X-ray Absorption Fine Structure of Free Selenium Clusters Although clusters have attracted great interest a s i n t e r m e d i a t e s t a t e s b e t w e e n a t o m i c a n d condensed states of matter [1], general methods for t h e s i z e - s e l e c t i v e s t r u c t u r a l a n a l y s i s o f n e u t r a l clusters have not been established so far. Recently w e m e a s u r e d t h e P h o t o e l e c t r o n P h o t o i o n Coincidence ( PEPICO ) spectra for selenium dimer S e 2 a n d l a r g e r s p e c i e s s u c h a s S e 5 [ 2 , 3 ] . T h e P E P I C O s p e c t r a r e v e a l t h a t t h e m u l t i p l y c h a r g e d i o n s a r e p r o d u c e d a s a r e s u l t o f d e - e x c i t a t i o n p r o c e s s e s f o l l o w i n g t h e K - s h e l l e x c i t a t i o n a n d f r a g m e n t e d t o a t o m i c i o n s w i t h z c h a r g e s , S e z + , owing to the Coulomb explosion (see Fig. 1 ). The b r a n c h i n g r a t i o s f r o m t h e p a r e n t c l u s t e r t o t h e daughter ions exhibit clear dependence on the size N o f t h e p a r e n t c l u s t e r s . I n p a r t i c u l a r , t h e b r a n c h i n g r a t i o s i n t h e n o n - r e s o n a n t a b s o r p t i o n r e g i o n a r e w e l l r e p r o d u c e d c o n s i d e r i n g t h a t t h e c h a r g e s w e r e r a n d o m l y d i s t r i b u t e d w i t h i n t h e c l u s t e r s b e f o r e t h e C o u l o m b e x p l o s i o n . B a s e d upon these findings, we proposed a new method for the size-selective EXAFS of neutral free clusters by utilizing the PEPICO measurements [3]. The basic relations we have suggested are the s i m u l t a n e o u s e q u a t i o n s o f t h e X - r a y a b s o r p t i o n co ef fi ci en ts pe r at om , α ( h ν ), pa ra me te ri ze d by z, as follows: I (h ν , z) = ∑ N A N B N (z) α ( h ν ) (1), wh er e I (h ν , z) is th e in te gr at ed PE PI CO in te ns it y for the z charged ion, A N is the abundance of parent c l u s t e r s c o n s i s t i n g o f N a t o m s a n d B N ( z ) i s t h e b r a n c h i n g r a t i o . S i n c e A N a n d B N ( z ) c a n b e estimated either from experiments or calculations, α ( h ν ) w a s d e d u c e d b y s o l v i n g e q . ( 1 ) f r o m t h e PEPICO measurements. In order to verify eq. (1) experimentally, we have c a r r i e d o u t s y n c h r o n o u s X A F S - P E P I C O measurements for an Se cluster beam at the X-ray un du la to r be am li ne BL 10 XU . Th e ne ut ra l cl us te r b e a m s w e r e p r o d u c e d b y a s u p e r s o n i c j e t e x p a n s i o n m e t h o d , i n t e r s e c t e d w i t h t h e X - r a y s a t right angles in the horizontal plane. The photoions p r o d u c e d b y a b s o r b i n g a n X - r a y p h o t o n w e r e e xt ra ct e d u p w a rd b y a co n st a n t e l e ct ri c fi e l d to a d e te ct o r, w h i l e th e p h o to e l e ct ro n s w e re e xt ra ct e d d o w n w a r d . T h e p r o c e d u r e s o f t h e s y n c h r o n o u s measurements are summarized in a flow chart (see Fig. 2 ). The XAFS spectrum was measured by the host computer that also controls the multi-channel- X-ray (a) X-ray absorption (b) Coulomb explosion Fig. 1. Schematic illustration of the X-ray absorption (a) and the Coulomb explosion (b) taking place in a cluster. 51 χ χ (k) 5 10 –0.02 0 0.02 k ( Å -1 ) Fig. 3. EXAFS spectra χ (k) for Se 2 (line) and “ Se 5 ” (circles). Makoto Yao and Kiyonobu Nagaya Kyoto University E-mail: yao @ scphys.kyoto-u.ac.jp anal yzer , by whic h PEPI CO spec tra are take n, as well as the angle of the X-ray monochromators and the gap of the undulator. In our experiment, Se 2 , Se 5 , Se 6 and Se 7 were p r o d u c e d a n d t h e i r r e l a t i v e a b u n d a n c e s w e r e 66:16:11:7, respectively. In Fig. 3 EXAFS spectrum χ ( k ) f o r “ S e 5 ” d e d u c e d f r o m e q . ( 1 ) i s s h o w n b y circles. Here “ Se 5 ” includes Se 5 , Se 6 and Se 7 , as in p r e v i o u s s t u d i e s [ 2 - 5 ] . A l t h o u g h t h e n u m b e r density of Se atoms in the cluster beam is smaller than that in the bulk Se samples by more than ten o r d e r s o f m a g n i t u d e , t h e E X A F S o s c i l l a t i o n i s c l e a r l y s e e n . T h e l i n e d e n o t e s t h e s m o o t h e d variation χ ( k ) for Se 2 in the cluster beam, which is i n g o o d a g r e e m e n t w i t h χ ( k ) f o r S e 2 i n t h e v a p o r phase [6] except for the scaling factor. The spacing of the EXAFS oscillation clearly demonstrates that the bond length of “ Se 5 ” is longer than that of Se 2 . More precisely, the former proves to be 2.35 Å and the latter 2.16 Å [7], which is fully consistent with a p i l e o f E X A F S d a t a f o r v a r i o u s f o r m s o f S e [ 8 ] . T h e r e f o r e , w e c o n c l u d e t h a t t h e p r o p o s e d s i z e - selective XAFS is a reliable and realistic method. F i g . 2 . A f l o w c h a r t o f t h e X A F S - P E P I C O synchronous measurements. Procedures by the host computer are shown on the left and those by the multi-channel-analyzer on the right. References [1] Clusters of Atoms and Molecules, ed. H. Haberland (Springer, 1994). [2] T. Hayakawa et al. , J. Phys. Soc. Jpn. 69 (2000) 2039. [3] T. Hayakawa et al. , J. Phys. Soc. Jpn. 69 (2000) 2850. [ 4 ] M . Y a o , T . H a y a k a w a , K . N a g a y a , K . H a m a d a , Y . O h m a s a , M . N o m u r a , J . Synchrotron Rad. 8 (2001) 542. [5] M. Yao et al. , Jpn. J. Appl. Phys. suppl. 38-1 (1999) 564. [6] S. Hoso kawa et al. , J. Chem . Phys . 97 (1992) 786. [ 7 ] K . N a g a y a , M . Y a o , T . H a y a k a w a , Y . Ohmasa, Y. Kajihara, M. Ishii, Y. Katayama, submitted in Phys. Rev. Lett. [ 8 ] M . I n u i e t a l . , J . P h y s . S o c . J p n . 5 7 (1988) 553. Commands Monochromator tuning Undulator gap tuning XAFS starts XAFS ends End Start PEPICO starts PEPICO ends Initialize MCA 52
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