r e c t i n g m y p o o r w r i t i n g piece b y p i e c e i n s p i t e of his b u s y schedule. M y advisory committee members, D r . A l l a n B e r t r a m , D r . M i k e Gerry, D r . A n t h o n y Merer, a n d D r . M o s h e S h a p i r o are a c k n o w l e d g e d for t h e i r g u i d a n c e d u r i n g t h e course. I w a n t to t h a n k D r . R a l p h S h i e l l a n d D r . X i a o k u n H u a g a i n for s h o w i n g m e i n t o t h i s field at U n i v e r s i t y of W a t e r l o o . T h e fellow s t u d e n t w h o m o v e d t o g e t h e r t o V a n c o u v e r f r o m W a t e r l o o , M o h a m e d M u s a , w a s h e l p f u l o n a l l k i n d s of c o m p u t e r p r o g r a m s . I w a n t t o t a k e t h i s c h a n c e t o t h a n k D r . W e i K o n g for her i n i t i a l d e s i g n of t h e m a c h i n e . I also t h a n k D r . J a m e s M a r t i n for h i s o r i g i n a l w o r k o n t h e i o n - p a i r p r o j e c t . I a p p r e c i a t e h e l p a n d d i s c u s s i o n f r o m m e m b e r s of t h e r e s e a r c h g r o u p : D r . V a l e r y M i l n e r , D r . Guillaume Bussiere, D r . T o d d Melville, D r . Denis R o l l a n d , and D r . Q u n Zhang. Denis and G u i l l a u m e were g r e a t h e l p d u r i n g t h e s e t u p of t h e l a b . T o d d a n d Q u n also w o r k e d for s o m e time on this project.  V a l e r y is t h e n e w l a b d i r e c t o r w h o is p u t t i n g effort t o m a k e t h e g r o u p  move forward. I feel a l o t of pressure f r o m fellow g r a d u a t e s t u d e n t s : S a r a h H a n n a , C h e n L i a n d X i a o j i X u . T h o s e y o u n g a n d h a r d - w o r k i n g researchers r e m i n d m e t h a t i t ' s t i m e t o m o v e o n . F i n a l l y I t h a n k m y p a r e n t s for t h e e a r l y e d u c a t i o n . I t h a n k m y f a t h e r for t e a c h i n g m e to be s t r o n g , a n d m y m o m for t e l l i n g m e t o be a n i c e p e r s o n .  Chapter 1 Introduction  1.1  Background of high-u ion-pair states and T I P P S  T h r e s h o l d I o n - P a i r P r o d u c t i o n S p e c t r o s c o p y ( T I P P S ) is a h i g h r e s o l u t i o n t h r e s h o l d p h o t o i o n ization technique process:  A-B(a)  w h i c h c a n p r o v i d e energetic a n d d y n a m i c i n f o r m a t i o n a s s o c i a t e d w i t h t h e  1  —> A (/3 ) +  +  + B~(f3~),  where a a n d  (3~ r e p r e s e n t t h e i n i t i a l a n d f i n a l  q u a n t u m states o f t h e s y s t e m . T I P P S is b a s e d o n t h e e x c i t a t i o n of n e u t r a l m o l e c u l e s to h i g h l y 2  v i b r a t i o n a l l y e x c i t e d (high-?;) i o n - p a i r states A {(5 ) +  limit.  +  — B~{(3~)  just below the dissociation  T h e s e states b e h a v e l i k e t h e h i g h - n R y d b e r g states u s e d for p u l s e d field i o n i z a t i o n -  zero k i n e t i c e n e r g y p h o t o e l e c t r o n ( P F I - Z E K E ) or mass a n a l y z e d t h r e s h o l d i o n i z a t i o n ( M A T I ) spectroscopy,  1.1.1  3,4  a n d c a n b e d e t e c t e d b y p u l s e d field d i s s o c i a t i o n .  R y d b e r g states a n d i o n - p a i r states  T h e q u a n t u m s t a t e of a n e l e c t r o n o r b i t i n g a n i o n - c o r e c a n b e d e f i n e d b y a set of q u a n t u m n u m b e r s n,  I, mi,  w h e r e n is t h e p r i n c i p a l q u a n t u m n u m b e r , I is t h e a n g u l a r m o m e n t u m  q u a n t u m n u m b e r a n d mi is t h e m a g n e t i c q u a n t u m n u m b e r . I f n is l a r g e e n o u g h , s u c h as w h e n i t is l a r g e r t h a n t h e p r i n c i p a l q u a n t u m n u m b e r s of a n y o t h e r e l e c t r o n s i n t h e s a m e a t o m or m o l e c u l e , t h e e l e c t r o n w i l l s p e n d m o s t of t h e t i m e at a l a r g e d i s t a n c e f r o m t h e i o n - c o r e , a n d t h e i n t e r a c t i o n b e t w e e n t h e e l e c t r o n a n d ion-core is d o m i n a t e d b y t h e C o u l o m b i c a t t r a c t i o n . S u c h a s t a t e is c a l l e d R y d b e r g state for w h i c h t h e i o n - c o r e c a n be c o n s i d e r e d as a frozen core of n e a r l y s p h e r i c a l s y m m e t r y , w i t h t h e size of a few B o h r r a d i i .  5  T h e e n e r g y levels of a R y d b e r g e l e c t r o n r e l a t i v e t o t h e i o n i z a t i o n l i m i t c a n b e by:  described  6  E n  '  1  '(n-dtf  =  " ( ^ p  =  (  L  1  )  w h e r e <5; is t h e q u a n t u m defect, a n d n * is t h e effective p r i n c i p a l q u a n t u m n u m b e r . T h e m a g n i t u d e of 5i p r o v i d e s a n i n d i c a t i o n of t h e e x t e n t t o w h i c h t h e w a v e f u n c t i o n of t h e R y d b e r g e l e c t r o n p e n e t r a t e s i n t o t h e ion-core r e g i o n . T y p i c a l values of 5i for m o l e c u l e s c o m p o s e d e n t i r e l y of f i r s t - r o w a t o m s are 5 = 1 . 0 - 1 . 5 , <5„ =0.4-0.8, a n d ~ 0 for nd a n d h i g h e r I f u n c t i o n s . n s  p  7  T h e v a l u e of R c a n b e c a l c u l a t e d f r o m :  R = —~~Roo where m  is t h e m a s s of e l e c t r o n , M = ^ ! ^ 7  e  tem with m constant.  c  c  is t h e r e d u c e d m a s s of t h e e l e c t r o n / i o n - c o r e  r e p r e s e n t i n g t h e mass of t h e i o n - c o r e , a n d i ? = 1 0 9 7 3 7 . 3 1 5 c m ~ o o  1  sys-  is t h e R y d b e r g  8  F r o m e q u a t i o n 1.1, t h e s e p a r a t i o n of successive energy levels i n a R y d b e r g series changes a p p r o x i m a t e l y as t h e inverse t h i r d p o w e r of n , or i n o t h e r w o r d s , t h e s t a t e d e n s i t y scales as n . 9  3  O n t h e o t h e r side, t h e r e l a t i v e cross s e c t i o n of o p t i c a l t r a n s i t i o n s f r o m valence s t a t e to h i g h - n R y d b e r g s t a t e s scales as n  - 3  .  1 0  T h e r e f o r e , for t r a n s i t i o n s t o h i g h - n R y d b e r g states, t h e average  t r a n s i t i o n s t r e n g t h p e r e n e r g y u n i t does n o t change.  It h a s t h e s a m e v a l u e for t r a n s i t i o n s t o  a b o v e a n d b e l o w t h e i o n i z a t i o n t h r e s h o l d . A s a r e s u l t , R y d b e r g s t a t e y i e l d is a d i r e c t r e f l e c t i o n of t h e p h o t o i o n i z a t i o n cross s e c t i o n j u s t a b o v e t h e t h r e s h o l d . T h e p r o p e r t i e s of h i g h - u i o n - p a i r states are s i m i l a r to h i g h - n R y d b e r g states i n m a n y w a y s . I n high-w i o n - p a i r states, t h e w e a k l y b o u n d o p p o s i t e charges - a n i o n a n d c a t i o n - o r b i t each o t h e r w i t h a l a r g e i n t e r n u c l e a r d i s t a n c e , a n d t h e a t t r a c t i o n b e t w e e n t h e m c a n also be expressed b y C o u l o m b p o t e n t i a l , l i k e t h e R y d b e r g s y s t e m . T h i s is d e m o n s t r a t e d i n F i g u r e 1.1, i n w h i c h the spectroscopically determined R K R p o t e n t i a l  1 1  of H F ( B E ) i o n - p a i r s t a t e at large i n t e r n u 1  clear d i s t a n c e m a t c h e s v e r y w e l l t h e s u p e r i m p o s e d C o u l o m b p o t e n t i a l . D u e t o t h e a s y m p t o t i c a t t r a c t i o n i n t h e i o n - p a i r states, t h e r e is a n i n f i n i t e n u m b e r of q u a s i b o u n d v i b r a t i o n a l levels b e l o w t h e i o n - p a i r t h r e s h o l d (like t h e R y d b e r g series b e l o w t h e ionization l i m i t ) .  1 2  T h e energies of these v i b r a t i o n a l levels r e l a t i v e t o t h e i o n - p a i r l i m i t have  0  1  3  2  4  5  S  R(H-F) (A)  F i g u r e 1.1: C o u l o m b i c a t t r a c t i o n i n H F ( 5 S ) i o n - p a i r s t a t e . T h e R K R d a t a o f t h e B T, 1  p a i r s t a t e a n d X !! 1  l  ion-  g r o u n d s t a t e a r e t a k e n f r o m r e f e r e n c e 11. A C o u l o m b p o t e n t i a l w i t h t h e  i o n - p a i r t h r e s h o l d as t h e a s y m p t o t i c e n e r g y is s u p e r i m p o s e d ( s o l i d l i n e ) a n d m a t c h e s w e l l w i t h t h e p o t e n t i a l o f t h e B !} 1  ion-pair state at large internuclear distance.  Chapter 1. the f o r m :  Introduction  1 2  K  '  J  = -(v  + j + i - s j y  (  w h e r e v a n d J a r e t h e v i b r a t i o n a l a n d r o t a t i o n a l n u m b e r s of t h e i o n - p a i r s y s t e m , R =  L  3  )  ^Roo  w i t h n as t h e r e d u c e d m a s s o f t h e i o n - p a i r s y s t e m , a n d Sj is t h e J specific q u a n t u m defect, like t h e 6[ t e r m i n e q u a t i o n 1.1. T h e t e r m v + J + 1 m e a s u r e s t h e n u m b e r of m o d e s i n t h e g i v e n i o n - p a i r r o v i b r a t i o n a l s t a t e a n d replaces t h e p r i n c i p a l q u a n t u m n u m b e r n a s s o c i a t e d w i t h R y d b e r g state.  T h e d e n o m i n a t o r o f e q u a t i o n 1.3 is m o r e c o m p l i c a t e d t h a n t h a t o f e q u a t i o n  1.1, since v s t a r t s f r o m 0 a n d J is i n d e p e n d e n t of v, w h i l e i n e q u a t i o n 1.1 n s t a r t s f r o m 1 a n d / has a v a l u e b e t w e e n 0 a n d n - 1 . S i n c e t h e r e d u c e d m a s s /J, of t h e i o n - p a i r s y s t e m is m u c h l a r g e r ( ~ 1 0 ) t h a n t h a t of a 3  R y d b e r g s t a t e , t h e d e n s i t y o f h i g h - v i o n - p a i r states is m u c h h i g h e r t h a n t h a t o f R y d b e r g states. B u t b o t h of t h e s t a t e d e n s i t i e s have a s i m i l a r f o r m r e g a r d i n g t o t h e s t a t e q u a n t u m n u m b e r (scales as v  3  for a n i o n - p a i r s t a t e a n d n  3  for R y d b e r g s t a t e ) , a n d l i k e t h e R y d b e r g s y s t e m , t h e  average cross s e c t i o n p e r e n e r g y u n i t is t h e same for t r a n s i t i o n s t o i o n - p a i r states a b o v e a n d below the ion-pair l i m i t .  1.1.2  Field ionization a n d  field  dissociation  W i t h t h e p r e s e n c e of a n e l e c t r i c f i e l d , t h e l o n g r a n g e i n t e r a c t i o n b e t w e e n a R y d b e r g e l e c t r o n a n d t h e i o n - c o r e is n o l o n g e r C o u l o m b i c a n d b e c o m e s m o r e c o m p l i c a t e d . I n a t o m i c u n i t s , i t is expressed as: V,  r z  = - - - F z r  (1.4)  w h e r e r is t h e d i s t a n c e of t h e R y d b e r g e l e c t r o n f r o m t h e i o n - c o r e , F i s t h e a p p l i e d e l e c t r i c field a n d - z is i t s d i r e c t i o n . T h i s is i l l u s t r a t e d i n F i g u r e 1.2. W i t h t h e e l e c t r i c field t u r n e d o n , t h e n  2  energetic d e g e n e r c y o f h y d r o g e n i c R y d b e r g states  is p a r t i a l l y r e m o v e d , since t h e o r i g i n a l s p h e r i c a l s y m m e t r y of t h e R y d b e r g e l e c t r o n is r e d u c e d t o c y l i n d r i c a l . C o n s e q u e n t l y , t h e p e r t u r b a t i o n leads t o a s p l i t t i n g o f t h e d e g e n e r a t e energy levels a n d I is n o longer a g o o d q u a n t u m n u m b e r . T h e energy levels a r e b e t t e r e x p r e s s e d as a f u n c t i o n of t h e p a r a b o l i c q u a n t u m n u m b e r s n\ a n d n  2  t h a n a f u n c t i o n o f I b e c a u s e t h e | nnin^m  > basis  F i g u r e 1.2:  P o t e n t i a l of a R y d b e r g e l e c t r o n i n e l e c t r i c  field.  S o l i d curves are the p o t e n t i a l  w h e n t h e field is off, a n d d a s h e d c u r v e s a r e t h e p o t e n i a l w h e n t h e field is o n . F i e l d s t r e n g t h is l O O V / c m . Z e r o p o i n t e n e r g y is t h e i o n i z a t i o n l i m i t .  f u n c t i o n s are a d a p t e d t o t h e c y l i n d r i c a l s y m m e t r y of t h e p r o b l e m . T h e | nn\ri2m are s u p e r p o s i t i o n s of t h e o r i g i n a l | nlm  > functions  > states. A t l o w t o m o d e r a t e e l e c t r i c fields, m r e m a i n s  a g o o d q u a n t u m n u m b e r as l o n g as t h e p o t e n t i a l o f t h e R y d b e r g states r e t a i n s c y l i n d r i c a l s y m m e t r y . T h e r e f o r e , h o m o g e n e o u s a n d c y l i n d r i c a l l y s y m m e t r i c e l e c t r i c fields d o n o t i n d u c e m m i x i n g . I n t h i s case, t h e e n e r g y levels are g i v e n i n a t o m i c u n i t s t o first o r d e r b y :  £ = - ^ 2 + ^ ( m - n  w h e r e n\ a n d n  2  are n o n n e g a t i v e integers a n d n\ — n  2  2  1 3  ) n  (1.5)  takes values r a n g i n g f r o m —(n — m)  to  ( n — m ) i n steps of 2. F o r n o n h y d r o g e n i c R y d b e r g a t o m , t h e s i t u a t i o n is m o r e c o m p l i c a t e d . T h e h i g h / states s t i l l c a n be d e s c r i b e d b y t h e a b o v e e q u a t i o n , as t h e y have l i t t l e p e n e t r a t i n g c h a r a c t e r (5i ~ 0 ) . s t r u c t u r e o f t h e e n e r g y levels is d o m i n a t e d b y t h e l i n e a r S t a r k effect.  The  T h e l o w I states t h a t  have n o n z e r o q u a n t u m defects are s e p a r a t e d f r o m t h e h i g h I h y d r o g e n i c m a n i f o l d a n d d i s p l a y a q u a d r a t i c S t a r k effect at l o w field s t r e n g t h s . I n a S t a r k m a p t h a t s h o w s t h e energy levels of a R y d b e r g e l e c t r o n i n e l e c t r i c field, for t h e s a m e p r i n c i p a l q u a n t u m n u m b e r n , t h e reddest shift (shift t o lowest energy) is t h e edge w i t h n\ — ri2 — — (n — 1) a n d m = 0, a n d t h e b l u e s t shift (shift t o h i g h e s t energy) is t h e edge w i t h n\ — n  2  = n — 1 and m =  0.  1 4  D u e t o t h e e n e r g y s p l i t t i n g a n d shift i n electric field, t h e b l u e S t a r k states of lower n m a y i n t e r a c t w i t h t h e r e d S t a r k states of h i g h e r n i f t h e e l e c t r i c f i e l d is s t r o n g e n o u g h t h a t t h e energy levels c a n pass t h e I n g l i s - T e l l e r l i m i t .  1 5  I n t e r a c t i o n s b e t w e e n R y d b e r g states w i t h t h e  s a m e m a g n e t i c q u a n t u m n u m b e r m b u t different p r i n c i p a l q u a n t u m n u m b e r n l e a d t o a v o i d e d crossings for n o n h y d r o g e n i c a t o m s .  If a n a v o i d e d c r o s s i n g is a p p r o a c h e d q u i c k l y as t h e field  is c h a n g e d , t h e R y d b e r g e l e c t r o n m a y cross t o a n o t h e r p o t e n t i a l c u r v e ( d i a b a t i c process).  If  t h e c r o s s i n g is a p p r o a c h e d s l o w l y e n o u g h , t h e R y d b e r g e l e c t r o n w i l l s t a y i n t h e s a m e p o t e n t i a l c u r v e ( a d i a b a t i c p r o c e s s ) . F o r a d i a b a t i c process, t h e q u a n t u m s t a t e of t h e R y d b e r g e l e c t r o n is c h a n g e d , w h i c h c a n o n l y h a p p e n w h e n t h e e l e c t r o n passes close t o t h e i o n - c o r e .  1 6  T h e t w o l i m i t i n g cases of d i a b a t i c a n d a d i a b a t i c processes c o r r e s p o n d t o v e r y different i o n i z a t i o n b e h a v i o r . F o r a c o m p l e t e l y a d i a b a t i c process, t h e c o n v e n t i o n a l s a d d l e p o i n t m o d e l  is useful t o p r e d i c t t h e f i e l d m a g n i t u d e necessary to i o n i z e a R y d b e r g s t a t e .  The  combined  C o u l o m b - S t a r k p o t e n t i a l of t h e R y d b e r g s t a t e ( e q u a t i o n 1.4) has t h e s a d d l e p o i n t at a z v a l u e of l/\fF  w h i c h c o r r e s p o n d s t o a p o t e n t i a l energy of - 2 \ / J F . A s a r e s u l t , t h e i o n i z a t i o n p o t e n t i a l  of t h e e l e c t r o n i s . l o w e r e d b y 2y/~F ( i n a t o m i c u n i t ) , or i n a c o n v e n i e n t way, t h e r e l a t i o n c o u l d be expressed a s :  1 5  IP  (1.6)  = IPo-Q.lVF  w h e r e I P a n d I P o are t h e i o n i z a t i o n p o t e n t i a l s w i t h a n d w i t h o u t e l e c t r i c f i e l d a n d i n c m  - 1  , and  F is t h e f i e l d m a g n i t u d e i n V / c m . F o r a c o m p l e t e l y d i a b a t i c process, i o n i z a t i o n of a p o p u l a t i o n of R y d b e r g states d i s t r i b u t e d e q u a l l y a m o n g a l l S t a r k s t a t e s of a g i v e n n v a l u e o c c u r s over a r a n g e of f i e l d s t r e n g t h s . T h e i o n i z a t i o n p o t e n t i a l is l o w e r e d b y A.Qy/F a n d 3.l^/F  for t h e b l u e s t a n d r e d d e s t shifts r e s p e c t i v e l y .  14  E x p e r i m e n t a l l y , n o n d i a b a t i c process w h i c h is a n i n t e r m e d i a t e p r o c e s s b e t w e e n a d i a b a t i c a n d d i a b a t i c processes is l i k e l y to be i m p o r t a n t , a n d t h e i o n i z a t i o n p o t e n t i a l of a R y d b e r g e l e c t r o n is lowered b y : IP  OVf  = IPQ -  (1.7)  w h e r e t h e a coefficient has a t y p i c a l v a l u e b e t w e e n 3.9 a n d 6 . 1 .  1 7  A v a l u e v e r y close t o 3.9 or 6.1  i n d i c a t e s a p r e d o m i n a n t l y d i a b a t i c or a d i a b a t i c b e h a v i o r of t h e R y d b e r g e l e c t r o n i n a n electric field, respectively. T h e l o w e r i n g of t h e i o n i z a t i o n t h r e s h o l d w i t h e l e c t r i c field has b e e n c o n f i r m e d i n m a n y R y d b e r g s y s t e m s , for e x a m p l e , i n N O a n d A r .  1 8  '  1 9  T h e high-w i o n - p a i r states e x h i b i t a s i m i l a r p r o p e r t y i n a n e l e c t r i c field, i.e., t h r e s h o l d is l o w e r e d b y t h e a p p l i c a t i o n of a n e l e c t r i c et al for t h e p r o c e s s H 2 + hu —• H  +  + H ~ i n electric  field.  the ion-pair  T h i s was first o b s e r v e d b y P r a t t  field.  17  T h e r e , a D C field of different  m a g n i t u d e s o n t h e o r d e r of a few h u n d r e d V / c m was a p p l i e d t o t h e r e a c t i o n r e g i o n . T h e H 2 m o l e c u l e was e x c i t e d t o t h e £ F S + ( u = 6 ) state b y t w o p h o t o n s , a n d t h e n was f u r t h e r e x c i t e d :  1  b y a t u n a b l e t h i r d p h o t o n to t h e i o n - p a i r t h r e s h o l d . B y m o n i t o r i n g t h e i o n - p a i r y i e l d s i g n a l as a f u n c t i o n of t h e t o t a l p h o t o n energy, t h e energetic t h r e s h o l d for i o n - p a i r f o r m a t i o n u n d e r different e l e c t r i c fields c o u l d b e m e a s u r e d . T h e y o b s e r v e d t h a t t h e i o n - p a i r t h r e s h o l d shifts t o  lower e n e r g y as t h e m a g n i t u d e of t h e e l e c t r i c field increases a c c o r d i n g t o e q u a t i o n 1.7, a n d a v a l u e of 5 . 7 ± 0 . 2 c m  _ 1  /(V/cm)  _ 1  w a s o b t a i n e d for t h e a coefficient. T h e i r a v a l u e w a s close to  t h e 6.1 l i m i t b e c a u s e a D C f i e l d was used for t h e i r e x p e r i m e n t a n d t h e i o n i z a t i o n process c o u l d b e r e g a r d e d as p r e d o m i n a n t l y a d i a b a t i c .  1.1.3  P F I - Z E K E photoelectron  spectroscopy  I n c o n v e n t i o n a l p h o t o e l e c t r o n spectroscopy, ergy, s u c h as H e I r a d i a t i o n at 2 1 . 2 e V .  2 0  t h e m o l e c u l e s are i o n i z e d b y a fixed p h o t o n e n -  E l e c t r o n s are p r o d u c e d w i t h different k i n e t i c energies,  l e a v i n g t h e p a r e n t ions i n v a r i o u s r o v i b r o n i c states. W i t h a t y p i c a l r e s o l u t i o n of 5 - 2 5 m e V or 4 0 200cm , - 1  t h e c o n v e n t i o n a l p h o t o e l e c t r o n s p e c t r o s c o p y u s u a l l y c a n n o t resolve t h e r o t a t i o n a l  2 1  levels of t h e p a r e n t i o n , e x c e p t for H constants.  2  a n d a few o t h e r s m a l l m o l e c u l e s w i t h l a r g e r o t a t i o n a l  2 2  T h e r e s o l u t i o n of p h o t o e l e c t r o n s p e c t r o s c o p y  c a n b e m u c h i m p r o v e d b y t h e t e c h n i q u e of  t h r e s h o l d p h o t o e l e c t r o n s p e c t r o s c o p y ( T P E S ) , i n w h i c h t h e p h o t o n e n e r g y is s c a n n e d a n d o n l y electrons w i t h s m a l l k i n e t i c energies are d e t e c t e d . T h e i n i t i a l m e t h o d e m p l o y e d for t h i s p u r p o s e was to e x t r a c t t h e e l e c t r o n s f r o m t h e i o n i z a t i o n r e g i o n w i t h a s m a l l e l e c t r i c field, a n d t r a n s p o r t t h e m as e i t h e r a p a r a l l e l b e a m , or s h a r p l y focus t h e m o n t o a s m a l l a p e r t u r e of a n e l e c t r o s t a t i c a n a l y z e r . I n t h i s process, energetic electrons w i t h n o n z e r o off-axis v e l o c i t i e s are efficiently rejected.  23  A n a l t e r n a t i v e m e t h o d i n v o l v e s a w a i t i n g p e r i o d r a n g i n g f r o m s e v e r a l h u n d r e d ns  t o s e v e r a l fis b e t w e e n p h o t o e x c i t a t i o n a n d s i g n a l d e t e c t i o n .  T h e w a i t i n g p e r i o d a l l o w s fast  electrons to d r i f t a w a y f r o m t h e i o n i z a t i o n r e g i o n a n d o n l y e l e c t r o n s w i t h s m a l l k i n e t i c energies are e x t r a c t e d t o t h e d e t e c t i o n s y s t e m .  2 3  T h i s t e c h n i q u e c a n achieve a r e s o l u t i o n o f a few c m  - 1  .  T h e e n e r g y r e s o l u t i o n is f u r t h e r i m p r o v e d b y t h e t e c h n i q u e o f P F I - Z E K E , w h i c h f r o m t h e n a m e o n l y d e t e c t s e l e c t r o n s w i t h zero k i n e t i c energy. P F I - Z E K E t a k e s a d v a n t a g e of t h e p r o p erties of h i g h - n R y d b e r g states i n e l e c t r i c field. I n s t e a d of d i r e c t l y i o n i z i n g t h e R y d b e r g states u s i n g a l a r g e D C field o f a few h u n d r e d V / c m , t h i s t e c h n i q u e uses a sequence of t w o e l e c t r i c field pulses. T h i s is i l l u s t r a t e d i n F i g u r e 1.3. T h e m o l e c u l e s A B are e x c i t e d t o h i g h - n R y d b e r g states or b e c o m e i o n i z e d t o A B  +  by tunable V U V photons.  A f t e r a s h o r t d e l a y t i m e , u s u a l l y a few  h u n d r e d ns, a s m a l l d i s c r i m i n a t i o n p u l s e ( c o u l d b e less t h a n l V / c m ) is a p p l i e d t o d r i v e a w a y t h e p r o m p t ions f o r m e d a b o v e i o n i z a t i o n t h r e s h o l d . T h e n at a l o n g e r d e l a y t i m e w h i c h is a b o u t several /xs, a s e c o n d p u l s e of l a r g e r m a g n i t u d e is a p p l i e d t o f i e l d i o n i z e t h e h i g h - n R y d b e r g states a n d e x t r a c t t h e t h r e s h o l d ions i n t o t h e d e t e c t o r . S i n c e o n l y t h r e s h o l d ions are d e t e c t e d , t h i s t e c h n i q u e c a n a c h i e v e v e r y h i g h r e s o l u t i o n , d o w n t o laser b a n d w i d t h s ( < l c m  - 1  ).  2 4  B e s i d e s s e r v i n g as t h e d i s c r i m i n a t i o n field, t h e first p u l s e i n P F I - Z E K E also ionizes t h e h i g h e s t - n R y d b e r g states. F r o m e q u a t i o n 1.7, t h e t h r e s h o l d s i g n a l c o m e s f r o m t h e energy range [IP  0  - a % / F i , IP  0  t r a c t i o n pulses.  - ay/F ], 2  where F  x  and F  2  are t h e m a g n i t u d e s o f d i s c r i m i n a t i o n a n d e x -  T h e r e f o r e , t h e b l u e edge ( h i g h energy edge) o f t h e s i g n a l w i l l shift w i t h t h e  m a g n i t u d e of t h e d i s c r i m i n a t i o n f i e l d F i , a n d t h e field-free i o n i z a t i o n p o t e n t i a l IP  0  c a n be  a c c u r a t e l y m e a s u r e d b y r e c o r d i n g t h e s p e c t r a w i t h a series of d i s c r i m i n a t i o n fields a n d t h e n e x t r a p o l a t i n g t o zero f i e l d . T h e h i g h r e s o l u t i o n of P F I - Z E K E m a k e s i t a n i m p o r t a n t a n d w i d e s p r e a d t e c h n i q u e t o det e r m i n e t h e m o l e c u l a r i o n i z a t i o n energies, a n d to s t u d y t h e s t r u c t u r e a n d r o v i b r a t i o n a l states of t h e m o l e c u l a r i o n s . ' 2 4  2 5  A v a r i a n t of P F I - Z E K E is M A T I (mass a n a l y z e d t h r e s h o l d i o n i z a t i o n ) s p e c t r o s c o p y .  4  This  t e c h n i q u e d e t e c t s p o s i t i v e ions i n s t e a d of t h e electrons. S i n c e i o n s a n d e l e c t r o n s are t h e c o u n t e r p a r t s of t h e s a m e i o n i z a t i o n process, a M A T I s p e c t r u m has t h e s a m e s t r u c t u r e s as t h e one r e c o r d e d b y P F I - Z E K E . F o r M A T I spectroscopy, a l a r g e r d i s c r i m i n a t i o n f i e l d (a few V / c m ) a n d a longer d e l a y t i m e b e t w e e n t h e t w o field pulses need t o b e a p p l i e d d u e t o t h e m u c h heavier mass of t h e ions c o m p a r e d t o electrons. S i n c e t h e b l u e edge of t h e s i g n a l gets e r o d e d t o m o r e e x t e n t w i t h a d i s c r i m i n a t i o n field of larger m a g n i t u d e a n d R y d b e r g states m i g h t d e c a y d u r i n g t h e d e l a y t i m e , t h i s t e c h n i q u e u s u a l l y has a lower s i g n a l t o noise r a t i o t h a n P F I - Z E K E . B u t since i t p r o v i d e s m a s s i n f o r m a t i o n , M A T I is useful i n s y s t e m s s u c h as r a d i c a l s a n d c l u s t e r s . ' 2 6  2 7  F o r b o t h P F I - Z E K E a n d M A T I , t h e ions c a n be p r o d u c e d i n different r o v i b r o n i c states. T h i s p r o v i d e s a g o o d s y s t e m for s t u d y i n g state-selected i o n - m o l e c u l e r e a c t i o n s , s u c h as t h e m o s t f u n d a m e n t a l one H  2  + H£  —• H^ + H .  2  8  The  ions p r e p a r e d i n a selected r o v i b r a t i o n a l  state b y field i o n i z a t i o n of h i g h - n R y d b e r g states c o l l i d e d w i t h a n e u t r a l H different energies. B y c o m p a r i n g t h e r e l a t i v e r a t i o of t h e p r o d u c t i o n H3  2  m o l e c u l a r b e a m of  w i t h H j , the relative  cross sections of the r e a c t i o n c o u l d be d e t e r m i n e d for different states of H j or as a f u n c t i o n of the c o l l i s i o n energy. 1.1.4  TIPP  spectroscopy  M o l e c u l a r p h o t o d i s s o c i a t i o n i n t o i o n - p a i r s AB + hi> —> A  + B~ is a basic p h o t o f r a g m e n t a t i o n  +  process, b u t not as m u c h research has been done on i t c o m p a r e d to p h o t o i o n i z a t i o n or phot o d i s s o c i a t i o n i n t o n e u t r a l fragments. T h i s is due to the very h i g h p h o t o n energies ( > 1 0 e V for m a n y d i a t o m i c molecules) needed to get to the i o n - p a i r t h r e s h o l d , a n d very low cross sections from the g r o u n d state t o the i o n - p a i r c o n t i n u u m . P r e v i o u s l y i o n - p a i r processes were s t u d i e d w i t h either fixed or t u n a b l e p h o t o n energy, b o t h w i t h a r e s o l u t i o n of a few h u n d r e d c m w o r k on H F ,  2 9  F2,  2 9  and C H 3 X ,  - 1  .  F o r studies w i t h fixed p h o t o n energy, such as the  the i n t e r n a l energy of the i o n - p a i r fragments was s t u d i e d  3 0  from analysis of t h e i o n fragment k i n e t i c energy. F o r studies w i t h t u n a b l e p h o t o n energy, such as the w o r k on H C 1 ,  3 1  HF,  3 2  HCN,  3 3  CF4,  34  C H X a n d C H , > - the i o n - p a i r y i e l d curve were 3  2  2  3 5  3 6  recorded as a f u n c t i o n of the p h o t o n energy, a l l o w i n g the measurement of i o n - p a i r t h r e s h o l d a n d cross sections at different energies. Since its i n v e n t i o n a few years ago, the h i g h resolution technique of T I P P S has been d e m o n s t r a t e d to be useful i n o b t a i n i n g energetic a n d d y n a m i c i n f o r m a t i o n a b o u t the i o n - p a i r dissocia t i o n processes i n several different m o l e c u l a r systems. S i m i l a r t o the technique of P F I - Z E K E w h i c h detects the field i o n i z a t i o n s i g n a l of high-w R y d b e r g states, T I P P S involves the field d i s s o c i a t i o n of high-w i o n - p a i r states:  AB + hv -y A  +  T h e whole process has three steps.  - B~  - » A+ +  B~  F i r s t , the molecules A B are e x c i t e d b y a pulsed laser  r a d i a t i o n f r o m the g r o u n d state to the high-w i o n - p a i r states A  +  — B~ j u s t below threshold.  Second, the p r o m p t fragment ions p r o d u c e d b y direct d i s s o c i a t i o n a n d fast p r e d i s s o c i a t i o n are repelled out of the r e a c t i o n region b y a s m a l l d i s c r i m i n a t i o n field pulse w i t h a m a g n i t u d e of a few V / c m . T h i r d , a larger field pulse w i t h larger m a g n i t u d e (several to tens of V / c m ) is a p p l i e d to field dissociate the high-w i o n - p a i r states a n d e x t r a c t the t h r e s h o l d ions i n t o the detection  s y s t e m . T h e m a g n i t u d e of the d i s c r i m i n a t i o n pulse a n d relative delay t i m e of the t w o pulses can be a d j u s t e d for different m o l e c u l a r systems t o achieve complete d i s c r i m i n a t i o n against the p r o m p t ions. B y o n l y d e t e c t i n g t h e t h r e s h o l d ions, T I P P S c a n achieve a s i m i l a r r e s o l u t i o n as P F I - Z E K E . T h i s is a great i m p r o v e m e n t over the c o n v e n t i o n a l i o n - p a i r technique. A s a n i l l u s t r a t i v e e x a m p l e , for the i o n - p a i r process 0 2  P ), 1/2  2  + hv —> 0 ( 5 ) + 0~( P /2 +  4  2  or  3  the T I P P spect r u m i n the t h r e h o l d region has two sets of c l e a r l y resolved peaks (see  1  F i g u r e 1.4), c o r r e s p o n d i n g t o f r a g m e n t a t i o n into 0  + 0 ~ ( P / ) and 0  +  2  3  2  +  + 0~( Pi/ ).  set has a sequence of peaks c o m i n g f r o m different r o t a t i o n a l levels of the 0  2  Each  2  molecules i n the  2  g r o u n d state. T h e energy s e p a r a t i o n between the peaks i n each set is e x a c t l y the energy gap between 0  2  r o t a t i o n a l levels. T h e s p i n - o r b i t s p l i t t i n g between 0 ~ ( P / ) a n d 0 ( P i / ) c a n be 2  3  _  2  2  2  d e t e r m i n e d f r o m t h e energy difference between t h e two sets of p e a k s , a n d the intensities reflect the relative cross sections t o t h e t w o i o n - p a i r channels 0  +  + 0 ~ ( P / ) and 0 2  3  2  +  +  0~( P / ). 