Rdsum6 -La mobilitC e t l a c o m p r e s s i b i l i t d du sodium adsorb6 s u r l a f a c e (110) du t u n g s t h e s o n t dCtermin6es p a r l a mesure de l a f o n c t i o n de c o r r 6 l a t i o n temp o r e l l e d e s f l u c t u a t i o n s du courant l o c a l d'gmission de champ pour d e s recouvrements e n sodium v a r i a n t de 2x1013 3 3x1014 atomes p a r cm2 e t d e s tempdratur e s de 170 5 500 K . Deux rdgimes de tempsrature a p p a r a i s s e n t clairement sdparmt des

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... s 06 l e s parambtres de d i f f u s i o n s o n t complbtement d i f f 6 r e n t s . Par comparaison avec l e s r g s u l t a t s obtenus p a r d i f f r a c t i o n d q 6 1 e c t r o n s l e n t s l a t r a n s i t i o n e n t r e l e s deux rcgimes a p p a r a I t comme une t r a n s i t i o n continue o r d r e local-ddsordre. Abstract -The m o b i l i t y and t h e c o m p r e s s i b i l i t y of sodium adsorbed on t h e (110) f a c e of tungsten have been determined by measuring t h e time c o r r e l a t i o n function of t h e l o c a l f i e l d emission c u r r e n t f l u c t u a t i o n s f o r coverages from 2x1013 t o 3x1014 atoms cm-2 and f o r temperatures from 170 t o 500K. Two temperature r e g imes c l e a r l y appear d i v i d i n g phases where t h e d i f f u s i o n parameters a r e completel y d i f f e r e n t . By comparison w i t h slow e l e c t r o n d i f f r a c t i o n r e s u l t s t h e t r a n s it i o n between b o t h regimes appears a s a continuous local-arder-disorder t r a n s i t i o n . The i n t r o d u c t i o n of t h e field-emission-fluctuation method t o i n v e s t i g a t e t h e mobil i t y and t h e c o m p r e s s i b i l i t y of a d s o r b a t e s 11,2 1 h a s r e c e n t l y given very i n t e r e s t i n g r e s u l t s concerning g a s on metal systems ( 3 , 4 , 5 , 6 1 . The improvement compared w i t h c l a s s i c a l spreading methods i s based upon t h e f a c t t h a t it i s a microscopic method d i r e c t l y g i v i n g t h e d i f f u s i o n coefF-icient D (through t h e c o r r e l a t i o n time T~ of d e n s i t y f l u c t u a t i o n s and t h e probed r q i o n a r e a A) and t h e c o m p r e s s i b i l i t y x (through t h e r e l a t i v e d e n s i t y f l u c t u a t i o n amplitude) of t h e adphase v e r s u s t h e mean coverage on a given f a c e without d e t a i l e d a n a l y s i s of a c o n c e n t r a t i o n p r o f i l e . I n t h i s s h o r t paper, u s i n g t h i s method; we p r e s e n t t h e r e s u l t s obtained i n one c a s e (Na on W(110)) belonging t o a n o t h e r c l a s s of a d s o r p t i o n system i . e . metal on metal system. One i n t e r e s t i n studying t h i s system comes from t h e knowledge of t h e adsorpt i o n bond c h a r a c t e r i s t i c s ( i . e . i o n i c type) and of t h e phase diagram obtained by slow e l e c t r o n d i f f r a c t i o n 17,8 1 . A c l a s s i c a l s t a i n l e s s s t e e l f i e l d emission microscope h a s been used, allowing an u l t i m a t e u l t r a -h i g h vacuum o f 1 t o 3x10-l1 Torr. Local c u r r e n t measurements a r e achieved u s i n g a probe-hole technique w i t h a t i p holder movable on bellows. The temperature of t h e t i p can be changed by c l a s s i c a l servo-loop-current from 8 0 K t o 3000 K. The evaporation source i s an a l k a l i metal d i s p e n s e r f u r n i s h e d by S.A.E.S. G e t t e r which i s u s u a l l y used i n t h e phototube i n d u s t r y . It i s a c o n t a i n e r f i l l e d up w i t h sodium chromate and a non evaporable agent Zr 16% -A 1 a l l o y named S t 101 t o prevent from g a s evaporation when h e a t i n g t h e source. Local c u r r e n t c o r r e l a t i o n f u n c t i o n s a r e measured i n a s i m i l a r way a s t h a t used by D i Foggio and Gomer t o study t h e d i f f u s i o n of hvdronen on W(110) (Fig. 1) The fowler-Nordheirn parameters 19 1 governing t h e l o c a l f i e l d emission c u r r e n t i have been measured u s i n g a l i n e a r r e g r e s s i o n program on Log ( i / v 2 ) v s where V i s a p p l i e d t i p v o l t a g e . Fig. 2 shows v a r i a t i o n s of t h e s e parameters w i t h Na coverage which i s o b t a i n e d from work f u n c t i o n v e r s u s coverage measurements on a macroscopic c r y s t a l 18 1. Article published online by EDP Sciences and available at http://dx.