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MEMORY EFFECTS IN THE FRAGMENTATION OF MOLECULES FOLLOWING SITE-SPECIFIC CORE
EXCITATION
W. Habenicht, L. Chewter, M. Sander, K. Müller-Dethlefs, E. Schlag
To cite this version:
W. Habenicht, L. Chewter, M. Sander, K. Müller-Dethlefs, E. Schlag. MEMORY EFFECTS IN THE FRAGMENTATION OF MOLECULES FOLLOWING SITE-SPECIFIC CORE EXCITATION.
Journal de Physique Colloques, 1987, 48 (C9), pp.C9-741-C9-744. �10.1051/jphyscol:19879127�. �jpa-
00227236�
JOURNAL DE PHYSIQUE
C o l l o q u e C9, s u p p l A m e n t au n 0 1 2 , Tome 48, d h c e m b r e 1 9 8 7
MEMORY EFFECTS IN THE FRAGMENTATION OF MOLECULES FOLLOWING SITE- SPECIFIC CORE EXCITATION
W. HABENICHT, L.A. CHEWTER, M. SANDER, K. M~~LLER-DETHLEFS a n d
E.W. SCHLAG
Institut fiir Physikalische und Theoretische Chemie, Technischen Universitdt Miinchen, Lichtenbergstrasse 4, 0-8046 Garching, F.R.G.
A b s t r a c t
T h e s i t e - s p e c i f i c e x c i t a t i o n u s i n g s o f t X - r a y s o f i n n e r s h e l l e l e c t r o n s a t two c a r b o n a t o m s i n t h e s a m e m o l e c u l e b u t w i t h v a r i e d c h e m i c a l e n v i r o n m e n t s i s d e s c r i b e d . R e s u l t i n g f r a g m e n t a t i o n p a t t e r n s d e m o n s t r a t e t h a t f r a g m e n t a t i o n i s d e p e n d e n t u p o n t h e s i t e o f e x c i t a t i o n .
I n t r o d u c t i o n
T h e q u e s t i o n a r i s e s w h e t h e r u p o n e x c i t a t i o n o f e l e c t r o n s o f a n i n n e r a t o m i c s h e l l t h e f r a g m e n t a t i o n o f t h e m o l e c u l e i s r a n d o m o r d e p e n d e n t o n t h e l o c a l e n v i r o n m e n t o f t h e e x c i t e d a t o m . The i n - f l u e n c e upon t h e s e n o n - b o n d i n g o r b i t a l s by t h e c h e m i c a l e n v i r o n m e n t ( " c h e m i c a l s h i f t " ) m a k e s i t f e a s i b l e t o e x c i t e a t o m s s e l e c t i v e l y w i t h i n t h e m o l e c u l e ; t h a t i s , t o e x c i t e t h e m o l e c u l e s i t e - s p e c i - f i c a l l y / I / . P a r t i a l f r a g m e n t a t i o n c r o s s - s e c t i o n s o n e x c i t a t i o n o f i n n e r s h e l l e l e c t r o n s h a v e b e e n shown t o d e p e n d o n t h e e x - c i t a t i o n w a v e l e n g t h by E b e r h a r d t e t a l . . M i i l l e r - D e t h l e f s e t a l . h a v e g o n e o n t o show by o b s e r v i n g f r a g m e n t a t i o n p a t t e r n s i n c o - i n c i d e n c e w i t h t h r e s h o l d p h o t o e l e c t r o n s ( T P E ) t h a t s u c h p r o c e s s e s a r e i n d e e d s i t e - s p e c i f i c . We r e p o r t h e r e t h e f r a g m e n t a t i o n p a t t e r n s o f t h e m o l e c u l e s CF2CH2, CF3COCH3, CF3CH2NH2 o n i o n i z a t i o n o f e l e c - t r o n s f r o m t h e C 1 s s h e l l u s i n g t h e TPE p h o t o i o n c o i n c i d e n c e t e c h n i q u e / 3 / .