2  1  2  D u e t o t h e s i m i l a r i t y between h i g h - n R y d b e r g state a n d high-t> i o n - p a i r state, the blue edge of T I P P S s i g n a l also shifts w i t h the m a g n i t u d e of t h e d i s c r i m i n a t i o n field. Therefore, b y r e c o r d i n g T I P P S s p e c t r a w i t h a series of different d i s c r i m i n a t i o n fields, one c a n measure the field-free i o n - p a i r t h r e s h o l d E ^  P  by extrapolation according to:  EIP  = E°  IP  -  (1.8)  aVF  where EIP is u s u a l l y t a k e n as the energy c o r r e s p o n d i n g to t h e h a l f m a x i m u m height of the blue edge of the T I P P S s i g n a l , a n d F is the m a g n i t u d e of t h e d i s c r i m i n a t i o n field. I n this way, the E®  P  value c o u l d be d e t e r m i n e d t o a n accuracy as g o o d as a f r a c t i o n of 1 c m . - 1  1.1.5  D e t e r m i n a t i o n of b o n d dissociation  energy  O n e i m p o r t a n t a p p l i c a t i o n of T I P P S is to determine t h e m o l e c u l a r b o n d d i s s o c i a t i o n energy DQ(A-B). AB  F r o m T I P P S , the i o n - p a i r t h r e s h o l d E ® c a n b e a c c u r a t e l y m e a s u r e d for the process  + hv —> A  P  +  + B ~ . I f t h e i o n i z a t i o n p o t e n t i a l of fragment A ( I P ( A ) ) a n d electron affinity of  fragment B ( E A ( B ) ) are also k n o w n , the b o n d d i s s o c i a t i o n energy DQ(A — B) c a n be c a l c u l t e d  i i—r—|—i—i—i—i—|—i—i—i—i—(—i—i—i—i  i  I  17.25  I  I  l _ l  17.26  I  !  I  I  17.27  I  I  1  I  I  I  L  17.28  |  i—i—i—r  III—II—I—  I  17.29  17.30  V U V Photon Energy (eV)  F i g u r e 1.4: T I P P s p e c t r u m o f O 2 . S p e c t r u m is t a k e n f r o m reference 1. A d i s c r i m i n a t i o n field of 2 . 1 V / c m was p u l s e d o n 0.3/xs after p h o t o e x c i t a t i o n . A n e x t r a c t i o n field o f 6 0 V / c m was p u l s e d o n 4.9/us l a t e r . T h e field-free i o n - p a i r t h r e s h o l d s for O ( A 0 + hv -+ 0 { S) 2  were m a r k e d above t h e s p e c t r u m .  +  4  + 0-( P , 2  3/2  2  A/2)  Chapter 1.  Introduction  according to: Do(A-B)  = Efj -IP(A)  (1.9)  + EA(B)  P  M o s t a t o m i c i o n i z a t i o n p o t e n t i a l s are k n o w n to a n a c c u r a c y b e t t e r t h a n 1 c m , as w e l l as m a n y - 1  a t o m i c electron a f f i n i t i e s . curacy ( ~ l c m  _ 1  37  Therefore, b y m e a s u r i n g the i o n - p a i r t h r e s h o l d E °  ) u s i n g T I P P S , the b o n d d i s s o c i a t i o n energy D${A  can be a c c u r a t e l y measured ( ~ l c m  - 1  ).  to a h i g h ac-  — B) of d i a t o m i c molecules  F o r p o l y a t o m i c molecules ABC,  3 8  I P  the s i t u a t i o n is more  c o m p l i c a t e d since the i o n i z a t i o n p o t e n t i a l or electron affinity of one or b o t h fragments u s u a l l y are not precisely k n o w n to ~ l c m  _ 1  .  B u t w h e n the d a t a is a v a i l a b l e , t h e b o n d d i s s o c i a t i o n  energy c a n be d e t e r m i n e d to a n accuracy of a few c m  - 1  .  3 9  F o r d i a t o m i c molecules, the b o n d d i s s o c i a t i o n energy DQ(A — B) m a y also be o b t a i n e d f r o m e x t r a p o l a t i o n of the s p e c t r o s c o p i c a l l y d e t e r m i n e d energy levels near the l o n g range dissociat i o n l i m i t i n t o n e u t r a l fragments A + B ,  4 0  '  4 1  w h i c h u n f o r t u n a t e l y are not always available. T h i s  m e t h o d c a n n o t be a p p l i e d t o t r i a t o m i c or p o l y a t o m i c molecules, since the energy levels of different v i b r a t i o n a l levels are p e r t u r b e d b y each other a n d thus m a k e the e x t r a p o l a t i o n impossible. A n o t h e r a l t e r n a t i v e to measure the b o n d d i s s o c i a t i o n energy is t h r o u g h t h e r m o c h e m i c a l reactions. F o r the r e a c t i o n RH + X ^  R + XH,  b y m e a s u r i n g the e q u i l i b r i u m constant, K,  one c a n c a l c u l a t e the value of free energy change, A G , f r o m w h i c h the e n t h a l p y change,  AH,  c a n be e x t r a c t e d . If the heats of f o r m a t i o n of R H , X a n d X H are k n o w n , the b o n d d i s s o c i a t i o n energy DQ(R — H) c a n be d e r i v e d . few h u n d r e d c m  - 1  4 2  T h i s m e t h o d n o r m a l l y has a n u n c e r t a i n t y no less t h a n a  .  O v e r t h e last t w e n t y years, the technique of H ( D ) a t o m p h o t o f r a g m e n t t r a n s l a t i o n a l spectroscopy has b e e n d e m o n s t r a t e d t o be effective to measure b o n d d i s s o c i a t i o n energies.  This  technique involves the p h o t o d i s s o c i a t i o n at fixed p h o t o n energy of molecules H - X ( X is u s u a l l y a free r a d i c a l ) i n t o n e u t r a l fragments H a n d X . T h e t r a n s l a t i o n a l ( a n d , where a p p r o p r i a t e , the angular) d i s t r i b u t i o n of the fragments can be measured b y r e c o r d i n g t h e T O F s p e c t r u m of H a t o m . T h e T O F s p e c t r u m provides i n f o r m a t i o n of t r a n s l a t i o n a l d i s t r i b u t i o n of the H fragment, w h i c h is r e l a t e d to the t r a n s l a t i o n a l energy of fragment X b y m o m e n t u m conservation.  The  b o n d d i s s o c i a t i o n energy DQ(H — X) c a n be c a l c u l a t e d f r o m :  D (H  - X )  0  = E  h v  -  + E {HX) INT  where t h e i n t e r n a l energy of H X (Ei (HX))  -  E (X) INT  -  E (H) K  (1.10)  E (X) K  c a n be ignored i f t h e s a m p l e gas is i n t r o d u c e d  NT  i n t o t h e r e a c t i o n region b y a s k i m m e d molecular b e a m .  T h e i n t e r n a l energy of X  (Ei (X)) NT  can be o b t a i n e d b y a n a l y z i n g t h e energy d i s t r i b u t i o n of the fragment. T h i s technique has been a p p l i e d to measure a n u m b e r of b o n d d i s s o c i a t i o n energies, a m o n g w h i c h are DQ{H — D (HCC 0  -  H )  4  4  D (H 0  -  ~ 1 0 t o a few h u n d r e d c m  SH) - 1  a n d D (H  45  0  -  NH2)  43  T h e y have a t y p i c a l u n c e r t a i n t y f r o m  CN) . 46  .  R e c e n t l y , i o n p a i r i m a g i n g spectroscopy ( I P I S ) was used as a n efficient technique t o s t u d y the i o n - p a i r d i s s o c i a t i o n p r o c e s s .  47  T h e molecules were dissociated i n t o i o n - p a i r s w i t h fixed p h o -  t o n energy, a n d k i n e t i c energies o f the fragments were a n a l y z e d b y the h i g h r e s o l u t i o n technique of two d i m e n t i o n a l v e l o c i t y m a p i m a g i n g .  4 8  F r a g m e n t s w i t h t h e same i n i t i a l v e l o c i t y vector were  m a p p e d onto t h e same p o i n t of the image detector. T h i s technique c o u l d b e a p p l i e d t o s t u d y the v i b r a t i o n a l , even r o t a t i o n a l s t r u c t u r e of the fragment i o n , such as C H j from C H 4 X .  4 9 , 5 0  It  was also used t o measure t h e i o n - p a i r t h r e h o l d , f r o m w h i c h t h e b o n d d i s s o c i a t i o n energy c a n be derived. T h e b o n d d i s s o c i a t i o n energy DQ(F — F) i n molecule F 2 w a s m e a s u r e d i n t h i s w a y w i t h a n uncertainty of ~ 1 0 c m  1.2  - 1  (1.606±0.001eV).  51  Recent work on T I P P S  I n a d d i t i o n t o t h e p r e v i o u s w o r k of T I P P S o n O 2 , H C 1 , 1  T I P P S has been a p p l i e d t o t h e following molecules:  and H F ,  3 8  H /D , 2  2  5 3  H S, 2  5 2  i n t h e last few years  3 9  HC1/DC1,  5 4  HF/DF,  H C N a n d ( H F ) . I n t h i s thesis, the results o n H C 1 / D C 1 ( C h a p t e r 3), H F / D F ( C h a p t e r 4 ) , H C N 2  ( C h a p t e r 5) a n d ( H F ) 2 ( C h a p t e r 6) w i l l be presented. T h e w o r k o n these molecules investigated three topics: B o r n - O p p e n h e i m e r b r e a k d o w n ( H C 1 / D C 1 , H F / D F ) , i o n - p a i r f o r m a t i o n m e c h a n i s m ( H C 1 / D C 1 , H F / D F , ( H F ) 2 ) , energetics a n d d y n a m i c s of i o n - p a i r f o r m a t i o n i n t r i a t o m i c a n d p o l y a t o m i c molecules ( H C N , ( H F ) 2 ) .  1.2.1  Investigation of B o r n - O p p e n h e i m e r breakdown i n diatomic  molecules  A s discussed i n t h e p r e v i o u s section, the accurately m e a s u r e d i o n - p a i r t h r e s h o l d E°  P  T I P P S c a n be used to calculate the b o n d d i s s o c i a t i o n energy DQ{A — B). the zero p o i n t energies Ezp - 1  E  -  1  F u r t h e r m o r e , since  of m a n y d i a t o m i c molecules are k n o w n to h i g h a c c u r a c y (a fraction  of 1 c m ) , the c l a s s i c a l b o n d d i s s o c i a t i o n energy D (A r a c y of ~ l c m  from  (see F i g u r e 1.5). B y m e a s u r i n g the D (A E  — B)  c a n be d e t e r m i n e d to a n a c c u -  — B) values of isotopomers such as  H C 1 / D C 1 a n d H F / D F , the p h e n o m e n o n of B o r n - O p p e n h e i m e r b r e a k d o w n c a n be s t u d i e d for these molecules i n the g r o u n d electronic state. T h e B o r n - O p p e n h e i m e r a p p r o x i m a t i o n is of f u n d a m e n t a l i m p o r t a n c e to m o l e c u l a r spectroscopy a n d provides the basis for the analysis of molecular, s p e c t r a . T o a n a l y z e spectroscopic d a t a , one assumes t h a t there is a n i n t r a m o l e c u l a r p o t e n t i a l defined b y the electronic state of the molecule, a n d solves for t h e m o t i o n of the nuclei i n t h a t p o t e n t i a l to o b t a i n the r o v i b r a t i o n a l energy levels for t h a t electronic state.  F o r most p r a c t i c a l purposes, the B o r n - O p p e n h e i m e r  a p p r o x i m a t i o n is a v e r y g o o d one, w i t h corrections o n the order of electron-nuclear mass r a t i o ( 1 0 ) . W h e n t h e B o r n - O p p e n h e i m e r a p p r o x i m a t i o n is a p p l i e d , isotopomers such as H C 1 a n d - 3  D C 1 are assumed to have the same p o t e n t i a l energy curve for a g i v e n electronic state. However, w h e n h i g h r e s o l u t i o n spectroscopic d a t a are available for different i s o t o p o m e r s , one cannot obt a i n fits to the d a t a u s i n g t h e same p o t e n t i a l f u n c t i o n for different i s o t o p o m e r s , a n d one needs to take i n t o account the b r e a k d o w n of the B o r n - O p p e n h e i m e r a p p r o x i m a t i o n .  5 5  O n e useful m o d e l to take this b r e a k d o w n i n t o account was p r o p o s e d b y W a t s o n .  5 6  For an  isotopomer (i) of t h e d i a t o m i c molecule ( A B ) , the effective h a m i l t o n i a n is expressed as:  where //$ is the r e d u c e d mass of isotopomer i, the s u m m a t i o n is over the two a t o m i c masses for isotopomer i, U^^(R)  is the effective p o t e n t i a l energy f u n c t i o n , a n d qj(R)  is a J-dependent  f u n c t i o n a r i s i n g f r o m n o n a d i a b a t i c c o u p l i n g w i t h other electronic states a n d w h i c h represents corrections for the failure of the electrons of finite mass to follow precisely the r o t a t i o n a l a n d v i b r a t i o n a l m o t i o n of t h e n u c l e i .  H (D )-hCl +  +  EA(Cl)  I I (D )+Cl~ 4  +  IP(H)(IP(D))  H(D)+C1  HC1/DC1  F i g u r e 1.5: B o r n C y c l e for H C 1 / D C 1 B o n d E n e r g i e s . T h e i o n - p a i r t h r e s h o l d v a l u e Ej  P  c a n be  used t o c a l c u l a t e t h e b o n d d i s s o c i a t i o n energy DQ(A — B) a n d t h e c l a s s i c a l b o n d d i s s o c i a t i o n energy D  E  ( A -  B).  T h e effective p o t e n t i a l energy is given by:  U?"(R)  = U o(R) B  +  TTl UA(R) e  +  M  A  where UBO{R)  m UB(R) e  (1.12)  M  B  is the B o r n - O p p e n h e i m e r p o t e n t i a l f u n c t i o n (defined w i t h the reference  UBo(Re)  = 0), a n d MA a n d MB are the masses of the two atoms i n i s o t o p o m e r i. F o r the case where the B o r n - O p p e n h e i m e r b r e a k d o w n terms are gathered i n t o the  rrieUj(R)  J=0,  c o r r e c t i o n terms to the  B o r n - O p p e n h e i m e r p o t e n t i a l f u n c t i o n . These c o r r e c t i o n t e r m s for each a t o m i c center describe the r a d i a l v a r i a t i o n of first-order a d i a b a t i c energy a n d ./-independent second order corrections from homogeneous i n t e r a c t i o n s . T h e d e t e r m i n a t i o n of the B o r n - O p p e n h e i m e r b r e a k d o w n terms i n the effective h a m i l t o n i a n (qA(R),  qB{R),  u (R), A  V-B(R))  are t y p i c a l l y done b y a s s u m i n g a  f u n c t i o n a l f o r m , whose t e r m s are t h e n fitted to available spectroscopic  data.  5 7 , 5 8  W h i l e this  a p p r o a c h w o r k s for the regions of the p o t e n t i a l where there is available d a t a , t y p i c a l l y for the b o t t o m of the p o t e n t i a l w e l l , the e x t r a p o l a t i o n of these functions outside the range of available d a t a m a y not lead to reliable results. O n e must e x t r a p o l a t e to R — oo to d e t e r m i n e the dissoc i a t i o n energy for a g i v e n isotopomer, w h i c h is outside the range of spectroscopic d a t a . B a s e d o n t h i s m o d e l , the B o r n - O p p e n h e i m e r b r e a k d o w n i n the g r o u n d state of H C 1 / D C 1 a n d H F / D F has been s t u d i e d b y c o n s t r u c t i o n of the p o t e n t i a l energy curves f r o m e x p e r i m e n t a l d a t a a n d e x t r a p o l a t i o n to the l o n g range d i s s o c i a t i o n l i m i t . u n c e r t a i n t y quoted) a n d 1 6 ± 9 c m  - 1  5 9 , 6 0  A difference of 8 . 8 c m  was p r e d i c t e d to exist between the D  E  - 1  (no  values of H C 1 a n d  D C 1 , a n d of H F a n d D F . B y a p p l y i n g the h i g h resolution technique of T I P P S to these two pairs of isotopomers, we w o u l d be able to measure the s m a l l energy difference a n d our result w o u l d provide a n e x p e r i m e n t a l test to the p r e d i c t e d values.  1.2.2  S t u d y of ion-pair formation  mechanism  T h e m e c h a n i s m for m o l e c u l a r p h o t o i o n - p a i r f o r m a t i o n c a n be either d i r e c t or indirect:'  A-B A-B  +hu  +hu -> A+ + B~ -> A-B**  -+A+  (direct) + B-  (indirect)  In the i n d i r e c t m e c h a n i s m , A - B * * u s u a l l y is a h i g h l y e x c i t e d R y d b e r g state, w h i c h predissociates into the i o n - p a i r c o n t i n u u m , w h i l e i n the direct m e c h a n i s m the c o n t i n u u m is excited.  The  i n d i r e c t m e c h a n i s m t y p i c a l l y d o m i n a t e s , as most F r a n c k - C o n d o n accessible e x c i t e d states have very l i t t l e i o n i c character at R « R  E  . T h e process of p r e d i s s o c i a t i o n i n t o i o n - p a i r s is not well  u n d e r s t o o d t h e o r e t i c a l l y , even for simple systems. A s a n e x a m p l e , for the i o n - p a i r f o r m a t i o n i n H C 1 , single p h o t o n e x c i t a t i o n f r o m the g r o u n d state w o u l d result i n S or I I R y d b e r g states. 1  states c a n be either X U  or A T,  2  2  1  XH l  T h e ion-core for these two R y d b e r g  (see F i g u r e 1.6). Since the X H  i o n i z a t i o n t h r e s h o l d is below  2  the i o n - p a i r t h r e s h o l d , a n d the F r a n c k - C o n d o n factors to the v i b r a t i o n a l l y e x c i t e d levels of X I I 2  state are very l o w , the favorable i o n - p a i r i n t e r m e d i a t e states are either a S or 1  state w i t h a n ion-core A H. 2  between  x  1  n Rydberg  F u r t h e r m o r e , i t was reasoned i n p r e v i o u s w o r k t h a t the c o u p l i n g  l l R y d b e r g state a n d V E i o n - p a i r state is weak, so t h a t the i o n - p a i r f o r m a t i o n i n 1  H C 1 proceeds t h r o u g h T, R y d b e r g states w i t h the A T, i o n - c o r e . l  2  31  B u t i t r e m a i n s unclear w h a t  R y d b e r g series converging to w h a t r o v i b a t i o n a l levels of the A T, ion-core act as the i n t e r m e d i a t e 2  states. B y r e c o r d i n g the h i g h resolution i o n - p a i r y i e l d a n d T I P P s p e c t r a of molecules, one w o u l d o b t a i n d e t a i l e d i n f o r m a t i o n a b o u t the energetic levels of some i n t e r m e d i a t e states. T h u s the i o n - p a i r f o r m a t i o n m e c h a n i s m c o u l d be s t u d i e d . T h e s t u d y of i o n - p a i r f o r m a t i o n m e c h a n i s m is s h o w n i n the w o r k o n H C 1 / D C 1 a n d H F / D F . In the w o r k o n H F / D F , resonance enhancement was found to be especially i m p o r t a n t w i t h i o n - p a i r signals c o m p a r a b l e t o p h o t o i o n i z a t i o n signals. M a n y s h a r p resonances were observed i n the h i g h r e s o l u t i o n s p e c t r a . It w o u l d be interesting to a t t e m p t to u n d e r s t a n d a n d assign those resonant states. Besides R y d b e r g states converging to ion-core A B , the e x c i t e d valence states formed b y +  the p r o m o t i o n of a valence electron i n t o a n e x c i t e d o r b i t a l (TT* or a*) c o u l d be the second type of i n t e r m e d i a t e s t a t e .  6 2  T h e m a i n difference between the e x c i t e d valence state a n d R y d b e r g  state is t h a t the former one has a larger internuclear distance t h a n t h a t of the i o n core i n the Rydberg state.  6 3  T h i s t y p e of m e c h a n i s m is even less u n d e r s t o o d a n d was found to p l a y a n  i m p o r t a n t role i n t h e processes of SF  6  + hv - » 5 F  5  +  + F~  a n d OCS  + hv -> CO  +  +  S~. '  64 65  F o r the p r o d u c t i o n of i o n A A  +  + B~,  +  f r o m molecule A B , besides the i o n - p a i r process AB + hv —>  the d i s s o c i a t i v e i o n i z a t i o n process AB + hv —> A  +  + B + e~ c a n also y i e l d the i o n A . +  T h e thresholds of the two processes are different b y the e l e c t r o n affinity of fragment B ( E A ( B ) ) . N o r m a l l y i n o u r i o n - p a i r e x p e r i m e n t s the p h o t o n energy is b e l o w the dissociative i o n i z a t i o n t h r e s h o l d , a n d the o n l y c h a n n e l to produce A  +  is t h r o u g h the i o n - p a i r c h a n n e l . However, if the  p h o t o n energy is above the dissociative i o n i z a t i o n t h r e s h o l d , i t w o u l d be i n t e r e s t i n g to check w h i c h process is the m a i n c h a n n e l . T h i s is s h o w n i n the w o r k of c h a p t e r 6 ( p r o d u c t i o n of H F H  +  f r o m ( H F ) 2 ) . U s u a l l y the d i s s o c i a t i v e i o n i z a t i o n process has a m u c h larger cross section t h a n the i o n - p a i r process. F o r e x a m p l e , the p h o t o d i s s o c i a t i o n of N 2 O y i e l d s a b o u t 1000 t i m e s of NjJ" as m u c h as  1.2.3  0~.  6 5  Ion-pair formation in triatomic and polyatomic  molecules  I o n - p a i r f o r m a t i o n i n t r i a t o m i c a n d p o l y a t o m i c molecules is even less u n d e r s t o o d t h a n d i a t o m i c molecules for several reasons. F i r s t , the i o n - p a i r cross sections for larger molecules ( 1 0 ~  3  or less  c o m p a r e d to p h o t o i o n i z a t i o n cross sections) are u s u a l l y lower t h a n i n d i a t o m i c molecules ( ~ 1 0 c o m p a r e d to p h o t o i o n i z a t i o n cross sections).  - 2  Second, the e x c i t e d states of larger molecules at  h i g h energies near t h e i r i o n - p a i r thresholds are u s u a l l y not k n o w n .  T h u s it w o u l d be  difficult to s t u d y the i o n - p a i r f o r m a t i o n m e c h a n i s m i n larger molecules.  more  T h i r d , for larger  molecules, one or b o t h of the i o n - p a i r fragments have more t h a n one a t o m a n d t h e y c a n be i n different r o v i b r o n i c levels, t h e r e b y m a k i n g the i o n - p a i r s p e c t r u m m o r e c o m p l i c a t e d to i n t e r p r e t . S h o w n i n F i g u r e 1.7 is the i o n - p a i r f o r m a t i o n process i n H C N . T h e p a r e n t molecule H C N c a n be at different r o t a t i o n a l levels i n the g r o u n d state even for a low r o t a t i o n a l t e m p e r a t u r e of m o l e c u l a r b e a m . A f t e r p h o t o e x c i t a t i o n , the C N ~ fragment c a n also be formed i n different states, a n d the C N ~ r o t a t i o n a l n u m b e r J ' is not necessarily r e l a t e d to the H C N r o t a t i o n a l n u m b e r J " since there are no specific a n g u l a r m o m e n t u m selection rules for p h o t o d i s s o c i a t i o n into fragments. T h e r e f o r e , there are m a n y different t r a n s i t i o n c o m b i n a t i o n s , a n d the energetic l i m i t s of a l l those t r a n s i t i o n s c a n be present i n the s p e c t r u m . A l t h o u g h i t w o u l d be u n l i k e l y to resolve every single peak, a h i g h r e s o l u t i o n T I P P s p e c t r u m is expected to show the energetic p a t t e r n of i o n - p a i r f o r m a t i o n i n t r i a t o m i c a n d p o l y a t o m i c  HCN  F i g u r e 1.7: I o n - p a i r f o r m a t i o n i n H C N . T h e r o t a t i o n a l energy levels H C N a n d C N  are s h o w n .  molecules.  F r o m the s i g n a l intensities at different energies, i t is also possible to s t u d y the  relative cross sections of dissociations i n t o fragments i n different levels. P r e v i o u s l y T I P P S was a p p l i e d to the t r i a t o m i c molecule of H 2 S . B o t h i o n - p a i r channels H  +  4- S H  -  and H ~ + S H  +  were s t u d i e d .  channels were measured to be 1 2 2 4 5 8 ± 3 c f n  2 , 6 6  _1  T h e field-free i o n - p a i r thresholds for these two  a n d 1 0 9 4 2 1 ± 5 c m , w h i c h y i e l d e d the same value _ 1  for the b o n d d i s s o c i a t i o n energy j D o ( H - S H ) w i t h i n e x p e r i m e n t a l u n c e r t a i n t y ( 3 1 4 4 6 ± 3 c m ~  1  a n d 3 1 4 4 7 ± 6 c m ~ ) . T h e results revealed different d y n a m i c s between the two i o n - p a i r channels. 1  W h i l e the S H  fragment i n the first channel was formed at low r o t a t i o n a l levels ( J <4), the S H  -  +  fragment i n the second c h a n n e l c o u l d be v i b r a t i o n a l l y e x c i t e d w i t h c o m p a r a b l e cross sections to v'=0  and  v'=l.  I n t h i s thesis p r o j e c t , T I P P S was a p p l i e d to two more t r i a t o m i c or p o l y a t o m i c  molecules  ( H C N a n d ( H F ) 2 ) , a n d the results are presented i n chapters 5 a n d 6.  References 1. M a r t i n J D D a n d H e p b u r n J W 1997 Phys. 2. H e p b u r n J W Chemistry  2003  The Encyclopedia  Lett. 79 3154  Rev.  of Mass  Spectrometry,  Vol 1:  Theory  and  Ion  e d P B A r m e n t r o u t (New Y o r k : E l s e v i e r ) p p 241-248  3. M i i l l e r - D e t h l e f s K , Sander M a n d Schlag E W 1984 Chem. 4. Z h u L a n d J o h n s o n P 1991 J. Chem.  Phys.  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T h e s a m p l e gas is i n t r o d u c e d b y a p u l s e d m o l e c u l a r b e a m i n t o the p h o t o e x c i t a t i o n r e g i o n u n d e r h i g h v a c u u m . 3. A n i o n d e t e c t i o n s y s t e m . A f t e r p h o t o d i s s o c i a t i o n , the i o n fragments are e x t r a c t e d into a T i m e - o f - F l i g h t mass spectrometer. D e t e c t i o n signals are a q u i r e d b y a L a b V i e w p r o g r a m . T h e basic c o m p o n e n t s of the a p p a r a t u s used i n t h i s p r o j e c t were m o v e d f r o m U n i v e r sity of W a t e r l o o i n 2001. T h e o r i g i n a l version of the a p p a r a t u s was b u i l t fifteen years ago for P F I - Z E K E experiments.  1  L a t e r o n some changes were made o n the v a c u u m s y s t e m a n d T O F  spectrometer for T I P P S e x p e r i m e n t s . D u r i n g the p e r i o d of t h i s thesis w o r k , the i o n detection 2  s y s t e m was m o d i f i e d , a n d new d a t a a c q u i s i t i o n software, L a b V i e w ( L a b o r a t o r y V i r t u a l I n s t r u ment E n g i n e e r i n g W o r k b e n c h ) ,  3  was i n s t a l l e d .  I n t h i s c h a p t e r a l l the m a j o r p a r t s of the a p p a r a t u s w i l l be discussed.  2.2  Laser system and V U V generation  F o r a l l the molecules s t u d i e d i n t h i s project, the i o n - p a i r thresholds lie above l O e V . T o p u m p the molecules f r o m t h e i r g r o u n d states t o their i o n - p a i r states b y single p h o t o n e x c i t a t i o n ( m u l t i p h o t o n e x c i t a t i o n requires i n t e r m e d i a t e resonances a n d c a n also p r o d u c e i n t e r f e r i n g n e u t r a l a n d ionic fragments t h r o u g h other r e a c t i o n channels), the w a v e l e n g t h of t h e l i g h t has to be i n the  F i g u r e 2.1: S c h e m a t i c of the e x p e r i m e n t a l setup. D e t a i l s of the T O F spectrometer are shown i n F i g u r e 2.2.  V U V region. A t u n a b l e laser t h a t operates at such short w a v e l e n g t h is not yet available. O n the other h a n d , the technology of visible dye lasers is w e l l developed. T h r o u g h the c o m b i n a t i o n of n o n l i n e a r optics a n d the technique of four-wave m i x i n g , a t u n a b l e V U V laser b e a m c a n be generated f r o m dye laser o u t p u t s .  2.2.1  N d - . Y A G pulsed dye  lasers  T h r o u g h o u t t h i s p r o j e c t t h e two dye lasers were p u m p e d b y the second a n d t h i r d h a r m o n i c o u t p u t of a S p e c t r a P h y s i c s N d : Y A G laser ( G C R - 4 ) . T h e N d : Y A G laser produces very short 4  (~8ns) laser pulses at 1064nm at a r e p e t i t i o n rate of 1 0 H z . T h e 1064nm light is d o u b l e d b y a K D P (KH2PO4) c r y s t a l to make the second h a r m o n i c at 5 3 2 n m , w h i c h c a n t h e n be further m i x e d w i t h the f u n d m e n t a l i n a second K D P c r y s t a l to generate the t h i r d h a r m o n i c at 355nm. T h e p u m p i n g scheme c a n be any of the possible configurations (532-355, 532-532, or 355-355) d e p e n d i n g o n the desired o u t p u t s from the dye lasers. N o r m a l l y the first c o n f i g u r a t i o n was a p p l i e d , a n d the energy of each p u m p pulse was adjusted to be a r o u n d l O O m J . E a c h of the t w o L a m b d a P h y s i k dye lasers ( M o d e l F L 3 0 0 2 ) has one oscillator, one p r e a m 5  plifier a n d one a m p l i f i e r .  T h e oscillator c a v i t y consists of one g r a t i n g ( 6 0 0 g r o o v e s / m m ) , a  cuvette w i t h f l o w i n g dye s o l u t i o n , a n d a n end m i r r o r . T h e dye laser c o u l d be t u n e d b y r o t a t i n g the angle between the g r a t i n g n o r m a l a n d c a v i t y axis. T h e exact l a s i n g w a v e l e n g t h c o u l d be c a l i b r a t e d to a p r e c i s i o n of 0.005nm u s i n g a hollow cathode o p t o g a l v a n i c l a m p of U or Fe filled w i t h noble gas N e or A r .  6  T h e dye molecules are generally organic c o m p o u n d s w i t h h i g h fluorescence efficiency  7  The  lasing w a v e l e n g t h for a given dye m a y be v a r i e d over tens of n m , d e p e n d i n g o n the dye p r o p e r ties a n d other c o n d i t i o n s . W h e n necessary two different dye solutions c o u l d be m i x e d to e x t e n d the t u n i n g range. A f t e r b e i n g c o u p l e d out of the oscillator cavity, the laser b e a m passes t h r o u g h the p r e a m p l i fier a n d m a i n a m p l i f i e r . T h e power is a m p l i f i e d b y a factor of a b o u t 10 b y the m a i n amplifier. N o r m a l l y the o u t p u t from the dye laser is about 1 5 - 3 0 m J / p u l s e . T h e frequency of t h e dye laser o u p u t c o u l d be d o u b l e d b y a K D P or B B O (/3-BaB204) c r y s -  t a l , d e p e n d i n g o n the w a v e l e n g t h .  B B O c r y s t a l s w o r k b e t t e r at shorter w a v e l e n g t h w i t h the  8  f r e q u e n c y - d o u b l i n g l i m i t at 2 0 5 n m . W h e n even shorter wavelengths ( ~ 2 0 0 n m ) are necessary, 9  the second h a r m o n i c of the dye laser o u t p u t can be m i x e d w i t h the r e s i d u a l f u n d m e n t a l i n a second B B O c r y s t a l .  1 0  I n t h i s w a y the use of B B O c r y s t a l c o u l d be e x t e n d e d d o w n to 197nm  at r o o m t e m p e r a t u r e , a n d 194.4nm i n a cooled c r y s t a l .  1 1  A f t e r frequency d o u b l i n g or even t r i p l i n g the two b e a m s are o v e r l a p p e d b o t h t e m p o r a l l y a n d s p a t i a l l y b y a d i c h r o i c m i r r o r . F o r effective o v e r l a p p i n g , the two b e a m s need to arrive at the same spot o n the d i c h r o i c m i r r o r simultaneously. T h i s means the l i g h t p a t h difference between the two laser b e a m s need to be m i n i m i z e d , n o r m a l l y i t is as s m a l l as a few c m .  The  d i c h r o i c m i r r o r is c o a t e d for h i g h reflection of a s m a l l range of w a v e l e n g t h ( ~ 2 0 n m ) a n d h i g h t r a n s m i s s i o n for other wavelengths. I n our e x p e r i m e n t one w a v e l e n g t h is fixed (2L>I corresponds to one t w o - p h o t o n resonant state of K r or X e ) , w h i l e the other w a v e l e n g t h is scanned. N o r m a l l y the fixed w a v e l e n g t h is reflected b y the d i c h r o i c m i r r o r . A f t e r o v e r l a p p i n g , the two laser beams are focused b y a convex lens ( f « 2 5 c m for v{)  to  interact w i t h a p u l s e d j e t of K r or X e gas to generate V U V p h o t o n s .  2.2.2  G e n e r a t i o n of V U V photons by four-wave  mixing  T h e t h e o r y of four-wave m i x i n g is based o n the n o n l i n e a r response of n o n m a g n e t i c m a t e r i a l s to s t r o n g laser electric fields. p o l a r i z a t i o n response, a p p l i e d field,  P(OJ)  where the x^  W h e n a n electric field is a p p l i e d to a m e d i u m , the i n d u c e d c a n be expressed as a T a y l o r series e x p a n s i o n i n t e r m s of t h e  E{w):  12  =  x  ( 1 )  • E(w)  + x  {2)  • E(OJ) • E(u)  + x  ( 3 )  • E{w)  •  E{u)  •  E(u)  + ...  (2.1)  tensor is the n t h order s u s c e p t i b i l i t y of the m e d i u m , a n d as n increases the  value decreases s u b s t a n t i a l l y . U n d e r o r d i n a r y c o n d i t i o n s , for e x a m p l e , w h e n a cw light source is used, we o n l y need to consider the linear t e r m x^  • E(ui).  H o w e v e r , under the s t r o n g  electric fields p r o d u c e d b y a p u l s e d laser, the n o n l i n e a r terms c a n b e c o m e significant a n d result i n the g e n e r a t i o n of frequencies at linear c o m b i n a t i o n s of the a p p l i e d frequencies.  For media  w i t h a center of s y m m e t r y , s u c h as K r a n d X e gas, the even order t e r m s are a l l zero, a n d the first n o n l i n e a r t e r m is the t h i r d order response. F o r three i n p u t frequencies v\, v  2  a n d V3, the  o u t p u t frenquency voutput c a n be a n y linear c o m b i n a t i o n s of the i n p u t frequencies. T h e o u t p u t frequency, together w i t h the three i n p u t fundmentals, oscillate i n the n o n l i n e a r m e d i u m , so this t y p e of frequency m i x i n g is c a l l e d four-wave m i x i n g . T h e specific t y p e of four-wave m i x i n g used i n t h i s p r o j e c t is t w o - p h o t o n resonant s u m frequency m i x i n g , w h i c h involves two i n p u t frequencies vi a n d u , a n d the generated frequency 2  is voutput at 2v\ + v .  T h e i n t e n s i t y of the o u t p u t frequency is d e s c r i b e d b y :  2  l a , = (x ) llju N F(bAk) [3) 2  2  (2.2)  2  where x ^ has the f o r m : ZgjZjjZjkZkg  (3) _ X  Here I ,  J„  Ul  2  h  3  ^  {iUg - vi)(Sl  jg  - 2v ){Sl l  - 2v  kg  x  -v )  K  2  •  ]  are the intensities of the i n p u t frequencies, a n d N is the n u m b e r density of the  n o n - l i n e a r m e d i u m . T h e phase m a t c h i n g factor F(bAk)  is a f u n c t i o n of bAk,  where b is the  confocal b e a m p a r a m e t e r of the focussed light, a n d Afc is the wavevector difference between the i n p u t a n d o u t p u t light (Afc =  fc(twtput)  — 2k{v{) — k(v ). 2  T h e value of F(bAk)  is n o n  zero o n l y i f the refractive i n d e x at v output is less t h a n the refractive i n d e x at v\ a n d v .  Such  2  b e h a v i o r is c a l l e d negative d i s p e r s i o n (in the n o r m a l case the refractive i n d e x increases w i t h the frequency), a n d regions of negative d i s p e r s i o n are f o u n d to the b l u e of resonance lines a n d above the i o n i z a t i o n p o t e n t i a l i n gases.  D u e to the pressure dependence of Afc, one cannot  s i m p l y increase N to i m p r o v e the o u t p u t intensity, b u t the N F(bAk) 2  t e r m as a whole.  F o r the x^ ^ t e r m , i n general, the more p o l a r i z a b l e the n o n l i n e a r m e d i u m is, the larger the 3  value x ^ ' w i l l be, so u s u a l l y rare gases such as K r or X e or m e t a l vapors are used as the 3  m e d i u m for four-wave m i x i n g . I n e q u a t i o n (2.3), Z  are the d i p o l e m a t r i x elements, fl g  are  the c o m p l e x t r a n s i t i o n frequencies for the x <— g t r a n s i t i o n s . E q u a t i o n shows t h a t the  x ^  xy  X  value c a n be g r e a t l y e n h a n c e d w h e n the i n p u t frequency, s u c h as 2is\, is t w o - p h o t o n resonant w i t h a t r a n s i t i o n i n the n o n l i n e a r m e d i u m . I n t h i s p r o j e c t , one i n p u t frequency v\ is chosen a n d fixed so t h a t 2v\ corresponds to one  T a b l e 2.1: F o u r - w a v e m i x i n g schemes i n t h i s p r o j e c t . Molecule  V U V energy / e V  HC1/DC1  14.36-14.61  M e d i u m resonance energy / c m X e 5 p 6 p ' [ l / 2 , 0] 89860.538  v\ / n m  1  2  /nm  222.57  360-380  X e 5 p 6 p [ l / 2 , 0] 80119.474  249.63  265-280  5  .  v  5  HF/DF  15.97-16.16  K r 4 p 5 p [ l / 2 , 0 ] 94093.662  212.55  276-288  HCN  15.10-15.25  K r 4 p 5 p [ l / 2 , 0 ] 94093.662  212.55  346-361  14.74-15.14  X e 5 p 6 p ' [ l / 2 , 0] 89860.538  222.57  310-345  15.11-15.86  K r 4 p 5 p [ l / 2 , 0 ] 94093.662  212.55  295-360  (HF)  5  5  5  2 5  t w o - p h o t o n resonance state i n the the rare gas. T h e other i n p u t frequency u is scanned so t h a t 2  the o u t p u t frenquency at 2v\ + u  2  is t u n a b l e . T h e setup of v\ a n d v , 2  a n d the energy range  of the generated V U V p h o t o n s for the different molecules s t u d i e d i n t h i s p r o j e c t are t a b u l a t e d i n T a b l e 2.1. T o generate V U V photons i n those energy ranges, a w i n d o w e d gas cell does not w o r k since no m a t e r i a l of s t r u c t u r a l thickness is t r a n s p a r e n t at s h o r t wavelengths. Therefore, a p u l s e d jet of K r or X e was used i n t h i s project. T h e K r or X e a t o m s at a b a c k i n g pressure of l - 2 b a r are i n t r o d u c e d i n t o the V U V chamber f r o m a p u l s e d valve ( G e n e r a l V a l v e ) w i t h 1 m m 1 3  d i a m e t e r nozzle l o c a t e d ~ 5 c m away f r o m the i n p u t laser beams. T h e V U V w a v e l e n g t h was c a l i b r a t e d b y r e c o r d i n g o p t o g a l v a n i c s p e c t r a i n a h o l l o w cathode discharge to c a l i b r a t e t h e t u n a b l e laser v , 2  i n the r e l a t i o n s h i p vvuv  =  + v. 2  6  a n d u s i n g the k n o w n X e resonance energy for 2v\  T h e u n c e r t a i n t i e s i n t h i s c a l i b r a t i o n come f r o m a s m a l l  u n c e r t a i n t y i n the a c t u a l value of 2v\, w h i c h c a n be s l i g h t l y different f r o m the t a b u l a t e d value due t o power b r o a d e n i n g of the X e resonance, a n d oscillations i n the c a l i b r a t i o n of the dye laser due to the m e c h a n i c s of the g r a t i n g drive (about ± 0 . 0 0 5 n m w i t h a p e r i o d of 1.5nm). effects c o m b i n e d give a n e s t i m a t e d error of ± 0 . 3 c m ~  1  i n our V U V c a l i b r a t i o n .  2  These  A f t e r the four-wave m i x i n g process, the generated V U V beams, together w i t h the f u n d mentals, propogate  i n t o a one-meter n o r m a l incidence m o n o c h r o m a t o r ,  w h i c h disperses a n d  refocuses the selected V U V b e a m i n t o the r e a c t i o n chamber to interact w i t h a molecular b e a m of the sample gas. A f t e r the i n t e r a c t i o n region the V U V i n t e n s i t y is recorded b y a m i c r o c h a n n e l plate detector.  2.3  Molecular beam and vacuum system  I n our e x p e r i m e n t s , t h e s a m p l e molecules are i n t r o d u c e d i n t o the r e a c t i o n c h a m b e r i n a pulsed m o l e c u l a r b e a m . O n e advantage of molecular b e a m is t h a t i t c a n achieve h i g h m o l e c u l a r d e n sity i n the r e a c t i o n region w i t h a r e l a t i v e l y low b a c k g r o u n d pressure. T h e sample molecules are e m i t t e d f r o m a p u l s e d valve ( G e n e r a l Valve) w i t h a 1 m m d i a m e t e r nozzle l o c a t e d 5 c m away from the V U V b e a m . T h e b a c k i n g pressure is u s u a l l y l - 2 b a r . T h e m a i n chamber is p u m p e d by a l O O O L / s t u r b o p u m p a n d the o p e r a t i o n pressure is o n the order of 1 0 t o r r w i t h a back_ 6  g r o u n d of 1 0 t o r r . T h e V U V chamber is under 1 0 - 1 0 t o r r w i t h the p u m p i n g of a two-stage _ 7  _ 3  _ 2  booster p u m p . T h e pressure i n the buffer zone between V U V c h a m b e r a n d the m o n o c h r o m a tor is a b o u t 1 0 t o r r , a n d the m o n o c h r o m a t o r itself is m a i n t a i n e d at 1 0 t o r r to protect the - 5  _ 7  g r a t i n g i n s t a l l e d inside f r o m c o n t a m i n a t i o n . D u e to the large pressure difference between the b a c k i n g gas source a n d the v a c u u m c h a m ber, the gas molecules e x p a n d very q u i c k l y i n t o the v a c u u m c h a m b e r w h e n the nozzle is open. D u r i n g the course of the e x p a n s i o n , the molecules collide w i t h each other a n d the o r i g i n a l l y r a n d o m m o l e c u l a r velocities become more a n d more aligned a l o n g the e x p a n s i o n axis as the m o l e c u l a r i n t e r n a l energy is converted i n t o t r a n s l a t i o n a l energy. A t some p o i n t of the e x p a n sion, the m o l e c u l a r b e a m becomes c o l l i s i o n free. F o r p h o t o e x c i t a t i o n at t h i s p o i n t , the p r o d u c t s p a t i a l a n d i n t e r n a l energy d i s t r i b u t i o n is not p e r t u r b e d b y i n t e r m o l e c u l a r reactions. T h e average speed t h a t the molecules finally reach, VQQ, c o u l d be c a l c u l a t e d f r o m :  1 4  (2.4)  where 7 = C /C p  v  (C ,C P  are the heat capacities of the gas species at constant pressure a n d  V  v o l u m e , r e s p e c t i v e l y ) , k is B o l t z m a n n ' s constant, m is the m o l e c u l a r weight of the sample gas, a n d To is the t e m p e r a t u r e of the gas reservoir. Since  is often greater t h a n the s o u n d velocity  i n the sample gas, the m o l e c u l a r b e a m is also designated as " s u p e r s o n i c m o l e c u l a r b e a m " . B y c o n v e r t i n g the i n t e r n a l energy i n t o t r a n s l a t i o n a l energy, t h i s t e c h n i q u e c a n lower the r o t a t i o n a l energy s u b s t a n t i a l l y a n d t h u s the n u m b e r of r o t a t i o n a l levels o c c u p i e d b y the molecules is m u c h less t h a n at r o o m t e m p e r a t u r e . I n this way the molecular s p e c t r a of m a n y species c o u l d be simplified and become interpretable. T h e n u m b e r d e n s i t y of sample molecules i n the i n t e r a c t i o n region c a n be a p p r o x i m a t e l y calculated according t o :  1 4  n = ^ -  2  (2.5)  where N is the flow rate (number of emerging molecule p e r u n i t t i m e ) , r is the distance from the nozzle orifice. T h e N value c a n be regarded as the average pressure i n the r e a c t i o n chamber m u l t i p l i e d b y the speed of the t u r b o p u m p . T h e t y p i c a l n u m b e r d e n s i t y i n our experiment is e s t i m a t e d to be o n the order of 1 0 / c m . 1 3  3  2  If a s k i m m e r is m o u n t e d i n the e x p a n s i o n p a t h , o n l y a c e n t r a l p o r t i o n of the molecular b e a m c a n go t h r o u g h . I n t h i s w a y the b e a m w i l l become even m o r e c o l l i m a t e d a n d allow further s i m p l i f i c a t i o n of the s p e c t r a . However, the distance between the nozzle a n d the r e a c t i o n region w i l l become m u c h larger w h e n a s k i m m e r is m o u n t e d o n the way. F r o m e q u a t i o n 2.5, the gas d e n s i t y decreases s u b s t a n t i a l l y at larger distance, t h u s the i o n s i g n a l w i l l become even weaker. F o r o u r e x p e r i m e n t s p r o b i n g i o n - p a i r processes, the cross sections are u s u a l l y at least two orders of m a g n i t u d e lower t h a n the p h o t o i o n i z a t i o n process p r o d u c i n g parent ions.  To  o b t a i n i o n - p a i r signals at detectable levels, no s k i m m e r was used.  2.4  Detection of the ion-pair signal  C o n c e p t u a l l y , the T I P P S e x p e r i m e n t a l setup is s i m i l a r to M A T I ( m a s s - a n a l y z e d t h r e s h o l d ionization) spectroscopy.  15  F o r M A T I spectroscopy, the molecules are e x c i t e d t h r o u g h either  s i n g l e - p h o t o n or m u l t i - p h o t o n process to the h i g h l y e x c i t e d R y d b e r g states, AB  +  - e~, t h e n  a sequence of two electric pulses is a p p l i e d . T h e first pulse d i s c r i m i n a t e s against any p r o m p t ions f o r m e d f r o m above-threshold processes. R y d b e r g states a n d the t h r e s h o l d ions A B  T h e second pulse field ionizes t h e w e a k l y b o u n d are detected b y a t i m e of flight mass spectrometer.  +  F o r T I P P S spectroscopy, t h e molecules are excited by single V U V p h o t o n s t o the h i g h l y v i b r a t i o n a l l y e x c i t e d i o n - p a i r states A t h r e s h o l d ions A  2.4.1  +  +  — B~, a n d two electric pulses are a p p l i e d to detect o n l y the  (or B ) f o r m e d f r o m field d i s s o c i a t i o n of t h e i o n - p a i r states. _  Time-of-Flight  spectrometer.  T h e T O F mass s p e c t r o m e t e r used for t h i s project is s h o w n i n F i g u r e 2.2. T h i s system c o u l d be used to detect b o t h p o s i t i v e a n d negative ions, b y s w i t c h i n g the p o l a r i t y of the e x t r a c t i n g a n d d e t e c t i n g voltages.  I n the detection s y s t e m there are four nickel m e s h electrode plates  ( P 1 , P 2 , P 3 , P 4 ) . T h e size of each plate is 7 x 5 c m  2  a n d the distance between adjacent plates is  about 2 c m . T h e V U V laser b e a m crosses the molecular b e a m i n the e x c i t a t i o n region between P 2 a n d P 3 . T h e two beams a n d the axis of the spectrometer are o r t h o g o n a l to each other. F o r t o t a l i o n - p a i r y i e l d s p e c t r a , one p u l s e d e x t r a c t i o n field is a p p l i e d to the e x c i t a t i o n region a few fis after p h o t o e x c i t a t i o n to e x t r a c t b o t h p r o m p t a n d t h r e s h o l d ions i n t o the T O F spectrometer (see F i g u r e 2.3).  F o r a T I P P s p e c t r u m , a s m a l l p u l s e d d i s c r m i n a t i o n field, u s u a l l y about 2-  l O V / c m is a p p l i e d at a delay t i m e of ~ 3 0 0 n s after p h o t o e x c i t a t i o n to d r i v e away any p r o m p t ions formed f r o m above t h r e s h o l d processes; t h e n at a delay t i m e of a few /xs, a second electric pulse of larger m a g n i t u d e ( u s u a l l y o n the order of tens of V / c m ) a n d opposite p o l a r i t y is a p p l i e d to field dissociate the l o n g l i v e d h i g h l y v i b r a t i o n a l l y excited i o n - p a i r states a n d e x t r a c t the t h r e s h o l d , ions i n t o the T O F t u b e .  T o collect T I P P s p e c t r a of every specific molecule s t u d i e d i n this  project ( H C 1 / D C 1 , H F / D F , H C N a n d (HF)2), the d i s c r i m i n a t i o n field m a g n i t u d e a n d relative delay t i m e of the two pulses were adjusted to guarantee complete d i s c r i m i n a t i o n against the p r o m p t ions. F u r t h e r m o r e , p o s i t i v e ions were recorded for a l l the molecules since t h e y u s u a l l y give r e l a t i v e l y higher s i g n a l to noise r a t i o t h a n the c o r r e s p o n d i n g negative i o n . F o r molecules H C 1 / D C 1 , H F / D F a n d H C N , the H / D +  +  i o n c a n o n l y come f r o m the i o n - p a i r c h a n n e l since  MCP  TOF  steering P4 P3  > HFbeam P2 PI F i g u r e 2.2: T i m e - o f - F l i g h t mass spectrometer.  the V U V energy, scanned over a n a r r o w range (see T a b l e 2.1) a r o u n d the i o n - p a i r threshold was not enough to excite the molecules to the dissociative i o n i z a t i o n c h a n n e l A B —> A e . _  +  + B +  F o r ( H F ) 2 , t h e V U V energy was scanned over a large range, a n d b o t h the i o n - p a i r channel  a n d the dissociative i o n i z a t i o n c h a n n e l are possible. Therefore, t h e negative signals ( F ~ a n d e~) also needed to be collected to investigate w h i c h channel the positive i o n H F H  +  originated  from. T h e space between P 3 a n d P 4 is the accelerating region.  T h e voltages a p p l i e d to t h i s  region were a d j u s t e d relative t o the e x t r a c t i o n field to satisfy the W i l e y - M c L a r e n c o n d i t i o n for space f o c u s i n g .  16  S m a l l voltages (a few V / c m ) were a p p l i e d to the steering electrodes after the  acceleration region to adjust the t r a j e c t o r y of the ions to o p t i m i z e the s i g n a l i n t e n s i t y from the m u l t i c h a n n e l plates at t h e e n d of the flight tube. T h e next stage is the field free T O F (time-of-flight) t u b e . I t ' s a r a t h e r s i m p l e design of a straight t u b e a b o u t 4 0 c m long. T h e mass r e s o l u t i o n of the spectrometer is a b o u t 100, w h i c h is enough to separate ions i n v o l v e d i n the i o n i z a t i o n a n d d i s s o c i a t i o n processes of s m a l l molecules i n our e x p e r i m e n t s . If higher resolution is needed for larger molecules i n the future, a more c o m p l e x T O F arrangement s u c h as a reflectron s y s t e m c o u l d be i n s t a l l e d .  1 7  F i n a l l y the ions h i t the front of the M C P ( m i c r o c h a n n e l plate) detector w h i c h is a stack of two pieces. E a c h M C P consists of a p a r a l l e l array of c h a n n e l electron m u l t i p l i e r s capable of p h o t o n , i o n , a n d electron detection a n d a m p l i f i c a t i o n . F o r n o r m a l o p e r a t i o n , a h i g h voltage of about 1000V is a p p l i e d to each M C P , w i t h the voltage of the o u t p u t surface p o s i t i v e relative to the i n p u t surface. F o r s u c h c o n d i t i o n one M C P c a n generate u p to 1 0 electrons for one incident 4  i o n . W h e n two M C P s are w i r e d i n series, a n a m p l i f i c a t i o n factor of 1 0 c a n be achieved. T h e 7  electron pulse generated b y the incidence of ions is collected b y a n anode, a n d the current t h e n generates a detectable voltage t h r o u g h a 50f2 resistor. T h e signals were sent to a boxcar ( S R 2 5 0 , S t a n f o r d R e s e a r c h S y s t e m s ) where t h e y were gated a n d i n t e g r a t e d . T h e gate w i d t h was set 18  at 20-50ns to m a t c h the F W H M of the T O F ion peak. T h e i n t e g r a t e d s i g n a l was displayed i n an oscilloscope a n d sent to the c o m p u t e r for d a t a collection. T h e i o n s i g n a l was recorded as a f u n c t i o n of the V U V energy, w h i c h was t u n e d by s c a n n i n g the wavelength of one of the dye lasers. B o t h laser s c a n n i n g a n d i o n signal recording were c o n t r o l l e d by a L a b V I E W p r o g r a m .  1  __l  Excitation  Total Yield  e ©  A  Discrimination  >  M S  Extraction  „ ©  o  „  3 3  „ ®  ©  w  F i g u r e 2.3: D e t e c t i o n scheme of the t h r e h o l d i o n - p a i r s i g n a l . A sequence of t w o field pulses is a p p l i e d . T h e b i g circles represent molecules at h i g h - ! / i o n - p a i r states, w h i l e s m a l l circles are the i o n fragments.  2.4.2  LabV I E W  program  T h e software of L a b V I E W  3  is a development e n v i r o n m e n t based o n the g r a p h i c p r o g r a m m i n g  language G w h i c h uses t e r m i n o l o g y , icons a n d relies o n g r a p h i c s y m b o l s r a t h e r t h a n t e x t u a l language to describe p r o g r a m m i n g actions. L a b V I E W c o n t a i n s comprehensive l i b r a r i e s for d a t a c o l l e c t i o n , a n a l y s i s , p r e s e n t a t i o n a n d storage. A l l lab V I E W p r o g r a m s have a front p a n e l a n d a b l o c k d i a g r a m . T h e front p a n e l is the g r a p h i c user interface w h i c h c o n t a i n s k n o b s , graphs a n d other controls.  T h e b l o c k d i a g r a m contains the g r a p h i c source code of the file w h i c h is  to c o n t r o l the i n p u t s a n d o u t p u t s on the front p a n e l .  T o scan t h e dye laser, the lab V I E W  c o m m a n d is sent t h r o u g h a G P I B cable to the I E E E - 4 8 8 interface of the dye laser. F o l l o w i n g the s c a n n i n g of the w a v e l e n g t h , the angle of the d o u b l i n g c r y s t a l is also r o t a t e d at the same t i m e a c c o r d i n g to the b u i l t - i n r e l a t i o n to give m a x i m u m power of second h a r m o n i c frenquency. A t each laser shot, the i n t e g r a t e d o u t p u t of i o n signal f r o m the b o x c a r is converted b y an A / D c a r d to a d i g i t a l s i g n a l , w h i c h is t h e n w r i t t e n to a d a t a file t h r o u g h the L a b V I E W p r o g r a m for further a n a l y s i s .  2.4.3  Ion-counting  technique  F o r some molecules s u c h as H C 1 , a l t h o u g h the i o n - p a i r cross section is m u c h lower t h a n p h o t o i o n i z a t i o n , there are s t i l l n u m e r o u s t h r e s h o l d ions ( H  +  a n d CT~) generated for each laser shot  w h e n the V U V energy corrsponds to the difference between c e r t a i n levels of the g r o u n d state a n d the i o n - p a i r l i m i t s . I n s u c h cases a general analogue t e c h n i q u e c o u l d be a p p l i e d ; t h a t is, the V U V energy was s c a n n e d at a speed of less t h a n 1 c m  - 1  per step, a n d the t h r e s h o l d ions  generated at each step were collected b y s e t t i n g the b o x c a r gate to t h e correct delay, a n d a d j u s t i n g the b o x c a r s e n s i t i v i t y to ensure t h a t the s i g n a l was not s a t u r a t e d . T h e averaged o u t p u t voltage of the b o x c a r t h e n reflected d i r e c t l y the n u m b e r of t h r e s h o l d ions generated, a n d was p l o t t e d as a f u n c t i o n of the V U V energy. However, for larger molecules such as H C N , the i o n - p a i r cross section is e x t r e m e l y low t h a t there is less t h a n one t h r e s h o l d i o n p r o d u c e d per laser shot ( s t a t i s t i c a l l y there was o n l y one t h r e s h o l d i o n for every one h u n d r e d laser shots at the most intense peak i n H C N T I P P  spectrum).  If general analogue d e t e c t i o n were used, at each step there w o u l d either be just  one t h r e s h o l d i o n or no s i g n a l at a l l . T h i s weak s i g n a l w o u l d be v e r y h a r d to detect from the b a c k g r o u n d noise, a n d w o u l d not reflect the relative cross sections t o i o n - p a i r states at different energies.  T o i m p r o v e the signal level, the i o n - c o u n t i n g technique was a p p l i e d .  The  V U V is also s c a n n e d , b u t at each energy p o s i t i o n we w a i t u n t i l a n u m b e r of laser shots were fired (for e x a m p l e , 100). A t h r e s h o l d voltage was set for the b o x c a r , a n d o n l y w h e n the o u t p u t voltage was above the t h r e s h o l d , one i o n signal was counted. I n t h i s way, the s i g n a l was clearly d i s t i n g u i s h e d f r o m the noise a n d thus the s i g n a l to noise r a t i o c o u l d be h i g h l y i m p r o v e d . A t each V U V energy, t h e t o t a l count of t h r e s h o l d ions was a c c u m u l a t e d , a n d t h a t n u m b e r , w h i c h reflects the i o n - p a i r t r a n s i t i o n s t r e n g t h , was p l o t t e d as a f u n t i o n of V U V energy. F r o m the above d i s c u s s i o n , the i o n - c o u n t i n g technique is not a p p l i c a b l e to the s i t u a t i o n w h e n there are more t h a n one ions p r o d u c e d for one laser shot.  I n such cases, the boxcar  o u t p u t w i l l be above the t h r e s h o l d voltage as l o n g as there is a n y i o n . Therefore, i t w o u l d not be able to t e l l t h e exact n u m b e r of ions p r o d u c e d a n d reflect t h e t r a n s i t i o n s t r e n g t h . I n the w o r k of t h i s thesis, T I P P s p e c t r a of D C 1 a n d H C N were recorded by i o n - c o u n t i n g technique due t o t h e i r low s i g n a l levels, w h i l e T I P P s p e c t r a of other molecules a n d a l l the t o t a l i o n y i e l d s p e c t r a were recorded i n the general analogue m o d e .  References 1. K o n g W 1993 Ph.D.  Thesis, U n i v e r s i t y of W a t e r l o o  2. M a r t i n J D D 1998 Ph.D.  Thesis, U n i v e r s i t y of W a t e r l o o  3. N a t i o n a l I n s t r u m e n t s C o r p o r a t i o n ,  http://www.ni.com/labview  4. S p e c t r a - P h y s i c s L a s e r s , a d e v i s i o n of N e w p o r t C o r p o r a t i o n ,  http://www.newport.com  5. L a m b d a P h y s i k , a s u b s i d i a r y of C o h e r e n t , Inc.,  http://www.lambdaphysik.com  6. D o v i c h i N J , M o o r e D S a n d K e l l e r R A 1982 Appl.  Opt. 21 1468  7. E x c i t o n , Inc.,  http://www.exciton.com  8. D m i t r i e v V G , G u r z a d y a n G G a n d N i k o g o s y a n D N 1999 Handbook Crystals,  of Nonlinear  Optical  3 r d revised e d i t i o n , ( B e r l i n , N e w Y o r k : Springer)  9. K a t o K 1986 IEEE  J. Quantum  Electron.  10. G l a b W L a n d Hessler J P 1987 Appl.  Q E - 2 2 1013  Opt. 26 3181  11. L o k a i P , B u r g h a r d t B a n d M i i c k e n h e i m W 1988 Appl. 12. H e p b u r n J W 1995 Laser Techniques in Chemistry  Phys.  B 45 245  ed A M y e r s a n d T R R i z z o ( N e w Y o r k :  W i l e y ) pp 149-184 13. P a r k e r H a n n i f i n C o r p o r a t i o n , 14. M i l l e r D R 1988 Atomic  http://www.parker.com  and Molecular  Vol 1 ed G Scoles ( N e w Y o r k :  Beam Methods,  O x f o r d U n i v e r s i t y Press) p p 14-53 15. Z h u L a n d J o h n s o n P 1991 J. Chem.  Phys.  16. W i l e y W C a n d M c L a r e n I H 1955 Rev.  Sci.  94 5769 Instrum.  26  1150  17. C o r n e t t D S, Peschke M , L a i H i n g K , C h e n g P Y , W i l l e y K F a n d D u n c a n M A 1992 Sci.  Instrum.  63  2177  18. S t a n f o r d R e s e a r c h Systems, Inc.,  http://www.thinksrs.com  Rev.  Chapter 3 Threshold Ion-Pair Production in HC1/DC1  3.1  Introduction  I n the w o r k of t h i s c h a p t e r , T I P P S was a p p l i e d to two isotopomers, H C 1 a n d D C 1 , i n a n effort to measure a c c u r a t e l y the difference between the d i s s o c i a t i o n energies of the two molecules.  The  H C 1 / D C 1 i s o t o p o m e r p a i r was chosen because there has been recent w o r k done u s i n g extensive results f r o m h i g h r e s o l u t i o n spectroscopy to m o d e l B o r n - O p p e n h e i m e r b r e a k d o w n effects i n this molecule.  1  F u r t h e r m o r e , the b o n d d i s s o c i a t i o n energy of H C 1 h a d been precisely measured a n d  thus p r o v i d e d a g o o d reference to the current r e s u l t .  2  B a s e d o n the m o d e l i n the previous chapter (section 1.2.1), C o x o n a n d H a j i g e o r g i o u e m ployed a l l h i g h r e s o l u t i o n spectroscopic d a t a for the X ^ 1  a n d B T, X  +  states of H C 1 a n d D C 1 i n  direct least-squares fits of the p o t e n t i a l energy curves for the two s t a t e s . B y u s i n g a n extensive, 1  m u l t i - i s o t o p o m e r d a t a set, t h e y were able to estimate not o n l y t h e B o r n - O p p e n h e i m e r p o t e n t i a l for the two states, b u t also B o r n - O p p e n h e i m e r b r e a k d o w n functions over a large r a d i a l range for b o t h the h y d r o g e n a n d chlorine centers. Since d a t a was available u p to v = 17 for the X state of H C 1 , a n d v = 24 for D C 1 , t h e y were able to fit the B o r n - O p p e n h e i m e r b r e a k d o w n terms to large R , i n c r e a s i n g the r e l i a b i l i t y of the e x t r a p o l a t i o n to R = oo.  T h e results of their fit  i n d i c a t e d t h a t the m a j o r c o r r e c t i o n t e r m for the p o t e n t i a l was / z # ( R ) , a n d the difference i n D for H C 1 a n d D C 1 was 8 . 8 c m -  1  (they found 3 7 1 9 4 c m -  1  for  e  £> (HCl)). e  G i v e n these very extensive spectroscopic results for H C 1 a n d D C 1 , where the highest v i b r a t i o n a l levels were w i t h i n 1 5 0 0 c m  - 1  of the d i s s o c i a t i o n l i m i t s , t h i s is a perfect test case for  c o m p a r i n g the results of the spectroscopic analysis w i t h o u r h i g h l y accurate d e t e r m i n a t i o n s of  b o n d d i s s o c i a t i o n energies for t h i s system. P r e v i o u s T I P P S results ( 3 7 2 3 2 . l ± 0 . 6 c m ) - 1  2  indi-  cated a s m a l l i n a c c u r a c y i n the £> (HCl) value d e t e r m i n e d b y C o x o n a n d H a j i g e o r g i o u , b u t e  it is i n t e r e s t i n g to c o m p a r e the p r e d i c t e d difference between the two isotopomers.  W i t h the  h i g h - r e s o l u t i o n c a p a b i l i t i e s of o u r T I P P S spectroscopic technique, it is possible to observe the p r e d i c t e d s m a l l difference of 8 . 8 c m , therefore, the present w o r k p r o v i d e s a n e x p e r i m e n t a l re- 1  sult t h a t s h o u l d be a rigorous test for the t h e o r e t i c a l l y p r e d i c t e d B o r n - O p p e n h e i m e r b r e a k d o w n i n H C 1 . T h i s is the p r i m a r y goal of t h i s work. A s T I P P S is based o n p h o t o i o n - p a i r f o r m a t i o n , the present w o r k also i n v e s t i g a t e d the det a i l e d d y n a m i c s of p h o t o i o n - p a i r f o r m a t i o n i n H C l / D C l . F o r the h y d r o g e n halides, the process of p h o t i o n - p a i r f o r m a t i o n was observed first for H F b y B e r k o w i t z et al,  3  b u t was not observed  for any other h y d r o g e n halides u n t i l Y e n c h a et al p u b l i s h e d the results of a s y n c h r o t r o n r a d i a t i o n s t u d y o n H C 1 a n d D C 1 . These authors c h a r a c t e r i z e d the i o n - p a i r p r o d u c t i o n process 4  b y m o n i t o r i n g the t o t a l C l ~ i o n signal as a f u n c t i o n of w a v e l e n g t h f r o m 75 t o 86 n m .  