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DECAY PROCESS OF THE CORE EXCITON IN ALKALI HALIDES
K. Tsutsumi, O. Aita, K. Ichikawa
To cite this version:
K. Tsutsumi, O. Aita, K. Ichikawa. DECAY PROCESS OF THE CORE EXCITON IN ALKALI HALIDES. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-919-C9-922.
�10.1051/jphyscol:19879163�. �jpa-00227277�
DECAY PROCESS OF THE CORE EXCITON IN ALKALI HALIDES
K. TSUTSUMI, 0 . AITA and K. ICHIKAWA
C o l l e g e o f Engineering, U n i v e r s i t y o f Osaka P r e f e c t u r e , Mozu, Sakai, Osaka 591, Japan
A b s t r a c t - The decay o f t h e L i I s c o r e e x c i t o n i n l i t h i u m h a l i d e s and t h e Na 2p c o r e e x c i t o n i n sodium h a l i d e s was i n v e s t i g a t e d b y m e a s u r i n g t h e c o n s t a n t - i n i t i a l - s t a t e (CIS) spectrum and t h e c o n s t a n t - f i n a l - s t a t e (CFS) spectrum. The decay p r o b a b i l i t y o f t h e c o r e e x c i t o n was e s t i m a t e d f r o m t h e p h o t o e l e c t r o n s p e c t r a and t h e a b s o r p t i o n s p e c t r a . I n t h e case o f sodium h a l i d e s t h e CIS and CFS s p e c t r a were measured w i t h two d i f f e r e n t i n c i d e n t a n g l e s o f t h e e x c i t i n g l i g h t and a l s o w i t h t w o d i f f e r e n t t e m p e r a t u r e s , i.e., room t e m p e r a t u r e and l i q u i d n i t r o g e n t e m p e r a t u r e . Two peaks were observed i n t h e CIS and CFS s p e c t r a o f NaCl and NaBr. The h i g h e r e n e r g y peak, which appears a t t h e same p h o t o n e n e r g y and shows t h e same t e m p e r a t u r e dependence as t h o s e o f t h e p r o m i n e n t e x c i t o n peak i n t h e a b s o r p t i o n spectrum, was i n t e r p r e t e d as o r i g i n a t i n g f r o m t h e decay o f t h e b u l k c o r e e x c i t o n . I t was f o u n d t h a t t h e e n e r g y p o s i t i o n and t h e i n t e n s i t y o f t h e l o w e r e n e r g y peak show t h e s t r o n g dependence on t h e p o l a r i z a t i o n o f t h e r a d i a t i o n i n c i d e n t on t h e sample s u r f a c e , and i t shows no d i s t i n c t e n e r g y s h i f t upon t h e c o o l i n g o f t h e samples. The l o w e r e n e r g y peak was i n t e r p r e t e d as o r i g i n a t i n g f r o m t h e decay o f t h e s u r f a c e c o r e e x c i t o n .
