I n n e r - S h e l l P h o t o e m i s s i o n f r o m F r e e M o l e c u l e s i n t h e M o l e c u l a r F r a m e P r o b e d b y t h e E l e c t r o n - I o n C o i n c i d e n c e M o m e n t u m I m a g i n g T e c h n i q u e I n n e r - S h e l l P h o t o e m i s s i o n f r o m F r e e M o l e c u l e s i n t h e M o l e c u l a r F r a m e P r o b e d b y t h e E l e c t r o n - I o n C o i n c i d e n c e M o m e n t u m I m a g i n g T e c h n i q u e Photoelectrons from molecules are scattered by the anisotropic molecular potential. One can study such scattering effects by measuring photoelectron emission in the molecular frame. The experimental te ch ni qu e we us e fo r su ch a st ud y is mo me nt um i m a g i n g , w h i c h i s b a s e d o n t h e t i m e o f f l i g h t m e a s u r e m e n t c o m b i n e d w i t h t h e m e a s u r e m e n t of th e position of detection for the charged particles . A m o m e n t u m i m a g i n g a p p a r a t u s w a s i n s t a l l e d o n Fig. 1. Photoelectron spectra of CO 2 for the molecular axis fixed in space parallel ( ∑ ), and perpendicular ( ∏ ) to the direction of the electric vector of the incident light, (a-c) , and without selection of the molecular axis (d) . beamline BL27SU a couple of years ago and was f i r s t u s e d f o r m e a s u r i n g t h r e e - d i m e n s i o n a l ( 3 - D ) momentum of fragment ions produced via the core e x c i t a t i o n o f C O 2 m o l e c u l e s [ 1 ] . O n e c a n a l s o e m p l o y t h i s t e c h n i q u e f o r m o l e c u l a r i n n e r - s h e l l p h o t o e l e c t r o n s . T h e m e a s u r e m e n t o f t h e 3 - D mom ent um of a pho toe lec tron in coi nci den ce wit h fr ag me nt i on s, e mi tt ed o ve r 4 π sr , a ll ow s us t o ob ta in photoelectron angular distributions in the molecular d ) ) ) h ν S 2 S 3 S 0 S 1 S 4 (a (b) (c (d) h ν = 307.1 eV h ν = 311.9 eV h ν = 319.4 eV = 319.4 eV, symmetry unresolve 0.01 0.00 0.01 0.00 0.04 0.02 0.00 0.04 0.02 0.00 20 15 10 5 0 –5 Σ Π Intensity (arbitrary units Relative Binding Energy (e V) 55 Fig. 1 F i g u r e 2 f r a m e , a s w e l l a s p h o t o e l e c t r o n s p e c t r a o f s e l e c t e d s y m m e t r i e s f o r t h e i o n i z a t i o n c o n t i n u u m . H e r e w e p r e s e n t s u c h a pho toe mis sio n stu dy of CO 2 mol ecu les and the argon dimers. In , we present CO 2 C1 s photoelectron sp ec tr a fo r se le ct ed sy mm et ry of th e io ni za ti on c o n t i n u u m w i t h r e s o l u t i o n s u f f i c i e n t t o r e s o l v e the satellite structure. This satellite structure is at tr ib ut ed to th e exc it at io n of a val en ce el ec tr on to the uno ccu pie d mol ecu lar orb ita l acc omp any ing core ionization. The energy scale represents the b i n d i n g e n e r g y r e l a t i v e t o t h e m a i n p h o t o l i n e , a n d t h e i n t e n s i t y i s n o r m a l i z e d s o t h a t t h e integrated area of each spectrum is unity. For ∑ a n d ∏ s y m m e t r i e s o f t h e i o n i z a t i o n c o n t i n u u m the molecular axis is parallel and perpendicular, respectively, to the electric vector of the incident li gh t. Th us , we ca n re so lv e th e sy mm et ri es of the con tin uum cha nne l by fil ter ing the fra gme nt i o n s e m i t t e d a t 0 o r 9 0 w i t h r e s p e c t t o t h e el ec tr ic ve ct or . Fr om th es e sy mm et ry -r es ol ve d p h o t o e l e c t r o n s p e c t r a , o n e c a n l e a r n t h a t t h e cr ea ti on of th e sa te ll it es is mo re ef fi ci en t fo r th e ∑ rather than the ∏ channel, stemming from the f a c t t h a t t h e s l o w p h o t o e l e c t r o n i n t e r a c t s m o r e s t r o n g l y w i t h t h e v a l e n c e e l e c t r o n s w h e n t h e p h o t o e l e c t r o n i s e m i t t e d a l o n g t h e m o l e c u l a r ax is [2]. p r e s e n t s p h o t o e l e c t r o n a n g u l a r d i s t r i b u t i o n s ( P A D ) f o r C 1 s i o n i z a t i o n o f C O 2 m o l e c