They  f o u n d t h a t i n t h e t h r e s h o l d r e g i o n , the p h o t o i o n - p a i r y i e l d s p e c t r u m showed a s t r o n g resonance s t r u c t u r e , s i m i l a r t o w h a t h a d been p r e v i o u s l y observed i n H F p h o t o i o n - p a i r y i e l d . I n a d d i t i o n to the e x p e r i m e n t a l w o r k , Y e n c h a et al c a l c u l a t e d the cross section for i o n - p a i r p r o d u c t i o n i n H C 1 a n d D C 1 b y m u l t i c h a n n e l q u a n t u m defect theory ( M Q D T ) . T h e agreement between the t h e o r e t i c a l results a n d the e x p e r i m e n t a l measurements was g o o d . A s t r i k i n g feature i n t h e i r s p e c t r a are the intense, s h a r p peaks, j u s t above the t h r e s h o l d i n b o t h H C 1 a n d D C 1 . T h e results of the M Q D T c a l c u l a t i o n were t h a t the cross sections for p h o t o i o n - p a i r f o r m a t i o n for H C 1 a n d D C 1 were quite s i m i l a r i n the t h r e s h o l d region, w h i l e e x p e r i m e n t showed p h o t o i o n - p a i r f o r m a t i o n to be s o m e w h a t less l i k e l y i n D C 1 . W e looked at the t o t a l p h o t o i o n - p a i r y i e l d s p e c t r a for H C 1 a n d D C 1 i n t h e t h r e s h o l d region as p a r t of the current study. O u r results are somewhat different f r o m the previous work. I n a d d i t i o n to the obvious r e s o l u t i o n i m p r o v e m e n t , we f i n d the s p e c t r a different between H C 1 a n d D C 1 , p a r t i c u l a r l y w i t h r e g a r d to cross sections, w h i c h we find to be m u c h lower i n D C 1 . U n l i k e i n the case of H F / D F (see chapter 4), the lack of resolved r o t a t i o n a l s t r u c t u r e i n the current results makes a definitive assignment of the resonant s t r u c ture p r o b l e m a t i c , b u t u s i n g the previous calculations of R y d b e r g resonances as a g u i d e ,  4,5  we  have successfully assigned m u c h of the observed s t r u c t u r e i n the t h r e s h o l d region. O u r current  assignment is i n basic agreement w i t h the previous w o r k , a l t h o u g h the details are different, p a r t i c u l a r l y w i t h respect to i n t e n s i t y of the resonances.  3.2  Experimental  F o r the present w o r k o n H C 1 a n d D C 1 , the t u n a b l e , p u l s e d V U V r a d i a t i o n was generated t h r o u g h resonant four-wave m i x i n g of dye laser r a d i a t i o n ( L a m b d a - P h y s i k F L 3002 dye lasers p u m p e d b y a S p e c t r a P h y s i c s G C R 4 N d : Y A G laser) i n a p u l s e d supersonic X e b e a m .  6  Two  different four-wave m i x i n g schemes were used to generate the 14.4eV (86nm) l i g h t necessary for these e x p e r i m e n t s . O n e of the i n p u t wavelengths was fixed either at 2 2 2 . 5 7 n m or 249.63nm c o r r e s p o n d i n g to a t w o - p h o t o n resonance i n X e ( 5 p 6 p ' [ l / 2 , 0 ] at 8 9 8 6 0 . 5 3 8 c m 5  at 8 0 1 1 9 . 4 7 4 c m ) . - 1  7  - 1  or 5 p 6 p [ l / 2 , 0 ] 5  T h e other wavelength was scanned either f r o m a b o u t 3 6 0 n m to 3 8 0 n m or  from 2 6 5 n m to 2 8 0 n m for the two different fixed wavelengths, r e s u l t i n g i n t u n a b l e V U V light at a p p r o x i m a t e l y 86 n m , w i t h a b a n d w i d t h of ~ l c m  - 1  .  T h e V U V l i g h t was separated f r o m the f u n d a m e n t a l b y a one m e t e r focal l e n g t h n o r m a l incidence m o n o c h r o m a t o r , w h i c h also focused the V U V b e a m i n t o the u n c o l l i m a t e d H C l / D C l gas jet. T h e V U V crossed the pulsed molecular b e a m of H C 1 or D C 1 e x p a n d e d f r o m a pulsed source w i t h a 1 m m d i a m e t e r nozzle ( G e n e r a l V a l v e , Series 9), at a b o u t 5 c m d o w n s t r e a m from the p i n h o l e . B o t h H C 1 (research p u r i t y , 99.999%, M a t h e s o n ) a n d D C 1 (99%, C / D / N Isotopes Inc.) gases were used d i r e c t l y w i t h a b a c k i n g pressure of ~ 2 0 p s i . T h e pressure i n the r e a c t i o n chamber was ~ 2 . 0 x l 0 t o r r w i t h the b e a m o n , w i t h a b a c k g r o u n d pressure of ~ 2 . 0 x l 0 t o r r . - 6  - 7  F o r the i o n - p a i r y i e l d s p e c t r a , a n e x t r a c t i o n field pulse of 3 5 V / c m was a p p l i e d to the i n t e r a c t i o n region 2/us after the p h o t o d i s s o c i a t i o n , a n d the p o s i t i v e i o n s i g n a l was recorded u s i n g gated i n t e g r a t i o n ( S t a n f o r d R e s e a r c h Systems S R 2 5 0 ) . F o r the T I P P s p e c t r a , a d i s c r i m i n a t i o n field pulse of d u r a t i o n 1/xs a n d m a g n i t u d e 2 V / c m was a p p l i e d 300ns after the V U V pulse to d i s c r i m i n a t e against any p r o m p t ions formed u p o n V U V e x c i t a t i o n . A f t e r a delay t i m e of 2.0/US after the V U V pulse, a p u l s e d field of 7 V / c m was a p p l i e d to field dissociate those l o n g - l i v e d w e a k l y b o u n d i o n - p a i r states a n d e x t r a c t the r e s u l t i n g p r o t o n s a n d deuterons i n t o the T O F .  B y u s i n g g a t e d d e t e c t i o n , we were able to c o m p l e t e l y d i s c r i m i n a t e against p r o m p t ions, a n d o n l y detect t h r e s h o l d signals. D u e to the different i o n intensities of H recorded i n different ways.  a n d D , t h e T I P P s p e c t r a of H C 1 a n d D C 1 were  +  F o r H C 1 , the H  +  +  ions were recorded b y g a t e d i n t e g r a t i o n of the  analogue s i g n a l f r o m the c h a n n e l plates. F o r D C 1 , because t h e D one order of m a g n i t u d e lower t h a n the H  +  s i g n a l was weaker (about  +  s i g n a l f r o m H C 1 ) , the T I P P s p e c t r a were recorded  using g a t e d i o n c o u n t i n g . T h i s was done b y u s i n g the gated i n t e g r a t o r as a d i s c r i m i n a t o r , w i t h the L a b V i e w s i g n a l c o l l e c t i o n software r e c o r d i n g a count if the o u t p u t of t h e gated integrator exceeded a fixed t h r e s h o l d o n a shot b y shot basis. W e f o u n d t h a t i t was never possible to e l i m i n a t e the H C 1 a n d H +  +  signals even w h e n pure  D C 1 was allowed to flow t h r o u g h the gas h a n d l i n g s y s t e m for several days. T h i s s i g n a l came from p r o t o n exchange w i t h the w a l l s of the gas h a n d l i n g s y s t e m , as we d i d not take the t i m e necessary to f u l l y d e u t e r a t e the s y s t e m , since we changed gases often i n o u r e x p e r i m e n t s , a n d used a v e r y low flow of D C 1 t h r o u g h the system.  H o w e v e r , this h a d a n advantage for the  present w o r k i n t h a t i t was possible to record b o t h T I P P a n d t o t a l p h o t o i o n - y i e l d s p e c t r a for b o t h isotopes u n d e r i d e n t i c a l c o n d i t i o n s . T o determine r e l a t i v e cross sections for t o t a l i o n - p a i r y i e l d , we c o u l d use the H C 1 / D C 1 +  3.3 3.3.1  +  signals to n o r m a l i z e the H  +  / D  +  yields.  Results and Discussion T I P P spectra  T h e T I P P s p e c t r a of H C 1 a n d D C 1 are s h o w n i n F i g u r e 3.1. F o r b o t h molecules, the spectra cover the e x c i t a t i o n range of H C l / D C X * 1 ! ! , v"=0,  J" <6) + h ^ -> H + / D + + C r ( 1 5 0 ) . T h e  s p e c t r a s h o w n i n figure 3.1 were collected u s i n g b o t h X e resonances as d e s c r i b e d i n the previous section, a n d t w o s p e c t r a have been overlapped for b o t h H C 1 a n d D C 1 . F o r the H C 1 T I P P S , s p e c t r a were r e c o r d e d at several different d i s c r i m i n a t i o n fields, a n d a n e x t r a p o l a t i o n to zero field y i e l d e d the same result as i n o u r previous w o r k ,  2  w i t h i n the error l i m i t s .  However, the  reduced s i g n a l to noise for the D C 1 T I P P S made t h i s procedure less r e l i a b l e , so a different  140 120  E DC1 TIPPS J"  VUV Energy - 116000/cm"  140 120  :  HC1 TIPPS J"=6  1 0  ' c 100 3 80  A 1  60 40  00  w  20 o 800  1000  1100  1200  1300  1400  VUV Energy - 115000/cm'  1  F i g u r e 3.1: T I P P s p e c t r a of D C 1 a n d H C 1 . T h e field-free i o n - p a i r thresholds for HCl/DCl(J") hu —» H /D +  +  + Ci ( 5o) -  1  were m a r k e d above the s p e c t r u m .  s p e c t r a have been s u p e r i m p o s e d . b e a m c o n d i t i o n s a n d electric  +  I n b o t h cases, t w o different  B o t h were recorded u n d e r t h e same c o n d i t i o n s i n t e r m s of  fields..The  difference between t h e t w o s p e c t r a is t h a t different X e  resonances are used i n t h e four-wave m i x i n g process used t o generate t h e t u n a b l e V U V (see text).  T a b l e 3.1: R e s u l t s f r o m of H C l / D C l T I P P s p e c t r a . E n e r g i e s i n c m c i a t i o n energy f r o m v = 0, D - 3  cm  c, Reference 8, for  - 1  3 5  .  8  .  D o is the b o n d disso-  is the classical B D E . T h e u n c e r t a i n t y i n the zero p o i n t energy  e  is less t h a n 1 0  - 1  N o t a t i o n s : a,  C 1 isotopomer.  3 5  C 1 isotopomer value (see reference 2). b, Reference 2.  T h e p u b l i s h e d value m u c h m o r e a c c u r a t e , so u n c e r t a i n t y  is not given here, d , R e s u l t of fit i n c l u d i n g B - 0 b r e a k d o w n (reference 1). E r r o r is s t a t i s t i c a l error of fit. See reference 1 for discussion of overall u n c e r t a i n t y . HC1 current  DC1  literature  2V/cm  116282.2±0.8  OV/cm  116287.4±0.9  B  0  10.48±0.06  10.44  0  35746.9±1.1  35748.2±0.8  D  116726.4±0.6 a  E  116288.7±0.6  Q  5.39  5.33±0.05  c  36161.3±0.9  6  1066.60  c  37194.0±0.7  37230.8±1.1  116731.6±0.7  6  c  1483.88  Go D  literature  current  37227.9±0.9  d  c  37185.2±0.7  d  a p p r o a c h was t a k e n to determine the relative thresholds for H C 1 a n d D C 1 . T h e four s p e c t r a s h o w n i n figure 3.1 were recorded under the same p u l s e d field c o n d i t i o n s , so the blue edges of a l l of the peaks i n these s p e c t r a s h o u l d be shifted b y the same a m o u n t f r o m the field free thresholds. T o get the value of the 2 V / c m thresholds for J " = 0 H C 1 or D C 1 , the h a l f height positions of the b l u e edges were measured for a l l the peaks t h a t were c l e a r l y resolved.  Since  the t h r e s h o l d energy for a given J" state is given by: Ethreshold = E J  I I =  = £/»  Q - E  rot  = 0  - B J{J  + 1) + D {J(J  0  0  + l))  (3.1)  2  the d a t a c o u l d be f i t t e d to a straight line to determine E J H ~ , the field-shifted t h r e s h o l d energy 0  for J " = 0, a n d Bo, the r o t a t i o n a l constant (the l i t e r a t u r e value of Do was u s e d ) . 8  T h i s plot  of the t h r e s h o l d d a t a is s h o w n i n figure 3.2, a n d resulted a 2 V / c m i o n - p a i r d i s s o c i a t i o n t h r e s h o l d of 116282.2 ± 0 . 8 c m  - 1  for H C 1 , w h i c h is i n g o o d agreement w i t h the value d e t e r m i n e d b y  e x t r a p o l a t i o n w h e n the 3 . 9 \ / F = 5 . 5 c m  - 1  field shift is t a k e n i n t o account. T h e results of this  d a t a a n a l y s i s are g i v e n i n table 3.1. T h e value o b t a i n e d for £>o(H-Cl) i n the present s t u d y is i n g o o d agreement w i t h p r e v i o u s l y  F i g u r e 3.2: D e t e r m i n a t i o n of i o n - p a i r thresholds for H C 1 a n d D C 1 . T h e b l u e edge of the T I P P S peaks for J " = 0 - 5 are p l o t t e d for H C 1 (squares) a n d D C 1 (triangles) for the s p e c t r a shown i n figure 3.1. T h e D C 1 t h r e s h o l d s have a l l been shifted u p w a r d s b y 4 0 0 c m . - 1  p u b l i s h e d r e s u l t , w h i c h was o b t a i n e d b y the more accurate m e t h o d of e x t r a p o l a t i o n to zero 2  field. Since w h a t m a t t e r s i n the present w o r k is the c o m p a r i s o n between H C 1 a n d D C 1 , we have not a t t e m p t e d t o i m p r o v e on our previous value for £>o(H-Cl), a n d s t i l l r e c o m m e n d t h a t i t be used. T h e u n c e r t a i n t y i n the d e t e r m i n a t i o n of Do comes three sources: t h e d e t e r m i n a t i o n of the i o n - p a i r d i s s o c i a t i o n t h r e s h o l d f r o m the d a t a s h o w n i n figure 3.2: ± 0 . 8 c m  - 1  for H C 1 , ± 0 . 6 c m  - 1  for D C 1 ; the electron affinity of C I : 2 9 1 3 8 . 3 i 0 . 5 c m ~ ; the c a l i b r a t i o n of V U V : ± 0 . 3 c m , a n d 1  the m o d e l dependence of o b t a i n i n g the H  3 5  C1/D  3 5  - 1  C 1 t h r e s h o l d f r o m t h e T I P P S of the m i x t u r e :  0.1cm" . 1  T h e result of t h i s a n a l y s i s gives the difference i n classical b o n d d i s s o c i a t i o n energy to be De(H-Cl)-£> (D-Cl) = 2 . 9 ± l . l c m e  i n D ), e  - 1  (the error due t o E A ( C 1 ) does not m a t t e r t o the difference  w h i c h is s m a l l e r t h a n the value of 8 . 8 c m  _ 1  obtained by Coxon and Hajigeorgiou.  1  In  the d a t a s h o w n i n figure 3.1, there is a s p e c t r u m of H C 1 a n d one of D C 1 t h a t were recorded o n the same d a y u n d e r i d e n t i c a l c o n d i t i o n s , t a k i n g advantage of the m i x t u r e of H C 1 a n d D C 1 present i n the m o l e c u l a r b e a m . If we consider o n l y these d a t a , w h i c h w o u l d m i n i m i z e errors due to w a v e l e n g t h c a l i b r a t i o n , field strengths a n d i o n density, we o b t a i n a s o m e w h a t different value for the difference i n D : e  D (H-Cl) - D (D-Cl) = 3.7±1.5cm . e  _ 1  e  T h e increased error i n  this d e t e r m i n a t i o n is caused b y the difficulty i n d e t e r m i n i n g the t h r e s h o l d energies, a n d the reduced a m o u n t of d a t a . T h e two different analyses do p r o d u c e a n u m b e r t h a t is i n agreement w i t h i n the u n c e r t a i n t y of each d e t e r m i n a t i o n . O u r r e c o m m e n d a t i o n for the value of  D (H-Cl) e  - D ( D - C l ) is t h e e r r o r - w e i g h t e d average of these two d e t e r m i n a t i o n s : 3 . 2 ± 1 . 0 c m . - 1  e  T h e r e is also a disagreement of 3 7 c m  _ 1  and 4 3 c m  - 1  i n the values of D  e  obtained by analy-  sis of the spectroscopic d a t a , a l t h o u g h t h a t is not s u r p r i s i n g g i v e n t h a t the least b o u n d levels considered i n the a n a l y s i s were s t i l l 1 5 0 0 c m  - 1  below the d i s s o c i a t i o n l i m i t . However, given the  extensive h i g h q u a l i t y spectroscopic d a t a available for H C l / D C l , t h i s difference does h i g h l i g h t the l i m i t a t i o n s of o b t a i n i n g precise b o n d d i s s o c i a t i o n energies f r o m spectroscopic  data.  Of  course, one c a n o b t a i n more accurate values for Do from spectroscopic measurements i f i t is possible to observe levels at or j u s t above the d i s s o c i a t i o n t h r e s h o l d , as was the case i n H F .  9  I n a recent s t u d y c a r r i e d o u t b y M i c h e l et al, a precise value was o b t a i n e d for the diss o c i a t i o n energy of H C 1  +  b y analysis of the lineshape of i n d i v i d u a l r o t a t i o n a l lines i n the  2  spectrum.  <— X Il3/2  AE.  2  +  j u s t above the X H / 2  3  These lines correspond to t r a n s i t i o n s t o A T,  1 0  2  r o t a t i o n a l levels  +  d i s s o c i a t i o n l i m i t , a n d allowed for a d e t e r m i n a t i o n of the H C 1 dissoci+  2  a t i o n energy t h a t h a d a n u n c e r t a i n t y of ± 0 . 5 c m . T h r o u g h a B o r n cycle, M i c h e l et al were _ 1  t h e n able t o derive a value for D o ( H - C l ) i n excellent agreement w i t h o u r p r e v i o u s l y p u b l i s h e d value. W e c a n , of course, use the current results i n a B o r n cycle t o o b t a i n the b o n d d i s s o c i a t i o n energies of b o t h H C 1 a n d D C 1 , thus p r o b i n g the B o r n - O p p e n h e i m e r b r e a k d o w n i n a n ionic +  +  system. C o m b i n i n g our results w i t h the i o n i z a t i o n energy of C I ( 1 0 4 5 9 0 ± 0 . 3 c m ~ ) , 1  i z a t i o n energies of H C 1 a n d D C 1 ( 1 0 2 8 0 1 . 5 ± l c m - for H C 1 a n d 1 0 2 8 3 6 . 1 ± l c m 1  c a n c a l c u l a t e the b o n d d i s s o c i a t i o n energies of H C 1 a n d D C 1 +  HCl /DCl (X n +  +  2  3 / 2  , J + = 3 / 2 ) + h i / -> H / D + C 1 + ( P ) . 3  are given i n table 3.2.  2  +  J  1 1  a n d the i o n for D C 1 )  1 2  we  for t h e d i s s o c i a t i o n processes  T h e results of these calculations  A s w o u l d be expected, there is g o o d agreement between our value for  J D O ( H - C 1 ) a n d t h a t o b t a i n e d b y M i c h e l et al, a l t h o u g h t h e i r a c c u r a c y is m u c h better since +  they d i r e c t l y d e t e r m i n e d the d i s s o c i a t i o n energy for the i o n i n t h e i r e x p e r i m e n t s . I n this case, the d e f i n i t i o n of Do for the i o n is the energy r e q u i r e d t o dissociate the g r o u n d r o t a t i o n a l level i n the HC1  2  n / 3  2  s p i n - o r b i t state i n t o H ( D ) + C l . U s i n g the k n o w n spectroscopic constants for +  a n d D C 1 , one c a n c a l c u l a t e the zero-point energies for these ions ( i n c l u d i n g the D u n -  +  +  h a m correction) a n d derive the classical b o n d d i s s o c i a t i o n energies. T h e difference i n classical b o n d d i s s o c i a t i o n energies has the opposite sign to the difference f o u n d for the n e u t r a l p a i r : £> (H-Cl+) - £> (D-Cl+) = e  e  -7.5i2.0cm- . 1  T h e r e have been r e l a t i v e l y few systems for w h i c h a n a t t e m p t has been m a d e to determine the effect of B o r n - O p p e n h e i m e r a p p r o x i m a t i o n b r e a k d o w n o n b o n d d i s s o c i a t i o n energies, as there have been r e l a t i v e l y few systems for w h i c h sufficiently a c c u r a t e d e t e r m i n a t i o n s of these energies have been m a d e .  T h u s , w h i l e the effects of B - 0 a p p r o x i m a t i o n b r e a k d o w n o n the  h i g h r e s o l u t i o n s p e c t r u m has been s t u d i e d i n d e t a i l for a n u m b e r of d i a t o m i c systems, such as BeH , +  1 5  LiH,  1 6  HBr,  8  a n d H I , ' t h e r e are o n l y a few systems where i t has been possible to look 8  at the effect of B - 0 a p p r o x i m a t i o n b r e a k d o w n o n D . e  F o r the H  2  T h e s e d a t a are s u m m a r i z e d i n table 3.3.  i s o t o p o m e r s , the b o n d energies used were the r e c o m m e n d e d values f r o m a review  T a b l e 3.2: D a t a for d e t e r m i n i n g the b o n d d i s s o c i a t i o n energies of H C 1 a n d D C 1 . Energies +  are i n c m  - 1  +  . N o t a t i o n s : a, Reference 2. b, Reference 12. c, Reference 10. d , C a l c u l a t e d from  spectroscopic constants g i v e n i n reference 13 u s i n g formulae i n reference 14. H C 1 HC1+  £>o(HCl/DCl)  current  literature  current  35746.9±1.1  35748.2±0.8°  36161.3±0.9  IE(HC1/DC1)  102801.5±1  D (HC1 /DC1+) +  0  37535.4±1.5  £» (HCl /DCl+) +  paper b y S t o i c h e f f ,  17  literature  102836.1±l  6  37536.7±0.5 1325.77  Go e  DC1 DC1+  C  h  37915.2±1.4 953.44  d  d  38868.6±1.4  38861.2±1.5  w i t h the zero-point energy c a l c u l a t e d f r o m spectroscopic constants. F o r  H e H , the results of a t h o r o u g h analysis b y C o x o n a n d H a j i g e o r g i o u was u s e d . +  Since t h i s  1 8  analysis i n c l u d e d d a t a f r o m q u a s i - b o u n d r o t a t i o n a l levels j u s t above t h e d i s s o c i a t i o n l i m i t , the D  e  values s h o u l d be accurate. I n a l l cases, the D  e  value for the h y d r i d e was larger t h a n the  c o r r e s p o n d i n g deuteride for a l l the n e u t r a l , w h i l e the reverse was t r u e for the cations.  3.3.2  Ion-pair yield spectra  T h e i o n - p a i r y i e l d s p e c t r a of H C 1 a n d D C 1 ( n o r m a l i z e d b y V U V i n t e n s i t y ) are s h o w n i n F i g u r e 3.3. T h e s p e c t r a were recorded w i t h a n e x t r a c t i o n field pulse of 3 5 V / c m a p p l i e d t o the r e a c t i o n region 2.0/xs after the p h o t o d i s s o c i a t i o n .  F o r y i e l d s p e c t r a , there was no d i s c r i m i n a t i o n field  pulse a p p l i e d , so b o t h t h r e s h o l d a n d p r o m p t ions were collected, a n d therefore the i o n i n t e n s i ties reflect the t o t a l cross s e c t i o n for the i o n - p a i r p r o d u c t i o n processes. F o r b o t h molecules, t h e s p e c t r a cover the range j u s t above the i o n - p a i r d i s s o c i a t i o n t h r e s h o l d . Since H  +  and D  +  were collected i n the same energy range u n d e r t h e same e x p e r i m e n t a l  c o n d i t i o n s , a n d we c o u l d n o r m a l i z e for relative H C 1 a n d D C 1 densities u s i n g the H C 1 a n d D C 1 +  +  T a b l e 3.3: S u m m a r y of classical b o n d energies £ > ( H - X ) for h y d r i d e s . e  Molecule  I? (H-X)/cm-  HC1  37230.8±1.1  Current  DC1  37227.9±0.9  Current  HC1+  38861.2±1.5  Current  DC1+  38868.6±1.4  Current  H  38297.25  16  HD  38295.93  16  D  38294.77  16  e  HC1  HC1+  H  2  Reference  Isotopomer  2  2  1  4  HeH+  16448.8  17  4  HeD+  16456.2  17  3  HeH+  16451.2  17  3  HeD+  16458.6  17  HeH+  F i g u r e 3.3: T o t a l p h o t o i o n - p a i r y i e l d s p e c t r a for D C 1 (top) a n d H C 1 ( b o t t o m ) . B o t h s p e c t r a have been n o r m a l i z e d b y V U V i n t e n s i t y a n d are o n the same r e l a t i v e scale.  N o t e t h a t the  relative scale for t h e D C 1 s p e c t r u m is e x p a n d e d by a factor of five c o m p a r e d w i t h H C 1 . T h e thresholds for a field of 3 5 V / c m are i n d i c a t e d above the s p e c t r a for different i n i t i a l r o t a t i o n a l levels.  signals, the relative cross sections for the i o n - p a i r channels were m e a s u r e d . T h e higher s i g n a l level for the H C 1 s p e c t r u m makes i t somewhat more reliable t h a n the D C 1 s p e c t r u m , b u t i t is clear t h a t the cross section for p h o t o i o n - p a i r f o r m a t i o n is s i g n i f i c a n t l y higher for H C 1 t h a n D C 1 i n the t h r e s h o l d region, i n contrast w i t h the earlier p u b l i s h e d w o r k . A l t h o u g h the resonances are r e l a t i v e l y s h a r p for b o t h H C 1 a n d D C 1 , there is no c l e a r l y resolved r o t a t i o n a l s t r u c t u r e i n any of t h e observed resonances, a l t h o u g h the resonances do show a n unresolved r o t a t i o n a l contour, a n d some h i n t s of r o t a t i o n a l s t r u c t u r e c a n be seen i n a few of the resonances. I n the a n a l y s i s of t h e i r d a t a , Y e n c h a et al relied o n ab initio nel q u a n t u m defect t h e o r y to assign the features they o b s e r v e d . p h o t o i o n - p a i r f o r m a t i o n was a p r e d i s s o c i a t i o n of b y the c o n t i n u u m of the V T, l  1  E  +  4  calculations and multichanT h e m e c h a n i s m p r o p o s e d for  R y d b e r g states w i t h a n A T, 2  i o n - p a i r state, a n d a l l resonances observed were assigned to this  +  t y p e of R y d b e r g states. T h e y identified three R y d b e r g series i n t h e i r s p e c t r a : nsa, nda,  i o n core  +  and  npa  w i t h the nscr a n d nda states d o m i n a t i n g . A l t h o u g h n R y d b e r g states are observed i n 1  the a b s o r p t i o n a n d a u t o i o n i z a t i o n s p e c t r a of H C 1 , Y e n c h a et of a r g u e d t h a t the heterogeneous 5  c o u p l i n g between these states a n d the V  1  S  +  4  i o n - p a i r state w i l l be weak, a n d thus one w o u l d  not expect to observe nXn R y d b e r g states i n the p h o t o i o n - p a i r y i e l d s p e c t r u m .  T o simulate  the observed s p e c t r a , Y e n c h a et al used q u a n t u m defects of 1.90, 1.55, a n d 0.83 for the so, a n d da R y d b e r g series, w h i c h were close to the ab initio al,  5  values r e p o r t e d b y L e f e b v r e - B r i o n et  ab initio t r a n s i t i o n m o m e n t s for t r a n s i t i o n s to the n\a s t a t e s ,  R y d b e r g states a n d t h e V ! ! 1  repulsive w a l l of t h e V  1  E  +  4  pa  5  a n d couplings between the  a n d i o n i z a t i o n c o n t i n u a e s t i m a t e d based o n ab initio values. T h e  state was adjusted t o enhance the intensities of the higher  R y d b e r g resonances, w h i c h have s m a l l F r a n c k - C o n d o n factors w i t h the V S ( u " = 0 ) 1  +  v'nsa ground  state. A s a r e s u l t , t h e most p r o m i n e n t peak i n the e x p e r i m e n t a l s p e c t r u m was assigned to the Asa(v=9)  state, w i t h the adjacent resonances, even the 4sa(v=8  or 10) ones, h a v i n g a m u c h  lower intensity. W h i l e we agree w i t h the general d y n a m i c s p r o p o s e d b y Y e n c h a et al, our det a i l e d assignments are somewhat different from theirs. I n the H C 1 s p e c t r u m s h o w n i n figure 3.3, there are several resonances t h a t m i g h t be assigned.  T o come u p w i t h the assignments given for four of the possible resonances, we used  the following m e t h o d o l o g y . T h e i o n i z a t i o n l i m i t s for the various v i b r a t i o n a l levels of the  A T, 2  +  state of H C 1 c o u l d be c a l c u l a t e d u s i n g the accurate value for the H C 1 i o n i z a t i o n e n e r g y +  the t e r m values for the v i b r a t i o n a l levels of the J 4 S 2  state of H C 1 ,  +  +  1 3  12  for the levels f r o m  and v =0 +  to 6. T o e x t r a p o l a t e t o higher levels, spectroscopic constants d e r i v e d f r o m the h i g h r e s o l u t i o n d a t a were used. T h e r o t a t i o n a l constants for the various R y d b e r g states were a s s u m e d to be the same as the constants for the c o r r e s p o n d i n g ionic states, a n d these were d e r i v e d f r o m the spectroscopic d a t a . A s s i g n m e n t s were m a d e such t h a t the effective q u a n t u m defects for the various R y d b e r g states were as close as possible to those q u o t e d i n Y e n c h a et at 1.90 for the so states, 1.56 for the pa states, a n d 0.84 for the da states. T o s i m u l a t e the s p e c t r u m s h o w n i n figure 4.4, we a s s u m e d a l i n e w i d t h of 2 5 c m  _ 1  for the 4scr(8) a n d 3da(7) b a n d s , a n d 5 0 c m  - 1  for the Apa{3)  a n d 4scr(9) b a n d s , a n d a r o t a t i o n a l t e m p e r a t u r e of 1 5 0 K . T h e q u a n t u m defects g i v e n i n table 3.4 were c a l c u l a t e d f r o m the b a n d origins: fi = n — \/R/(IE  — E), where IE is the l i m i t for the  R y d b e r g series d e t e r m i n e d f r o m spectroscopy, E is the f i t t e d b a n d o r i g i n , a n d n is the p r i n c i p a l q u a n t u m n u m b e r of the R y d b e r g state. T h e relative intensities of the b a n d s s h o w n i n figure 3.4 were a d j u s t e d t o p r o v i d e g o o d agreement w i t h the m e a s u r e d s p e c t r a . If a l l four R y d b e r g states h a d the same c o u p l i n g w i t h the V strongest, based o n the ab initio  5  1  E  +  i o n - p a i r state, t h e 3da(7) resonance w o u l d be the  t r a n s i t i o n strengths a n d c a l c u l a t e d F r a n c k - C o n d o n factors.  W e h a d to assume a n e n h a n c e d c o u p l i n g (relative to t h a t for the 3da(7) 4S<T(9) state, 4 0 x for the 4str(8) state, a n d 8x for the 4pa(3) i n q u a l i t a t i v e agreement w i t h the ab initio  state) of lOOx for the  state. T h i s e n h a n c e d c o u p l i n g is  r e s u l t s , w h i c h h a d t h e c o u p l i n g between the Asa  state a b o u t 13 t i m e s stronger t h a n the 3da state.  4  P r e s u m a b l y , the rest of the  enhancement  results f r o m larger F r a n c k - C o n d o n factors between the h i g h v 4sa states a n d the V  1  E  +  state,  or a reduced c o u p l i n g w i t h the i o n i z a t i o n c o n t i n u u m . T h e agreement between the s i m u l a t e d a n d the m e a s u r e d s p e c t r u m i n figure 3.4 is reasonable.  M o s t of the features are r e p r o d u c e d , a n d the b a n d contours agree w i t h the measured  s p e c t r u m . T h e r e are s t i l l features i n the s p e c t r u m t h a t are not a c c o u n t e d for i n o u r s i m u l a t i o n , to the b l u e of the assigned b a n d s . T h e s i g n a l / n o i s e of the s p e c t r u m makes d r a w i n g conclusions a b o u t these e x t r a features s o m e w h a t difficult, b u t i t w o u l d a p p e a r t h a t o t h e r R y d b e r g states are c o n t r i b u t i n g to the p h o t o i o n - p a i r y i e l d . T h e other possible  X  E R y d b e r g state w o u l d be a  3/cr state, w h i c h has a c a l c u l a t e d t r a n s i t i o n m o m e n t c o m p a r a b l e t o t h a t for the 3da  state.  5  F i g u r e 3.4: S i m u l a t i o n ( b o t t o m ) of H C 1 t o t a l p h o t o i o n - p a i r y i e l d s p e c t r u m (top). A s s i g n m e n t s are as d e s c r i b e d i n t h e t e x t . T h e n u m b e r i n brackets after the R y d b e r g state d e s i g n a t i o n is t h e vibrational quantum number.  Since the q u a n t u m defect for such a state w o u l d have t o be close to zero, i t w o u l d have to be a h i g h l y v i b r a t i o n a l l y e x c i t e d series, v =9 +  or higher. O t h e r p o s s i b i l i t i e s w o u l d be U R y d b e r g 1  states, such as npn a n d ndn states, w h i c h have s t r o n g c a l c u l a t e d t r a n s i t i o n m o m e n t s i n H C 1 , a l t h o u g h t h e y are e x p e c t e d t o couple w e a k l y to the i o n - p a i r c o n t i n u u m . T h e s i t u a t i o n w i t h D C 1 is less clear.  