0 50 100 150 200 250 25 24 23 22 21 20 19 18 17 16 15 Binding Energy ( eV ) Normalized Intensity ( arb. units) Out-of-plane vibration ν 2 : a 2 ” (b) Non resonant E 2 A 1 ’ B 2 E’ A 2 E” X 2 A 2 ’ D 2 E’ C 2 A 2 ” (a) B 1 s → 2 a 2 ” Nuclear Motion of Polyatomic Molecules Probed by High-resolution Resonant Auger Spectroscopy W h e n a n i n n e r - s h e l l e l e c t r o n o f a n a t o m c o m p o s i n g a m o l e c u l e i s p r o m o t e d t o a n u n o c c u p i e d m o l e c u l a r o r b i t a l , a r e s o n a n t A u g e r d e c a y t a k e s p l a c e o n a f e m t o s e c o n d t i m e s c a l e . W i t h i n t h i s s h o r t t i m e s c a l e , h o w e v e r , n u c l e a r m o t i o n c a n p r o c e e d i n t h e c o r e - e x c i t e d s t a t e . N u c l e a r m o t i o n i n a p o l y a t o m i c m o l e c u l e i s o f p a r t i c u l a r i n t e r e s t b e c a u s e m u l t i - d i m e n s i o n a l nuclear motion may be mediated by core excitation. The nuclear motion caused in the core-excited state t h e n m e d i a t e s n u c l e a r m o t i o n i n t h e A u g e r f i n a l states and thus may govern dissociation pathways. T h e n u c l e a r m o t i o n i n t h e c o r e - e x c i t e d s t a t e p r o c e e d s i n c o m p e t i t i o n w i t h t h e r e s o n a n t A u g e r decay and thus should be reflected in the resonant Au ge r sp ec tr um . To pr ob e th e nu cl ea r mo ti on in the core-excited state as well as in the Auger final state in the resonant Auger spectra, we installed a high-resolution electron spectroscopy apparatus on beamline BL27SU [1]. The apparatus consists of a state-of-the-art SES2002 electron energy analyzer unoccupied molecular orbital 2 a 2 ”. We probed out- o f - p l a n e n u c l e a r m o t i o n f o r t h e d e f o r m a t i o n f r o m D 3 h to C 3 v following the core excitation, by means of resonant Auger spectroscopy. The spectrum (b) in Fig. 1 represents the direct photoemission from the six valence orbitals in BF 3 , whereas the spectrum (a) is recorded at the B 1 s → 2 a 2 ” resonance. The electron emission for the C, D, and E bands is strongly enhanced by the B 1 s → 2 a 2 ” e x c i t a t i o n , s u g g e s t i n g t h a t t h e p a r t i c i p a t o r A u g e r d e c a y t a k e s p l a c e . I n t h e r e s o n a n t l y e n h a n c e d spectrum, one can see long progressions of out-of- p l a n e v i b r a t i o n s i n t h e A u g e r f i n a l s t a t e s . T h e h i g h l y e x c i t e d o u t - o f - p l a n e v i b r a t i o n s a r e a d i r e c t reflection of the out-of-plane nuclear motion in the core-excited state. The H 2 O molecule has a bent structure of C 2 v sym met ry. The two low est uno ccu pie d mol ecu lar o r b i t a l s 4 a 1 a n d 2 b 2 a r e t h e a n t i b o n d i n g c o u n t e r p a r t s o f t h e t w o O H b o n d i n g o r b i t a l s 3 a 1 a n d 1 b 2 . T h e O 1 s - 1 4 a 1 c o r e - e x c i t e d s t a t e i s known to be dissociative, whereas the O 1 s -1 2 b 2 core-excited state has a shallow potential minimum so that the vibrational structure can be seen in the O 1 s → 2 b 2 resonance [5]. We demonstrated that one can control the two-dimensional nuclear motion o f t h e A u g e r f i n a l s t a t e b y c h a n g i n g t h e n u c l e a r Fig 1. Valence-electron spectra of BF 3 (a) after excitation of the B 1s – 1 2a 2 ” state and (b) direct photoemission. ( G a m m a d a t a - S c i e n t a ) , a g a s c e l l , a n d a di ff er en ti al ly pumped experimental chamber, and allows us to observe the resonant Auger s p e c t r a o f g a s e o u s