INTRODUCTION
R e c e n t l y , we have i n v e s t i g a t e d t h e n o n r a d i a t i v e decay p r o c e s s e s o f t h e L i I s c o r e e x c i t o n i n l i t h i u m h a l i d e s 11-31 and t h e Na 2p c o r e e x c i t o n i n sodium h a l i d e s / 4 / b y u s i n g t h e t e c h n i q u e o f t h e r e s o n a n t p h o t o e m i s s i o n . I t has been f o u n d t h a t t h e valence-band s p e c t r u m as w e l l as t h e Auger peak i s enhanced a t t h e p h o t o n e n e r g y c l o s e t o t h e e x c i t a t i o n e n e r g y o f t h e L i I s c o r e e x c i t o n o r t h e Na 2p c o r e e x c i t o n . TO e x p l a i n t h e s e phenomena t h e f o l l o w i n g two decay p r o c e s s e s have been proposed
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879163
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/1,4/; A c o r e h o l e and a p h o t o e x c i t e d e l e c t r o n which f o r m t h e c o r e e x c i t o n recombine d i r e c t l y w i t h t h e e n e r g y t r a n s f e r r e d t o a v a l e n c e e l e c t r o n . T h i s p r o c e s s i s c a l l e d t h e " K ( e x c i t o n ) - V p r o c e s s " f o r l i t h i u m h a l i d e s o r t h e "Lp,3(exciton)-V p r o c e s s " f o r sodium h a l i d e s , where K ( e x c i t o n ) o r L 2 , 3 ( e x c i t o n ) denotes t h e i n i t i a l s t a t e w i t h t h e K h o l e o r t h e L2,3 h o l e acconpanied b y a bound e l e c t r o n ( c o r e e x c i t o n ) and V denotes t h e f i n a l s t a t e w i t h a h o l e i n t h e v a l e n c e band. T h i s p r o c e s s g i v e s r i s e t o t h e r e s o n a n t enhancement o f t h e c o n s t a n t - i n i t i a l - s t a t e (CIS) spectrum w i t h t h e i n i t i a l s t a t e a t t h e peak o f t h e v a l e n c e band around t h e e x c i t a t i o n p h o t o n e n e r g y o f t h e c o r e e x c i t o n , because t h e f i n a l s t a t e o f t h i s p r o c e s s has t h e same c o n f i g u r a t i o n as t h a t o f t h e d i r e c t p h o t o e x c i t a t i o n f r o m t h e v a l e n c e band a t t h e e x c i t a t i o n p h o t o n e n e r g y o f t h e c o r e e x c i t o n . I n t h e second p r o c e s s , t h e c o r e h o l e f o r m i n g a c o r e e x c i t o n recombines w i t h a v a l e n c e e l e c t r o n t r a n s f e r r i n g t h e e n e r g y t o a n o t h e r v a l e n c e e l e c t r o n w h i c h i s e x c i t e d t o t h e c o n t i n u u m s t a t e . T h i s p r o c e s s i s c a l l e d t h e I1K(exciton)-VV p r o c e s s " o r t h e "L2,3(exciton)-VV p r o c e s s " and g i v e s r i s e t o t h e enhancement i n t h e c o n s t a n t - f i n a l - s t a t e (CFS) s p e c t r u m w i t h t h e f i n a l s t a t e c o r r e s p o n d i n g t o t h e k i n e t i c e n e r g y o f t h e Auger e l e c t r o n .
T h i s paper i s concerned w i t h t h e s t u d y o f t h e decay p r o c e s s o f t h e c o r e e x c i t o n i n l i t h i u m h a l i d e s and sodium h a l i d e s . I n t h e c o u r s e o f t h e s t u d y , however, t h e e x t r a peak was o b s e r v e d around t h e e x c i t a t i o n e n e r g y o f t h e L 3 e x c i t o n ( 1 ) r e g i o n i n t h e CIS and CFS s p e c t r a o f NaCl and NaBr. I n t h e case o f N a I o n l y a s i n g l e peak was f o u n d i n t h e CIS and CFS s p e c t r a , t h o u g h t h e a b s o r p t i o n spectrum shows two s h a r p e x c i t o n peaks. To see t h e s u r f a c e e f f e c t on t h e c o r e e x c i t o n we measured t h e CIS and CFS s p e c t r a w i t h two d i f f e r e n t i n c i d e n t a n g l e s o f t h e e x c i t i n g l i g h t on t h e sample s u r f a c e . A l s o t h e t e m p e r a t u r e dependence o f t h e CIS and CFS s p e c t r a were measured a t room t e m p e r a t u r e (RT) and l i q u i d n i t r o g e n t e m p e r a t u r e (LNT) t o d i s t i n g u i s h t h e s u r f a c e c o r e e x c i t o n c l e a r l y f r o m t h e b u l k c o r e e x c i t i o n .