W h i l e resonance s t r u c t u r e is evident i n the spec-  t r u m s h o w n i n figure 3.3, p r o b l e m s w i t h baseline f l u c t u a t i o n s caused b y the lower s i g n a l level make i t difficult t o d r a w conclusions a b o u t the d e t a i l e d s t r u c t u r e . W i t h i n these l i m i t a t i o n s , some possible assignments for these resonances are also p r o v i d e d i n t a b l e 3.4. T h e lowest e n ergy resonance is f a i r l y w e l l defined, a n d we s i m u l a t e d its s t r u c t u r e u s i n g l i t e r a t u r e s p e c t r a l constants,  13  a l i n e w i d t h of 2 5 c m , a n d a r o t a t i o n a l t e m p e r a t u r e of 1 5 0 K (see figure 3.5). - 1  The  r e s u l t i n g q u a n t u m defect is the same as t h a t for the H C 1 4scr(9) state, a n d r a t i o of intensities D C l [ 4 s < r ( l l ) ] / H C l [ 4 s c r ( 9 ) ] was the same as the r a t i o of F r a n c k - C o n d o n factors. F o r the other possible resonances, there is a g o o d i n d i c a t i o n t h a t the b a n d s are r e d - s h a d e d , as one w o u l d expect, b u t a g a i n there w o u l d appear to be some e x t r a s t r u c t u r e b e y o n d the four bands we have t e n t a t i v e l y assigned. W i t h i n the l i m i t a t i o n s of this assignment, i t w o u l d appear t h a t the D C 1 resonances are consistent w i t h the H C 1 ones. B e y o n d a s s i g n i n g the resonances, there is a significant difference between the cross sections for H C 1 a n d D C 1 i n t h e t h r e s h o l d region. T h i s is at o d d s w i t h t h e c a l c u l a t e d results, where the cross sections at t h r e s h o l d were f o u n d to be nearly e q u a l , a n d is also at o d d s w i t h the previous e x p e r i m e n t a l results, where t h e cross section for D C 1 was d e s c r i b e d as b e i n g a b o u t one-half t h a t for H C 1 . T h e r e s u l t s s h o w n i n figure 3.3 i n d i c a t e t h a t there is r o u g h l y one order of m a g n i t u d e difference between the m a x i m u m p h o t o i o n - p a i r y i e l d cross sections j u s t above t h r e s h o l d for H C 1 a n d D C 1 . W e have no e x p l a n a t i o n for t h i s discrepancy, other t h a n to note t h a t i n the previous e x p e r i m e n t , there was a p o s s i b i l i t y t h a t the C I  -  s i g n a l a t t r i b u t e d to D C 1 c o u l d have  been c o m i n g f r o m t h e u n a v o i d a b l e H C 1 c o n t a m i n a t i o n . W e p r e s u m e t h a t the previous work d i d not look at H  +  or D  +  s i g n a l because of the p r o b l e m of higher energy second order light c o m i n g  from the s y n c h r o t r o n , w h i c h is not a p r o b l e m i n our e x p e r i m e n t s . O u r assignments for H C 1 are also somewhat different f r o m the p r e v i o u s w o r k i n t h a t we chose t o keep the q u a n t u m defects for the R y d b e r g resonances i n a g i v e n series as close as  F i g u r e 3.5: S i m u l a t i o n ( b o t t o m ) of lowest energy resonance i n the D C 1 p h o t o i o n - p a i r y i e l d s p e c t r u m (top).  T a b l e 3.4: A s s i g n m e n t of R y d b e r g resonances i n H C l / D C l p h o t o i o n - p a i r y i e l d s p e c t r a . Energies are i n c m  - 1  . N o t a t i o n : a, Reference 4.  HCl  Resonance  Assignment  M  Calculated  116330  4so{v=8)  1.8978  116500  117220  4so(v=9)  1.8962  117250  117100  4po(v=3)  1.5602  NA  116680  3da(v=7)  0.8400  NA  116720  Asa(v=ll)  1.8962  116620  116830  Zda{v=9)  0.825  NA  116830  4pa(v=9)  .1.583  NA  117135  3do{v=10)  0.846  NA  117135  4pa{v=4)  1.562  NA  117340  4sa(v=l2)  1.900  117320  DCl  0  possible to one a n o t h e r . T h i s choice resulted i n the assignment of the largest resonance to the 4pa R y d b e r g s t a t e , w i t h a n A T, 2  +  core. W e s h o u l d a g a i n stress t h a t we used effective  (v=3)  q u a n t u m defects, a n d d i d n o t c a r r y out a f u l l M Q D T c a l c u l a t i o n . W h i l e some o t h e r 4po states were assigned i n the p r e v i o u s w o r k , their cross sections were s m a l l c o m p a r e d w i t h the 4s<r a n d 3dcr resonances. H o w e v e r , we do not r e g a r d these differences i n assignment as significant, a n d feel t h a t our w o r k p o i n t s to the same basic m e c h a n i s m p r o p o s e d i n the earlier w o r k .  References 1. C o x o n J A a n d H a j i g e o r g i o u P G 2000 J. Mol. Spectrosc. 2. M a r t i n J D D a n d H e p b u r n J W 1998 J. Chem.  Phys.  203 49  109 8139  3. B e r k o w i t z J , C h u p k a W A , G u y o n P - M , H o l l o w a y J H a n d S p o h r R 1971 J. Chem.  Phys.  54 5165 4. Y e n c h a A J , K a u r D , D o n o v a n R J , K v a r a n A , H o p k i r k A , L e f e b v r e - B r i o n H a n d K e l l e r F 1993 J. Chem.  Phys.  99 4986  5. L e f e b v r e - B r i o n H , D e h m e r P M a n d C h u p k a W A 1987 J. Chem. 6. H e p b u r n J W 1995 Laser Techniques  in Chemistry  Phys.  88 811  ed A M y e r s a n d T R R i z z o ( N e w Y o r k :  W i l e y ) p p 149-184 7. M o o r e C E 1971 Atomic  Energy  Levels,  Vols.  I-III ( W a s h i n g t o n , D . C . : U . S . N a t i o n a l  B u r e a u of S t a n d a r d s ) 8. C o x o n J A a n d H a j i g e o r g i o u P G 1991 J. Mol. Spectrosc.  150 1  9. Z e m k e W T , C o x o n J A a n d H a j i g e o r g i o u P G 1991 Chem. 10. M i c h e l M , K o r o l k o v M V a n d W e i t z e l K - M . 2002 Phys. 11. N I S T C h e m i s t r y w e b b o o k :  Phys.  Chem.  Lett. 177 412  Chem.  Phys.  4 4083  http://webbook.nist.gov/chemistry  12. D r e s c h e r M , B r o c k h i n k e A , B d w e r i n g N , H e i n z m a n n U a n d L e f e b v r e - B r i o n H 1993 J . Chem.  Phys.  99 2300  13. Saenger K L , Z a r e R N a n d M a t h e w s C W 1976 J. Mol. 14. H e r z b e r g G a n d H u b e r K - P 1950 Molecular Diatomic  Molecules  Spectra  6 1 216  Spectrosc.  and Molecular  Structure  I. Spectra  ( N e w Y o r k : V a n N o s t r a n d ) p g . 109  15. C o x o n J A a n d C o l i n R 1997 J. Mol.  Spectrosc.  181  215  16. D u l i c k M , Z h a n g K Q , G u o B a n d B e r n a t h P F 1998 J. Mol. 17. Stoicheff B P 2001 Can.  J. Phys.  Spectrosc.  79 165  18. C o x o n J A a n d H a j i g e o r g i o u P G 1999 J. Mol.  Spectrosc.  193 306  188 14  of  Chapter 4 Threshold Ion-Pair Production in H F / D F  4.1  Introduction  In t h i s c h a p t e r , t h e t o t a l i o n - p a i r y i e l d a n d T I P P S s p e c t r a of H F a n d D F w i l l be presented. S i m i l a r t o t h e w o r k of H C l / D C l , T I P P S was a p p l i e d t o H F / D F i n a n a t t e m p t t o investigate the B o r n - O p p e n h e i m e r b r e a k d o w n i n the g r o u n d electronic state, a n d to s t u d y i n d e t a i l the m e c h a n i s m of p h o t o i o n - p a i r f o r m a t i o n i n H F / D F . T o investigate the B o r n - O p p e n h e i m e r b r e a k d o w n , i t is necessary to k n o w the D  e  values of  b o t h H F a n d D F t o h i g h accuracy. W h i l e the H F value is available f r o m p r e v i o u s T I P P S w o r k ( w h i c h m e a s u r e d the i o n - p a i r t h r e s h o l d to be 1 2 9 5 5 7 . 7 ± l c m 49362.2±lcm~ ), 1  1  a n d t h u s the D  _ 1  e  value to be  the D F value is not e x a c t l y k n o w n . I n the earlier w o r k p e r f o r m e d b y Zemke  et al, new h y b r i d p o t e n t i a l energy curves based o n e x p e r i m e n t a l p o t e n t i a l s were c o n s t r u c t e d for 2  the X  1  E  +  g r o u n d state of H F / D F . F o r the molecule of H F , some q u a s i b o u n d r o t a t i o n a l states  were observed near t h e d i s s o c i a t i o n t h r e s h o l d . T h e energies a n d l i n e w i d t h s of these q u a s i b o u n d states were c a l c u l a t e d f r o m the h y b r i d p o t e n t i a l for three different D and 4 9 3 7 1 c m . - 1  e  values: 49355, 49363  B y c o m p a r i n g the c a l c u l a t e d a n d observed line p o s i t i o n s a n d w i d t h s , the  D  e  value of H F was g i v e n as 4 9 3 6 2 ± 5 c m ~ . T h i s value is a l m o s t the same as the one c a l c u l a t e d 1  from reference 1 a n d the c u r r e n t T I P P S result ( 4 9 3 6 1 . 6 ± 0 . 9 c m ) . F o r the molecule of D F , no _ 1  such q u a s i b o u n d states was observed, a n d the D  e  value given b y Z e m k e et al ( 4 9 3 4 6 ± 8 c m ) _ 1  is s l i g h t l y different f r o m the result presented here ( 4 9 3 4 9 . 2 ± 0 . 9 c m ) . - 1  F o r the m e c h a n i s m of i o n - p a i r p r o d u c t i o n i n H F / D F , i t has been d e m o n s t r a t e d t h a t the i o n - p a i r cross s e c t i o n i n the t h r e s h o l d region is c o m p a r a b l e t o the p h o t o i o n i z a t i o n cross section,  w h i l e u s u a l l y the i o n - p a i r cross section is at least two orders of m a g n i t u d e lower. T h e h i g h i o n - p a i r cross section i n the t h r e s h o l d region is due to s t r o n g resonance enhancement, w h i c h was observed i n the p r e v i o u s w o r k of B e r k o w i t z et aP a n d Y e n c h a et al , a n d was confirmed i n 1  previous T I P P S w o r k o n H F . I n t h e w o r k of b o t h B e r k o w i t z et al a n d Y e n c h a et al, the ex1  p e r i m e n t s were c a r r i e d o u t at significantly lower r e s o l u t i o n t h a n t h i s w o r k , a n d over a broader range of p h o t o n energies.  T h e s e lower resolution studies showed t h a t for b o t h H F a n d D F ,  the p h o t o i o n - p a i r y i e l d s p e c t r a are d o m i n a t e d b y a s h a r p resonance j u s t above the i o n - p a i r t h r e s h o l d , followed b y a series of weaker resonances at higher energies, w i t h the cross section at 17eV a b o u t t w o orders of m a g n i t u d e smaller t h a n t h a t of the i n i t i a l resonance at 16.07eV. B e r k o w i t z et al p r o p o s e d a n i n d i r e c t m e c h a n i s m , w i t h i o n - p a i r s f o r m e d t h r o u g h predissocia3  t i o n of R y d b e r g states converging to low v i b r a t i o n a l levels of t h e H F / D F ( A ' n ) state. I n the +  +  2  p h o t o i o n i z a t i o n w o r k b y G u y o n et al, R y d b e r g states converging to low v i b r a t i o n a l levels, such 5  as v = 1 a n d 2 of b o t h c o m p o n e n t s 1/2 a n d 3 / 2 of the X H  state were also f o u n d to c o n t r i b u t e  2  to the f o r m a t i o n of H F / D F ( X n , v = 0) i o n s i g n a l t h r o u g h resonant a u t o i o n i z a t i o n . Y e n c h a +  +  2  et al p r o p o s e d a different m e c h a n i s m of i o n - p a i r f o r m a t i o n based o n M Q D T c a l c u l a t i o n s , w i t h 4  the most i m p o r t a n t resonances assigned to the R y d b e r g states w i t h h i g h l y v i b r a t i o n a l l y e x c i t e d A Y, 2  +  i o n cores. I n recent w o r k c a r r i e d out at C R Y R I N G ,  i o n - p a i r f o r m a t i o n i n H F (e~ + HF  —> H  +  + F~)  6  a h i g h r e s o l u t i o n s t u d y o n resonant  found the same resonance s t r u c t u r e as was  observed i n t h e earlier p h o t o i o n i z a t i o n e x p e r i m e n t s , a n d so no refinement of the assignments or m e c h a n i s m was p r o p o s e d .  B y a p p l y i n g T I P P S to H F / D F , the c u r r e n t s t u d y w o u l d reveal  d e t a i l e d i n f o r m a t i o n a b o u t the d y n a m i c s a n d m e c h a n i s m a b o u t p h o t o i o n - p a i r f o r m a t i o n i n t h i s prototypical system.  4.2  Experimental  In t h i s w o r k , the molecules of H F / D F were e x c i t e d b y a p u l s e d coherent V U V light source i n the p h o t o n energy range c o r r e s p o n d i n g to the t h r e s h o l d for f o r m i n g i o n - p a i r s (15.97 to 16.16eV). T h e coherent V U V r a d i a t i o n was generated t h r o u g h resonant four-wave m i x i n g v = 2u\ + u i n 2  a p u l s e d supersonic K r b e a m .  7  O n e i n p u t wavelength v\ was fixed at the 212.55nm so t h a t 2vi  corresponds to the 4 p 5 p [ l / 2 , 0 ] resonance at 9 4 0 9 3 . 6 6 2 c m 5  - 1  i n K r . T h e other wavelength v 8  2  was scanned r o u g h l y f r o m 2 7 6 n m to 2 8 8 n m , r e s u l t i n g i n t u n a b l e V U V l i g h t at a p p r o x i m a t e l y 16e V, w i t h a b a n d w i d t h of ~ l c m  - 1  . T h e V U V wavelength was c a l i b r a t e d b y u s i n g optogalvanic  spectroscopy i n a h o l l o w c a t h o d e discharge to c a l i b r a t e v ? 2  a n d u s i n g the k n o w n K r resonance  energy for 2v\. T h e V U V l i g h t was separated from the f u n d a m e n t a l b y a one meter focal l e n g t h n o r m a l incidence m o n o c h r o m a t o r , w h i c h also focused the V U V i n t o a n u n c o l l i m a t e d p u l s e d jet of H F or D F (from a G e n e r a l V a l v e Series 9 p u l s e d source) a b o u t 5 c m d o w n s t r e a m f r o m the nozzle. B o t h H F ( U . H . P . grade, M a t h e s o n ) a n d D F (99%, C a m b r i d g e Isotope L a b o r a t o r i e s , Inc.) gases were used d i r e c t l y w i t h o u t further p u r i f i c a t i o n , a n d the s t a g n a t i o n pressure i n the source was a b o u t l b a r . T h e pressure i n t h e r e a c t i o n chamber is ~ 2 . 0 x l 0 t o r r w i t h the m o l e c u l a r b e a m _ 6  on, w i t h a b a c k g r o u n d of ~ 2 . 0 x l 0 t o r r . _ 7  For the t o t a l i o n - p a i r y i e l d s p e c t r a , a n e x t r a c t i o n field pulse of 3 5 V / c m was a p p l i e d to the i n t e r a c t i o n region 2^s after the laser pulse, a n d the p o s i t i v e ions were detected i n a t i m e of flight mass spectrometer. F o r the T I P P s p e c t r a , a d i s c r i m i n a t i o n field pulse of 2-6 V / c m a n d 1/xs d u r a t i o n was a p p l i e d 300ns after the laser to repel a n y ions f o r m e d f r o m above t h r e s h o l d processes. A t a delay t i m e of 2ps after the laser, a n e x t r a c t i o n field pulse of 3 5 V / c m was a p p l i e d to field dissociate l o n g - l i v e d R y d b e r g - l i k e i o n - p a i r states a n d e x t r a c t t h e r e s u l t i n g H  +  or D  +  ions i n t o the mass spectrometer.  4.3  Results and Discussion  4.3.1  T I P P S spectra a n d B o n d dissociation energies of H F / D F  T h e T I P P s p e c t r u m o f H F is s h o w n i n F i g u r e 4.1. T h e s p e c t r u m covers the e x c i t a t i o n range of HF(X Y.,v" X  = 0, J" < 5) + hv -> H  D F { X T , , v" = 0, J" <6) L  +  + hv^D  +  + F'^SQ). + F-fSo)  T h e D F s p e c t r u m for the e x c i t a t i o n range is s h o w n i n F i g u r e 4.2.  A s s t a t e d i n t h e i n t r o d u c t i o n , the p r i m a r y m o t i v a t i o n of t h i s w o r k is to investigate the B o r n - O p p e n h e i m e r b r e a k d o w n i n H F / D F g r o u n d state.  T o achieve t h i s g o a l , the exact v a l -  300  250  200 2  g  1 J"=0  .§> 150 CO 100  50  0 800  1000  I  I  I  1200  1400  1600  V U V energy - 128000 /cm"  1800  1  F i g u r e 4.1: T I P P s p e c t r u m of H F . A d i s c r i m i n a t i o n field of 2 V / c m m a g n i t u d e a n d Ifis d u r a t i o n was p u l s e d o n 300ns after p h o t o e x c i t a t i o n . T h e e x t r a c t i o n field of 3 5 V / c m was p u l s e d o n 2p,s after p h o t o e x c i t a t i o n . F i e l d d i s s o c i a t i o n ranges for different J" are i n d i c a t e d o n the s p e c t r u m .  300  1600  1800  2000  2200  V U V energy - 128000 /cm"  2400  1  F i g u r e 4.2: T I P P s p e c t r u m of D F . A d i s c r i m i n a t i o n field of 2 V / c m m a g n i t u d e a n d 1/xs d u r a t i o n was pulsed o n 300ns after p h o t o e x c i t a t i o n . T h e e x t r a c t i o n field of 3 5 V / c m was p u l s e d o n 2/xs after p h o t o e x c i t a t i o n . F i e l d d i s s o c i a t i o n ranges for different J" are i n d i c a t e d o n the s p e c t r u m .  ues of the i o n - p a i r t h r e s h o l d s need to be m e a s u r e d , a n d t h e n the d i s s o c i a t i o n energies can be c a l c u l a t e d . T o measure the i o n - p a i r thresholds, a s i m i l a r t e c h n i q u e t o t h e H C 1 / D C 1 case was a p p l i e d ; t h a t is, T I P P signals were collected w i t h different m a g n i t u d e s of d i s c r i m i n a t i o n pulses a n d t h e n e x t r a p o l a t i o n was a p p l i e d to get the field-free i o n - p a i r t h r e s h o l d . H o w e v e r , d u r i n g the course of t h i s e x p e r i m e n t , b i g differences were f o u n d to exist between H F / D F a n d H C 1 / D C 1 . I n the T I P P s p e c t r a o f H C 1 / D C I , there is one clearly resolved p e a k observed for each r o t a t i o n a l level of the g r o u n d state, a n d the blue edge of each p e a k shifts w i t h t h e m a g n i t u d e of the d i s c r i m i n a t i o n field a c c o r d i n g to expected S t a r k i o n i z a t i o n b e h a v i o u r ( I o n - P a i r T h r e s h o l d EIP  = E°  IP  - ay/F,  w i t h 3.9 < a < 6 . 1 ) .  10  T h e T I P P s p e c t r a of H F / D F o n the other h a n d are  d o m i n a t e d b y s t r o n g resonance p e a k s , a n d direct t r a n s i t i o n s t o t h e i o n - p a i r q u a s i - c o n t i n u u m were weak. R e s o n a n c e signals c a n be identified f r o m the fact t h a t t h e i r blue edges do not shift w i t h the d i s c r i m i n a t i o n field. T o be observed i n T I P P s p e c t r a , t h e resonance states have to be w i t h i n the n a r r o w d e t e c t i o n w i n d o w [Ej  P  — a\fF~i, E®  — a \ / F 2 ] , where F\ a n d F  P  2  are the  m a g n i t u d e s of d i s c r i m i n a t i o n a n d e x t r a c t i o n pulses. W i t h the c u r r e n t set u p of pulse fields, the d e t e c t i o n w i n d o w has a range of a b o u t 3 0 c m 8cm  - 1  - 1  a n d the u p p e r edge of the w i n d o w lies a b o u t  below the i o n - p a i r t h r e s h o l d .  A l t h o u g h not o b v i o u s i n the H F / D F T I P P s p e c t r a , there m i g h t be c o n t r i b u t i o n of direct t r a n s i t i o n t o the t o t a l T I P P S s i g n a l . T o get the exact values of i o n - p a i r t h r e s h o l d s , i t was necessary to find those d i r e c t t r a n s i t i o n s . F i g u r e 4.3 shows H F T I P P s p e c t r a for J " = 0 , 1 a n d 3 w i t h d i s c r i m i n a t i o n pulses of 2, 4 a n d 6 V / c m (direct t r a n s i t i o n is too weak c o m p a r e d to resonance signal for J"=2).  T o o p t i m i z e the signal-to-noise r a t i o s , each s p e c t r u m was scanned seven times  under the same field to give a n average signal. Shift i n the b l u e edge was observed for the peak t h a t comes f r o m d i r e c t t r a n s i t i o n . T h i s is c l e a r l y d e m o n s t r a t e d i n t h e J " = l s p e c t r u m : w h i l e resonance p e a k s are always at the same energetic p o s i t i o n s , there is shift w i t h electric field for signal c o m i n g f r o m d i r e c t t r a n s i t i o n to i o n - p a i r states. T h i s is s i m i l a r to w h a t we observed i n the T I P P s p e c t r a of H / D . 2  2  n  F r o m the shift of T I P P S s i g n a l , the field-free i o n - p a i r t h r e s h o l d E]  P  to be 129557.1 ± 0 . 9 , 129557.3 ± 0 . 9 , a n d 129556.9 ± 0 . 9 c m A n average value of 129557.1 ± 0 . 9 c m  - 1  1  of H F was e x t r a p o l a t e d  for J " = 0 , 1 a n d 3 (see F i g u r e 4.4).  was o b t a i n e d for the i o n - p a i r t h r e s h o l d of H F . S i m i l a r  T a b l e 4.1: E n e r g e t i c results f r o m H F / D F T I P P s p e c t r a . E n e r g i e s are i n c m m a j o r u n c e r t a i n t i e s for t h e F?  IP  - 1  . T h e r e are two  values: ± 0 . 3 c m ~ for wavelength c a l i b r a t i o n a n d ± 0 . 8 c m 1  for  _ 1  e x t r a p o l a t i o n t o zero electric field. Do is the b o n d d i s s o c i a t i o n energy f r o m v" = 0, a n d D  e  is  the classical B D E . HF  DF  F?  129557.1 ± 0 . 9  130135.0 ± 0 . 9  Current  DQ  47310.8 ± 0 . 9  47858.8 ± 0.9  Current  D  49361.6 ± 0 . 9  49349.2 ± 0 . 9  Current  49362 ± 5  49346 ± 8  2  IP  E  D  E  Reference  measures were t a k e n for D F (see F i g u r e s 4.5 a n d 4.6), a n d t h e i o n - p a i r t h r e s h o l d value was d e t e r m i n e d t o be 130135.0 ± 0 . 9 c m . _ 1  These results are g i v e n i n T a b l e 4.1. T h e H F i o n - p a i r  t h r e s h o l d agrees w i t h t h e previous m e a s u r e m e n t ,  1  w h i l e the D F value is r e p o r t e d for the first  time. C o m b i n e d w i t h t h e i o n i z a t i o n energy of H ( D )  1 2  a n d electron affinity of F ,  d i s s o c i a t i o n energies Do of H F / D F c o u l d b e d e t e r m i n e d . energies b e i n g a v a i l a b l e ( 2 0 5 0 . 7 7 1 c m  _1  -1  for D F ) ,  m i n e the classical b o n d d i s s o c i a t i o n energies of these t w o molecules. e  higher t h a n t h e i r r e s u l t .  the b o n d  F u r t h e r m o r e , w i t h the zero p o i n t  for H F a n d 1 4 9 0 . 3 0 4 c m  very close t o the f i t t i n g result of Z e m k e et al, w h i l e t h e D  1 3  1 4  we c a n also deter-  Our D  e  value for H F is  value for D F is a few wavenumbers  2  T o p r o v i d e a m o r e accurate measurement of B o r n - O p p e n h e i m e r b r e a k d o w n effects, a c o m p a r i s o n of H F a n d D F T I P P S recorded under i d e n t i c a l c o n d i t i o n s u s i n g a m i x e d b e a m of H F a n d D F gave t h e difference  i n classical b o n d d i s s o c i a t i o n energy of D (H-F)  1 2 . 4 ± 0 . 5 c m , w i t h t h e error l i m i t c o m i n g from t h e ± 0 . 3 c m _ 1  e  - 1  - D (D-F) e  =  uncertainty i n V U V calibration  of t h e t w o s p e c t r a . O u r result provides a b e n c h m a r k for s t u d y i n g these effects i n t h i s simple system.  1520  1290  1  3  0  STv/i™  SP6Ctra  1  "  d  3  " "  h  1310  0  V U V energy - 128000 / c m "  1  M s of 2 (red), 4  (green)  F i g u r e 4.4: E x t r a p o l a t i o n to determine the d a t a has been shifted u p i n energy by BJ"(J"  field-free i o n - p a i r t h r e s h o l d for H F . T h e + 1) - D(J"(J" + l)) . 2  J"=l  F i g u r e 4.5: D F T I P P s p e c t r a for J"=0 a n d 6 V / c m (blue).  a n d 2 w i t h d i s c r i m i n a t i o n fields of 2 (red), 4 (green)  20 0.0  1  — — — —I— — — I 1  1  1  1  1  1  0.5  1  1  1  1.0  F  1  I  1  1  1  1  I  1.5 d  1 / 2  1  1  2.0  / (V/cm)  1  1  I  1  1  1  1  2.5  1/2  F i g u r e 4.6: E x t r a p o l a t i o n t o determine the field-free i o n - p a i r t h r e s h o l d for D F . T h e J"=2 has been shifted u p i n energy by BJ"(J"  + 1) - D{J"{J"  +  3.(  l)) . 2  da  4.3.2  T o t a l ion-pair yield spectra a n d mechanism of ion-pair formation  T h e t o t a l i o n - p a i r y i e l d s p e c t r a of H F / D F covering the same energy ranges as the T I P P spect r a ( F i g u r e 4.1 a n d 4.2) are s h o w n i n F i g u r e s 4.7 a n d 4.8. d o m i n a t e d b y some s h a r p resonances. s t r o n g , such as 1 2 9 5 6 0 c m  A t energy p o s i t i o n s w h e r e the i o n - p a i r y i e l d signal is  for H F a n d 1 3 0 1 2 0 c m  - 1  w i t h t h a t of parent i o n H F / D F . +  +  A s c a n be seen, b o t h s p e c t r a are  - 1  for D F , the y i e l d of H + / D + is c o m p a r a b l e  T h i s contrasts w i t h the n o r m a l case, where i o n - p a i r y i e l d  is orders of m a g n i t u d e s m a l l e r t h a n the parent i o n y i e l d . A l t h o u g h t h e y cover m u c h shorter energy ranges t h a n the l i t e r a t u r e w o r k  4  (15.97-16.09eV  c o m p a r e d t o 16.0-16.9eV for H F ) , our h i g h - r e s o l u t i o n s p e c t r a show m u c h more d e t a i l of the signal i n the t h r e s h o l d region (roughly 16.0-16. l e V ) . W h i l e there is o n l y one single intense peak i n t h e t h r e s h o l d r e g i o n i n the previous p u b l i s h e d w o r k , there are tens of s h a r p resonance peaks 4  observed i n o u r c u r r e n t s p e c t r u m . B y a n a l y z i n g the p o s i t i o n s of those s h a r p resonances, the m e c h a n i s m of H F / D F i o n - p a i r f o r m a t i o n i n the t h r e s h o l d region c o u l d be s t u d i e d i n d e t a i l .  300  800  lOOO  VUV F i g u r e 4.7:  1 2 0 0  1 4 0 0  1 6 0 0  energy - 128000 / c m  1 8 0 0  1  T o t a l i o n - p a i r y i e l d s p e c t r u m o f H F . A n e x t r a c t i o n field of 3 5 V / c m was pulsed o n 2/zs after p h o t o e x c i t a t i o n . N o  d i s c r i m i n a t i o n field was a p p l i e d . A s s i g n m e n t s of the resonances are labelled as N -J" (J"). +  J  I  I  I  1600  I  I  1800  VUV F i g u r e 4.8:  I  I  I  I  I  I  I  2000  I  I  2200  energy - 128000 /cm"  1  I  I  2400  1  T o t a l i o n - p a i r y i e l d s p e c t r u m o f D F . A n e x t r a c t i o n field o f 3 5 V / c m was pulsed o n 2/.is after p h o t o e x c i t a t i o n . N o  d i s c r i m i n a t i o n field was a p p l i e d . A s s i g n m e n t s of the resonances are labelled as N -J" {J"). +  Chapter 4.  Threshold Ion-Pair Production  in  HF/DF  4.3.2.1 A s s i g n m e n t of resonance peaks B y u s i n g t h e resonances observed i n the T I P P s p e c t r a , we c a n d e t e r m i n e the energy levels of some resonance p e a k s observed i n the t o t a l i o n - p a i r y i e l d s p e c t r a . Because o u r current w o r k provides a n a c c u r a t e energy for the i o n - p a i r d i s s o c i a t i o n t h r e s h o l d , we c a n p r o v i d e definitive assignments for the J " for a n y resonances t h a t are observed i n T I P P S . T h i s is based on the fact t h a t any resonances observed i n T I P P S result f r o m o p t i c a l l y allowed t r a n s i t i o n s to states w h i c h t h e n couple to t h e p s e u d o - c o n t i n u u m of the i o n - p a i r states l y i n g j u s t b e l o w the d i s s o c i a t i o n l i m i t . Since the i o n - p a i r t h r e s h o l d for H F (or D F ) is at a fixed energy, the e x c i t a t i o n energy range t h a t reaches this fixed energy b a n d is d i s t i n c t for a l l J" levels above J " = l (there is a s m a l l overlap between J"=0  a n d J " = l ranges for the field strengths used). F r o m the resonances  observed i n T I P P S , one c a n o b t a i n the energies of the u p p e r states r e l a t i v e t o J"=0  simply by  a d d i n g the i n i t i a l r o t a t i o n a l energy to the p h o t o n energy for the observed resonance.  These  energies are g i v e n i n T a b l e 4.2. D u e to the a s y m m e t r i c shape a n d r e l a t i v e l y large w i d t h ( F W H M u p to 5 c m ) of the resonance s i g n a l , a n d the ± 0 . 3 c m - 1  - 1  u n c e r t a i n t y i n V U V c a l i b r a t i o n , the  values given i n T a b l e 4.2 have a n u n c e r t a i n t y of about ± 0 . 5 c m . T h e r e f o r e , final states w i t h i n - 1  a difference of less t h a n 1 c m  - 1  are regarded as the same resonance.  T o p r o c e e d f u r t h e r w i t h the assignment, we c a n s t a r t w i t h t h e D F s p e c t r u m , w h i c h has fewer resonances t h a n t h a t of H F . O n e c a n i m m e d i a t e l y see t h a t there are two levels i n D F t h a t are seen i n several t r a n s i t i o n s i n T a b l e 4.2:  one at 1 3 0 1 0 4 c m , the other one at - 1  1 3 0 1 2 3 c m . C o m b i n i n g w i t h some other resonance peaks i n the t o t a l i o n - p a i r y i e l d s p e c t r u m , - 1  we o b t a i n e d a sequence of e x c i t e d state levels w i t h energies of 130104, 130123, 130154, 130198 a n d 1 3 0 2 5 3 c m . T h i s sequence of levels corresponds closely to w h a t one w o u l d expect for the - 1  five lowest r o t a t i o n a l levels of one D F X n / +  2  1  2  state, w i t h core r o t a t i o n a l n u m b e r N  +  equals 0  to 4. T h i s is based o n the a p p r o x i m a t i o n t h a t R y d b e r g states have the same r o t a t i o n a l energy p a t t e r n as t h e p u r e ion-core. F o r a n electronic state of H i / 2 , the r o t a t i o n a l energy levels c o u l d be expressed a s : 2  Ei/2 =  + B {(J 1/2eff  + \f  - 1]  1 5  (4.1)  T a b l e 4.2: energies.  Resonances seen i n T I P P s p e c t r a of H F a n d D F , a n d c o r r e s p o n d i n g final state E n e r g i e s are i n c m  - 1  .  E r r o r i n p o s i t i o n is a b o u t ± 0 . 5 c m . - 1  F i n a l state energy is  relative t o J " = 0 . N o t a t i o n : ov indicates a resonance t h a t lies i n t h e energy range of overlap between J " = 0 a n d J " = l . DF  HF  J "  Resonance  F i n a l State  J "  Resonance  F i n a l State  0  130123.6  130123.6  0  129526.9  129526.9  QOV  130104.0  130104.0™  1  129485.0  129526.1  130104.0  130125.7  1  129471.6  129512.7  1  130081.4  130103.1  2  129416.9  129540.2  2  130058.8  130123.9  2  129403.3  129526.6  2  130039.0  130104.1  3  129292.8  129539.2  3  129989.5  130119.7  3  129284.0  129530.4  4  129131.4  129541.7  4  129126.5  129536.8  4  129119.8  129530.1  4  129112.1  129522.4  OU  where A ff  — A — 2B is t h e effective s p i n - o r b i t s p l i t t i n g between t h e two s p i n components  e  2  n / 2 a n d I l i / 2 , Bi/2eff 2  3  of N  +  = B(l  — B/A)  is the effective r o t a t i o n a l c o n s t a n t , a n d J has the value  for I I / 2 state. Therefore, the energy gap between adjacent r o t a t i o n a l levels has a  +1/2  2  1  p a t t e r n of 3:5:7:9- • • (the c o r r e s p o n d i n g  2  n / 3  2  state has a p a t t e r n of 5:7:9:11- • • s t a r t i n g f r o m  7V+=0). T h e o r e t i c a l l y the r o t a t i o n a l s t r u c t u r e for a R y d b e r g state w i t h a n / 2  s c r i b e d i n H u n d ' s case (c) or ( e ) .  16  B u t if we a p p r o x i m a t e the low N  +  1  2  core s h o u l d be de-  r o t a t i o n a l s t r u c t u r e of  the R y d b e r g state to be s i m i l a r to t h a t of the i o n core, the R y d b e r g states converging to i o n core w o u l d also have t h e p a t t e r n of 3:5:7:9- • •. T h a t is w h y the sequence of etc. was assigned as R y d b e r g states w i t h  2  ni/  2  2  n / 1/  2  130104cm  - 1  i o n core.  A s c a n be seen i n F i g u r e 4.8 a consistent assignments c a n b e a r r i v e d at for a n u m b e r of observed resonances as t r a n s i t i o n s to the above sequence, w i t h t w o s t r o n g peaks at 130123 a n d 130133cm  - 1  each assigned to two different possible t r a n s i t i o n s . T h e o v e r a l l results are also  t a b u l a t e d i n T a b l e 4.3. F o r the s p e c t r a of H F , s i m i l a r measures were t a k e n t o do the a n a l y s i s , b u t the assignment is m o r e c o m p l i c a t e d due to the presence of more s t r u c t u r e i n b o t h the T I P P a n d t o t a l y i e l d s p e c t r a . F u r t h e r m o r e , due to the r e l a t i v e l y large r o t a t i o n a l constant of H F 17.577cm  - 1  for X R. /2 2  +  i o n (B  e  =  electronic s t a t e ) , i t is u n l i k e l y t h a t there are two resonances observed 1 7  x  i n T I P P s p e c t r u m b e l o n g i n g to the same r o t a t i o n a l sequence of the i o n , considering the n a r r o w d e t e c t i o n w i n d o w ( ~ 3 0 c m ) used i n the e x p e r i m e n t . F r o m the resonances listed i n T a b l e 4.2, - 1  we c a n pick u p t h e two values of 1 2 9 5 2 6 c m  - 1  a n d 1 2 9 5 4 0 c m , b o t h of w h i c h were observed - 1  for a few t r a n s i t i o n s i n T I P P s p e c t r u m . However, w h e n l o o k i n g for c o r r e s p o n d i n g peaks at the same p o s i t i o n s i n the t o t a l i o n - p a i r y i e l d s p e c t r u m , for the resonance of 1 2 9 5 2 6 c m , o n l y - 1  a t r a n s i t i o n f r o m J"=2  was found at 1 2 9 4 0 4 c m . - 1  T h i s renders a lot of u n c e r t a i n t y for the  assignment of t h i s resonance. F o r the resonance of 1 2 9 5 4 0 c m , a few t r a n s i t i o n s f r o m different - 1  J" levels were observed i n t h e t o t a l y i e l d s p e c t r u m ( 1 2 9 4 1 7 c m J " = 3 , and 129130cm  - 1  f r o m J"=4).  - 1  129294cm  - 1  from  T o assign t h i s resonance t o one r o t a t i o n a l sequence of  R y d b e r g states converging t o a c e r t a i n v i b r a t i o n a l level of H F 129540cm  f r o m J"—2,  - 1  +  i o n , one c a n assume t h a t the  resonance has core r o t a t i o n a l n u m b e r J V , t h e n one looks for higher r o t a t i o n a l lev+  T a b l e 4.3: L i s t of p r e d i c t e d a n d observed resonances i n D F i o n - p a i r y i e l d s p e c t r u m . Energies are i n c m  - 1  , w i t h p r e d i c t e d values followed b y observed resonances i n b r a c k e t s . N o t a t i o n : sh  indicates o b s e r v e d line is a shoulder. J"\N+-J"  -2  -1  1  130104  130123  (130104)  (130123)  130082  130101  130132  130176  (130081)  (130104s/i)  (130133)  (130177)  130039  130058  130089  130133  130188  (130038)  (130059)  (130090)  (130133)  (130187)  129993  130024  130068  130123  (129992)  (130024)  (130068)  (130123)  129937  129981  130036  (129938)  (129981)  (130038)  0  1  2  3  4  2  0  els of 7V++1, 7V++2 etc.  (lower r o t a t i o n a l levels such as N -l +  c a n n o t be observed since they lie  below the d e t e c t i o n w i n d o w ) . T h e energetic values of these higher levels c o u l d p r e s u m a b l y be c a l c u l a t e d based o n t h e a l r e a d y k n o w n B  e  value of H F  +  i o n . T h e n we c a n l o o k for t r a n s i t i o n s  to those higher levels. T h e i d e a is t h a t since a few t r a n s i t i o n s were observed c o r r e s p o n d i n g to the resonance at 1 2 9 5 4 0 c m , i t is reasonable to expect s i m i l a r t r a n s i t i o n s to the next res- 1  onance i V + l . +  F r o m t h i s a n o t h e r r o t a t i o n a l level was d e r i v e d , w i t h energy at  ( w i t h t r a n s i t i o n s , of 1 2 9 4 2 4 c m J"=5).  - 1  from J " = 3 , 1 2 9 2 5 9 c m  129669cm  f r o m J " =4, a n d 1 2 9 0 5 4 c m  - 1  - 1  - 1  from  T h e r e are no resonances w i t h even higher core r o t a t i o n a l n u m b e r o b t a i n e d , perhaps  due to the low cross sections associated w i t h those higher resonances, or r e l a x a t i o n from those h i g h r o t a t i o n a l resonances. If the two levels at 1 2 9 5 4 0 c m  - 1  and 129669cm  - 1  were assumed to  belong to the same r o t a t i o n a l sequence, they w o u l d have core r o t a t i o n a l n u m b e r s of 3 a n d 4 a c c o r d i n g to t h e i r energy gap. Since o n l y two resonances were f o u n d i n the above sequence, the assignments are not as c o n v i n c i n g as the D F sequence. T h e other issue is t h a t m a n y s t r o n g t r a n s i t i o n s i n H F i o n - p a i r y i e l d s p e c t r u m r e m a i n unassigned. T o go further w i t h the assignment, one need to look for resonances not observed i n the T I P P s p e c t r u m (refer to T a b l e 4.2).  W i t h careful c a l c u l a t i o n  w h i c h s t a r t e d f r o m the a t t e m p t t o assign some of the most intense resonance peaks, such as 129579, 129560 a n d 1 2 9 4 9 5 c m  - 1  as t r a n s i t i o n s to the same sequence, a n o t h e r sequence of res-  onances was d e r i v e d , w i t h energy levels as 129579, 129618, 129683 a n d 1 2 9 7 7 4 c m  ( N + = 0 , 1,  - 1  2 a n d 3). M a n y of the s t r o n g peaks i n the t o t a l y i e l d s p e c t r u m c a n be assigned as t r a n s i t i o n s to this sequence, w i t h 1 2 9 5 7 9 c m 129495cm  - 1  - 1  identified as t r a n s i t i o n Q ( 0 ) or Q ( l ) , 1 2 9 5 6 0 c m  as Q ( 2 ) ,  - 1  as P ( 2 ) a n d so o n . T h e d e t a i l e d assignment c a n be seen i n F i g u r e 4.7 a n d the re-  sults are also g i v e n i n T a b l e 4.4. Some other s t r o n g peaks i n H F y i e l d s p e c t r u m w h i c h are not r e l a t e d to t h i s sequence c a n be identified as t r a n s i t i o n s to resonances l i s t e d i n T a b l e 4.2, w i t h the peak 1 2 9 4 7 2 c m 129404cm  - 1  - 1  identified as 1 2 9 5 1 3 c m  as 1 2 9 5 2 7 c m  - 1  - 1  <- J " = l , 1 2 9 3 8 9 c m  +  2  core are ~ 2 9 0 c m  - 1  lower  5  as 1 2 9 5 1 3 c m  - 1  <-  J"=2,  <- J " = 2 .  So far three r o t a t i o n a l sequences of H F / D F have i o n core at the X ILi/2  - 1  +  R y d b e r g states have been assigned. T h e y a l l  state (energies of c o r r e s p o n d i n g R y d b e r g states w i t h a n  A' n / 2  3  2  a n d thus below t h r e s h o l d for p r o d u c i n g i o n - p a i r s , see T a b l e 4.5).  T a b l e 4.4: L i s t of p r e d i c t e d a n d observed resonances i n H F i o n - p a i r y i e l d s p e c t r u m . Energies are i n c m , w i t h p r e d i c t e d values followed b y observed resonances i n b r a c k e t s . N o t a t i o n : sh indicates observed line is a s h o u l d e r . - 1  J"\N+-J"  -2  -1  0  1  2  129579  129618  129683  (129579)  (129616s/i)  (129682)  129538  129577  129642  129733  (sh)  (129579s/i)  (sh)  (129733)  129456  129495  129560  129651  (129456)  (129495)  (129560)  (sh)  129372  129437  129528  (129371)  (129436)  (sh)  129273  129364  (129274)  (129364)  0  1  2  3  4  T h e o v e r a l l agreement between c a l c u l a t e d a n d observed peak p o s i t i o n s is v e r y g o o d ; therefore we believe i t is reasonable t o have t h e above sequences, especially t h e t w o t h a t involve 4 or 5 energy levels. I t is t h e first t i m e t h a t such k i n d of r o t a t i o n a l sequences have been d e r i v e d as t h e i n t e r m e d i a t e resonance states i n i o n - p a i r f o r m a t i o n processes. A t t h e same t i m e w e are aware t h a t a t t h i s p o i n t i t is i m p o s s i b l e t o c a l c u l a t e every resonance level a n d assign i t t o one sequence, d u e t o t h e large n u m b e r of resonances present i n o u r s p e c t r u m a n d t h e lack of enough h i g h r e s o l u t i o n d a t a for t h e H F / D F excited states. B u t as o u r p u r p o s e is t o u n d e r s t a n d t h e m e c h a n i s m for i o n - p a i r f o r m a t i o n i n H F / D F , t h e c u r r e n t t e n t a t i v e assignment w i l l b e useful i n p r o v i d i n g some i n s i g h t a b o u t t h i s process. F r o m t h e energetic p a t t e r n o f Hi/2 2  r o t a t i o n a l constants B ff e  state (refer t o e q u a t i o n 4.1), t h e values of effective  of t h e three r o t a t i o n a l sequences c o u l d b e c a l c u l a t e d . F o r t h e t w o  sequences w i t h 4 o r 5 levels, B /f values were c a l c u l a t e d t o b e 6 . 3 ± 0 . 1 c m e  13.0±0.1cm  - 1  - 1  ( D F sequence) a n d  ( H F sequence), respectively.  F u r t h e r m o r e , i f t h e effective r o t a t i o n a l constant o f t h e R y d b e r g states i s assumed t o b e a b o u t t h e same as t h e i o n core, one c a n calculate t h e v i b r a t i o n a l level o f t h e i o n core. F o r H F sequence, t h e closest effective r o t a t i o n a l constant of H F  +  i o n core is 1 2 . 9 c m  sequence, t h e closest effective r o t a t i o n a l constant o f D F spectroscopic from H F  +  constants of H F  +  +  i o n core is 6 . 2 c m  are available i n l i t e r a t u r e ,  a n d corrected for t h e change i n reduced m a s s .  1 8  For D F  (v =6).  - 1  +  - 1  w h i l e those o f D F  (v =10). +  The  were d e r i v e d  +  1 5  F r o m T a b l e 4.5 o n e c a n see t h a t for b o t h H F a n d D F sequences i d e n t i f i e d i n o u r i o n p a i r s p e c t r a , v e r y s i m i l a r B*JJ H F / D F ( J f U.i/ ) +  +  2  2  values c o u l d b e f o u n d a n d those B^j  a t h i g h v i b r a t i o n a l levels.  values c o r r e s p o n d t o  However, i t is n o t c e r t a i n w h e t h e r or n o t we  c a n s i m p l y assign those i d e n t i f i e d sequences t o R y d b e r g states w i t h H F / D F ( X n ! / ) i o n at +  +  2  2  those h i g h v i b r a t i o n a l levels. A t t h i s m o m e n t i t is n o t clear h o w close t h e effective r o t a t i o n a l constants are for R y d b e r g states a n d p u r e i o n core, especially w h e n t h e R y d b e r g electron has a l o w p r i n c i p a l q u a n t u m n u m b e r . A c t u a l l y i f t h e value of B~£jf is n o t t h a t close t o B j;, e  w o u l d get a v e r y different m a t c h .  For H F , if ± 2 c m  - 1  difference exists between t h e t w o v a l -  ues, t h e v i b r a t i o n a l level o f t h e p u r e i o n core c a n v a r y f r o m v =4 +  (B+  =11.4cm- ). 1  one  (5^=14.5cm  - 1  ) to  v =8 +  T a b l e 4.5: E n e r g e t i c s of H F / D F a n d assignment of R y d b e r g sequencess i n the t o t a l i o n - p a i r y i e l d s p e c t r a . E n e r g i e s are i n c m a n d relative to H F / D F ( X E , u " = 0, J" = 0). T h e H F i o n i z a t i o n energy t o f o r m HF^X ^^ was c a l c u l t e d f r o m the X !!^^ l i m i t . D F i o n i z a t i o n energies were d e r i v e d f r o m H F energetics using the B o r n - O p p e n h e i m e r a p p r o x i m a t i o n for the H F / D F ( X E ) a n d H F / D F ( X n ) p o t e n t i a l curves, a n d therefore are not so precise. - 1  1  2  1  +  +  2  2  H+—F" / D+—F" HF /DF (X n +  +  2  3 /  2 , ^ = 0 , i V = =0) +  +  H F / D F ( J f n 2 , < ; = 0 , / V = =0) +  +  2  1 /  +  J  +  O b s e r v e d R y d b e r g sequence  HF  DF  Reference  129557.1 ± 0.9  130135.0 ± 0 . 9  Current  129422.4 ± 1  129567  19  129700.7  129825  129579  130104  129618  130123  129683  130154  129774  130198 130253  A s s i g n e d i o n core  X n  eff  13.0±0.1  6.3±0.1  12.9  6.2  B  Closest  B+  ff  2  1 / 2  Ion core w i t h closest  X U ,v+  n*  2.71  2  1/2  ,  iV+=0-3  = 6  X n 2  1 / 2  x n ,v+ 2  1/2  2.51  ,  iV =0-4 +  =  w  Table  4.6:  Franck-Condon  factors  for  transitions  HF+/DF+(X n 2  1 / 2  ,D )  <-  +  HF/DF(X E,w"=0). 1  molecule  v  HF DF  =0  v+ = 1  v  0.674  0.253  0.060  0.011  0.589  0.294  0.090  0.022  +  = 2  +  v+ = 3  If it is t r u e t h a t R y d b e r g states w i t h H F / D F ( X H / ) at h i g h v i b r a t i o n a l levels are more +  favorable t h a n low v i b r a t i o n a l levels (v =0 +  +  2  1  2  or 1) as the resonance states, i t w o u l d be s i m i l a r to  the resonance assignment i n H C 1 / D C 1 i o n - p a i r y i e l d s p e c t r a . It c o u l d also e x p l a i n the difference between H F i o n - p a i r y i e l d s p e c t r u m a n d the H F p h o t o i o n i z a t i o n s p e c t r u m i n the same energy range (see F i g u r e 4.9).  W h i l e most of the strong resonances i n i o n - p a i r s p e c t r u m c o u l d be  found i n the i o n i z a t i o n s p e c t r u m , the l a t t e r s p e c t r u m also has m a n y other resonances w h i c h m a y come f r o m R y d b e r g states w i t h i o n core at low v i b r a t i o n a l levels. U n f o r t u n a t e l y , the above analysis is not s u p p o r t e d b y c a l c u l a t i o n s . T o i l l u s t r a t e t h i s p o i n t a n d to further u n d e r s t a n d the i o n - p a i r d y n a m i c s of H F / D F , two c a l c u l a t i o n s were p e r f o r m e d to see w h a t k i n d of v i b r a t i o n a l levels of H F / D F ( X n ! / ) are favored as the i n t e r m e d i a t e states, +  i.e., w h a t v i b r a t i o n a l levels of H F / D F +  +  +  2  2  i o n are more easily accessed f r o m the H F / D F g r o u n d  states a n d s t r o n g l y c o u p l e d to the i o n - p a i r states. T h e first c a l c u l a t i o n used the c o m p u t a t i o n program Level 7.5  2 0  to c a l c u l a t e the F r a n c k - C o n d o n factors to H F / D F +  +  i o n at different v i b r a -  t i o n a l levels. T h e results show t h a t for b o t h H F a n d D F , the relative cross section drops q u i c k l y as v  increases. T h i s agrees w i t h e x p e r i m e n t a l results i n H F / D F p h o t o i o n i z a t i o n s p e c t r a .  +  5 , 1 9  A second c a l c u l a t i o n was p e r f o r m e d to see w h i c h v i b r a t i o n a l levels cross the i o n - p a i r curve. T h e spectroscopically d e t e r m i n e d R K R d a t a  2 1  of H F ( 5 E ) i o n - p a i r state was fitted w i t h a 1  R i t t n e r m o d e l p o t e n t i a l a n d extended to shorter internuclear distance u s i n g e q u a t i o n :  V M X (R) = A xexp(-R/p x) M  M  - R"  1  - [(%  + a )/2R \ x  - (a OL /R )  l  M  X  7  - (C x/R ) M  6  22  (4.2)  T h e f i t t i n g result (see F i g u r e 4.10) shows t h a t the i o n - p a i r c u r v e is l i k e l y to cross the H F / D F X n curve at low v i b r a t i o n a l levels, such as t> =0 or 1. +  +  2  +  W h i l e not c e r t a i n , such  4  r-  800  1000  1200  1400  1600  V U V energy - 128000 / c m ' F i g u r e 4.9: C o m p a r i s o n of H  +  and H F  +  1  i n the t h r e s h o l d region.  1800  crossing indicates t h a t low v i b r a t i o n a l states of H F / D F ( X I I ) have b e t t e r chance to couple +  +  2  to the i o n - p a i r states. If it is true t h a t R y d b e r g states w i t h i o n core H F / D F ( X I I ) at low v i b r a t i o n a l levels are +  +  2  c r i t i c a l i n m a k i n g i o n - p a i r s , i t w o u l d e x p l a i n t h e h i g h i n t e n s i t y of the i o n - p a i r s i g n a l . A s m e n t i o n e d earlier, t h e i o n - p a i r s i g n a l of H F / D F is comparable to t h e p a r e n t i o n near the threshold w h i l e u s u a l l y i o n - p a i r signals are orders of m a g n i t u d e lower.  T h i s m i g h t be due to the very  close values of i o n - p a i r thresholds a n d i o n i z a t i o n thresholds (for H F , the i o n - p a i r t h r e s h o l d is 16.06eV w h i l e the i o n i z a t i o n t h r e s h o l d is 1 6 . 0 4 e V ) . 19  W i t h i n such a close energy range, there  w o u l d be a large n u m b e r of R y d b e r g resonances of H F / D F ( X n ) +  +  2  i o n at low v i b r a t i o n a l  levels available to couple to the i o n - p a i r states a n d thus g r e a t l y enhance the i o n - p a i r signal. It w o u l d also give us some h i n t s about the r e l a t i v e l y different p a t t e r n between the t o t a l i o n - p a i r y i e l d s p e c t r a of H F a n d D F (see F i g u r e s 4.7 a n d 4.8). s p e c t r u m has some r o t a t i o n a l branches, i.e.,  T h e H F ion-pair yield  the y i e l d s i g n a l seems to appear at the energy  r o u g h l y corresponds t o t h e detection w i n d o w for each g r o u n d r o t a t i o n a l state i n T I P P spect r u m . T h i s is obvious for J" > 3 (signal is strong a n d o v e r l a p p e d for J " = 0 , 1 a n d 2).  There  is no such p a t t e r n s h o w n i n D F i o n - p a i r y i e l d s p e c t r u m . T h i s is i n t e r e s t i n g a n d possibly i n dicates t h a t H F a n d D F do not follow e x a c t l y the same m e c h a n i s m i n m a k i n g i o n - p a i r s . O n e e x p l a n a t i o n is t h a t w h i l e H F i o n - p a i r t h r e s h o l d lies r o u g h l y halfway between the energy l i m i t s of H F ( J V n +  2  3 / 2  ,u =0)  a n d rlF (X n. ,v =0),  +  above the D F + ( X n 2  +  1 / 2  2  l/2  the D F i o n - p a i r t h r e s h o l d is a b o u t  +  1  , i ; = 0 ) l i m i t (see T a b l e 4.5). T h i s means t h a t w h i l e energetically R y +  d b e r g states converging to H F ( X n / , w = 0 ) c a n couple to i o n - p a i r states, +  states i n D F c a n n o t .  300cm-  2  1  +  2  corresponding  T h i s w o u l d e x p l a i n the different p a t t e r n s between H F a n d D F i o n - p a i r  y i e l d s p e c t r a i f R y d b e r g states w i t h H F / D F ( X n ) i o n at low v i b r a t i o n a l levels are favored +  +  2  as the i n t e r m e d i a t e states i n m a k i n g ion-pairs. If h i g h v i b r a t i o n a l levels (v we w o u l d not e x p e c t such b i g difference between the H F a n d D F s p e c t r a .  +  > 3) are favored,  R . H  F  (Angstroms)  F i g u r e 4.10: H F P o t e n t i a l curves. T h e dashed v e r t i c a l lines represent t h e F r a n c k - C o n d o n region for t r a n s i t i o n s f r o m t h e H F g r o u n d v i b r a t i o n a l state. T h e s t a r s y m b o l s are t h e R K R d a t a of the B  1  ^  i o n - p a i r state a n d t h e i m p o s e d solid line is t h e f i t t e d R i t t n e r p o t e n t i a l .  Chapter 4.  Threshold  Ion-Pair Production  in  HF/DF  4.3.2.2 C o m p a r i s o n w i t h p r e v i o u s assignments W h i l e o u r t o t a l i o n - p a i r y i e l d s p e c t r a agree w i t h previous w o r k ,  3 , 4  i n t h a t the i o n - p a i r f o r m a t i o n  i n H F / D F is g r e a t l y e n h a n c e d b y resonance states, there was no i n d i c a t i o n of a n y t h i n g b u t a single resonance at t h r e s h o l d i n a n y of the previous w o r k .  A s a r e s u l t , a l l p r e v i o u s analyses  were based o n a single R y d b e r g resonance d o m i n a t i n g the i o n - p a i r y i e l d s p e c t r u m . In the i n i t i a l w o r k b y G u y o n et al of almost t h i r t y years a g o , t w o resonances were observed 5  near H F i o n - p a i r t h r e s h o l d . T h e two low r e s o l u t i o n ( a b o u t 20 m e V , or 1 6 0 c m ) - 1  r o u g h l y c o r r e s p o n d the energy ranges [129100, 1 2 9 3 5 0 ] c m  a n d [129400, 1 2 9 6 5 0 ] c m  -1  s p e c t r u m . T h e y were assigned to R y d b e r g states w i t h i o n core at X n / , 2  v  +  3  +  v  +  2  = 1, respectively, a n d p r i n c i p a l q u a n t u m n u m b e r of R y d b e r g n=6.  those two resonances matches w e l l w i t h H F  resonances  = I and  -1  i n our X U.i/ , 2  2  T h e s e p a r a t i o n between  s p i n - o r b i t s p l i t t i n g w h i c h is a b o u t  290cm . - 1  1 7  B u t t h e i r assignment w o u l d not be able to e x p l a i n the b e t t e r resolved resonance structures i n o u r h i g h r e s o l u t i o n s p e c t r u m . F o r e x a m p l e , there are m a n y s h a r p resonance peaks i n the range of [129400, 1 2 9 6 5 0 ] c m . - 1  If t h e y were a l l from t r a n s i t i o n s to some resonances converging to the  same v i b r o n i c state, a l l those peaks i n t h a t 2 5 0 c m  - 1  range w o u l d have to be r e l a t e d to each  other b y the same r o t a t i o n a l sequence. T h i s is not w h a t we observed. Since the present results show m a n y s h a r p resonances i n the t h r e s h o l d region, one c a n hope to p r o v i d e a m o r e definitive assignment for the R y d b e r g states i n v o l v e d i n p h o t o i o n - p a i r f o r m a t i o n t h r o u g h a n analysis of the observed r o t a t i o n a l s t r u c t u r e . In the w o r k b y Y e n c h a et al, the strongest resonances near i o n - p a i r t h r e s h o l d were assigned 4  to R y d b e r g states w i t h A H 2  +  i o n core at h i g h v i b r a t i o n a l levels ( w = 1 7 , 1 8 for H F a n d v +  +  =  2 2 , 2 3 , 2 4 for D F ) . T h i s is different f r o m our assignment. A c t u a l l y , the ui Xe value used i n t h e i r e  w o r k ( 2 0 c m ) is r a t h e r different f r o m the value given i n reference 5 ( 8 8 . 4 c m ) a n d t h i s w o u l d - 1  - 1  lead to some m i s c a l c u l a t i o n of the v i b r a t i o n a l levels of the A T, 2  sequence w i t h A Y, 2  +  +  state. F u r t h e r m o r e , R y d b e r g  ion-core has a r o t a t i o n a l p a t t e r n of 2:4:6- • •, a n d t h a t is not w h a t was  observed i n the c u r r e n t s t u d y . T o s u m m a r i z e t h e discussion o n H F / D F i o n - p a i r p r o d u c t i o n m e c h a n i s m , we have observed s t r o n g resonances i n b o t h T I P P a n d t o t a l y i e l d s p e c t r a , a n d the energy levels of m a n y resonances  were c a l c u l a t e d a n d three r o t a t i o n a l sequences were derived f r o m those resonances. A l l of t h e m were assigned to R y d b e r g states w i t h H F / D F ( . X ' n / 2 ) i o n cores. T h e effective r o t a t i o n a l +  +  2  1  constants of those sequences were found to be close to t h a t of p u r e H F / D F ( X n / ) i o n +  core at h i g h v i b r a t i o n a l levels.  +  2  1  2  However, other calculations show t h a t R y d b e r g states w i t h  H F / D F ( X n ) i o n core at low v i b r a t i o n a l levels are more favored as the intermediate states +  +  2  in H F / D F ion-pair production.  References 1. M a r t i n J D D a n d H e p b u r n J W 2000 Faraday  Discuss.  1 1 5 416  2. Z e m k e W T , C o x o n J A a n d H a j i g e o r g i o u P G 1991 Chem.  Phys.  412  Lett. 177  3. B e r k o w i t z J , C h u p k a W A , G u y o n P - M , H o l l o w a y J H a n d S p o h r R 1971 J. Chem.  Phys.  5 4 5165 4. Y e n c h a A J , H o p k i r k A , G r o v e r J R , C h e n g B - M , L e f e b v r e - B r i o n H a n d K e l l e r F 1995 J. Chem.  Phys.  1 0 3 2882  5. G u y o n P - M , S p o h r R , C h u p k a W A a n d B e r k o w i t z J 1976 J. Chem.  Phys.  6 5 1650  6. D j u r i c N , D u n n G H , A l - K h a l i l i A , D e r k a t c h A M , N e a u A , R o s e n S, S h i W , V i k o r L , Z o n g W , L a r s s o n M , L e P a d e l l e c A , D a n a r e d H a n d af U g g l a s M 2001 Phys.  Rev.  A 64  022713 7. H e p b u r n J W 1995 Laser Techniques in Chemistry  ed A M y e r s a n d T R R i z z o ( N e w Y o r k :  W i l e y ) p p 149-184 8. M o o r e C E 1971 Atomic  Energy  Levels,  Vols.  I-III ( W a s h i n g t o n , D . C . : U . S . N a t i o n a l  B u r e a u of S t a n d a r d s ) 9. D o v i c h i N J , M o o r e D S a n d K e l l e r R A 1982 Appl.  Opt. 2 1 1468  10. G a l l a g h e r T F 1994 Rydberg Atoms ( C a m b r i d g e : C a m b r i d g e U n i v e r s i t y Press) 11. S h i e l l R C , H u X K , H u Q J a n d H e p b u r n J W 2000 Faraday  Discuss.  1 1 5 331  12. J o h n s o n W R a n d Soff G 1985 At. Data Nucl. Data  Tables v o l 33 p p 4 0 5 - 4 4 6  13. B l o n d e l C , C a c c i a n i P , D e l s a r t C a n d T r a i n h a m R 1989 Phys. 14. C o x o n J A a n d H a j i g e o r g i o u P G 1990 J. Mol. Spectrosc. 15. B e r n a t h P F 1995 Spectra of Atoms  and Molecules  Rev.  A 40 3698  142 254  ( N e w Y o r k : O x f o r d U n i v e r s i t y Press)  p g 320 16. L e f e b v r e - B r i o n H 1990 J. Chem.  Phys.  93 5898  17. G e w u r t z S, L e w H a n d F l a i n e k P 1975 Can. J. Phys.  53 1097  18. C o s b y P C , H e l m H a n d L a r z i l l i e r e M 1991 J. Chem.  Phys.  19. M a n k A , R o d g e r s D a n d H e p b u r n J W 1994 Chem. 20. L e R o y R J 2002 LEVEL  Phys.  94 92 Lett. 219 169  7.5 ( U n i v e r s i t y of W a t e r l o o C h e m i c a l P h y s i c s R e s e a r c h R e p o r t  CP-655) 21. D i L o n a r d o G a n d D o u g l a s A E 1973 Can. J. Phys. 22. B r u m e r P a n d K a r p l u s M 1973 J. Chem.  Phys.  51 434  58 3903  Chapter 5 Threshold Ion-Pair Production in H C N  5.1  Introduction  In the w o r k of t h e last two chapters, i t has been d e m o n s t r a t e d t h a t b y a p p l y i n g T I P P S to d i a t o m i c molecules, the i o n - p a i r thresholds c o u l d be a c c u r a t e l y m e a s u r e d a n d the i o n - p a i r f o r m a t i o n m e c h a n i s m i n the t h r e s h o l d region c o u l d be investigated i n d e t a i l . It is therefore i n teresting to k n o w w h e t h e r t h i s technique c a n also be a p p l i e d to m o r e c o m p l e x d s y s t e m , such as t r i a t o m i c molecules. F o r t r i a t o m i c molecules, the i o n - p a i r p r o d u c t has one fragment c o n t a i n i n g two a t o m s , a n d t h i s fragment c o u l d be i n different r o v i b r o n i c states. T h e r e f o r e , the s p e c t r a of t r i a t o m i c molecules are e x p e c t e d to be more c o m p l i c a t e d t h a n d i a t o m i c molecules. I n the w o r k of t h i s chapter, T I P P S was a p p l i e d to the molecule of H C N . P r e v i o u s l y T I P P S has been a p p l i e d to another t r i a t o m i c molecule H 2 S , for b o t h channels H  +  + SH , ' -  1  2  and H  _  + SH . +  2  I o n - p a i r thresholds i n b o t h processes have been m e a s u r e d , a n d  t h e y y i e l d e d the same value for t h e b o n d d i s s o c i a t i o n energy D o ( H - S H ) . T h e results revealed different d y n a m i c s between the two i o n - p a i r channels.  W h i l e the S H  c h a n n e l was f o r m e d at low r o t a t i o n a l levels ( J < 4 ) , the S H  +  -  fragment  i n the first  fragment i n the second channel  c o u l d be v i b r a t i o n a l l y e x c i t e d w i t h c o m p a r a b l e cross sections for v'=0  and  v'=l.  F o r p h o t o i o n - p a i r p r o d u c t i o n i n H C N , the first y i e l d s p e c t r u m was m e a s u r e d b y B e r k o w i t z et al almost forty years a g o . T h e i r w o r k was c a r r i e d out at s i g n i f i c a n t l y lower r e s o l u t i o n ( ~ 0 . 0 1 e V 3  or 1 0 0 c m ) a n d over a m u c h broader range of p h o t o n energies (70-83nm or - 1  Both H  +  14.94-17.71eV).  a n d C N ~ i o n s i g n a l were m o n i t o r e d , a n d the two s p e c t r a showed the same s t r u c t u r e  w i t h i n e x p e r i m e n t a l u n c e r t a i n t y . A s p h o t o n energy increases the i o n i n t e n s i t y rises to the m a x -  i m u m r o u g h l y at 7 8 n m (or 15.9eV) a n d t h e n g r a d u a l l y decreases. F r o m the offset of the s p e c t r a t h e y o b t a i n e d the v a l u e of H C N i o n - p a i r t h r e s h o l d : 1 5 . 1 8 ± 0 . 0 2 e V . T h e i r s p e c t r a also i n d i c a t e d a m e c h a n i s m of p r e d i s s o c i a t i v e i o n i z a t i o n , i.e., the i o n - p a i r process goes t h r o u g h e x c i t e d states of H C N , r a t h e r t h a n d i r e c t t r a n s i t i o n to the i o n - p a i r c o n t i n u u m . A l t h o u g h there is no recent w o r k p e r f o r m e d o n the i o n - p a i r p r o d u c t i o n process i n H C N , there has b e e n a large n u m b e r of w o r k done o n t h e d i s s o c i a t i o n of H C N i n t o n e u t r a l p r o d u c t s w i t h C N fragment i n g r o u n d or e x c i t e d electronic s t a t e s .  T h e w o r k m o s t r e l a t e d to us are  4 - 1 1  those g i v i n g the v a l u e of b o n d d i s s o c i a t i o n energy Z ? o ( H - C N ) . U s i n g the t e c h n i q u e of H ( D ) atom photofragment  t r a n s l a t i o n a l spectroscopy,  M o r l e y et al have m e a s u r e d the £>o(H-CN)  v a l u e . T h e y r e c o r d e d the t r a n s l a t i o n a l energy s p e c t r a of the H a t o m s r e s u l t i n g f r o m p h o t o l y s i s 7  of j e t - c o o l e d  H C N molecules at 121.6nm.  T h e i r spectra displayed structure which provided  i n f o r m a t i o n a b o u t the energy d i s p o s a l i n t o the r o v i b r a t i o n a l states of the C N fragment. T h e y f o u n d o u t t h a t f r a g m e n t a t i o n i n t o H + C N ( A , v'=0)  is t h e d o m i n a n t d i s s o c i a t i o n c h a n n e l ,  w h i l e there was n o C N ( X ) p r o d u c t observed. T h e i r s p e c t r a showed the energetic p a t t e r n of the C N ( A , v'=0)  fragment for different r o t a t i o n a l levels. F r o m t h a t t h e y m e a s u r e d t h e d i s s o c i a t i o n  energy of H C N i n t o H + C N ( J 4 , V'=0, energy i n t o H + C N ( A \ v'=0,  N'=0).  N'=Q),  w h i c h was t h e n used to c a l c u l a t e the d i s s o c i a t i o n  T h e y r e p o r t e d the D ( H - C N ) t o be 43740 ± 0  150cm , - 1  w i t h the u n c e r t a i n t y r e l a t e d m a i n l y to the u n c e r t a i n t y i n the m e a s u r e d T O F distance. L a t e r o n t h i s value was i m p r o v e d b y C o o k et al u s i n g s i m i l a r t e c h n i q u e . 43710±70cm . - 1  10  T h e y r e p o r t e d a value of  B y a p p l y i n g o u r h i g h r e s o l u t i o n technique of T I P P S to t h e H C N molecule,  we were h o p i n g to p r o v i d e a m o r e accurate value of D o ( H - C N ) , a n d some i n s i g h t i n t o i o n - p a i r f o r m a t i o n i n the t h r e s h o l d r e g i o n .  5.2  Experimental  T h e H C N molecules were e x c i t e d b y a p u l s e d coherent V U V l i g h t source i n the p h o t o n energy range c o r r e s p o n d i n g to the t h r e s h o l d for f o r m i n g i o n - p a i r s ( 1 5 . 1 8 ± 0 . 0 2 e V ) . T h e coherent V U V r a d i a t i o n was generated t h r o u g h resonant four-wave m i x i n g v = 2v\ + v  2  i n a p u l s e d supersonic  Kr beam.  1 2  O n e i n p u t w a v e l e n g t h v\ was fixed at ~ 2 1 2 . 5 5 n m so t h a t 2v\ corresponds to the  4 p 5 p [ l / 2 , 0 ] resonance at 9 4 0 9 3 . 6 6 2 c m 5  - 1  in K r .  1 3  T h e other w a v e l e n g t h v was scanned a r o u n d 2  3 5 0 n m , r e s u l t i n g i n a t u n a b l e V U V light b e a m at a p p r o x i m a t e l y 15.2eV, w i t h a b a n d w i d t h of ~lcm  - 1  . T h e V U V w a v e l e n g t h was c a l i b r a t e d b y u s i n g o p t o g a l v a n i c spectroscopy i n a hollow  cathode discharge to c a l i b r a t e v , 2  a n d u s i n g the k n o w n K r resonance energy for 2v\.  14  a c t u a l value of 2v\ c a n be s l i g h t l y different f r o m the t a b u l a t e d value 9 4 0 9 3 . 6 6 2 c m  - 1  The  due to  power b r o a d e n i n g of the K r resonance. T h e r e is also a s m a l l u n c e r t a i n t y i n the c a l i b r a t i o n of V2 due to the o s c i l l a t i o n s of the g r a t i n g drive i n the dye l a s e r . give a n e s t i m a t e d error of ± 0 . 3 c m  - 1  15  T h e s e c o m b i n e d uncertainties  i n the V U V c a l i b r a t i o n .  T h e V U V l i g h t was s e p a r a t e d f r o m the f u n d a m e n t a l b y a one meter f o c a l l e n g t h n o r m a l incidence m o n o c h r o m a t o r , w h i c h also focused the V U V i n t o a n u n c o l l i m a t e d p u l s e d jet of H C N (from a G e n e r a l V a l v e Series 9 p u l s e d source) a b o u t 5 c m d o w n s t r e a m f r o m the nozzle. H C N gas ( C P grade, T e x - L a Gases Inc.)  