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Ni 2p-3d EMISSION SPECTRUM : EXISTENCE OF AN X-RAY RESONANT STATE
F. Vergand, D. Fargues, C. Bonnelle
To cite this version:
F. Vergand, D. Fargues, C. Bonnelle. Ni 2p-3d EMISSION SPECTRUM : EXISTENCE OF AN X-RAY RESONANT STATE. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-923-C9-926.
�10.1051/jphyscol:19879164�. �jpa-00227278�
JOURNAL DE PHYSIQUE
Colloque C9, supplement au n012, Tome 48, decembre 1987
Ni 2p-3d EMISSION SPECTRUM : EXISTENCE OF AN X-RAY RESONANT STATE
F. VERGAND, D. FARGUES'~) and C. BONNELLE
Laboratoire de Chimle Physique, 11, Rue Pierre et Marie Curie, F-75231 Paris Cedex 05. France
RESUME : Les d i f f e r e n c e s o b s e r v e e s e n t r e l e s s p e c t r e s d u n i c k e l mas- s i f e t d ' a g r e g a t s d e n i c k e l d e d i f f e r e n t e s t a i l l e s s o n t d i s c u t e e s . Un d t a t X r e s o n n a n t e s t o b s e r v e p o u r l e s a g r e g a t s . On c o n c l u t q u e c e t B t a t r e s o n n a n t 2 p e x i s t e p o u r c e s s y s t g m e s d e p e t i t e d i - m e n s i o n c a r l ' e n v i r o n n e m e n t du n i c k e l a g i t comme u n e b a r r i g r e d e po- t e n t i e l c o n f i n a n t l e s e l e c t r o n s 3d sur l e s a t o m e s d e n i c k e l .
ABSTRACT : D i f f e r e n c e s o b s e r v e d b e t w e e n t h e s p e c t r a o f b u l k N i and N i " c l u s t e r s o f v a r i o u s s i z e s embedded i n s n X - z e o l i t e a r e d i s c u s s e d . An X-ray r e s o n a n t s t a t e i s o b s e r v e d f o r t h e c l u s t e r s . T t i s c o n c l u - d e d t h a t t h i s 2 p - r e s o n a n t s t a t e e x i s t s b e c a u s e n i c k e l h a s i n t h e s e s m a l l s i z e s y s t e m s a n e n v i r o n n e m e n t w h i c h a c t s a s a p o t e n t i a l b a r r i e r c o n f i n i n g t h e 3d e l e c t r o n s o n t h e N i a t o m s .
I n b u l k N i , t h e s i n g l e - e x c i t e d s t a t e h a v i n g a 2p h o l e and a n e l e c t r o n i n t h e n o r m a l l y e m p t y d s t a t e s h a s no q u a s i - a t o m i c c h a r a c - t e r . T h i s s t a t e m e n t i s s u p p o r t e d by t h e f o l l o w i n g f a c t s : 1) t h e N i 2p p h o t o a b s o r p t i o n i s r e l a t i v e l y w e l l d e s c r i b e d i n a band m o d e l w i t h t h e t r a n s i t i o n p r o b a b i l i t i e s and t h e 2 p h o l e l i f e - t i m e e f f e c t t a - ken i n t o a c c o " n t . [ l ] ; 2 ) no X-ray r e s o n a n c e l i n e i s o b s e r v e d i n t h e b u l k N I 2p e m l s s l o n ; t h e s e e m i s s i o n s i n v o l v i n g t h e d i r e c t r a d i a t i v e r e c o m b i n a t i o n of t h e e x c i t e d e l e c t r o n t o t h e 2p h o l e a r e o b s e r v a b l e o n l y if t h e s i n g l e - e x c i t e d s t a t e i s s t r o n g l y l o c a l i z e d
1 2 1 .
On t h e o t h e r h a n d , Coulomb c o r r e l a t i o n e f f e c t s a r e e x p e c t e d when a h o l e i s c r e a t e d i n a n y l e v e l o f a q u a s i - f i l l e d d b a n d .
Among t h e t r a n s i t i o n m e t a l s , n i c k e l i s t h e o n e f o r w h i c h t h e s e c o r r e - l a t i o n e f f e c t s a r e e x p e c t e d t o b e t h e m o s t n o t i c e a b l e [3]
.
