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THE POLARISABILITY OF IRIDIUM NEUTRAL ATOMS AND THEIR VAN DER WAALS
INTERACTION WITH A TUNGSTEN SURFACE MEASURED BY F.I.M
J. Bardon, M. Audiffren
To cite this version:
J. Bardon, M. Audiffren. THE POLARISABILITY OF IRIDIUM NEUTRAL ATOMS AND THEIR VAN DER WAALS INTERACTION WITH A TUNGSTEN SURFACE MEASURED BY F.I.M.
Journal de Physique Colloques, 1984, 45 (C9), pp.C9-245-C6-249. �10.1051/jphyscol:1984941�. �jpa-
00224421�
JOURNAL DE PHYSIQUE
Colloque C9, supplément au n°12, Tome 45, décembre 1984 page C9-245
THE POLARISABILITY OF IRIDIUM NEUTRAL ATOMS AND THEIR VAN DER WAALS INTERACTION WITH A TUNGSTEN SURFACE MEASURED BY F.I.M,
J. Bardon* and M. Audiffren
CRMC2-CNRS, Campus de Luminy, case 913, 13S88 Marseille cedex 09, France
*Also Université de Vrovence, U.E.R. de Physique, 13003 Marseille, France
RESUME : Nous décrivons la mesure, en microscopie ionique de champ, de la polarisa- bilité a d'adatomes neutres d'iridium et de leur constante d'interaction de Van der Waals C avec une surface de Tungstène. Les valeurs trouvées sont : a = 8 + 1 A^ et C = 10 + 3 eV A3.
ABSTRACT : We describe the measurement, by Field Ion Microscopy, of the polarisabi- lity a of neutral Iridium atoms and of their Van der Waals interaction constant C with a Tungsten surface. The values obtained are a = 8 + 1 13 and C = 10 + 3 eV A3.
INTRODUCTION
The static polarisability of neutral atoms is an important quantity involved in many phenomena related to atomic beams. It has been theoretically calculated for many atoms /1,2/, but reliable experimental data are scarce and mainly limited to the noble gases /3/ and alkali metals /4/. The reason is the difficulty of obtaining the strong gradient of electric field and the good spatial resolution of neutral atoms detectors that are needed. Some attempts have been made first by Drechsler /5/
a long time ago by using a FEM tip to produce the electric field gradient and a glass plate as a detector but the accuracy was poor ; Bermond /6/ measured the pola- risability of Pb using a F.E.M. tip also as a detector.
We thought that the Field Ion Microscopy was ideally suited for such measure- ments because field gradients and spatial resolution are inherent to this instrument.
The F.I.M. tip was used to obtain the field gradient and to determine the location of Ir atoms reaching the tip. Thus, we have measured the polarisability of neutral Ir atoms and also the Van der Waals constant of attraction between these atoms and the tungsten surface of the tip.
EXPERIMENTAL
The principle of the measurement and the experimental device are schematized in Fig. 1. A source of Iridium atoms is set on the side of a tungsten field ion microscope tip, far enough for the velocities of the incoming atoms to be conside- red as parallel. In the absence of an electric field during the deposition, one could expect that all the Ir atoms would impinge on the <Sunny> part (S-part) of the tip limited by the xOz plane. When an electric field is applied to the tip, the electric field gradient curves the atomic trajectories and some atoms must arrive into the geometrically shadowed region (D-part).
Experimental details have been described elsewhere
11/.The source temperature was 2200 ± 30K and was monitored during the depositions by a Twocolour pyrometer, giving an excellent reproducibility for the number of deposited atoms.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984941
JOURNAL DE PHYSIQUE
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F i g u r e 1 a
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- F i g u r e - --
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1 - b -F i g . 1 a ) P r i n c i p l e of t h e measurement ; w i t h o u t i n t e r a c t i o n t h e atoms incoming w i t h v e l o c i t i e s p a r a l l e l t o t h e Oy a x i s s h o u l d s t r i k e t h e t i p i n t h e S - p a r t o n l y . F i g . 1 b) Example of a p a t t e r n o b t a i n e d a f t e r one d e p o s i t i o n ; each d o t c o r r e s p o n d s t o one adatom. The sum of d
i s
t h e p a r a m e t e r which was used t o d e t e r m i n e t h e p o l a - r i s a b i l i t y .The l o c a l i z a t i o n o f t h e L l i n e / 7 / was made by u s i n g t h e t o t a l i t y o f t h e pro- j e c t i o n s w i t h and w i t h o u t t h e e l e c t r i c f i e l d a p p l i e d . The t o t a l number o f adatoms i n t h e D-part was 516 and 2820 i n t h e S - p a r t .
