DISSOCIATION DYNAMICS OF INNER-SHELL EXCITED MOLECULES BY MEANS OF PRECISE MEASUREMENTS OF RECOIL MOMENTA OF FRAGMENT IONS

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W h e n o n e o f t h e i n n e r - s h e l l e l e c t r o n s o f a n atom in a molecule is promoted to a vacant valence molecular orbital or to a Rydberg orbital, either an A u g e r p r o c e s s o r a s e r i e s o f A u g e r p r o c e s s e s generally occur in the femtoseconds time scale and multiply charged ions are produced. These parent ions normally dissociate rapidly into fragments due t o C o u l o m b r e p u l s i o n a m o n g s e v e r a l p o s i t i v e l y c h a r g e d c e n t e r s . R e c e n t r e s e a r c h h a s d e m o n s t r a t e d th at nu cl ea r mo ti on ca n pr oc ee d i n th e co re -h ol e state before the Auger decay occurs. This nuclear m o t i o n s h o u l d b e r e f l e c t e d i n t h e a n g u l a r c o r r e l a t i o n a m o n g p r o d u c t s o f d i s s o c i a t i o n t h a t occurs following subsequent Auger decay. To probe such nuclear motion in the study of d i s s o c i a t i o n d y n a m i c s , w e h a v e i n s t a l l e d a n appar atus at beaml ine BL27S U which enabl es us t o t a k e d i r e c t m e a s u r e m e n t s o f m o m e n t u m distribution of all the fragment ions. This technique c o n s i s t s o f a t i m e - o f - f l i g h t m e a s u r e m e n t u s i n g p o s i t i o n - s e n s i t i v e t w o - d i m e n s i o n a l d e t e c t o r s w i t h delay-line anodes. Combining this apparatus with the source of very high resolution linearly polarized s o f t X - r a y s [ 1 ] , w e h a v e b e e n a b l e t o s t u d y t h e d i s s o c i a t i o n d y n a m i c s o f t h e l i n e a r t r i a t o m i c m o l e c u l e CO 2 following the excitation of either the C1 s [2] or O1 s [3] electron into 2 π υ or σ * molecular orbital in u n p r e c e d e n t e d d e t a i l . W e f o c u s e d o n t h e t h r e e - bo dy br ea ku p of th e tr ip ly ch ar ge d mo le cu la r io n, CO 2 3 + → C + + O + + O + , following the C1 s → 2 π υ e x c i t a t i o n . T h e r e s u l t s o f t h e m e a s u r e m e n t s a r e best represented by so-called Newton diagrams as shown in Fig. 1 . In these diagrams, the magnitude of the linear momentum of the first O + is normalized to unity and the vector is placed on the x-axis in the n e g a t i v e d i r e c t i o n . T h e r e s p e c t i v e l i n e a r m o m e n t u m DISSOCIATION DYNAMICS OF INNER-SHELL EXCITED MOLECULES BY MEANS OF PRECISE MEASUREMENTS OF RECOIL MOMENTA OF FRAGMENT IONS of C + and the second O + are then pl otted rel ative to t h i s u n i t v e c t o r i n t h e p o s i t i v e a n d n e g a t i v e y d i r e c t i o n s , r e s p e c t i v e l y , s o a s t o s a t i s f y t h e m o m e n t u m conservation requirements. The separate observation of the two Renner- Teller split states claims for a salient feature in the p r e s e n t s t u d y . T h e C 1 s - 1 2 π υ c o r e - h o l e s t a t e i s p r e d i c t e d t o b e s p l i t i n t o t w o s t a t e s v i a v i b r o n i c co up li ng wi th th e be nd in g vi br at io n; on e in wh ic h the π orbital lies in the bending plane (panel a , in- p l a n e ) a n d t h e o t h e r i n w h i c h i t e x t r u d e s p e r p e n d i c u l a r l y f r o m t h e b e n d i n