With- t h e a i m o f d e t e r m i n i n g t h e i m p o r t a n c e o f i n t r a a t o m i c i n t e r a c t i o n s on t h e b u l k N i s p e c t r a l c h a r a c t e r i s t i c s , i t i s i n t e r e s t i n g t o i n v e s t i g a t e i f p h y s i c o - c h e m i c a l c o n d i t i o n s e x i s t i n w h i c h t h e N i 3d s t a t e s c a n h a v e some q u a s i - a t o m i c b e h a v i o u r . T h u s we h a v e a n a l y z e d t h e d e n s i t i e s o f s t a t e s o f s m a l l p a r t i c l e s . I n f a c t , a t o m i c c h a r a c t e r i s t i c s a r e e x p e c - t e d t o a p p e a r a s t h e s i z e d e c r e a s e s .We r e p o r t h e r e o u r r e s u l t s o n t h e X-ray s p e c t r a l d e n s i t i e s o f embedded N i s m a l l p a r t i c l e s . We d i s c u s s t h e c h a n g e s o b s e r v e d with t h e s i z e r e l a t i v e t o t h e b u l k . I n f o r m a t i o n s on t h e i m p o r t a n c e o f i n - t e r a c t i o n s o f i n t e r a t o m i c a n d i n t r a a t o m i c t y p e a r e d e d u c e d f o r t h e a n a l y z e d p a r t i c l e s a n d c o n s e q u e n t l y f o r t h e b u l k .
11. EXPERIMENTAL RESULTS
The s a m p l e s u s e d w e r e N i 1 0 C a z 0 Na4 H~~
x
z e o l i t e t r e a t e d(''present address : Laboratoire M. Letort. CNRS, BP 104. F-54600 Villers-les-Nancy. France
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879164
C9-924 JOURNAL
DE
PHYSIQUEin order to obtain an almost homogeneous dispersion of NiO clusters lo- cated in sodalites or in supercages Details of preparation and charac- terisation are described elswhere
~ 4 3 .
Let us recall that from FMR ex- periments these clusters are superparamagnetic. Static magnetism shows that the mean diameter of the NiO clusters in sodalite is less than 0.65 nm (corresponding to clusters of about 8 atoms) and 1 nm (about 5 0 atoms) in supercages. A mono granular layer of the zeolite is stuck onto a cooled copper target.Spectra are performed In a 5 0 0 mm radius bent crystal vacuum spectrometer.
A
(1050) beryl crystal is used in the first order of reflection. The detector is a Ar-CH flow qas counter. The energy of primary electrons is chosen in order to minimize satellite emission and self absorption in the target. In the spectral range studied (845-860 eV), the resolving power is about 1750C5J,
mainly as a result of the angular width of the reflection curve of the beryl crystal. Neverthe- less, the experimental accuracy is better than 0.leV for the measu- rement of energies. The spectra of clusters are compared to that of bulk nickel obtained under the same experimental conditions.The Ni 2p -3d emissions of 5 0 atoms and 8 atoms clusters are shown fig. 1-a
,
3 / 2 and 2-a respectively. The maximum (labelledx)"
Photon energy (eV) Photon energy ( e V )
Figure 1 : Ni 2p -3d emission Figure 2 : a: Ni 2p -3d emission spectra of: a: 5 o 3 I 2 atoms clusters spectrum of 8 atom 3/2clusters a': bulk Ni b: difference between C ~ U S - b: difference between cluster and ter and bulk Ni spectra bulk Ni spectra.
of the emission coincides ( * 0.08 eV) with that of bulk nickel (curve 1 . The main characteristic of the clusters spectra is the appea- rance of fine structure L6IC7.l which is more marked for the 5 0 atoms spectrum than for that of the 8 atoms. For a better chacterization of the clusters spectra, we show the difference curves between the spec- tra of the clusters and of t h e , b u l k nickel in each case (fig. lb.;2b).
Towards lower spectral energies
,
the 5 0 atom cluster emis- sion is slightly narrowed as clearly seen by negative values on the spectra difference for energies lower than Ex.
In contrast for the 8 atoms clusters,no contraction is observed but positive and negativev a l u e s o n t h e s p e c t r a d i f f e r e n c e o n e a c h s i d e o f E g d e n o t e a c l e a r c h a n g e i n t h e s h a p e o f t h e c l u s t e r e m i s s i o n .
