RESONANT 4f PHOTOEMISSION AND Yb LIII SEXAFS STUDY OF Yb/Pd (111) INTERFACE

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RESONANT 4f PHOTOEMISSION AND Yb LIII SEXAFS STUDY OF Yb/Pd (111) INTERFACE

E. Beaurepaire, C. Brouder, B. Carrière, D. Chandesris, G. Krill, J. Lecante, P. Légaré

To cite this version:

E. Beaurepaire, C. Brouder, B. Carrière, D. Chandesris, G. Krill, et al.. RESONANT 4f PHO-

TOEMISSION AND Yb LIII SEXAFS STUDY OF Yb/Pd (111) INTERFACE. Journal de Physique

Colloques, 1987, 48 (C9), pp.C9-939-C9-942. �10.1051/jphyscol:19879168�. �jpa-00227282�

RESONANT 4f PHOTOEMISSION AND Y b LE SEXAFS STUDY OF Yb/Pd (111) INTERFACE

E. BEAUREPAIRE, C. BROUDER", B. C A R R I E R E * ~ , D. CHANDESRIS*"", G. KRILL*

,

J. LECANTE* * * and P

. ^LBGARB*

^{* *}

^'

LMSES, CNRS UA-306, 4, Rue Blaise Pascal, F-67070 Strasbourg Cedex, France

* ~ a b o r a t o i r e d e Physique des Solides, CNRS UA-155, universite d e Nancy I, B P 239, F-54506 Vandoeuvre-les-Nancy Cedex, France ' " ~ a b o r a t o i r e de Cristallographie et d'Etude des Surfaces, CNRS UA-795, 4 , Rue Blaise Pascal, F-67070 Strasbourg Cedex.

France

'""LURE, CNRS LP-08, F-91405 Orsay Cedex, France

* * * * Laboratoire de Catalyse et d e Chimie des Surfaces,

CNRS UA-423, 4, Rue Blaise Pascal, F-67070 Strasbourg Cedex, France

Resume

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Nous avons utilisk des mesures de seuil LIII de surface et de photo- emission W pour etudier l'etat de valence d'atomes d'ytterbium deposes en films minces (1 ?i 25 monocouches) sur le Pd(ll1) B 300 K et B 77 K. Ces r4-

sultats montrent que l'interdiffusion entre ces deux elements est importante 300 K, ce qui donne lieu B la formation d'alliages Yb-Pd B la surface.

Par contre, l'ytterbium reste divalent et similaire au metal pour des couches d4pos4es 2 77 K.

Abstract

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Surface Yb L .-edge and W photoemission have been used to study the valence state of ~b'IIatoms deposited on Pd(ll1) at 300 K and 77 K for coverages between 1 and 25 monolayers. The results are interpreted by a strong interdiffusion of the species at 300 K, going with the formation of Yb-Pd alloys. In opposition, Yb remains divalent and metal-like for layers deposi- ted at 7 7 K.

I - INTRODUCTION

In the field of research devoted to interface formation, Synchrotron Radiation appeared to be of particular interest ; two approaches are generally developed : either the study of local environment by SEXAFS or of electronic structure and growing process by photoemission.

Besides their specific interest, Rare Earth (RE) Transition Metal (TM) interfaces may be considered as model systems, taking advantage of the sensitivity of the RE valence state to modifications of the local chemical surrounding. Therefore, LIII X ray absorption spectroscopy and photoemission are believed to be particularly relevant for the study of RE/TM interfaces 11, 21 since these techniques have been widely used to such a purpose in the case of intermetallic alloys 131.

Me Present here Yb LIII surface X-ray absorption spectroscopy (SXAS) and 4f photo- emission for thin Yb layers deposited on a Pd(ll1) substrate at 300 K and 7 7 K.

Our results clearly show that interdiffusion leads to the formation of Yb-Pd alloys near the surface at room temperature. This mechanism has been quenched at low temperature (77 K) where Yb metal-like layers grow on the Pd substrate.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879168

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PHYSIQUE

I1

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EXPERIMENTAL

Experiments have been c a r r i e d a t L.U.R.E. (ACO s t o r a g e r i n g f o r photoemission and D C I f o r SXAS).

I n b o t h c a s e s , P d ( l l 1 ) s u r f a c e was c l e a n e d by c y c l i n g ~ r + bombardment and a n n e a l i n g (up t o 600°C). The growth of Yb l a y e r s h a s been checked by a q u a r t z c r y s t a l m o n i t o r and Auger s p e c t r o s c o p y .

111 - RESULTS

Some t y p i c a l p h o t o e m i s s i o n s p e c t r a (hv = 160 eV) a r e d i s p l a y e d on F i g . 1 f o r v a r i o u s Yb c o v e r a g e s a t 300 K ( l a ) and 77 K ( I b ) . Due t o t h e s o - c a l l e d "cooper minimum", Pd 4d s t a t e s do n o t c o n t r i b u t e s i g n i f i c a n t l y t o t h e s p e c t r a t h a t a r e l a r g e l y domina- t e d by t r a n s i t i o n s f r a n Yb 4f s t a t e s even f o r a d s o r b a t e s i n t h e monolayer range. Com- p a r i s o n w i t 3 r e f r r e n c e s p e c t r a 1 4 1 confirms t h a t f e a t u r e s c h a r a c t e r i s t i c of yb3+