The d e p o s i t i o n s were made up t o 0 . 3 V/A w i t h p o s i t i v e and n e g a t i v e f i e l d s . The r e s u l t s were t h e same, g i v i n g e v i d e n c e t h a t t h e i n t e r a c t i o n i s e f f e c t i v e l y a f u n c t i o n of t h e s q u a r e o f t h e f i e l d . These measurements a l s o show t h a t , a s e x p e c t e d from t h e Saha-Langmuir r e l a t i o n , t h e number of i o n s e m i t t e d by t h e s o u r c e i s n e g l i - g i b l e . We were p r e v e n t e d from u s i n g h i g h e r n e g a t i v e v a l u e s of t h e f i e l d b e c a u s e o f t h e i o n i z a t i o n of t h e incoming n e u t r a l atoms by t h e FEM c u r r e n t .
0
With a p o s i t i v e t i p , t h e f i e l d was v a r i e d up t o 0.65 V I A . I t was c a l i b r a t e d a g a i n s t t h e B I V and t h e b e g i n n i n g o f t h e f i e l d e v a p o r a t i o n . We assumed a v a l u e of 4 . 5
v / l
f o r t h e BIV. By i n c r e a s i n g t h e f i e l d , t h e t o t a l number N t and Nd t h e number i n t h e D-zone of observed I r adatoms i n c r e a s e up t o 0.58v/I.
They d e c r e a s e s h a r p l y above t h i s v a l u e . T h i s d e c r e a s ei s
due t o t h e i o n i z a t i o n of t h e atoms inco- ming i n t h e v i c i n i t y of t h e t i p . T h i s i n t e r p r e t a t i o n i s confirmed by t h e f a c t t h a t t h e r a t i o Nd/Nt o f t h e number of adatoms i n t h e D-zone t o t h e t o t a l number a l s o d e c r e a s e s , t h e atoms incoming i n t h e D-zone b e i n g exposed t o t h e h i g h e l e c t r i c f i e l d f o r a l o n g e r time t h a n t h e o t h e r o n e s .Between two d e p o s i t i o n s , t h e adatoms were f i e l d d e s o r b e d . However t h e f i e l d needed t o d e s o r b
I r
adatoms i s of t h e same magnitude t h a n t h e e v a p o r a t i o n f i e l d of t u n g s t e n . To p r e v e n t a r a p i d growth of t h e t i p , we u s e d t h e method d e s c r i b e d by Tsong and Walko181
: h e a t i n g t h e t i p a t 300 K w i t h t h e imaging f i e l d a p p l i e d d r i v e s t h e adatoms t o t h e l e d g e s of t h e f a c e s where t h e y a r e immediately f i e l d d e s o r b e d .For e a c h v a l u e of t h e e l e c t r i c f i e l d , t e n d e p o s i t i o n s were made. The mean number of atoms d e p o s i t e d i n t h e D-part was 90 and t h e mean t o t a l number of adatoms i n a 32"
-
wide zone around t h e a p e x of t h e t i p was450.
I r i d i u m i s w e l l s u i t e d t o t h e s e measurements due t o t h e h i g h v a l u e s of t h e f i e l d s needed t o i o n i s e t h e n e u t r a l atoms and t o d e s o r b t h e adatoms. The same expe- r i m e n t s w i t h t a n t a l u m and t u n g s t e n a r e l i m i t e d t o a n a r r o w e r range of e l e c t r i c f i e l d ( 0 . 3 VIA) and micrographs t a k e n a t f i e l d f u r t h e r down from of t h e B I V a r e more
d i f f i c u l t t o o b s e r v e w i t h t h e twocolour s u p e r p o s i t i o n method / 9 / . RESULTS
As a l r e a d y d e s c r i b e d f o r t a n t a l u m atoms / 7 / , I r i d i u m adatoms e x i s t i n t h e D- zone a f t e r t h e d e p o s i t i o n s w i t h o u t e l e c t r i c f i e l d . The r a t i o of t h e number of t h e s e adatoms t o t h e t o t a l number i n a 3Z0 zone around t h e a p e x o f t h e t i p i s 0.11.