On t h e h i g h e n e r g y s i d e , b o t h s p e c t r a s h o w a s t r u c t u r e l a - b e l l e d € i n t h e r a n g e a r o u n d 8 5 3 e V , i . e . i n t h e 2 p p h o t o a b s o r p t i o n r a n g e o f b u l k n i c k e l . H o w e v e r f o r t h e 5 0 a t o m s c l u 2 t g r s , t h e s t r u c - t u r e € a p p e a r s a s a s t r o n g c o n t r i b u t i o n o f p o s i t i v e v a l u e s w h i c h c o i n - c i d e s w i t h t h e p e a k A o f N i 2 p p h o t o a b s o r p t i o n w h i l e f o r 8 a t o m s c l u s t e r s t h e c o n t r i b u t i o n c o r r i L 6 0 n d i n g t o E i s w e a k e r a n d a p p e a r s s h i f t e d t o w a r d s h i g h e r e n e r g i e s b y a b o u t 0 . 5 e V . T h e s t r u c t u r e & e v i - d e n c e s t h e e x i s t e n c e o f a n X - r a y r e s o n a n t e m i s s i o n i n t h e s e s y s t e m s . DISCUSSION
F o r b u l k N i , t h e d o c c u p i e d s p e c t r a l d e n s i t i e s a s s e e n b y XES a n d XPS a r e k n o w n t o b e n a r r o w e r t h a n t h e t h e o r e t i c a l d DOS c a l - c u l a t e d f o r t h e m e t a l i n t h e g r o u n d s t a t e . I n b o t h e x p e r i m e n t s , t h e s p e c t r a l d e n s i t i e s d e s c r i b e a h o l e i n t h e v a l e n c e b a n d w e i g h t e d b y t r a n s i t i o n p r o b a b i l i t i e s . T h e y r e t a i n t h e c h a r a c t e r o f a c o n t i n u o u s b a n d a n d n o f e a t u r e i s s e e n i n t h e X - r a y 2 p - 3 d e m i s s i o n ( c f . f i g u r e s )
T h e X - r a y h i g h - e n e r g y s a t e l l i t e o f b u l k N i ' c a n b e d u e t o t h e . c r e a t , i o n o f d o u b l y - e x c i t e d o r i o n i z e d s t a t e s w i t h 2 p a n d 3 d h o - l e s b y s h a k e - u p e f f e c t f o l l o w e d b y a r a d i a t i v e r e c o m b g i a t l o n t o a f i n a l s t a t e w i t h t w o 3 d h o l e s . On t h e o t h e r h a n d , w h e n t h e e n e r g y E o f i n c i d e n t i o n i z i n g p a r t i c l e s i s h i g h e r t h a n t h e 2 p t h r e s h o l d , O
t h e i n t e r m e d i a t e s t a t e w i t h 2 p a n d 3 d h o l e s c a n '/'be c r e a t e d b y 2 p - 2 p 3 d C o s t e r - K r o n i g t g i g s i t i o n ; t h i s s t a t e d e c a y s t o t h e s a A k 2
f i 8 g f
s t a t e w i t h t w o 3 d h o l e s m a k i n g t h e s h a k e - u p a n d C o s t e r - K r o n i g s a t e l l i t e s i n d i s t i n g u i s h a b l e . A s a l r e a d y u n d e r l i n e d C 8 ] , t h e s a t e l l i t e i s a c o n t i n u o u s e m i s s i o n b a n d a n d n o f e a t u r e c h a r a c t e - r i s t i c o f t r a n s i t i o n s f r o m a q u a s i - a t o m i c s t a t e w i t h t w o 3 d h o l e s i s s e e n i n t h e b u l k N i X - r a y s p e c t r u m w h a t e v e r t h e v a l u e o f E o ( f i g u r e l ) . T h e A u g e r t r a n s i t i o n s h a v i n g t h e s a m e f i n a l s t a t e s h o w a n a n a l o - g o u s b e h a v i o u r . [ 9 ]I n c o n t r a s t , i n t h e c l u s t e r s p e c t r a , s t r u c t u r e s a r e o b s e r v e d which v a r y a c c o r d i n g t o t h e s i z e .