E x p e r i m e n t a l s e t - u p
The e x p e r i m e n t s w e r e p e r f o r m e d a t t h e h i g h e n e r g y t o r o i d a l mono- c h r o m a t o r (HETGM) a t BESSY, B e r l i n . The H E T G M c o v e r s t h e w a v e l e n g t h f r o m 62 t h r o u g h 1 5 A ( 2 0 0 t h r o u g h 8 0 0 e V ) a n d h a s a r e s o l u t i o n o f a b o u t 0 . 5 eV n e a r t h e C 1 s i o n i z a t i o n e d g e
/ a / .
O u r m o l e c u l a r beam U H V c h a m b e r c o n t a i n s a n e l e c t r o n d e t e c t o r c o m p r i s i n g a n a c c e l e r a t i o n a n d d r i f t r e g i o n a n d a 900 m o d i f i e d M a t s u d a a n a l y z e r a n d a t i m e - o f - f l i g h t (TOF) m a s s s p e c t r o m e t e r f o r t h e i o n s . The X - r a y l i g h t beam, TOF a x i s a n d t h e skimmed s u p e r s o n i c m o l e c u l a r beam a r e a l l m u t u a l l y p e r p e n d i c u l a r . The e m i t t e d e l e c t r o n s a n d p h o t o i o n s a r e e x t r a c t e d by a n e l e c t r i c f i e l d ( l o V / c m ) . T h e e l e c t r o n a n a l y z e r was s e t t o f a c i l i t a t e d e t e c t i o n o f t h r e s h o l d e l e c t r o n s o n l y , i . e . e l e c t r o n s w i t h n o k i n e t i c e n e r g y ( w i t h i n a b a n d w i d t h o f 0 . 3 e V ) . The r e s o l u t i o n o f t h e Tot' m a s s s p e c t r o m e t e rArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879127
CY-742 JOURNAL DE PHYSIQUE
i s a b o u t l o o . The f r a g m e n t a t i o n m a s s s p e c t r a i n c o i n c i d e n c e w i t h t h e TPE. w e r e o b t a i n e d by u s i n g a t i m e - t o - a m p l i t u d e c o n v e r t e r ( T A C ) a n d a p u l s e h e i g h t a n a l y z e r (PHA). The TAC was s t a r t e d by t h e TPE s i g n a l p u l s e a n d s t o p p e d by t h e i o n s i g n a l p u l s e . T h e u s u a l r e c o r d i n g t i m e was 6 0 m i n u t e s .
R e s u l t s
F i g u r e 1 s h o w s t h e TPE s p e c t r u m o f CF2CH2 n e a r t h e C 1 s i o n i z a t i o n t h r e s h o l d . The f r a g m e n t i o n s p e c t r a r e c o r d e d i n c o i n c i d e n c e w i t h t h e e l e c t r o n s f r o m C H a n d C F i o n i z a t i o n a r e g i v e n i n f i g u r e 2 a n d 3 , r e s p e c t i v e l y . I n b o t h c a s e s we o b s e r v e t h e f r a g m e n t s : C , .
.