EXPERIMENTAL PROCEDURE
P h o t o e l e c t r o n s p e c t r a were o b t a i n e d w i t h an u l trah'igh-vacuum p h o t o e l e c t r o n s p e c t r o m e t e r w i t h a 2-m g r a z i n g i n c i d e n c e monochromator o f a m o d i f i e d Rowland-mount t y p e . S y n c h r o t r o n r a d i a t i o n f r o m an e l e c t r o n s t o r a g e r i n g a t t h e I n s t i t u t e f o r S o l i d S t a t e P h y s i c s o f t h e U n i v e r s i t y o f Tokyo was used as a l i g h t source. The e n e r g y o f t h e p h o t o e l e c t r o n s was measured w i t h a double-stage e l e c t r o s t a t i c e l e c t r o n - e n e r g y a n a l y z e r o f t h e c y l i n d r i c a l m i r r o r t y p e .
Samples were p r e p a r e d i n s i t u b y e v a p o r a t i o n on$o g o l d s u b s t r a t e s . The t h i c k n e s s . o f t h e samples was e s t i m a t e d t o be a b o u t 100 A w i t h an o s c i l l a t i n g - q u a r t z t h i c k n e s s gauge.
The a n g l e between t h e e x c i t i n g l i g h t and t h e e n t r a n c e a x i s o f t h e e l e c t r o n - e n e r g y a n a a l y z e r was f i x e d a t 45: w h i l e t h e i n c i d e n t a n g l e o f t h e e x c i t i n g l i g h t on t h e sample s u r f a c e was v a r i e d . The i n c i d e n t r a d i a t i o n i s p o l a r i z e d w i t h t h e e l e c t r i c f i e l d v e c t o r p e r p e n d i c u l a r t o t h e d i r e c t i o n o f p r o p a g a t i o n o f t h e i n c i d e n t r a d i a t i o n i n t h e h o r i z o n t a l p l a n e and t h e i n c i d e n t a n g l e s were s e t a t 0" and 45" i n t h e p r e s e n t e x p e r i m e n t on sodium h a l i d e s as i l l u s t r a t e d i n F i g . 1. I n t h e case o f l i t h i u m h a l i d e s t h e i n c i d e n t a n g l e was f i x e d a t 45"
I n t h e case o f sodium h a l i d e s t h e a b s o r p t i o n measurements were p e r f o r m e d b y u s i n g s y n c h r o t r o n r a d i a t i o n f r o m a 750 MeV s t o r a g e r i n g a t t h e I n s t i t u t e f o r M o l e c u l a r Science. A p l a n e - g r a t i n g monochromator e q u i p p e d w i t h a c y l i n d r i c a l f o c u s i n g m i r r o r and a t o r o i d a l r e f o c u s i n g m i r r o r was used. Samples were p r e p a r e d i n s i t u b y e v a p o r a t i o n o n t o c o l l o d i o n s u b s t r a t e s c o a t e d w i t h a t h i n aluminum f i l m . RESULTS AND DISCUSSION
The p r o b a b i l i t i e s o f t w o decay p r o c e s s e s mentioned above were e s t i m a t e d f r o m t h e p h o t o e l e c t r o n s p e c t r a and t h e a b s o r p t i o n s p e c t r a . They have been g i v e n e l s e w h e r e /2,4/. I n c o n t r a s t t o t h e r e s u l t s o b t a i n e d b y Haensel e t a l . / 5 / and Sugawara and Sasaki
161,
t h e decay p r o b a b i l i t y t h r o u g h t h e K ( e x c i t o n ) - V V p r o c e s s o r ( 1 ) The c o r e e x c i t o n i n t h e Na L2,3 a b s o r p t i o n s p e c t r a o f sodium h a l i d e s s h o u l d be t r e a t e d w i t h t h e i n t e r m e d i a t e c o u p l i n g scheme. I n t h e p r e s e n t case, however, we use t h e n o m e n c l a t u r e i n t h e u s u s a l s p e c t r o s c o p i c n o t a t i o n f o r s i m p l i c i t y .Sample
To
yQ E:ggyfll
Energy 45. Analyzer
Analyzer
4ncident
,
Light/
/
Surf ace Normal
/
Surf ace Incident Normal Light
Fig.