The  was used d i r e c t l y 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 , a n d  the s t a g n a t i o n pressure i n the source was a b o u t l b a r . T h e pressure i n the r e a c t i o n chamber was ~ 2 . 0 x l 0 t o r r w i t h t h e m o l e c u l a r b e a m o n , w i t h a b a c k g r o u n d of ~ 2 . 0 x l 0 t o r r . - 6  - 7  F o r t h e t o t a l i o n - p a i r y i e l d s p e c t r u m , a n e x t r a c t i o n field pulse of 7 V / c m was a p p l i e d to the i n t e r a c t i o n region 2/xs after the laser pulse a n d the H flight mass spectrometer.  +  ions were detected w i t h a t i m e of  F o r the T I P P s p e c t r u m , a d i s c r i m i n a t i o n field pulse of 2 V / c m a n d  l ^ s d u r a t i o n was a p p l i e d 300ns after the laser was fired to r e p e l a n y ions f o r m e d f r o m above t h r e s h o l d processes. A t a delay t i m e of 2/xs after the laser, a n e x t r a c t i o n field pulse of 7 V / c m was a p p l i e d t o field dissociate l o n g - l i v e d R y d b e r g - l i k e i o n - p a i r states a n d e x t r a c t the resultant H  +  ions i n t o t h e mass spectrometer.  5.3  Results and Discussion  T h e t o t a l i o n - p a i r y i e l d s p e c t r u m of H C N n o r m a l i z e d b y the V U V i n t e n s i t y is s h o w n i n F i g u r e 5.1. T h e s p e c t r u m covers the energy range r o u g h l y f r o m 121800 to 1 2 3 2 0 0 c m , c o r r e s p o n d i n g - 1  to the r i s i n g edge of H C N i o n - p a i r y i e l d s p e c t r u m i n reference 3. T h e r e are no s h a r p resonances  present i n t h i s s p e c t r u m , b u t i t does show some r e l a t i v e l y b r o a d s t r u c t u r e s , w h i c h according to reference 3 m i g h t be due to some v i b r a t i o n a l bands of e x c i t e d states of H C N . A t energy positions where t h e i o n - p a i r y i e l d s i g n a l is s t r o n g , such as 1 2 2 6 0 0 c m " , the y i e l d of H 1  a b o u t 0.015% of the p a r e n t i o n H C N + (in the case of H S -> H 2  +  +  is  + S H " , the H + : H S + r a t i o is 2  ~ 0 . 1 % ) . W i t h i n the energy range of F i g u r e 5.1 are the t h r e s h o l d s c o r r e s p o n d i n g to e x c i t a t i o n 1  to different i o n - p a i r l i m i t s H levels of H C N ( A ' E , v"=0,  +  + C N ~ ( X E , v'=Q, 1  J') f r o m the i n i t i a l l y p o p u l a t e d r o t a t i o n a l  Therefore it was i m p o s s i b l e to d e t e r m i n e where these i o n - p a i r  J").  1  +  l i m i t s e x a c t l y lie since t h i s s p e c t r u m does not show any a p p a r e n t r o v i b r a t i o n a l p a t t e r n of either H C N molecule or the C N  fragment.  -  T h e T I P P s p e c t r u m of H C N is s h o w n i n F i g u r e 5.2, also n o r m a l i z e d b y V U V intensity. T h e s p e c t r u m covers the energy range f r o m 121840 to 1 2 2 8 4 0 c m . T h e o v e r a l l T I P P s i g n a l starts - 1  to increase at a b o u t 1 2 2 2 0 0 c m , becomes more or less steady for a few h u n d r e d wavenumbers - 1  a n d t h e n decreases s l i g h t l y at higher energy. D u e to the v e r y low i n t e n s i t y of the i o n - p a i r s i g n a l , the H  +  ions were r e c o r d e d u s i n g b o x c a r d e t e c t i o n i n i o n - c o u n t i n g m o d e . T h e V U V energy was  scanned at steps of 0 . 4 c m , a n d at each step, the s i g n a l was a c c u m u l a t e d for one t h o u s a n d - 1  laser shots. It was e s t i m a t e d t h a t at the most intense peak the average n u m b e r of t h r e s h o l d ions p r o d u c e d for one t h o u s a n d laser shots was a b o u t 10 counts ( c o m p a r e d to 200 counts of threshold H  +  ions for H S —> H 2  +  + SH ) . -  1  T o c o n f i r m t h a t we were c o u n t i n g the ' r e a l ' i o n  shots at s u c h low level of s i g n a l , we repeated s c a n n i n g the T I P P s p e c t r u m i n the range f r o m 122300 to 1 2 2 8 0 0 c m  - 1  (see F i g u r e 5.3), almost every single p e a k was r e p r o d u c i b l e a l t h o u g h the  relative i n t e n s i t y of each peak m i g h t v a r y ( w i t h i n a factor of 2). T h i s makes us confident a b o u t the peak p o s i t i o n s i n the T I P P s p e c t r u m . F i g u r e 5.2 shows t h a t the T I P P s p e c t r u m is c o m p l i c a t e d a n d f u l l of s t r u c t u r e . are more t h a n 50 not c l e a r l y resolved peaks i n the range f r o m 122200 to 1 2 2 8 0 0 c m . - 1  There This  is u n d e r s t a n d a b l e , t a k i n g i n t o account the r e l a t i v e l y s m a l l r o t a t i o n a l constants of b o t h H C N molecule a n d C N  -  fragment ( < 2 c m ) - 1  1 6  '  1 7  .  A s p e c t r a l range of 5 0 0 c m  - 1  c o u l d cover m a n y  t r a n s i t i o n s t o different i o n - p a i r l i m i t s H — C N ( u ' = 0 , J') f r o m H C N at different i n i t i a l r o t a +  t i o n a l levels J".  -  E a c h p e a k i n F i g u r e 5.2 is likely to be a c o m b i n a t i o n of a few such t r a n s i t i o n s .  W i t h the setup of t h e t w o field pulses i n t h i s w o r k , most peaks have a t y p i c a l w i d t h ( F W H M )  F i g u r e 5.1:  a, I o n - p a i r y i e l d s p e c t r u m of H C N f r o m reference 3.  converted f r o m A to c m  - 1  a n d therefore is not evenly spaced,  t r u m of H C N f r o m the current w o r k .  T h e h o r i z o n t a l axis was  b, T o t a l i o n - p a i r y i e l d spec-  A n e x t r a c t i o n field of 7 V / c m was p u l s e d o n 2ps  p h o t o e x c i t a t i o n . N o d i s c r i m i n a t i o n field was a p p l i e d .  after  F i g u r e 5.2: T I P P s p e c t r u m of H C N . A d i s c r i m i n a t i o n field of 2 V / c m m a g n i t u d e a n d lps d u r a t i o n was p u l s e d o n 300ns after p h o t o e x c i t a t i o n . T h e e x t r a c t i o n field of 7 V / c m was p u l s e d o n 2/zs after p h o t o e x c i t a t i o n .  2300  2400 2500 2600 2700 VUV Energy - 120000 /cm"1  F i g u r e 5.3: T I P P s p e c t r a of H C N w i t h two different V U V i n t e n s i t i e s . T h e i n t e n s i t y of t h e s t r o n g V U V is a b o u t t w i c e t h a t o f the weak V U V . B o t h s p e c t r a are n o r m a l i z e d b y t h e r e l a t i v e VUV  intensities. T h e T I P P s p e c t r u m w i t h t h e s t r o n g V U V is shifted u p for c o m p a r i s o n . F o r  b o t h s p e c t r a , a d i s c r i m i n a t i o n field of 2 V / c m m a g n i t u d e a n d 1/xs d u r a t i o n was p u l s e d o n 300ns after p h o t o e x c i t a t i o n . T h e e x t r a c t i o n field of 7 V / c m was p u l s e d o n 2/us after p h o t o e x c i t a t i o n .  of 3 c m , w h i c h is a b o u t half of the field detection w i n d o w a ( f 2 — s/F\) w i t h 2 a n d 7 V / c m - 1  v  for j F i a n d F , 2  respectively, a n d a values between 3.9 a n d 6 . 1 .  /  1 8  It has been suggested i n the previous w o r k t h a t the H C N i o n - p a i r process goes t h r o u g h the R y d b e r g states of H C N + w h i c h couple to the i o n - p a i r s t a t e s . T o test w h e t h e r the T I P P s i g n a l 3  is d o m i n a t e d b y resonance enhancement or m a i n l y comes f r o m direct t r a n s t i o n to the i o n - p a i r pseudo c o n t i n u u m , several peaks i n F i g u r e 5.2 w i t h above average i n t e n s i t y were r a n d o m l y selected a n d s c a n n e d w i t h different d i s c r i m i n a t i o n fields of 2, 4 a n d 6 V / c m . However, likely due to the e x t r e m e l y low i n t e n s i t y of the t h r e s h o l d signals, i t w a s v e r y difficult t o t e l l w h e t h e r there was shift of t h e blue edges expected for i o n i z a t i o n b e h a v i o u r .  1 8  F i g u r e 5.4:  S i m u l a t i o n I of the H C N T I P P s p e c t r u m .  result of s i m u l a t i o n 1.  T h e b o t t o m trace is the e x p e r i m e n t a l s p e c t r u m , w h i l e the t o p one is the  V U V Energy - 120000 /cm F i g u r e 5.5: S i m u l a t i o n I I of H C N T I P P s p e c t r u m . T h e b o t t o m trace is t h e e x p e r i m e n t a l s p e c t r u m , w h i l e the t o p one of s i m u l a t i o n 2.  T o o b t a i n useful energetic a n d d y n a m i c i n f o r m a t i o n , s i m i l a r measures were t a k e n to analyse a n d s i m u l a t e the T I P P s p e c t r u m s i m i l a r to our previous w o r k o n H 2 S .  T o do the s i m u l a t i o n ,  1  a G a u s s i a n l i n e shape f u n c t i o n was assumed for each t r a n s i t i o n f r o m H C N molecule at g r o u n d state to t h e i o n - p a i r states.  T h e r o t a t i o n a l energy levels of H C N were c a l c u l a t e d f r o m its  precisely k n o w n r o v i b r a t i o n a l c o n s t a n t s H=3.40xl0  cm  - 1 2  - 1  for H  1 2  C  1 4  (B=1.478221840cm ,  16  -1  D=2.91047xl0 cm , - 6  - 1  and  N w h i c h was the o n l y isotopmer considered i n our s i m u l a t i o n ) ,  the r e l a t i v e p o p u l a t i o n d i s t r i b u t i o n c o u l d also be c a l c u l a t e d b y a s s u m i n g a n a p p r o x i m a t e r o t a t i o n a l t e m p e r a t u r e . F o r C N ( X E , v'=0), a r o t a t i o n a l c o n s t a n t of 1 . 8 7 1 5 8 ± 0 . 0 0 0 9 0 c m _  1  +  was t a k e n f r o m the recent w o r k of T J L e e et al ,  a l t h o u g h the results of different theoretical  17  approaches c a n v a r y f r o m 1.86 to 1 . 8 9 c m the value of 1 . 8 9 1 0 7 c m .  1 9  assumed t o be 6 x l 0  - 1  - 1  - 6  cm  -1  w h i l e e x p e r i m e n t a l result of a few dacades ago gave  - 1  T h e c e n t r i f u g a l d i s t o r t i o n constant of C N based o n the values of C N  -  is not available b u t was  i o n a n d C N free r a d i c a l .  +  2 0  To match  the p e a k p o s i t i o n s i n t h e T I P P s p e c t r u m , w h i c h is o u r first c o n s i d e r a t i o n a n d p r i o r i t y i n d o i n g the s i m u l a t i o n , one t e n t a t i v e value of i o n - p a i r t h r e s h o l d E ° p (for t r a n s i t i o n f r o m H C N ( X E , 1  v"=0,  to i o n - p a i r H  J"=0)  +  + C N ( A " E , v'=0, -  1  J'=0))  +  was a s s u m e d a n d t h e n adjusted to  give the best agreement. T h e results are s h o w n i n F i g u r e s 5.4 a n d 5.5. F r o m the f i t t i n g we o b t a i n e d the r o t a t i o n a l t e m p e r a t u r e of H C N as 2 0 0 K w h i c h is reasonable for the b e a m c o n d i t i o n s i n the experiment. T h e best f i t t i n g of t h e T I P P s p e c t r u m gave a value of 1 2 2 2 3 8 . 3 c m  - 1  for the H C N i o n - p a i r t h r e s h o l d i n a n electric field of 2 V / c m . C o r r e c t e d  b y the field shift t e r m a\/2 ( w i t h 3.9 < a < 6 . 1 ) , the field-free H C N i o n - p a i r t h r e s h o l d E ° 18  was d e t e r m i n e d to be 1 2 2 2 4 6 ± 4 c m . - 1  term and ~ 0 . 3 c m ~0.015cm  - 1  - 1  T h e r e is a n u n c e r t a i n t y of ± 1 . 5 c m  i n V U V c a l i b r a t i o n ; the r o t a t i o n a l constant of C N  a n d c o u l d c o n t r i b u t e a n error of ~ 2 . 7 c m  - 1  -  - 1  p  f r o m the ay/2  has a n u n c e r t a i n t y of  (for J ' = 1 2 - 1 4 ) t o the final value. O u r  result agrees w i t h the one given i n reference 3, b u t has a m u c h i m p r o v e d precision. T o c a l c u l a t e the b o n d d i s s o c i a t i o n energy £>o(H-CN) f r o m the i o n - p a i r t h r e s h o l d E ° p , one needs to k n o w t h e i o n i z a t i o n energy of H a t o m a n d t h e e l e c t r o n affinity of C N r a d i c a l . W h i l e the I P ( H ) value is precisely k n o w n , there is r e l a t i v e l y large u n c e r t a i n t y i n the value 2 1  of E A ( C N ) .  2 2  '  2 3  If one uses the result of E A ( C N ) = 3 . 8 6 2 ± 0 . 0 0 4 e V d e t e r m i n e d i n reference 23  u s i n g the technique of p h o t o e l e c t r o n spectroscopy of the negative i o n , the £>o(H-CN) value c a n  be o b t a i n e d as 4 3 7 1 7 ± 3 2 c m , w i t h the u n c e r t a i n t y m a i n l y f r o m t h e E A ( C N ) value. T h i s value - 1  agrees w i t h the result f r o m reference 10 ( 4 3 7 1 0 ± 7 0 c m ) a n d has a smaller uncertainty. W h e n _ 1  i n the future a m o r e precise value of E A ( C N ) is measured, one w i l l be able to calculate the £>o(H-CN) value t o even better accuracy f r o m o u r E°-  value.  P  F u r t h e r m o r e , f r o m the s i m u l a t i o n one c a n also o b t a i n d y n a m i c a l i n f o r m a t i o n a b o u t the r e l ative b r a n c h i n g r a t i o i n t o different r o t a t i o n a l levels of the C N  -  product.  The C N  -  fragment  p r o d u c e d i n o u r e x p e r i m e n t has to be v i b r a t i o n a l l y cold due to its r e l a t i v e l y large f u n d m e n t a l v i b r a t i o n a l frenquency ( 2 0 3 5 ± 4 0 c m ) , - 1  2 3  b u t i t w o u l d be i n t e r e s t i n g to look i n t o its r o t a t i o n a l  d i s t r i b u t i o n . I n o u r s i m u l a t i o n s a set of parameters w h i c h represent the relative strengths of different t r a n s i t i o n s were i n c l u d e d so t h a t the fitting w o u l d agree w i t h the o v e r a l l shape of the s p e c t r u m . T o fit those p a r a m e t e r s , two different approaches were a p p l i e d . I n the first a p p r o a c h ( F i g u r e 5.4), consecutive C N  -  r o t a t i o n a l levels J' were g r o u p e d i n threes (012, 345, 678 a n d so  on), a n d one cross s t r e n g t h was assumed for each group regardless of the i n i t i a l H C N r o t a t i o n a l level J". (J'-J")  T h e second a p p r o a c h ( F i g u r e 5.5) was to assume one cross s t r e n g t h for each A J f r o m -10 t o 10. T h e two approaches gave the same value for H C N i o n - p a i r t h r e s h o l d  a n d s i m i l a r p a t t e r n of b r a n c h i n g r a t i o i n t o C N  E°-  p  (see F i g u r e 5.6).  fragments at different r o t a t i o n a l levels  -  B o t h approachs show t h a t H C N i o n - p a i r process has a m a x i m u m t r a n s i t i o n  s t r e n g t h at a b o u t J ' = 1 2 - 1 4 . S u c h p a t t e r n is close to the p o p u l a t i o n d i s t r i b u t i o n of C N  frag-  -  ment w i t h a r o t a t i o n a l t e m p e r a t u r e of 9 0 0 K . T h e relative f r a g m e n t a t i o n r a t i o of H C N i n t o H + C N ~ ( J ' ) is different from the two i o n p a i r channels of H 2 S ( H SH  +  + SH  -  and H  _  + S H ) . A s w h a t m e n t i o n e d i n the i n t r o d u c t i o n , the +  fragment is m a i n l y r o t a t i o n a l l y c o l d a n d the S H  -  +  fragment c a n be v i b r a t i o n a l l y e x c i t e d .  W h i l e i n the c u r r e n t s t u d y i t was observed t h a t the f o r m a t i o n of r o t a t i o n a l l y e x c i t e d C N  -  fragment is favored i n the i o n - p a i r process of H C N . T h e d i s t r i b u t i o n of C N i n g e x c i t e d states.  -  r o t a t i o n indicates t h a t the i o n - p a i r process goes t h r o u g h b e n d -  T h e e x c i t a t i o n of the b e n d i n g m o d e w i l l i n d u c e a n increase i n the r o t a -  t i o n a l e x c i t a t i o n of the C N fragment w h i c h eventually manifests itself i n the p r o d u c t i o n CN  -  of  at h i g h r o t a t i o n a l levels. S i m i l a r r o t a t i o n a l p a t t e r n s have been observed i n n e u t r a l H C N  dissociation. ' 7  10  I n the w o r k of reference 7, the relative t r a n s i t i o n s t r e n g t h to C N ( ^ 4 ) ' q at w  =  F i g u r e 5.6: R e l a t i v e b r a n c h i n g r a t i o i n t o C N  -  fragment at different r o t a t i o n a l levels. T h e star  s y m b o l s are t h e f i t t i n g results of the first a p p r o a c h w h i l e the t r i a n g l e s are f r o m t h e second approach.  T h e s o l i d line is t h e c a l c u l a t i o n of p o p u l a t i o n d i s t r i b u t i o n of C N  r o t a t i o n a l t e m p e r a t u r e of 9 0 0 K .  -  based o n a  different r o t a t i o n a l levels was f o u n d to resemble the p o p u l a t i o n d i s t r i b u t i o n of C N w i t h a r o t a t i o n a l t e m p e r a t u r e of 700 ± 1 5 0 K . T h e r o t a t i o n a l l y e x c i t e d C N r a d i c a l was reasoned to be the p r e d i s s o c i a t i o n p r o d u c t of bent H C N at electronic state 3 A',  for w h i c h the p o t e n t i a l en-  1  ergy surface is available f r o m ab initio  calculation.  24  T h e H C N i o n - p a i r d i s s o c i a t i o n process  is e x p e c t e d t o follow s i m i l a r m e c h a n i s m , a l t h o u g h further u n d e r s t a n d i n g w o u l d require more knowledge of H C N e x c i t e d states, especially the m u l t i d i m e n t i o n a l p o t e n t i a l energy surfaces i n the h i g h energy region t h a t p r o d u c e ion-pairs (15.2eV). T o s u m m a r i z e , we have recorded the h i g h resolution i o n - p a i r y i e l d a n d T I P P s p e c t r a of H C N i n this work. structure.  A l t h o u g h the i o n - p a i r s i g n a l is very low, the T I P P s p e c t r u m has a lot of  S i m u l a t i o n s were p e r f o r m e d to m a t c h the T I P P s p e c t r u m .  F r o m the s i m u l a t i o n s  we have d e r i v e d t h e precise value of the i o n - p a i r t h r e s h o l d of H C N , w h i c h was t h e n used to calculate the b o n d d i s s o c i a t i o n energy £>o(H-CN). T h e s i m u l a t i o n s also d e m o n s t r a t e t h a t r o t a t i o n a l l y hot C N  -  fragment is favored as the p r o d u c t w i t h the cross s t r e n g t h peaks at a b o u t  J'=12-14.  References 1. S h i e l l R C , H u X K , H u Q J a n d H e p b u r n J W 2000 J. Phys. 2. H u Q J 2000 M.Sc.  Chem.  A 104 4339  Thesis U n i v e r s i t y of W a t e r l o o  3. B e r k o w i t z J , C h u p k a W A a n d W a l t e r T A 1969 J. Chem. 4. S t e i n I a n d G e d a n k e n A 1978 J. Chem. 5. Lee L C 1980 J. Chem.  Phys.  Phys.  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M a r t i n J D D 1998 Ph.D.  Opt. 2 1 1468  Thesis U n i v e r s i t y of W a t e r l o o  16. M a k i A , Q u a p p W , K l e e S, M e l l a u G C a n d A l b e r t S 1996 J. Mol. 17. L e e T J a n d D a t e o C E , 1999 Spectrochimia 18. G a l l a g h e r T F 1994 Rydberg  Spectrosc.  1 9 4 189  Acta Part A 55 739  Atoms ( C a m b r i d g e : C a m b r i d g e U n i v e r s i t y Press)  19. H u b e r K P a n d H e r z b e r g G 1979 Constants  of Diatomic  Molecules  (New York:  Van  Nostrand Reinhold) 20. N I S T C h e m i s t r y W e b B o o k h t t p : / / w e b b o o k . n i s t . g o v / c h e m i s t r y / 21. J o h n s o n W R a n d Soff G 1985 At. Data Nucl.  Data  22. K l e i n R , M c G i n n i s R P a n d Leone S R 1983 Chem.  Tables v o l 33 p p 4 0 5 - 4 4 6 Phys.  Lett. 1 0 0 475  23. B r a d f o r t h S E , K i m E H , A r n o l d D W a n d N e u m a r k D M 1993 J. Chem. 24. P e r i c M , D o h m a n n H , P e y e r i m h o f f S D a n d B u e n k e r R J 1987 Z. Phys.  Phys. D 5 65  9 8 800  Chapter 6 Production of H F H + from ( H F )  6.1  2  Introduction  A s the last p a r t of t h i s thesis project, T I P P S was a p p l i e d to a h y d r o g e n b o n d e d d i m e r — ( H F ) 2 , i n i t i a l l y a i m e d t o s t u d y the i o n - p a i r process (HF)2 -+ H F H  + F . L i k e t h e w o r k o n H C N , by  +  -  recording the h i g h r e s o l u t i o n t o t a l y i e l d a n d t h r e s h o l d s p e c t r a of the H F H  i o n , i t is possible  +  to o b t a i n the energetic t h r e s h o l d of the above process, a n d the r o v i b r a t i o n a l d i s t r i b u t i o n of the H F H + fragment. T h e d i s s o c i a t i o n energy of (HF)2 into two H F monomers (£>o) is 1 0 3 8 c m . - 1  ary geometry of the d i m e r has been investigated by ab initio  calculations  2  The station-  1  a n d microwave  e x p e r i m e n t s ' . T h e ( H F ) d i m e r at e q u i l i b r i u m is p l a n a r (see F i g u r e 6.1). T h e b o n d l e n g t h of 3  4  2  H - F i n the two H F u n i t s are very close to t h a t of H F m o n o m e r ( 0 . 9 1 7 0 A ) ,  5  a n d the hydrogen  b o n d e d H a t o m is s l i g h t l y off the F - F axis. T h e r e are r e l a t i v e l y large uncertainties i n the six f u n d a m e n t a l v i b r a t i o n a l frequencies of ( H F ) , w i t h differences u p to 2 0 0 c m 2  - 1  between  theo-  r e t i c a l a n d e x p e r i m e n t a l r e s u l t s . T h e r o t a t i o n a l structures i n the t w o H - F s t r e t c h i n g bands 2  of ( H F ) have been investigated b y i n f r a r e d a n d microwave a b s o r p t i o n e x p e r i m e n t , 2  induced  fluorescence.  - 1  2  a n d laser-  7  Since the frequencies of the s t r e t c h i n g modes ( w « 3 9 0 0 c m ) (HF)  6  2  of b o t h H F monomers i n  are greater t h a n the dissociation energy of ( H F ) i n t o t w o H F m o n o m e r s , e x c i t a t i o n of 2  those two modes c a n result i n states w e l l above the d i s s o c i a t i o n l i m i t of ( H F ) . D u r i n g the 2  past two decades, a lot of t h e o r e t i c a l  8 - 1 0  and e x p e r i m e n t a l ' 6  1 1 - 1 6  w o r k have been performed to  s t u d y the p r e d i s s o c i a t i o n rates, the photofragment r o v i b r a t i o n a l d i s t r i b u t i o n , as w e l l as t u n n e l -  HF+ + e-  H+ + F "  15  HFH •+ F + e  H  \O.97±O01 14±2" \ H  HFH  1  + F"  10 CD CD  cz  H v  63+6"  0.924±0.005 2.72+0.03 H §-926+0.005 0+  H F (+HF)  (HF)  2  F i g u r e 6.1: ( H F ) 2 energetics a n d m o l e c u l a r s t r u c t u r e s o f ( H F ) 2 a n d H F H . +  i n A. S h a d e d a r e a represents t h e t u n i n g range of t h e V U V energy.  B o n d lengths a r e  i n g between the two equivalent conformers of ( H F ) 2 (labelled as H F - H b F b a n d F H - F ( , H f , ) . a  T h e e q u i l i b r i u m s t r u c t u r e of H F H from different w o r k ,  1 8 - 2 0  +  a  a  i o n i n the M i g r o u n d electronic state is available  with R _ = 0 . 9 7 ± 0 . 0 l A and ZHFH=114±2°. i f  1 7  a  T h e two s t r e t c h i n g  F  m o d e frequencies have b e e n a c c u r a t e l y measured b y h i g h r e s o l u t i o n i n f r a r e d a b s o r p t i o n spectroscopy  (vi=3348.7078±0.36cm- , •y =3334.6895±0.26cm- ), 1  quency is a b o u t U 2 = 1 3 9 0 ± 1 5 c m . - 1  1  3  1 9  18  a n d the b e n d i n g m o d e fre-  T h e p o t e n t i a l energy surface u p t o 1.5eV above the zero  p o i n t energy are also available b y v a r i a t i o n a l c a l c u l a t i o n s p e r f o r m e d i n J a c o b i c o o r d i n a t e s . These discrete v i b r a t i o n a l levels have a n e s t i m a t e d u n c e r t a i n t y of tens of c m  - 1  1 9 , 2 1  .  It was s h o w n i n t h e w o r k of N g a n d T i e d e m a n n et al t h a t ( H F ) 2 a n d other higher clusters c o u l d be f o r m e d i n the e x p a n s i o n of H F molecular b e a m . ' 2 2  2 3  P h o t o i o n i z a t i o n of the H F b e a m  c o u l d y i e l d i o n species H F H , ( H F ) 2 H etc., w h i l e the parent ions ( H F ) + ( w i t h n > 1) were not +  observed.  For H F H  +  +  i o n , the y i e l d curve is s m o o t h a n d shows v e r y l i t t l e s t r u c t u r e . F r o m the  onset of the y i e l d curve the appearance p o t e n t i a l of H F H  +  was m e a s u r e d to be 15.65±0.04eV.  T h i s value was r e g a r d e d as the d i s s o c i a t i o n t h r e s h o l d to f o r m p r o d u c t s H F H  +  + e~ + F , a n d  was t h e n used to c a l c u l a t e the p r o t o n affinity of H F (the e n t h a l p y change AH  for the process  H F + H + - > H F H + ) . T h e y o b t a i n e d a value of 9 5 . 5 ± 1 . 4 k c a l / m o l . H o w e v e r , a m u c h higher value for H F p r o t o n affinity was r e p o r t e d b y Foster a n d B e a u c h a m p , w h o s t u d i e d the p r o t o n exchange r e a c t i o n C H s  +  + H F —> CH4 + H F H .  2  B y m e a s u r i n g the  +  e q u i l i b r i u m constant K, t h e y were able to calculate the free energy change A G for t h i s r e a c t i o n . W i t h a n e s t i m a t e d v a l u e of e n t r o p y change A S , the e n t h a l p y change AH T h e AH  c o u l d be o b t a i n e d .  value is t h e difference between the p r o t o n affinities of H F a n d CH4. U s i n g the k n o w n  p r o t o n affinity of CH4, t h e p r o t o n affinity of H F was o b t a i n e d as 1 1 2 ± 2 k c a l / m o l . T h e r e are also some ab initio  c a l c u l a t i o n results a b o u t H F p r o t o n affinity. B y u s i n g the  m e t h o d of M R D - C I (multireference single- a n d d o u b l e - e x c i t a t i o n c o n f i g u r a t i o n i n t e r a c t i o n ) , P e t s a l a k i s et al gave a p r o t o n affinity of 1 1 6 . 5 k c a l / m o l for H F (no u n c e r t a i n t y q u o t e d ) , is v e r y close to S C F (self-consistent-field)  c a l c u l a t i o n results ( 1 1 6 . 3  25  and  19  which  116.9kcal/mol ). 26  A l l these c a l c u l a t i o n results agree reasonably w e l l w i t h the value o b t a i n e d b y Foster a n d Beauchamp.  2 4  If the p r o t o n affinity value for H F o b t a i n e d f r o m t h e r m o c h e m i c a l r e a c t i o n a n d ab initio c a l -  c u l a t i o n s is correct, the d i s s o c i a t i o n t h r e s h o l d of (HF)2 i n t o H F H 14.52eV, a n d the i o n - p a i r t h r e s h o l d of ( H F ) (EA(F)=27432.440±0.025cm-  1  + e  +  -  + F w o u l d be a b o u t  (to f o r m H F H + + F ) w o u l d be a b o u t -  2  11.22eV  or ~ 3 . 4 0 e V ) . These t h r e s h o l d s are also s h o w n i n F i g u r e 6.1. 2 7  In the c u r r e n t w o r k , the (HF)2 d i m e r was e x c i t e d w i t h t u n a b l e V U V p h o t o n s i n t h e energy range f r o m 14.7 t o 15.9eV. I n i t i a l l y the goal was t o s t u d y t h e i o n - p a i r c h a n n e l H F H + -I- F , -  a l t h o u g h the H F H + e  -  i o n c o u l d also be p r o d u c e d f r o m the dissociative i o n i z a t i o n c h a n n e l H F H  +  +  + F . A n o t h e r g o a l of t h i s w o r k was to s t u d y the m e c h a n i s m s of these t w o d i s s o c i a t i o n  channels a n d c o m p a r e t h e i r cross sections. a t i o n thresholds, the H F H  Since the V U V energy is above t h e two dissoci-  c o u l d be p r o d u c e d at h i g h v i b r a t i o n a l levels, w h i c h w o u l d allow  +  the s t u d y of f r a g m e n t a t i o n r a t i o i n t o p r o d u c t s i n different v i b r a t i o n a l states. since the p o t e n t i a l surface of H F H vibrational bands i n H F H  +  +  has been c a l c u l a t e d ,  1 9 , 2 1  Furthermore,  one c o u l d assign the different  s p e c t r u m recorded b y the h i g h r e s o l u t i o n technique of T I P P S / P F I -  Z E K E . O u r results w o u l d resolve the discrepency between the H F p r o t o n affinity values from previous p h o t o i o n i z a t i o n technique a n d other methods.  6.2  Experimental  T h e d i m e r s of ( H F ) 2 were e x c i t e d b y a pulsed coherent V U V l i g h t source w h i c h was generated t h r o u g h resonant four-wave m i x i n g v = 2v\ + v  2  i n a pulsed supersonic K r or X e gas b e a m .  2 8  O n e i n p u t w a v e l e n g t h v\ was fixed so t h a t 2v\ corresponds to either K r 4 p 5 p [ l / 2 , 0 ] resonance 5  at 9 4 0 9 3 . 6 6 2 c m  - 1  or X e 5 p 6 p ' [ l / 2 , 0 ] resonance at 8 9 8 6 0 . 5 3 8 c m . 5  - 1  2 9  T h e other wavelength v  2  was scanned over l O O n m (590-720nm) to t u n e the final V U V energy i n the l o n g range (14.715.9eV) necessary for t h i s e x p e r i m e n t . T h e t a b u l a t e d resonance value was used for 2v\, w h i l e the w a v e l e n g t h of v  2  was c a l i b r a t e d i n two short ranges ( w i t h each range ~ 1 0 n m ) u s i n g o p t o -  galvanic spectroscopy i n a hollow cathode d i s c h a r g e .  30  T h e o p t o g a l v a n i c c a l i b r a t i o n was t h e n  e x t r a p o l a t e d to the w h o l e range a n d was found to agree w i t h the r e a d i n g given b y a B u r l e i g h W A 4 5 0 0 wavemeter to ± 0 . 0 1 n m .  T h e final absolute V U V energy was e s t i m a t e d to have a n  u n c e r t a i n t y of ± 1 . 5 c m , w i t h a b a n d w i d t h of ~ l c m - 1  - 1  .  T h e V U V l i g h t was separated f r o m the f u n d a m e n t a l b y a one meter focal l e n g t h n o r m a l  incidence m o n o c h r o m a t o r , w h i c h also focused the V U V i n t o a n u n c o l l i m a t e d pulsed jet of H F f r o m a G e n e r a l V a l v e Series 9 pulsed source about 5 c m d o w n s t r e a m f r o m the nozzle. T h e H F gas ( C P grade, M a t h e s o n G a s P r o d u c t s , Inc.) was 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 , a n d the s t a g n a t i o n pressure i n t h e source was about l b a r . T h e pressure i n the r e a c t i o n chamber was ~ 8 x l 0 t o r r w i t h the m o l e c u l a r b e a m o n , w i t h a b a c k g r o u n d pressure of ~ 2 . 0 x l 0 t o r r . - 6  - 7  For the t o t a l i o n y i e l d s p e c t r u m , a n e x t r a c t i o n field pulse of 3 5 V / c m was a p p l i e d to the i n t e r a c t i o n region 5/iS after the laser pulse a n d the H F H flight mass s p e c t r o m e t e r .  +  ions were detected w i t h a t i m e of  F o r the P F I s p e c t r u m , a d i s c r i m i n a t i o n field pulse of l O V / c m a n d  1/us d u r a t i o n was a p p l i e d 300ns after the laser to repel a n y ions f o r m e d f r o m above t h r e s h o l d processes. A t a delay t i m e of 5fis after the laser, a n e x t r a c t i o n field pulse of 3 5 V / c m was a p p l i e d to field dissociate l o n g - l i v e d R y d b e r g states a n d e x t r a c t the r e s u l t a n t H F H  +  i o n i n t o the mass  spectrometer.  6.3  Results and Discussion  T h e i n t e r a c t i o n of a m o l e c u l a r b e a m of H F w i t h V U V p h o t o n s at 15.90eV y i e l d s the time-offlight s p e c t r u m i n F i g u r e 6.2. B o t h H F  +  a n d H F H , together w i t h ions f r o m larger complexs +  were observed. T h e i o n i z a t i o n p o t e n t i a l of H F is 1 6 . 