,CH2,(CH,), C2
,..,
C2H2, CF, C2F, C2F2 ( a l l f r a g m e n t s s i n g l y c h a r g e d e x c e p t w h e r e s p e c i a l l y n o t e d ) . The s p l i t t i n g o f some p e a k s ( v e r y c l e a r l y s e e n f o r t h e two p e a k s a s s o c i a t e d w i t h H) o r i g i n a t e s f r o m t h e v e l o c i t y o f t h e i o n s d u e t o t h e f r a g m e n t a t i o n e n e r g y . O n l y i o n s e m i t t e d i n a f o r w a r d a n d b a c k w a r d d i r e c t i o n r e a c h t h e d e t e c t o r , t h e r e m a i n d e r a r e d i s c r i m i n a t e d a g a i n s t b y t h e a p e r t u r e s . F o r b o t h C F ( f i g . 2 ) a n d C H ( f i g . 3 ) e x c i t a t i o n t h e CF f r a g m e n t i s t h e m o s t d o m i n a n t a n d t h e C2F f r a g m e n t p e a k i n t e n s i t i e s a r e t h e s a m e . H o w e v e r , t h e o b s e r v a t i o n o f t h e CHx, C2H2 a n d C2F2 f r a g m e n t s d e p e n d s s t r o n g l y o n t h e e x c i t a t i o n s i t e ; i n c o i n c i d e n c e w i t h t h e T P E ' s f r o m C F ( C H ) 1 s e x c i t a t i o n we o b s e r v e more C H x a n d C2H2 ( C 2 F 2 ) f r a g m e n t s t h a n f o r C H ( C F ) 1 s e x c i t a t i o n . F i g . 4 s h o w s t h e TPE s p e c t r u m o f CF3COCH3. We i n v e s t i g a t e d t h e f r a g m e n t a t i o n c h a n n e l s i n c o i n c i d e n c e w i t h t h e T P E ' s f r o m C F , C O a n d C H 1 s i o n - i z a t i o n ( f i g 5 , 6 a n d 7 , r e s p . ) . The w i d t h o f t h e p e a k a t 9 ~ s e c i s b i g g e r f o r C F a n d C H c o m p a r e d t o C O e x c i t a t i o n . We a t t r i b u t e t h i s t o t h e a d d i t i o n a l a p p e a r a n c e o f t h ec o 2 +
f r a g m e n t ( f i g . 5 a n d 7 , r e s p . ) . The p e a k a t 1 6 . 5 u s e c c o u l d b e e i t h e r C2F o r CH3CO. From t h e c o m p a r i s o n o f t h e m a s s s p e c t r a f o r C F ( f i g . 5 ) a n d C H ( f i g . 7 ) e x c i t a t i o n-
we o b s e r v e no CF3CO i n f i g . 7-
we c o n c l u d e t h a t t h e p e a k a t 1 6 . 5 p s e c i s d u e t o C2F. A g a i n we o b s e r v e s i t e - s p e c i f i c f r a g m e n t a t i o n s : C H e x c i t a t i o n p r o d u c e s more CF3, C2F a n d C2F2 f r a g m e n t s t h a n C F e x c i t a t i o n .F i g u r e 8 , 9 a n d 1 0 show t h e t h r e s h o l d e l e c t r o n s p e c t r u m a n d t h e m a s s s p e c t r a o f CF3CH2NH2 a t C F a n d a t C N i o n i z a t i o n , r e s p e c t i v e l y . F o r C F ( f i g . 9 ) e x c i t a t i o n we o b s e r v e more CF f r a g m e n t s t h a n f o r C H ( f i g . l o ) e x c i t a t i o n b u t , i n t e r e s t i n g l y , t h e CH2NH2 a b u n d a n c e i s t h e s a m e . A v e r y s h a r p p e a k , w h i c h c a n b e a s s i g n e d t o NH3 ( w i t h v e r y l i t t l e k i n e t i c e n e r g y ) i s f o u n d f o r C N e x c i t a t i o n . F o r CF3CH2NH2 t h e f r a g m e n t a t i o n d e p e n d e n c e o n t h e s i t e o f e x c i t a t i o n s e e m s t o b e more c o m p l i c a t e d t h e n f o r t h e o t h e r m o l e c u l e s .
C o n c l u s i o n s
Our m e a s u r e m e n t s show t h a t t h e f r a g m e n t a t i o n d e p e n d s on t h e s i t e o f e x c i t a t i o n w i t h i n a m o l e c u l e . The k i n e t i c e n e r g y o f t h e f r a g - m e n t s i s i m p o r t a n t f o r t h e u n d e r s t a n d i n g o f t h e d y n a m i c s i n v o l v e d , i . e . t h e e n e r g y r e d i s t r i b u t i o n f o l l o w i n g t h e i n n e r - s h e l l e x c i - t a t i o n . CF+ i s a n e x t r e m e l y s t a b l e f r a g m e n t a n d a p p e a r s i n a l l c o i n c i d e n c e s p e c t r a . F + , h o w e v e r , i s n o t o b s e r v e d . A p o s s i b l e e x p l a n a t i o n f o r t h i s may b e t h e h i g h e l e c t r o n a f f i n i t y o f F.