g p l a n e ( p a n e l b , out -of -pl ane ). How eve r, the sep ara te obs erv ati on o f t h e s e s t a t e s h a s n e v e r b e e n a c h i e v e d . T h e p r e s e n t t e c h n i q u e e n a b l e s u s t o o b t a i n t h i s separation successfully. If only the events of which the vector product of the linear momenta of two O + fragments is parallel to the E vector and the linear momentum of the C + fragment is perpendicular to the E vector are selected from the recorded data, t h e s e l e c t e d e v e n t s f o r m a n e n s e m b l e o f t h e dissociation events from the core-hole state whose transition moment is perpendicular to the plane of t h e b e n d i n g m o t i o n ( p a n e l a ) . I n c o n t r a s t , i f t h e e v e n t s o f w h i c h t h e v e c t o r p r o d u c t o f l i n e a r m o m e n t a of the two O + fragments is perpendicular to the E vector and the linear momentum of the C + fragment i s p a r a l l e l t o t h e E v e c t o r a r e s e l e c t e d , the n the o b t a i n e d e n s e m b l e i s t h a t o f t h e d i s s o c i a t i o n ev en ts from the core- hole state whose trans ition momen t is parallel to the plane of the bending motion (panel b ). For in- pla ne exc ita tio n, the isl and s of C + and s e c o n d O + h a v e c l e a r l y s h o w n t o p o s s e s s c o n s i d e r a b l y long tails, whereas those for out-of- plane excitation d o n o t , i n d i c a t i n g t h a t t h e m o l e c u l e s a r e b e n t c o n s i d e r a b l y i n t h e f o r m e r e x c i t a t i o n j u s t b e f o r e -1 0 1 1 0 -1 Normalized Momentum Normalized Momentum C + + O + + O + + C 1 s → → π π ∗ ∗ (in-plane) (a) F i g . 1 . N e w t o n d i a g r a m s f o r t h e t h r e e f r a g m e n t i o n s , f o r m e d b y t h r e e - b o d y b r e a k u p o f t h e t r i p l y c h a r g e d molecular ion CO 2 3+ . (a) for C1s -1 2 π υ (in-plane) excitation, (b) for C1s -1 2 π υ (out-of-plane) excitation. d i s s o c i a t i o n , w h e r e a s t h e l a t t e r i s n o t . W e c a n i n t e r p r e t t h i s a s reflection of the nuclear motion in the c o r e - e x c i t e d s t a t e s t o w a r d t h e e q u i l i b r i u m g e o m e t r y , r e p r e s e n t i n g t h e f i r s t d i r e c t v e r i f i c a t i o n o f t h e o c c u r r e n c e o f l i n e a r a n d b e n t structures for the C1 s -1 2 π υ core-hole states ( Renner-Teller split pair). References [ 1 ] H . O h a s h i e t a l . , J . E l e c t r o n Spectrosc. Rel. Phenom. - in press. [ 2 ] N . S a i t o , M . L a v o l l e e , Y . Muramatsu, F. Heiser, U. Becker, H. C h i b a , K . K u b o z u k a , A . C z a s c h , T . We be r, R. Mo sh am me r, H. Sch mi dt - Böcking, I. Koyano and K. Ueda, to be published in Phys. Rev. Lett. [3] Y. Muramatsu, K. Ueda, N. Saito, H . C h i b a , A . C z a s c h , T . W e b e r , R . M o s h a m m e r , O . J a g u t z k i , H . S c h m i d t - B ö c k i n g , M . L a v o l l é e , K . K u b o z u k a a n d I . K o y a n o , t o b e published in Phys. Rev. Lett. C + + O + + O + + -1 0 1 1 0 -1 Normalized Momentum Normalized Momentum C 1 s → → π π ∗ ∗ (out-of-plane) (b) Norio Saito a , Kiyoshi Ueda b and Inosuke Koyano c (a) National Institute of Advanced Industrial Science and Technology (b) Tohoku University (c) Himeji Institute of Technology E-mail: koyano @ sci.himeji-tech.ac.jp