m o l e c u l e s w i t h a n u n p r e c e d e n t e d r e s o l u t i o n w h e n t h e y a r e c o m b i n e d w i t h a h i g h - r e s o l u t i o n s o f t X - r a y monochromator at BL27SU [2]. We present here two typical showcases o f o u r o b s e r v a t i o n s ; t h e r e s o n a n t A u g e r spectra of the boron trifluoride molecule BF 3 [3] and those of the water molecule H 2 O [4]. The BF 3 molecule in the ground state has a planar structure of D 3 h symmetry, whereas i t h a s t r i g o n a l p y r a m i d a l s t r u c t u r e o f C 3 v sy mm et ry in th e co re -e xc it ed st at e in wh ic h a B 1 s e l e c t r o n i s p r o m o t e d t o t h e l o w e s t 46 (d) 536.12 eV (2,0) Binding Energy ( eV ) (c) 535.95 eV (1,0) (b) 535.77 eV (0,0) (a) 535.41 eV (off resonance) 20.0 19.5 19.0 18.5 18.0 17.5 17.0 30 24 18 14 36 40 33 29 26 22 42 39 35 Intensity (arbitrary units) 0 0 0 0 × 6 × 1 . 7 References [1 ] Y. Sh im iz u et al ., J. El ec tr . Sp ec tr os c. Re la t. Phenom. 114-116 (2001) 63. [2 ] H. Oh as hi et al ., Nu cl . In st ru m. Me th . A 46 7- 468 (2001) 533. [3] K. Ueda, to be published in Surface Rev. Lett.; K . U e d a , A . D e F a n i s , K . O k a d a e t a l . , t o b e published in Chem. Phys. [4] A. De Fanis et al ., J. Phys. B 33 (2002) L23. motion in the O 1 s -1 2 b 2 core-excited state. Th e el ec tr on sp ec tr a re co rd ed at fo ur ph ot on e n e r g i e s a c r o s s t h e O 1 s → 2 b 2 r e s o n a n c e a r e presented in Fig. 2 . Here the largest contribution to t h e s p e c t r a l w i d t h c o m e s f r o m t h e D o p p l e r w i d t h due to the thermal motion of the sample molecules. The spectra cover the binding energy region 17.0 - 20.0 eV, where the electron emission from the 1 b 2 o r b i t a l i s p r e s e n t . T h e b o t t o m s p e c t r u m ( a ) r e p r e s e n t s t h e d i r e c t p h o t o e m i s s i o n s p e c t r u m , whereas the other three spectra (b), (c), and (d) are recor ded appro ximat ely at the energ ies of the ( ν 1 , ν 2 ) = (0,0), (1,0) and (2, 0) vibrational components, respectively, of the O 1 s → 2 b 2 band. When the Auger final state is populated via the O 1 s → 2 b 2 ( 0 , 0 ) e x c i t a t i o n ( s p e c t r u m ( b ) ) , t h e v i b r a t i o n a l structure of the ν 2 mode with spacing of ~ 200 meV i s p a r t i a l l y r e s o l v e d . T h e v i b r a t i o n a l s t r u c t u r e , however, becomes less resolved at the O 1 s → 2 b 2 (1,0) and (2,0) excitations (spectra (c) and (d)). To discover the reason for this, we carried out ab initio c a l c u l a t i o n s . T h e r e s u l t s s h o w r e a s o n a b l e agreement with the experimental spectra as can be seen in Fig. 2 . It is clear from the ab initio spectra that more and more vibrational components with a mixture of the ν 1 and ν 2 modes are populated with the inc rea se in the exc ita tio n ene rgy . In thi s way w e c o n f i r m t h a t i t i s p o s s i b l e t o c o n t r o l t h e t w o - dimensional nuclear motion in the 1 b 2 -1 Auger final s t a t e b y t u n i n g t h e i n c i d e n t e n e r g y t o d i f f e r e n t portions of the O 1 s -1 2 b 2 core-excited state. Fig. 2. Measured and calculated resonant Auger s p e c t r a o f H 2 O d e c a y t o t h e 1 b 2 - 1 A u g e r f i n a l state at various excitation energies across the O 1 s → 2 b 2 b a n d , c o n t i n u o u s a n d d a s h e d l i n e s . T h e c a l c u l a t e d v i b r a t i o n a l c o m p o n e n t s a r e represented by vertical bars. Kiyoshi Ueda Tohoku University E-mail: ueda @ tagen.tohoku.ac.jp 47
home
- JP
- EN