F u r t h e r work u s i n g m u l t i p l e c o i n c i d e n c e t e c h n i q u e s f o r t h e m e a s u r e - ment o f Auger e l e c t r o n s a n d c o r r e l a t e d i o n i c f r a g m e n t s i s r e q u i r e d t o c o n f i r m t h e i n d i c a t i o n o f t h e s e "memory e f f e c t s n i n f r a g m e n - t a t i o n p r o c e s s e s .
ELECTRONS: MOLECULE CF2tH2
Fig. 1: Threshold P h o t o e l e c t r o n spectrum o f C F Z C H ~
0.0 10.0 20.0 30.0 40.D
TIME OF FLIGHT IMICROSECONDSI
0.0 10.0 20.0 30.0 40.0
TlME OF FLIGHT IMICROSECONDSI
Fig. 2 : C o i n c i d e n c e s f o r C F (1s) excitation. Flg. 3: C o i n c i d e n c e s f o r C H (Is) excitation.
ELiGRONS MOLECULE CFICOCH3
Fig. 4: Threshold Photoelectron s p e c t r u m o f CF3COCH3.
0.0 10.0 20.0 30.0 '0.0 0.0 10.0 20.0 30.0 40.0
TIME OF FLIGHT IMICROSECONDSI TIME OF FLIGHT IMICROSECONDSI
Fig. 5 : C o i n l d e n c e s for C F (1s) excltation. Fig. 6: Coincidences for C o (Is) excltatlon.
0.0 10.0 20.0 30.0 60 0
TlME OF FLIGHT IMICROSECONDS)
Fig. 7: Coincidences f o r CH (1s) excitation.
JOURNAL DE PHYSIQUE
ELECTRONS: MOLECULE CF3CH211H2
Fig. 8: 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 u m of C F 3 C H z N H 2 .
0.0 iD.0 20.0 30.0 0.0 10.0 20.0 30.0
TIME OF FLIGHT lM!CROSECONDSI TIME OF FLIGHT IMICROSECONDSI 40.0
F i g . 9: C o i n c i d e n c e s f o r C F ( 1 s ) e x c i t a t i o n . Fig. 1 0 : Coincidences f o r C N ( i s ) e x c i t a t i o n .
T h i s r e s e a r c h w a s f u n d e d by t h e B u n d e s m i n i s t e r i u m f i i r F o r s c h u n g und T e c h n o l o g i e , g r a n t n o . 05366 FAI.
/ 1 / K . S i e g b a h n , C . N o r d l i n g , G . J o h a n s s o n , J . Hedmann, P.F.
H e d e n , K . H a m r i n , U . G e l i u s , T. B e r g m a r k , L . O . W e r m e , R . M a n n e , Y . B a e r , "ESCA A p p l i e d t o F r e e M o l e c u l e s n , N o r t h H o l l a n d P u b l . C o . , A m s t e r d a m , 1 9 6 9
/2/ W . E b e r h a r d t , T.K. Sham, R . C a r r , S. Kummacher, M . S t r o n g i n , S . L . W e r g a n d , D . W e s n e r , P h y s . Rev. L e t t . 5 0 ( 1 9 8 3 ) 1 0 3 8 / 3 / K . M f i I l e r - D e t h l e f s , M . S a n d e r , L . A . C h e w t e r , E.W. S c h l a g , J .
P h y s . Chem. 8 8 ( 1 9 8 4 ) 6 0 9 8 4 BESSY J a h r e s b e r i C h t 1 9 8 4