-Schematic illustrations of the relation between the radiation incident on the sample surface and the axis of the electron-energy analyzer at the incident angles of 0" (a) and
45"(b). The electric field vector of the incident radiation is in the horizontal plane.
NaCL
CIS
31 32 33 34 35
PHOTON ENERGY (eV)
l ~ ~ ~ ~ l ~ ~ ~ . l ~ ~ ~ ~ l ~ ~ ~ ~ " l
31 32 33 34 35
PHOTON ENERGY(&)
Fig., 2
-CIS and CFS spectra of NaCl Fig. 3
-CIS, CFS and absorption obtained with the exciting light spectra obtained at
RT(dashed lines) incident at the angles of 0" (dashed and at LNT (solid lines). These CIS lines) and
45"(solid lines) together and CFS spectra were obtained at the viith the Na
L2,3absorption spectrum, incident angle of 0 :
The vertical lines are drawn at the
L2-and LQ-core exciton peaks.
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t h e L z , ~ ( e x c i t o n ) - V V p r o c e s s i s g r e a t e r t h a n t h a t t h r o u g h t h e K(exciton)-V p r o c e s s o r L z , ~ ( e x c i t o n ) - V p r o c e s s b o t h i n l i t h i u m h a l i d e s and i n sodium h a l i d e s . Moreover, i t was f o u n d t h a t t h e k i n e t i c e n e r g y o f t h e e j e c t e d e l e c t r o n i n t h e K ( e x c i t o n ) - V V p r o c e s s o r t h e L z , ~ ( e x c i t o n ) - V V p r o c e s s i s s l i g h t l y l a r g e r t h a n t h a t o f t h e o r d i n a r y Auger e l e c t r o n /1,2,4/.
The CIS s p e c t r a w i t h t h e i n i t i a l s t a t e a t t h e peak o f t h e v a l e n c e band and t h e CFS s p e c t r a w i t h t h e f i n a l s t a t e c o r r e s p o n d i n g t o t h e k i n e t i c e n e r g y o f t h e Auger e l e c t r o n were measured f o r NaF, NaC1, NaBr and NaI w i t h t w o d i f f e r e n t i n c i d e n c t a n g l e s o f t h e e x c i t i n g l i g h t around t h e e x c i t a t i o n p h o t o n e n e r g y o f Na 2p c o r e e x c i t o n . The r e s u l t s f o r NaCl a r e shown i n F i g . 2 t o g e t h e r w i t h t h e a b s o r p t i o n spectrum. The CIS and CFS s p e c t r a shown b y s o l i d l i n e s and dashed l i n e s were o b t a i n e d a t t h e i n c i d e n t a n g l e o f 45" and 0: r e s p e c t i v e l y . The e n e r g y p o s i t i o n o f t h e m o s t i n t e n s e peak B i n t h e CIS and CFS s p e c t r a agrees w i t h t h a t o f t h e L2-core e x c i t o n peak i n t h e a b s o r p t i o n spectrum, w h i l e t h e p o s i t i o n o f . t h e l o w e r e n e r g y peak i s d i f f e r e n t f r o m t h a t o f t h e L3-core e x c i t o n peak i n t h e a b s o r p t i o n spectrum.
Moreover, t h e i n t e n s i t y r a t i o o f t h e l o w e r e n e r g y peak t o t h e h i g h e r e n e r g y one i n t h e CIS and CFS s p e c t r a depends on t h e i n c i d e n t a n g l e o f t h e e x c i t i n g l i g h t and i s d i f f e r e n t f r o m t h a t o f t h e L3- t o t h e L2-core e x c i t o n peaks i n t h e a b s o r p t i o n spectrum.