0 4 e V ,  31  therefore the H F  +  signal in Figure  6.2 was f r o m r o t a t i o n a l l y e x c i t e d H F molecules (J" > 7). T h e r e l a t i v e i n t e n s i t y of H F  +  against H F H  decreases f r o m 16.02 to 15.87eV, the H F  +  +  is s h o w n i n F i g u r e 6.3.  A s V U V energy  s i g n a l drops q u i c k l y . Since the i o n y i e l d a n d P F I  s p e c t r a were r e c o r d e d i n the energy range f r o m 14.7 t o 15.9eV, there was n o c o n t a m i n a t i o n of the H F H  +  signal from H F  +  considering the relative n a r r o w w i d t h (~30ns) of the boxcar  detection gate. T h e r e are no peaks c o r r e s p o n d i n g to the species of ( H F ) + i n F i g u r e 6.2, w h i c h is consistent w i t h the o b s e r v a t i o n i n previous p h o t o i o n i z a t i o n w o r k s . ' 2 2  2 3  T h i s was c o n f i r m e d b y the T O F  s p e c t r u m recorded f r o m a H F m o l e c u l a r b e a m m i x e d w i t h A r (see F i g u r e 6.4). W h e n the V U V energy was below the i o n i z a t i o n p o t e n t i a l of A r ( 1 2 7 1 0 9 . 8 0 c m  -1  or ~ 1 5 . 7 6 e V ) , there was no 3 2  (HF) H 2  +  (HF) tf 3  0 i  i  i  i  i  200  i  i  i  I  400  i  i  i  I — i — u — i *~r i — i — i — i  600  800  1000  Time F i g u r e 6.2: I o n species p r o d u c e d f r o m p h o t o i o n i z a t i o n of H F m o l e c u l a r b e a m a t 15.90eV.  peak of m / e = 40. W h e n the V U V energy was above A r i o n i z a t i o n p o t e n t i a l , a peak of mass 40 was observed. Since b o t h ( H F ) J a n d A r  +  have m / e = 40, F i g u r e 6.4 showed t h a t there was  no ( H F ) ^ p r o d u c e d . T h e t o t a l i o n y i e l d s p e c t r u m of H F H  n o r m a l i z e d w i t h V U V i n t e n s i t y is shown i n F i g u r e  +  6.5. T h e i o n s i g n a l increases s u b s t a n t i a l l y as V U V energy rises above 15.65eV. It confirms the observation i n p r e v i o u s p h o t o i o n i z a t i o n w o r k , ' 2 2  for the appearance p o t e n t i a l of H F H . +  2 3  a n d w h y t h e y gave a value of 15.65±0.04eV  However, our y i e l d s p e c t r u m c l e a r l y shows t h a t there  is also i o n s i g n a l at V U V energies as low as 14.8eV. T h i s indicates t h a t the appearance energy of H F H  +  is a c t u a l l y m u c h lower t h a n the value given i n p r e v i o u s w o r k , a n d t h a t the h i g h i n -  tensity of H F H  +  s i g n a l above 15.65eV p r o b a b l y comes f r o m the F r a n k - C o n d o n favored v e r t i c a l  t r a n s i t i o n s f r o m the m o l e c u l a r g r o u n d state i n t o closely spaced h i g h v i b r a t i o n a l levels of the ion. A n o t h e r n o t i c a b l e p o i n t is t h a t w h i l e the y i e l d curve i n p r e v i o u s w o r k is s m o o t h a n d shows no s t r u c t u r e , there are m a n y very sharp peaks i n our h i g h r e s o l u t i o n s p e c t r u m . resolved s h a r p peaks have a t y p i c a l w i d t h ( F W H M ) of ~ 2 c m  - 1  These well  .  T o check w h e t h e r the s h a r p peaks present i n the y i e l d s p e c t r u m are f r o m t r a n s i t i o n s to resonance states, a short range of the s p e c t r u m was collected w i t h different a m p l i t u d e s of disc r i m i n a t i o n pulses of 6, 10 a n d 1 6 V / c m (see F i g u r e 6.6), no shift was observed for the blue edges of the s i g n a l e x p e c t e d for S t a r k b e h a v i o r .  33  Therefore, i t is c e r t a i n those peaks are due  to resonant t r a n s i t i o n s t o p r e d i s s o c i a t i n g states. T o f u r t h e r u n d e r s t a n d those resonance peaks i n the H F H  y i e l d s p e c t r u m , there are a few  +  issues need to be c l a r i f i e d . F i r s t , is ( H F ) the o n l y source for p r o d u c t i o n of H F H 2  a m o n g the two c o m p e t i t i v e processes t h a t c a n produce H F H  To produce H F H  +  +  ion? Second,  f r o m ( H F ) : the dissociative  +  2  i o n i z a t i o n process a n d the i o n - p a i r process, w h i c h is d o m i n a n t ? m e c h a n i s m for H F H  +  T h i r d , w h a t is the d e t a i l e d  production? i o n f r o m a H F molecular b e a m , there are a few possible sources. Beside  ( H F ) 2 , some other clusters s u c h as ( H F )  3  are also generated i n the e x p a n s i o n of H F b e a m .  P h o t o e x c i t a t i o n of ( H F ) c o u l d y i e l d different i o n p r o d u c t s t h r o u g h (HF) —> ( H F ) H 3  3  +  2  + e~  +  F a n d ( H F ) 5 - > H F H + + e~ + F + H F . T h e t h r e s h o l d t o m a k e H F H + i o n f r o m ( H F ) is higher 3  t h a n the t h r e s h o l d from ( H F )  2  b y the b i n d i n g energy of ( H F )  2  a n d H F ( ~ 0 . 1 e V ) . B u t since  F i g u r e 6.4: T O F s p e c t r u m f r o m H F b e a m m i x e d w i t h A r .  F i g u r e 6.5: T o t a l i o n y i e l d s p e c t r u m of H F H . +  A n e x t r a c t i o n field of 3 5 V / c m was p u l s e d o n  5/is after p h o t o e x c i t a t i o n . N o d i s c r i m i n a t i o n field was a p p l i e d .  2.0 r 1.8  -  1.6 ~  7  1.4  Q 0  -  t— — — — 1  15.824  1  1  1  I  1  15.825  1  1  1  I  I  15.826  I—I  I  I  1  15.827  1  1  1  I  '  '  15.828  V U V energy /eV F i g u r e 6.6: P F I s i g n a l of H F H + w i t h different d i s c r i m i n a t i o n fields.  '  HFH  c o u l d be p r o d u c e d i n different v i b r a t i o n a l levels a n d there are r e l a t i v e l y large uncer-  +  tainties i n the energetics of the v i b r a t i o n a l levels, one w o u l d not be able to t e l l w h i c h peak i n the s p e c t r u m is f r o m w h i c h p a r e n t n e u t r a l . However, if i t is assumed t h a t ( H F ) 3 c o u l d be the c o m m o n source to m a k e ( H F ) 2 H  +  a n d H F H , one w o u l d expect t h a t at c e r t a i n p h o t o n energies +  c o r r e s p o n d i n g t o specific resonances of (HF)*., b o t h ( H F ) 2 H  +  and H F H  c o u l d be p r o d u c e d as  +  the p r o d u c t of u n i m o l e c u l a r d i s s o c i a t i o n of (HF)*;. A s the r e s u l t , some peaks i n ( H F ) H 2  HFH  y i e l d s p e c t r a w o u l d overlap. F i g u r e 6.7 shows the y i e l d s i g n a l of ( H F ) H  +  2  i n the energy range f r o m 15.80 t o 15.87eV. W h i l e the H F H the ( H F ) 2 H  +  +  +  and  and H F H  +  +  s p e c t r u m shows m a n y resonances,  curve is r a t h e r s m o o t h . Therefore, the ( H F ) 3 c o m p l e x is v e r y u n l i k e l y to p r o d u c e  HFH+. O n the other h a n d , i t is necessary to check w h e t h e r H F H t h r o u g h the process H F * + HF—> H F H w o u l d expect t h a t H F H  +  +  +  is p r o d u c e d from collision  + e~ + F . If t h i s c o l l i s i o n process is i m p o r t a n t , one  i n t e n s i t y increases s u b s t a n t i a l l y w i t h t h e c o l l i s i o n t i m e . T h i s phe-  n o m e n o n was not observed, w h i l e a c t u a l l y the i o n i n t e n s i t y seemed t o be stable d u r i n g the w a i t i n g p e r i o d of a few fis after p h o t o e x c i t a t i o n . T h i s is s h o w n i n F i g u r e 6.8. T h e i n t e n s i t y of the H F H  +  i o n s i g n a l stays more or less stable as the delay t i m e of the e x t r a c t i o n pulse varies  f r o m 400ns to 4 /xs. A n o t h e r w a y to check the c o l l i s i o n effect is to c o m p a r e the s p e c t r u m of H F H of H F . Since H F * is the source to make H F +  +  i o n , one c a n collect H F  and H F H  +  w i t h that  +  s i g n a l at  +  the same t i m e . F i g u r e 6.9 shows t h a t there is no clear correspondence between the s p e c t r a of HF+  and H F H + . For the p r o d u c t i o n of H F H  +  f r o m ( H F ) 2 , there are two c o m p e t i t i v e channels. O n e is the  dissociative i o n i z a t i o n c h a n n e l ( H F ) 2 —• H F H (HF)  2  +  + e~ + F . T h e other one is the i o n - p a i r c h a n n e l  -> H F H + + F , for w h i c h the t h r e s h o l d is 3.40eV ( E A value of F a t o m ) -  2 7  lower t h a n  the first c h a n n e l . T o test w h i c h c h a n n e l is the m a j o r one to p r o d u c e H F H , the p o l a r i t y of the +  e x t r a c t i o n field a n d d e t e c t i o n voltage were i n v e r t e d to check the negative s i g n a l ( e T h e r e was no o b s e r v a t i o n of F , w h i l e detecting of e _  _  _  or F ) . _  s i g n a l gave t h e same s p e c t r u m as H F H  +  positive i o n (see F i g u r e 6.10). S i m i l a r comparisons were p e r f o r m e d i n t w o other short energy regions a n d led to the same c o n c l u s i o n . Therefore, the dissociative i o n i z a t i o n is the d o m i n a n t  J  1  15.80  I  I  I  I  15.82  I  I  I  I  I  i  15.84  i  l  i  15.86  V U V energy /eV F i g u r e 6.7: C o m p a r i s o n of y i e l d s p e c t r a of H F H  +  and  (HF)2H . +  i  i  480  500  520  540  560  Time F i g u r e 6.8: T O F s p e c t r u m of H F H  +  i o n w i t h different d e l a y t i m e of t h e e x t r a c t i o n pulse.  F i g u r e 6.9: C o m p a r i s o n of the y i e l d s p e c t r a for H F  +  and H F H + .  c h a n n e l to p r o d u c e H F H  +  f r o m (HF)2 over the p h o t o n energy range s t u d i e d .  T h e v e r y weak t r a n s i t i o n to the i o n - p a i r states is at least p a r t l y due to the v e r y few n u m ber of (HF)?, resonance states available. Since the i o n - p a i r p o t e n t i a l is 3.40eV lower t h a n the c o r r e s p o n d i n g dissociative i o n i z a t i o n p o t e n t i a l , there are m a n y fewer resonance states c o u p l e d to the i o n - p a i r c h a n n e l t h a n to the dissociative i o n i z a t i o n c h a n n e l . T h e P F I s p e c t r u m of H F H  +  i n the same energy range as the t o t a l i o n y i e l d s p e c t r u m is  s h o w n i n F i g u r e 6.11, also n o r m a l i z e d to the V U V intensity. T h e o v e r a l l s t r u c t u r e of the P F I s p e c t r u m shows s i m i l a r p a t t e r n to the t o t a l y i e l d s p e c t r u m , a n d every p e a k observed i n P F I s p e c t r u m was also seen i n the t o t a l y i e l d s p e c t r u m . T o m a k e sure t h a t we were not r e c o r d i n g some p r o m p t H F H  +  ions for the P F I s p e c t r u m , a M A T I s p e c t r u m of A r was recorded under  s i m i l a r gas pressure w i t h even smaller d i s c r i m i n a t i o n fields (see F i g u r e 6.12). It was confirmed t h a t such e x p e r i m e n t a l c o n d i t i o n s guaranteed complete d i s c r i m i n a t i o n against p r o m p t  HFH  +  ion. A c t u a l l y since resonance enhancement is a b i g factor i n b o t h t h e y i e l d a n d P F I signals, it is not s u r p r i s i n g t h a t some peaks appear at the same p o s i t i o n s i n b o t h s p e c t r a . A t the same t i m e , there are some peaks present i n the y i e l d s p e c t r u m t h a t are m i s s i n g i n the P F I s p e c t r u m . T h i s is better s h o w n i n F i g u r e 6.13 where the t o t a l y i e l d a n d P F I s p e c t r a of shorter energy range are c o m p a r e d . T h a t is because those resonances lie above the thresholds of c e r t a i n d i s s o c i a t i o n l i m i t s a n d therefore do not couple to m a k e P F I s i g n a l . T h e resonances observed i n the s p e c t r a are l i s t e d i n T a b l e 6.1.  15  10  e  5  0  -  15.80  15.82  15.84  15.86  V U V energy /eV F i g u r e 6.10: C o m p a r i s o n of the H F H  +  and e  signals.  I  I  I  I  I  14.8  I  15.0  I  I  I  1  15.2  VUV F i g u r e 6.11: P F I s p e c t r u m of H F H . +  I  I  I  I  '  '  15.4  energy  '  I  15.6  1 9  -  2 1  I  I  I  I  I  15.8  /eV  A d i s c r i m i n a t i o n field of l O V / c m m a g n i t u d e a n d 1/is d u r a t i o n was p u l s e d o n 300ns after  p h o t o e x c i t a t i o n , a n d a n e x t r a c t i o n field of 3 5 V / c m was p u l s e d o n 5/JS after p h o t o e x c i t a t i o n . v i b r a t i o n a l levels of H F H + .  I  T h e s t a r signs are the c a l c u l a t e d  T o p r o d u c e P F I signals, besides the necessity for the resonance states of (HF)?, to couple to the d i s s o c i a t i o n c h a n n e l , energetically the resonance states have t o be j u s t ( a b o u t 12 to 2 4 c m ~ w i t h the c u r r e n t field s t r e n g t h s )  33  below c e r t a i n dissociative i o n i z a t i o n l i m i t s w i t h H F H  +  1  prod-  uct i n different r o v i b r o n i c states. Therefore, the peak positions i n t h e P F I s p e c t r u m also reflect various dissociative i o n i z a t i o n l i m i t s a l t h o u g h some l i m i t s m a y be unaccessible due to the lack of c o u p l i n g resonance states at the c o r r e s p o n d i n g energy. F r o m t h i s p o i n t a c o m p a r i s o n of the P F I s p e c t r u m w i t h the energetic spacing of the H F H  +  i o n is necessary to o b t a i n any energetic  information. I n the P F I s p e c t r u m ( F i g u r e 6.11) the v i b r a t i o n a l levels of H F H  +  from theoretical calculations '' 19  are l a b e l l e d w i t h star s y m b o l s . T h e y are shifed i n energy to m a t c h t o the best w i t h the energetic p a t t e r n of P F I s p e c t r u m . B u t since b o t h (HF)2 a n d H F H  +  c o u l d be r o t a t i o n a l l y excited, i t  is not possible t o e x a c t l y determine the v i b r a t i o n a l b a n d heads. Therefore, i f a n u n c e r t a i n t y of ± 2 0 0 c m  - 1  is allowed for assignments of P F I signal to different H F H  the m a t c h is reasonable. F r o m t h i s , the H F H for the process (HF)2~-+ H F H  +  +  +  v i b r a t i o n a l levels,  appearance p o t e n t i a l is given as 14.50±0.03eV  + e~ + F . T h e p r o t o n affinity of H F was t h e n c a l c u l a t e d t o  be 5 . 0 7 ± 0 . 0 3 e V (or 1 1 6 . 9 ± 0 . 7 k c a l / m o l ) , w h i c h agrees w e l l w i t h the result i n references 19, 25 and  26, a n d t h u s resolve the discrepancy between the results f r o m p r e v i o u s p h o t o i o n i z a t i o n  work ' 2 2  2 3  a n d other l i t e r a t u r e v a l u e s  1 9 , 2 5  ' . 2 6  R e g a r d i n g the m e c h a n i s m of the dissociative i o n i z a t i o n process, the following process is proposed: (1) p h o t o e x c i t a t i o n H - F — H - F + hv -> ( H - F ) * — H - F (2) i n t r a m o l e c u l a r r e a c t i o n ( H - F ) * — H - F -> ( H - F - H — F ) * (3a) dissociative i o n i z a t i o n ( H - F - H — F ) * - c H F H + + F + e~ (3b) d i s s o c i a t i o n ( H - F - H — F ) * -> H F H * + F T h e first step is t h e a b s o r p t i o n of the V U V p h o t o n b y one H - F m o n o m e r i n the d i m e r c o m plex. T h e h y d r o g e n b o n d is r e l a t i v e l y weak a n d w o u l d easily get b r o k e n i f i t is the one g e t t i n g e x c i t e d . T h e second step is t h r o u g h i n t r a m o l e c u l a r r e a c t i o n , one H a t o m is transfered from the o r i g i n a l l y s t r o n g l y b o n d e d F a t o m t o the other F a t o m . T h i s step is necessary to produce the final fragments of H F H  +  a n d F . T h e t h i r d step is the d i s s o c i a t i o n of ( H - F - H — F ) * . F o r the y i e l d  20  30  40  V U V energy - 128500 /cm" F i g u r e 6.12:  50  1  M A T I s p e c t r a of A r w i t h different d i s c r i m i n a t i o n f i e l d s . T h e d i s c r i m i n a t i o n field  of lfis d u r a t i o n was p u l s e d o n 300ns after p h o t o e x c i t a t i o n , a n d a n e x t r a c t i o n field of 3 5 V / c m was p u l s e d o n 5/zs after p h o t o e x c i t a t i o n .  2.5  2.0  total yield §  Sb •»-(  1.5  + J  j y k J in ftjuui.  PFI  0.5  0.0 i  15.00  i  i  i  i  15.05  i  i  i  i  i  15.10  i  i  i  i  i  15.15  i  i  i  i  i  15.20  i  i  ' ' ' ' ' ' ' 15.25  V U V energy /eV F i g u r e 6.13: C o m p a r i s o n of t o t a l y i e l d a n d P F I s p e c t r a for H F H + .  15.30  T a b l e 6.1: R e s o n a n c e s seen i n H F H  +  t o t a l y i e l d s p e c t r u m F i g u r e 6.5.  s p o n d i n g f i n a l states are n o t k n o w n . E n e r g i e s are i n c m u n c e r t a i n t y of ± 1 . 5 c m  _ 1  -  1  E n e r g y levels of corre-  a n d relative to 1 2 0 0 0 0 c m  - 1  with an  f r o m c a l i b r a t i o n . N o t a t i o n : * i n d i c a t e s a resonance also observed i n  P F I s p e c t r u m F i g u r e 6.11.  peak  intensity  peak  intensity  peak  intensity  peak  intensity  peak  intensity  -248  0.2  -222  0.1  24  0.2  94  0.3  97  0.2  peak  intensity  898  0.1  1217  0.8  1251  0.2  1393  0.8  1528  0.2  1552  0.1  1886  0.6  1910  0.3  1979  0.1  1995  0.4  2051  0.5  1745  0.4  2088*  2.0  2136*  1.2  2155*  0.2  2170*  0.4  2679*  0.2  2789  0.1  2939  0.1  3013*  0.4  3028*  0.2  3060*  0.3  3219  0.1  3249*  0.1  3280*  0.1  3294*  0.1  3367*  0.1  3445*  0.2  3450  0.1  3465*  0.2  3585*  0.1  3600*  0.2  3642*  0.1  3649  0.1  3695*  0.3  3716*  0.3  3783*  0.1  3799*  0.1  3806*  0.1  3863*  0.1  3877*  0.1  3884*  0.1  3908  0.1  3930*  0.2  3935*  0.1  3941  o.i •  3974*  0.2  3983*  0.1  3990*  0.1  4021*  0.2  4039  0.6  4054  0.1  4111  0.2  4144  0.6  4170  0.1  4221  0.1  4229  1.7  4244  0.7  4251  0.1  4255  0.3  4259  0.2  4265  0.1  4269  0.3  4335  1.4  4356*  12.8  4385*  4.6  4425  0.1  4497  2.0  4510  0.1  4712  0.1  4729  0.1  4744  0.2  4821  1.1  4871  1.8  4955  0.1  5007  0.2  5173  0.1  5200  0.1  5220  0.2  5292  0.1  5300  0.1  5307  0.2  5315  0.6  5422  0.1  5470  0.1  5519  0.1  5627  0.2  5647  3.1  5712  0.4  5796  0.3  5835  0.4  5879  0.2  6183  1.5  6211  0.5  6239  0.7  6246  0.7  6281*  3.2  6290  1.5  6303*  6.0  6315  1.5  6319*  2.5  6349  0.7  6355*  6.0  6362*  2.0  6367*  1.0  6369  1.1  6381*  2.2  6391  0.6  6406  0.6  6416  1.1  6428*  12.0  6442*  6.3  6518  0.5  6520*  0.5  6529*  0.6  6558  0.8  6586  0.3  6599  0.6  6602  0.6  6615  0.6  6621  0.5  6664  0.6  6729*  0.8  6798  0.8  6869  1.8  6931*  10.0  7070  1.3  7084  4.5  7130*  1.0  7145  0.9  7147*  1.2  7177  2.3  7187*  1.0  7217  3.0  7248  2.3  7260*  6.0  7280*  8.0  7314  3.2  7317*  2.0  7363  0.4  7376*  3.3  7394*  6.6  7413  6.8  7433  3.8  7447*  0.6  7455*  3.2  7478*  14.2  7486*  3.5  7530*  44.4  7543*  20.4  7548*  60.0  7583*  2.2  7593*  23.6  7599*  45.7  7640*  8.7  7648* . 6.4  7656*  4.9  7668*  24.2  7675  5.0  7684*  1.8  7694*  2.5  7697*  5.8  7704*  1.5  7711*  1.5  7715*  8.0  7726*  1.8  7732  2.5  7775  3.8  7788  3.4  7843  7.0  7856  6.5  7927  5.0  7968*  7.0  signal, prompt H F H  +  ions are generated d i r e c t l y v i a r e a c t i o n 3a. F o r the P F I s i g n a l , R y d b e r g  H F H * states are generated b y r e a c t i o n 3b w h i c h are t h e n field i o n i z e d to m a k e t h r e s h o l d H F H ion.  I n 3b, F a t o m has t o leave t h e s y s t e m before e  _  to m a k e R y d b e r g H F H * .  +  B u t for step  3a, i t is h a r d to t e l l w h e t h e r the F a t o m leaves before or at the same t i m e as the e . _  One  m i g h t argue t h a t i f t h e y were p r o d u c e d at the same t i m e , t h e y w o u l d c o m b i n e to f o r m the F ~ ion w h i c h is m u c h m o r e stable. However, considering the f a i r l y large distance of between the F a t o m a n d the e~ i n ( H - F - H — F ) * , there is not m u c h chance t h a t F a t o m w o u l d c a p t u r e the e~.  References 1. P i n e A S a n d H o w a r d B J 1986 J. Chem.  Phys.  84 590  2. C o l l i n s C L , M o r i h a s h i K , Y a m a g u c h i Y a n d Schaefer H F 1995 J. Chem.  Phys.  103  6051 a n d c i t e d references 3. H o w a r d B J , D y k e T R a n d K l e m p e r e r W 1984 J. Chem.  Phys.  8 1 5417 a n d c i t e d  references 4. G u t o w s k y H S, C h u a n g C , K e e n J D , K l o t s T D a n d E m i l s s o n T 1985 J. Chem.  Phys.  83 2070 5. H e r z b e r g G a n d H u b e r K - P 1950 Molecular Diatomic  Molecules  Spectra and Molecular  Structure  I. Spectra  of  (New York: V a n Nostrand)  6. P i n e A S a n d L a f f e r t y W J 1983 J. Chem.  Phys.  78 2154  7. Y u Z , H a m m a m E a n d K l e m p e r e r W 2005 J. Chem.  Phys.  122 194318  8. H a l b e r s t a d t N , B r e c h i g n a c P , B e s w i c k J A a n d S h a p i r o M 1986 J. Chem. 9. Z h a n g D H , W u Q a n d Z h a n g J Z H 1995 J. Chem.  Phys.  84 170  Phys.  119 286  102 124  10. V i s s e r s G W M , G r o e n e n b o o m G C a n d v a n der A v o i r d A 2003 J. Chem. 11. P i n e A S, L a f f e r t y W J a n d H o w a r d B J 1984 J. Chem.  Phys.  Phys.  81 2939  12. H u a n g Z S, J u c k s K W a n d M i l l e r R E 1986 J. Chem. 13. v o n P u t t k a m e r K a n d Q u a c k M 1989 Chem.  85 3338  Phys.  139 31  Phys.  14. M a r s h a l l M D , B o h a c E J a n d M i l l e r R E 1992 J . Chem.  Phys.  97 3307  15. S u h m M A , F a r r e l l J T , M c l l r o y A a n d N e s b i t t D J 1992 J. Chem. 16. K l o p p e r W , Q u a c k M a n d S u h m M A 1998 J. Chem. 17. H o u g e n J T a n d O h a s h i N 1985 J. Mol. Spectrosc. 18. Schafer E a n d S a y k a l l y R J 1984 J. Chem.  Phys.  Phys.  97 5341  108 10096  Phys.  109 134 81 4189  19. P e t s a l a k i s I D , T h e o d o r a k o p o u l o s G , W r i g h t J S a n d H a m i l t o n I P 1990 J. Chem.  Phys.  92 2440 20. B o t s c h w i n a P 1983 Proceedings Geometric  and Electronic  of a NATO  Structure  Advanced  Study Institute  on Molecular  Ions:  ed J B e r k o w i t z a n d K O G r o e n e v e l d ( N e w Y o r k :  Plenum) 21. B u n k e r P R , J e n s e n P , W r i g h t J S a n d H a m i l t o n I P 1990 J. Mol.  Spectrosc.  144 310  22. N g C Y , T r e v o r D J , T i e d e m a n n P W , C e y e r S T , K r o n e b u s c h P L , M a h a n B H a n d L e e Y T 1977 J. Chem.  Phys.  67 4235  23. T i e d e m a n n P W , A n d e r s o n S L , C e y e r S T , H i r o o k a T , N g C Y , M a h a n B H a n d Lee Y T 1979 J. Chem.  Phys.  71 605  24. Foster M S a n d B e a u c h a m p J L 1975 Inorg. 25. L i s c h k a H 1973 Theor.  Chim.  Chem.  14 1229  Acta 31 39  26. M a v r i d i s A a n d H a r r i s o n J F 1982 J. Chem.  Soc. Faraday  Trans. 2 78 447  27. B l o n d e l C , C a c c i a n i P , D e l s a r t C a n d T r a i n h a m R 1989 Phys. 28. H e p b u r n J W 1995 Laser Techniques W i l e y ) pp 149-183  in Chemistry  Rev.  A 40 3698  ed A M y e r s a n d T R R i z z o ( N e w Y o r k :  29. M o o r e C E 1971 Atomic  Energy  Levels,  Vols.  I-III  (Washington, D . C . : U.S. National  B u r e a u of S t a n d a r d s ) 30. D o v i c h i N J , M o o r e D S a n d K e l l e r R A 1982 Appl.  Opt. 2 1 1468  31. M a n k A , R o d g e r s D a n d H e p b u r n J W 1994 Chem.  Phys.  Lett. 2 1 9 169  32. M i n n h a g e n L 1973 J. Opt. Soc. Am. 6 3 1185 33. G a l l a g h e r T F 1994 Rydberg Atoms  (Cambridge: C a m b r i d g e University Press)  Chapter 7 Concluding Remarks and Future Work  I n the w o r k of t h i s thesis, T I P P S has been a p p l i e d t o H C 1 / D C 1 , H F / D F , H C N a n d ( H F ) . 2  It was d e m o n s t r a t e d t h a t as a h i g h resolution p h o t o i o n i z a t i o n technique, T I P P S c a n provide useful i n f o r m a t i o n a b o u t energetics, m e c h a n i s m a n d d y n a m i c s i n i o n - p a i r f o r m a t i o n processes. F o r energetics, the i o n - p a i r thresholds c o u l d be measured to a n a c c u r a c y of ~ l c m d i a t o m i c molecules ( H C 1 / D C 1 , H F / D F ) , a n d a few c m  - 1  _ 1  for  for t r i a t o m i c molecules ( H C N ) . F r o m  the i o n - p a i r t h r e s h o l d one c a n calculate b o n d d i s s o c i a t i o n energies to a n unprecedented accuracy, a n d investigate t h e B o r n - O p p e n h e i m e r b r e a k d o w n i n isotopomers of d i a t o m i c molecules (HC1/DC1, H F / D F ) . T h e i o n - p a i r f o r m a t i o n m e c h a n i s m can be s t u d i e d . Resonance enhancement c a n p l a y a n i m p o r t a n t role i n the p r o d u c t i o n of i o n - p a i r s . F o r H C 1 / D C 1 , some r e l a t i v e l y b r o a d structures were observed i n the i o n - p a i r y i e l d s p e c t r u m , w h i c h were assigned to R y d b e r g states converging to some v i b r o n i c states of H C 1 / D C 1 ( A E ) . +  +  2  +  F o r H F / D F , s h a r p resonances were observed  i n b o t h the T I P P a n d i o n - p a i r y i e l d spectra. T h e energetic levels of some resonances c o u l d be d e t e r m i n e d , t h u s a l l o w i n g d e t a i l e d analyses of those resonances. T h r e e r o t a t i o n a l sequences of R y d b e r g states converging to H F / D F ( n ! / ) were d e r i v e d as the i n t e r m e d i a t e states i n +  +  2  2  i o n - p a i r f o r m a t i o n . F o r ( H F ) 2 , d e t a i l e d m e c h a n i s m about the p r o d u c t i o n of H F H  +  has been  investigated. S h a r p peaks i n b o t h i o n y i e l d a n d t h r e s h o l d s p e c t r a were assigned to resonance states of ( H F ) . 2  T h e i o n - p a i r f o r m a t i o n d y n a m i c s c a n be s t u d i e d , as s h o w n i n the w o r k o n H C N . T h e C N ~ ( u ' , J') fragments were f o u n d to be r o t a t i o n a l l y hot w i t h a J' m a x i m u m near 12-14. T h e i o n - p a i r f o r m a t i o n process of H C N is likely to go t h r o u g h a bent electronic state, w i t h the  e x c i t a t i o n of t h e b e n d i n g v i b r a t i o n a l m o d e i n d u c i n g t h e p r o d u c t i o n of C N ~ i n h i g h r o t a t i o n a l levels. However, t h i s t e c h n i q u e does have some difficulties. F i r s t , t h e i o n - p a i r cross sections are u s u a l l y v e r y low c o m p a r e d to those for p h o t o i o n i z a t i o n process. F o r d i a t o m i c molecules, it is a b o u t t w o orders of m a g n i t u d e lower ( H F / D F is a n e x c e p t i o n i n w h i c h the i o n - p a i r signal is greatly enhanced b y resonance states). F o r larger molecules, the i o n - p a i r s i g n a l is even weaker (for H C N , the i o n - p a i r s i g n a l H  +  is a b o u t 0.015% of the parent i o n H C N + ) . F o r ( H F ) , the 2  i o n - p a i r s i g n a l is negligible c o m p a r e d to the dissociative i o n i z a t i o n process. O n e has to use the i o n - c o u n t i n g t e c h n i q u e w h e n t h e i o n - p a i r signal is very low ( D C 1 a n d H C N ) to a c c u m u l a t e the t h r e s h o l d i o n s i g n a l over t h o u s a n d s of laser shots, w h i c h takes a l o n g t i m e . Second, i n order t o s t u d y i o n - p a i r f o r m a t i o n m e c h a n i s m s , one needs to k n o w the p o t e n t i a l energies of the resonance states at h i g h energies. U n f o r t u n a t e l y t h i s u s u a l l y is not clearly k n o w n , especially for t r i a t o m i c a n d p o l y a t o m i c molecules. T h e r e f o r e , there are some u n c e r t a i n ties i n d e t e r m i n i n g the i o n - p a i r f o r m a t i o n m e c h a n i s m , even for d i a t o m i c molecules. T h i r d , for t r i a t o m i c a n d p o l y a t o m i c molecules, T I P P s p e c t r a b e c o m e more c o m p l i c a t e d since one or b o t h of the i o n - p a i r fragments c a n be i n different energy levels ( H C N ) . T h i s c a n lead to difficulty i n d e t e r m i n i n g the exact i o n - p a i r t h r e s h o l d . It w o u l d be even more difficult if the energetic levels of the fragment are not w e l l k n o w n . F u r t h e r m o r e , t h e presence of resonances w o u l d m a k e exact measurement of the energetic l i m i t s t o a n a c c u r a c y of a few c m  - 1  almost  impossible (such as ( H F ) 2 ) . F u t u r e w o r k c o u l d be p e r f o r m e d i n the following aspects. F o r d i a t o m i c molecules, there are m a n y m o l e c u l a r systems w h e r e the i o n - p a i r f o r m a t i o n processes have not b e e n s t u d i e d . Therefore, there is s t i l l a lot of w o r k one c a n do. F o r t r i a t o m i c or p o l y a t o m i c molecules, there exists m a n y i n t e r e s t i n g systems t h a t one c a n study, a n d there m i g h t be some ways to i m p r o v e the s i g n a l level.  F i r s t , since the i o n - p a i r  signal is u s u a l l y m u c h weaker t h a n the parent i o n , the M C P detector w o u l d get s a t u r a t e d w i t h the parent i o n w h e n one tries to increases the i o n - p a i r s i g n a l . O n e possible w a y to solve this p r o b l e m is to use a q u a d r u p o l e mass spectrometer w h i c h c a n select the i o n species to detect. If o n l y the i o n - p a i r s i g n a l is detected, it w o u l d be difficult to s a t u r a t e the M C P a n d therefore  w o u l d a l l o w a m p l e r o o m to i m p r o v e the i o n - p a i r s i g n a l ( w h i c h of course requires higher V U V i n t e n s i t y ) . S e c o n d , t o collect t h e signal b y i o n - c o u n t i n g t e c h n i q u e , one needs to fire m a n y laser shots (for e x a m p l e , one t h o u s a n d ) at each energy. R i g h t now t h e N d : Y A G laser used for t h i s project has a r e p e t i t i o n rate of 1 0 H z a n d laser pulses of a b o u t 8ns w i d e . T h i s means t h a t there is a b o u t 100ms d u r a t i o n between two successive laser shots. Since i t o n l y takes tens of fxs for p h o t o e x c i t a t i o n a n d d a t a c o l l e c t i o n , a lot of t i m e is wasted between two laser pulses. If i n the future, a laser s y s t e m w i t h a higher r e p e t i t i o n rate is i n s t a l l e d , i t w o u l d take shorter t i m e to collect a T I P P s p e c t r u m . F u r t h e r m o r e , for large molecules a n d i o n fragments, a l t h o u g h i t is h a r d to resolve the r o t a t i o n a l energy levels, one w o u l d expect to get i n f o r m a t i o n a b o u t v i b r a t i o n a l levels, w h i c h c u r r e n t l y are not k n o w n for m a n y ionic species. F i n a l l y , i t w o u l d be i n t e r e s t i n g to do state-selected i o n - i o n or i o n molecule reactions. Since T I P P c a n p r o d u c e i o n fragments i n c e r t a i n r o v i b r o n i c states, one m i g h t like t o s t u d y the react i v i t y of ions i n those specific states.  

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