( 4 f I 3 + 4f + e ) - r e s p . yb2+ (4f l 4 -t 4 f 1 3 + e )- a r e observed f o r b i n d i n g e n e r g i e s between 5 and 15 eV - r e s p . 0 and 4 eV-. T e shape of yb3+ spectrum i s g i v e n by a suc- c e s s i o n of a t o m i c l f 1 2 m u l t i p l e f s 151. Ybqi spectrum c o n s i s t s i n p r i n c i p l e of t h e s p i n o r b i t d o u b l e t 4f

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s e p a r a t e d by 1.26 eV w i t h t h e r a t i o 314. T h i s i s q u a l i t a t i v e l y o b s e r v e a / 2 f o r 4f712 77 X l a y e r s , b u t a n a d d i t i o n a l c o n t r i b u t i o n s h o u l d c l e a r l y b e t a k e n i n t o a c c o u n t f o r t h i c k e s t l a y e r s a t 300 K. I n order to s o r t ^out t h e s e c o n t r i b u t i o n s , t h e s p e c t r a have been f i t t e d by f o l d i n g t h e a t o m i c l i n e s h a p e w i t h a Doniach-Sunjic f u n c t i o n and t h e i n s t r u m e n t a l r e s o l u t i o n ( g a u s s i a n , FWHM % 0 . 5 eV). These l i n e s were indexed a s b u l k yb3+, b u l k yb2+ and s u r f a c e yb2+ i n Table I

and F i g . 1 . The b u l k o r s u r f a c e c h a r a c t e r i s f i x e d by t h e i n t e n s i t y dependence v e r s u s photon energy.

Fig. 1

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Photoemission s p e c t r a (hv = 160 eV) f o r Yb d e p o s i t e d a t 300 K ( a ) and 77 K (b) on ~ d ( l l 1 ) . Dashed l i n e : volume c o n t r i b u t i o n ; d o t t e d l i n e : s u r f a c e c o n t r i b u - t i o n ( s e e t e x t and t a b l e I ) .

l 3 12b 1 2 s

0.9 m l 57.1 0 42.9

1.5 ml 56.8 0 43.2

3 ml 55.5 0 44.5

4.5 ml 3 7 . 3 12.7 5 0

7 . 5 ml 18 47.5 34.5

20 m l - 7 7 K 0 80 20

Yb Pd (a) 48.5 0 5 1 . 5

~ b J d 4 ( a ) 37.8 12.7 49.5

3+ 2+

Table I - R e l a t i v e i n t e n s i t y of Yb ( I ~ ) , b u l k yb2+ ( I Z b ) and s u r f a c e Yb

( I p s ) c o n t r i b u t i o n s measured by photoemission f o r d i f f e r e n t Yb c o v e r a g e s on

P d ( l l 1 ) . ( a ) Ref. / 4 / ; hv = 120 eV.

yield method) are due to the 2p -t 5d transi ions. Th large shift 7 eV) between the threshold associated respectively to Yb5* and Yb

5+

configurations allows the estimation of their relative weight using standard fitting routines 161.

This is expressed by the fractional valence v quoted in Fig. 2.

I I

8910 8920 89SO 8940 8960 PHOTON E N E R G Y I.VI

Fig. 2

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Yb L II,SXAS for various Yb coverages on Pd(ll1). The line is a fit (see text) alfowmg determination of Yb valence.

The case of Yb layers deposited on Pd at 7 7 K is apparently the simplest one. Yb atoms remain essentially divalent, what is coherent with the creation of an almost abrupt interface covered by divalent-Yb metal. Additional results would be necessa- ry to precise this picture in the submonolayer range.

The main point at 300 K (Fig. la) is that the trivalent contribution, very impor- tant at low coverages, decreases for increasing Yb layer depth. Since Yb is known to be trivalent in YbPd compounds for x > 1 , this is the indication of strong diffusionprocess betwee5 Yb and Pd. In fact, spectra obtained for the increasing coverages 0 . 5 ml, 0.9 - 3 . 5 ml and 4.5 ml agree with those of the intermetallic series YbPd Yb Pd and YbPd / 4 / . Thicker Yb layers (up to 12.5 ml not shown here) are s2:ll aif$erent from Yb metal (compare eg with 20 ml Yb at 7 7 K) and could result from Yb richer metastable alloys formation / 7 / .

It should be noted that what we called the surface contribution is very important in our spectra (and also those of 1 4 1 ) . Assuming an electron mean free path of

% 6 8 , one should conclude that Yb electronic state is modified beyond the first top layer. The mechanism responsible for this effect (lattice expansion or atomic concentration variations near the surface) is not clear at the moment.

The Yb LIII SXAS results of Fig. 2 confirm the diffusion process between Yb and Pd at room temperature. But we observe that the relative intensity of ~ b 3 + contri- bution, though it follows the same trend that when measured by photoemission,

is always significantly larger. This can be qualitatively understood by the weaker surface sensitivity of SEXAFS (% 100 8)unfavourable to the divalent Yb layers close to the surface. A more quantitative comparison of SXAS and photoemission results is under progress in order to get informations on deepest Yb layers.

JOURNAL DE PHYSIQUE

REFERENCES

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and Marchal, G, submitted to Phys. Rev. B.

1 3 1 See eg "Valence Instabilities", P. Wachter and H. Boppart eds, North Holland (1 982).

/ 4 / Domke, M., Laubschat, C., Sampathkumaran, E.V., Prietsch, M., Mandel, T., Kaindl, G. and Riddelmann, H.U., Phys. Rev. B

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