u l e s i n t h e m o l e c u l a r f r a m e , w i t h t h e m o l e c u l a r a x i s o r i e n t e d a l o n g , a t 4 5 , a n d perpendicular to the electric vector of the incident l i g h t . T h e m a j o r f e a t u r e s o f t h e s e m e a s u r e d P A D s a r e f a i r l y a c c u r a t e l y r e p r o d u c e d b y c a l c u l a t i o n s e m p l o y i n g a r e l a x e d - c o r e H a r t r e e - Fock approach, as can be seen in the figure. In co nt ra st to th e we ll -k no wn PA D fo r N 1 s io ni za ti on of N 2 , which exhibits a rich structure dominated by the f-wave ( = 3) at the shape resonance [3], the PAD for C 1 s photoionization of CO 2 is quite u n s t r u c t u r e d o v e r t h e e n t i r e o b s e r v e d r a n g e F i g . 2 . P o l a r p l o t s o f C 1 s p h o t o e l e c t r o n an gu la r di st ri bu ti on s of CO 2 mo le cu le s in a molecular frame, at electron kinetic energies of 6.2, 9.4, 14.6, 23.3 and 32.2 eV. The dots and curves correspond to measurements and c a l c u l a t i o n s , r e s p e c t i v e l y . T h e d i r e c t i o n o f the electric vector of the incident light, E, is hor izo nta l. The mol ecu lar axi s is at 0 , 45 , and 90 with respect to E. 56 E O + C O + 6 . 2 e V 9 . 4 e V 1 4 . 6 e V 2 3 . 3 e V × 2 × 2 × 2 × 4 3 2 . 2 e V Fi g. 3 F i g . 3 . P o l a r p l o t s o f A r 2 p 1 / 2 a n d 2 p 3 / 2 p h o t o e l e c t r o n a n g u l a r d i s t r i b u t i o n s o f a r g o n d i m e r s i n t h e m o l e c u l a r f r a m e . T h e dependence on the relative orientation between t h e d i m e r a x i s a n d t h e e l e c t r i c v e c t o r o f t h e in ci de nt li gh t is re mo ve d by in te gr at in g ov er the all possible relative orientations. a c r o s s t h e s h a p e r e s o n a n c e , i l l u s t r a t i n g t h a t m o r e t h a n o n e p a r t i a l w a v e c o n t r i b u t e t o t h e f o r m a t i o n o f t h e s h a p e r e s o n a n c e , a n d t h a t these waves interfere with each other [4]. In , we pr es en t PA Ds fo r th e ar go n 2 p -shel l ioni zatio n of argon dimer s. Here the P A D ’ s d e p e n d e n c e o n t h e d i r e c t i o n o f t h e e l e c t r i c v e c t o r o f t h e i n c i d e n t l i g h t h a s b e e n r e m o v e d b y i n t e g r a t i n g o v e r a l l r e l a t i v e or ie nt at io ns of th e di me r ax is wi th re sp ec t to th e electric vector. The photoemission events belonging to the dimers have been filtered out w i t h t h e m o m e n t u m i m a g i n g o f t w o A r + i o n s b y r e q u i r i n g m o m e n t u m c o n s e r v a t i o n f o r t h e fragments; the event is attributed to the dimer on ly if th e tw o Ar + ha ve op po si te mo me nt um vector. Comparison with ab initio calculation is necessary to interpret the process, and this will be performed in the near future. I n t h e f u t u r e w e i n t e n d t o s y s t e m a t i c a l l y ex te nd th is el ec tr on -i on co in ci de nc e mo me nt um im ag in g te ch ni qu e to a ph ot oe mi ss io n st ud y of three-dimensional polyatomic molecules. Alberto De Fanis a , Norio Saito b and Kiyoshi Ueda c (a) SPring-8 / JASRI (b) National Institute of Advanced Industrial Science and Technology (c) Tohoku University E-mail: defanis @ spring8.or.jp References [ 1 ] s e e f o r e x a m p l e Y . M u r a m a t s u e t a l . , P h y s . Rev. Lett. 88 (2002) 133002. [2] A. De Fanis et al. , Phys. Rev. Lett. 89 (2002) 023006. [3] E. Shigemasa et al. , Phys. Rev. Lett. 74 (1995) 359. [ 4 ] N . S a i t o , A . D e F a n i s , K . K u b o z u k a , M . Mac hid a , M. Tak aha shi , H. Yos hid a , I.H . Suz uki , A. Ca ssi mi , A. Cza sch , L. Sch mid t , R. Dö r ne r , K. Wang , B. Zimmermann , V. McKoy , I. Koyano and K. Ueda , J. Phys. B : At. Mol. Opt. Phys. 36 (2003) L25. 57 2 p 1 / 2 ( k i n e t i c e n e r g y = 0 . 9 e V ) 2 p 3 / 2 ( k i n e t i c e n e r g y = 3 . 1 e V ) d i m e r a x i s d i m e r a x i s
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