F o r b o t h c l u s t e r s , a r e s o n a n t e m i s s i o n (
&
) i s o b s e r v e d . S u c h a r a d i a t i v e p r o c e s s i s p r e s e n t o n l y i f t h e l i f e t i m e o f t h e e x c i - t e d s a t e 1s o f t h e s a m e o r d e r a s t h e c o r e h o l e l i f e t i m e ( h e r e1
. .9 . 1 0 - 6 s . s l n c e t h e N i 2P l e v e l i s 0 . 7 eV w i d e ) . T h i s e m i s s i o n c h a r a c t e r i z e s t h e l o c a l i s 8 t $ o n d u e t o t h e s i z e e f f e c t t h a t s u f f e r s a n
e l e c t r o n e x c i t e d i n t h e e m p t y d d e n s i t i e s i n t h e p r e s e n c e o f t h e c o r e - h o l e .
F o r t h e 5 0 a t o m c l u s t e r s , t h e r e s o n a n t e m i s s i o n c o i n c i d e s e x a c t l y w i t h t h e b u l k p h o t o a b s o r p t i o n p e a k . M o r e o v e r n o 3 d t r a n s i - t i o n c h a r a c t e r i s t i c o f a N i 3 d - l i g a n d o r b i t a l m i x i n g i s o b s e r v e d . T h i s s u g g e s t s t h a t t h e N i c l u s t e r - z e o l i t e i n t e r a c t i o n i s w e a k . T h e c o n t r a c t i o n o f t h e e m i s s i o n s p e c t r u m c a n t h e n b e r e l a t e d t o t h e d e - c r e a s e i n t h e m e a n c o o r d i n a t i o n n u m b e r i n a g r e e m e n t w i t h c a l c u l a t i o n s a n d p h o t o e m i s s i o n r e s u l t s . I n t h i s m o d e l , t h e r e a r e t h e N i - N i i n t e r - a c t i o n s w h i c h e s s e n t i a l l y g o v e r n t h e d i s t r i b u t i o n o f N i 3 d f i l l e d s t a t e s . T h e n t h e e m i s s i o n s t r u c t u r e s , o t h e r s t h a n & , c a n b e i n t e r p r e - t e d b y a p a r t i a l r e m o v i n g o f f i l l e d 3 d s t a t e s d e g e n e r a c y d u e t o t h e s i z e e f f e c t . O n t h e o t h e r h a n d , t h e w a l l s o f t h e s u p e r c a g e f o r m a r e p u l s i v e p o t e n t i a l b a r r i e r w h i c h c o n f i n e s t h e 3 d e l e c t r o n s i n t h e c l u s t e r l e a d i n g t o a l o c a l i s a t i o n o f 3 d s t a t e s . T h i s l o c a l i s a t i o n f a - v o u r s t h e d e g e n e r a c y r e m o v i n g o f 3 d f i l l e d o r b i t a l s a n d s t a b i l i z e s t h e e x c i t e d s t a t e p r e c u r s o r o f t h e r e s o n a n t e m i s s i o n .