The i n c i d e n t a n g l e dependence o f t h e s p e c t r a l f e a t u r e s i s s i m i l a r t o t h e r e s u l t o b t a i n e d b y Rehder e t a l . / 7 / and s u g g e s t s t h e e x i s t e n c e o f t h e s u r f a c e c o r e e x c i to n . F u r t h e r e v i d e n c e on t h e s u r f a c e c h a r a c t e r i s o b t a i n e d f r o m t h e t e m p e r a t u r e dependence o f t h e s p e c t r a . They a r e shown i n F i g . 3 f o r t h e i n c i d e n t a n g l e o f 0: where dashed- and s o l i d - l i n e s r e p r e s e n t t h e s p e c t r a o b t a i n e d a t RT and LNT, r e s p e c t i v e l y . When t h e sample i s c o o l e d f r o m RT t o LNT, t h e m a i n peak i n t h e CIS and CFS s p e c t r a s h i f t s t h e same amount o f t h e e n e r g y as t h a t i n t h e a b s o r p t i o n s p e c t r u m w h i c h i s b e l i e v e d t o show t h e b u l k p r o p e r t i e s , w h i l e t h e l o w e r e n e r g y peak i n t h e CIS and CFS s p e c t r a shows n o d i s t i n c t energy s h i f t upon t h e c o o l i n g o f t h e sample. Thus, t h e peak B i n t h e CIS and CFS s p e c t r a i s a t t r i b u t e d t o t h e n o n r a d i a t i v e decay o f t h e b u l k c o r e e x c i t o n , w h i l e t h e l o w e r e n e r g y peak i s caused b y t h e decay o f t h e s u r f a c e c o r e e x c i t o n .
N e x t we c o n s i d e r t h e i n c i d e n t a n g l e dependence o f t h e peaks due t o t h e s u r f a c e c o r e e x c i t o n o b s e r v e d i n t h e CIS s p e c t r a . I n t h e s u r f a c e l a y e r o f t h e s i n g l e c r y s t a l o f sodium h a l i d e s s y m m e t r i c a l d i s t r i b u t i o n o f t h e Na 2p wave f u n c t i o n i s d i s t u r b e d , e s p e c i a l l y i n t h e d i r e c t i o n p e r p e n d i c u l a r t o t h e s u r f a c e . Even i n t h e s u r f a c e l a y e r o f t h e e v a p o r a t e d f i l m t h e Na 2p wave f u n c t i o n m i g h t e x h i b i t more o r l e s s t h e s i m i l a r p r o p e r t i e s . We b e l i e v e t h a t t h i s g i v e s r i s e t o t w o k i n d s o f t h e s u r f a c e c o r e e x c i t o n s , i . e . . t h e s u r f a c e c o r e e x c i t o n formed w i t h t h e Na 2p wave f u n c t i o n i n t h e d i r e c t i o n p e r p e n d i c u l a r t o t h e s u r f a c e (Sp e x c i t o n ) and t h e s u r f a c e c o r e e x c i t o n formed w i t h t h e Na 2p wave f u n c t i o n i n t h e d i r e c t i o n p a r a l l e l t o t h e s u r f a c e ( S s e x c i t o n ) . Then, t h e e l e c t r i c f i e l d v e c t o r p e r p e n d i c u l a r t o t h e s u r f a c e ( p - p o l a r i z e d l i g h t ) can e x c i t e t h e S s u r f a c e c o r e e x c i t o n . and t h e e l e c t r i c f i e l d v e c t o r p a r a l l e l t o t h e s u r f a c e ( s - p o ? a r i z e d l i g h t ) can e x c i t e t h e Ss-surface c o r e e x c i t o n . While, s - p o l a r i z e d l i g h t may n o t a p p r e c i a b l y c o n t r i b u t e t o t h e e x c i t a t i o n o f t h e S p - s u r f a c e c o r e e x c i t o n , and v i c e v e r s a . T h i s g i v e s r i s e t o t h e a n g l e dependence o f t h e peaks due t o t h e s u r f a c e c o r e e x c i t o n i n t h e CIS s p e c t r a .
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