JOURNAL
DE
PHYSIQUEI n t h e 8 a t o m s c l u s t e r s , a l l t h e a t o m s a r e s u r f a c e o n e s a n d e a c h N i h a s o n l y f o u r N i n e x t n e i g h b o u r s . C o n s e q u e n t l y t h e i n t e r a c - t i o n s b e t w e e n n i c k e l a n d z e o l i t e , m a i n l y o x y g e n o f t h e s o d a l i t e w a l l s , m u s t b e t a k e n i n t o a c c o u n t . I n d e e d t h e r e l a t i v e l y l a r g e a n d l i t t l e s t r u c t u r e d l i n e o b s e r v e d f o r t h i s s y s t e m c a n b e e x p l - a i n e d b y i n t e r a c - t i o n s b e t w e e n t h e N i d s t a t e s a n d t h e d e n s i t i e s o f s t a t e s o f t h e s o - l i d z e o l i t e . H o w e v e r t h e e m i s s i o n i s d i f f e r e n t f r o m t h a t o f N i 0 : n o c r y s t a l f i e l d s p l i t t i n g o f t h e 3 d s t a t e s i s o b s e r v e d . On t h e o t h e r h a n d , t h e h i g h e r e n e r g y f e a t u r e , E , i s s h i f t e d i n t h e d i r e c t i o n o f t h e N i p h o t o a b s o r p t i o n p e a c k i n N i O i n s p i t e o f t h e f a c t t h a t n o such f e a t u r e i s s e e n i n t h e 3 d e m i s s i o n o f N i O . T h e n t h e s p e c t r u m i s c h a - r a c t e r i s t i c o f N i c l u s t e r s i n w h i c h a l l t h e a t o m s - h a v e o x y g e n n e i g h - b o u r s a t d i s t a n c e s c l e a r l y l a r g e r t h a n i n t h e d e f i n e d c o m p o u n d . I n s u c h a s y s t e m , t h e s p e c t r a l d e n s i t y i s n o t d o m i n a t e d b y t h e N i - N i i n - t e r a c t i o n s b u t b y t h e i n t e r a c t i o n s b e t w e e n t h e N i 3 d s t a t e s a n d t h e w a l l s o f t h e c a g e a s r e v e a l e d b y t h e s p e c t r a l c h a r a c t e r i s t i c s : b r o a - d e n i n g o f t h e 3 d f i l l e d s t a t e d i s t r i b u t i o n a n d s h i f t o f t h e . r e s o n a n t e m i s s i o n .
CONCLUSION
The s t r u c t u r e s o b s e r v e d i n t h e X - r a y e m i s s i o n b a n d f o r em- b e d d e d N i c l u s t e r s d e p e n d s t r o n g l y o n t h e e n v i r o n m e n t . T h i s s h o w s t h a t i n t e r a c t i o n s o f t h e i n t e r a t o m i c t y p e p l a y a p r e d o m i n a n t r o l e o n t h e s h a p e o f s p e c t r a l d e n s i t i e s f o r s u c h s y s t e m s . I n p a r t i c u l a r t h e c h a r a c - t e r i s t i c s o f t h e X - r a y r e s o n a n t s t a t e v a r i e s w i t h t h e e n v i r o n m e n t p r o - v i n g t h a t t h i s s t a t e i s d u e t o t h e p o t e n t i a l b a r r i e r i n d u c e d b y t h e c o n j u g a t e e f f e c t o f t h e p a r t i c l e s i z e a n d t h e c a g e w a l l s .
C o n v e r s e l y , t h e i n t r a a t o m i c c o r r e l a t i o n s do n o t d o m i n a t e a n d o n l y p a r t i a l a t o m i c c h a r a c t e r i s p r e s e n t i n t h e X - r a y s p e c t r a l d e n s i t i e s o f t h e embedded p a r t i c l e s . T h e n a p u r e l y a t o m i c m o d e l c a n - n o t b e u s e d t o d e s c r i b e t h e m . A f o r t i o r i , s u c h a m o d e l i s n o t v a l i d t o d e s c r i b e t h e b u l k N i s p e c t r a , a s a l r e a d y e x p e c t e d f r o m t h e a b s e n c e o f a t o m i c c h a r a c t e r i n t h e X - r a y a n d A u g e r s p e c t r a o f t h i s m e t a l . REFERENCES
1 F.SZMULOWICZ, D.M.PEASE, P h y s . R e v . B 1 7 , 3 3 4 1 ( 1 9 7 8 )
2 C.BONNELLE, C o n f e r e n c e P r o c e e d i n g s o f 7 X - r a y a n d I n n e r - S h e l l , P r o c e s s e s i n A t o m s , M o l e c u l e s a n d S o l i d s " , p . 3 1 7 ( 1 9 8 4 )
3 G.TREGLIA, F.DUCASTELLE, D.SPANJAARD, J . P h y s . - 4 3 , 3 4 1 ( 1 9 8 2 ) . a n d r e f e r e n c e s t h e r e i n .
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1 5 2 6 ( 1 9 8 0 ) .
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4 6 (1984).6 D.FARGUES, F . VERGAND, E. B E L I N , C . BONNELLE, D. O L I V I E R , L . BONNEVIOT, M.CHE, S u r f . S c i . 1 0 6 , 2 3 9 ( 1 9 8 1 ) .
7 D.FARGUES, T h e s e P a r i s 1 9 8 4 .
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