T h i s phenomenon was modelised w i t h a Van d e r w a a l s a t t r a c t i o n , assuming a n i n t e r a c t i o n e n e r g y of t h e form W =
-
c / z 3 . Ci s
t h e c o n s t a n t of i n t e r a c t i o n and z t h e d i s t a n c e of t h e atom t o t h e s u r f a c e of t h e t i p . The Maxwellian d i s t r i b u t i o n of t h e v e l o c i t i e s of t h e atoms l e a v i n g t h e s o u r c e was t a k e n i n t o a c c o u n t . The c a l c u l a - t i o n i s t h r e e d i m e n s i o n a l a c c o r d i n g t o t h e r e s u l t s of Bermond /11/ who d e m o n s t r a t e d t h e importance of non p l a n a r t r a j e c t o r i e s ."3 The v a l u e of t h e c o n s t a n t t h a t we have o b t a i n e d
i s
C = 10+
3 eV A.
We u s e d t h e same t y p e of c a l c u l a t i o n t o d e t e r m i n e t h e v a l u e of t h e p o l a r i s a - b i l i t y . The t o t a l energy was t a k e n a s t h e sum of t h e Van d e r Waals e n e r g y and t h e p o l a r i s a t i o n energy 1/2 a ~ 2 . The t i p was r e p r e s e n t e d by an h y p e r b o l o I d superimposed t o a cone 1121 ; t h i s r e p r e s e n t a t i o n e n a b l e d u s t o o b t a i n t h e s p a t i a l r e p a r t i t i o n of t h e e l e c t r i c f i e l d a n a l y t i c a l l y . The v a l u e of t h e form f a c t o r was 0.1. Some t e s t s have demonstrated t h a t t h i s v a l u e can v a r y from 0.06 t o 0.18 w i t h o u t n o t i c e a b l e e f f e c t s on t h e r e s u l t s . With t h i s model, we c a l c u l a t e d a q u a n t i t y p r o p o r t i o n n a l t o t h e sum of t h e d i s t a n c e s of t h e adatoms t o t h e L - l i n e f o r d i f f e r e n t v a l u e s of
a
and F. A c o r r e c t i o n was made t o t r a n s f o r m t h e t r u e c a l c u l a t e d d i s t a n c e s t o a s t e r e o g r a - p h i c p r o j e c t i o n a c c o r d i n g t o Muller and Tsong / 9 / .A f i r s t a t t e m p t t o e x p e r i m e n t a l l y measure t h e r a t i o N d / N t v e r s u s t h e f i e l d f a i l e d t o g i v e s u f f i c i e n t l y p r e c i s e r e s u l t s . The s t a t i s t i c a l s c a t t e r of t h e e x p e r i - m e n t a l p o i n t s was such t h a t a p r o h i b i t i v e number of d e p o s i t i o n s would have been n e c e s s a r y .
1
F i g . 2 : Sum of t h e d i s t a n c e s of t h e F i g . 3 : T o t a l number o f adatoms v e r s u s adatoms i n t h e D-zone t o t h e
L
l i n e e l e c t r i c f i e l d . e x p e r i m e n t a l p o i n t s , v e r s u s t h e e l e c t r i c f i e l d : e x p e r i -----
t h e o r e t i c a l c u r v e sm e n t a l p o i n t s ,
----
t h e o r e t i c a l c u r v e sC9-248
3 0 U R N A L DE PHYSIQUE
Thus, we measured t h e d i s t a n c e s d t o t h e L - l i n e of a l l t h e adatoms i n t h e
D-
zone and c a l c u l a t e d t h e sum of t h e s e d i s t a n c e s f o r a l l t h e micrographs t a k e n a f t e r d e p o s i t i o n s a t same f i e l d . R e s u l t s a r e g i v e n i n F i g . 2 t o g e t h e r w i t h t h e c a l c u l a t e d sums o f d i s t a n c e s . Fig.3
shows t h e t o t a l number o f adatoms v e r s u s t h e f i e l d .A s e x p e c t e d , t h e d i s p e r s i o n o f t h e e x p e r i m e n t a l r e s u l t s i s more i m p o r t a n t i n F i g .
3
t h a n i n F i g . 2 . But t h e i m p o r t a n t p o i n ti s
t h a t t h e two c u r v e s a g r e e w i t h each o t h e r . I t i s l i k e l y t h a t a n e r r o r i n t h e model o r an i m p o r t a n t e f f e c t of t h e d i f f e r e n c e s i n t h e l o c a l r a d i u s o f c u r v a t u r e would g i v e i m p o r t a n t d i s c r e p a n c i e s between t h e two f i g u r e s .O 3 The b e s t f i t f o r t h e v a l u e of
a
i n F i g . 2i s
o b t a i n e d f o r a = 8 A.
DISCUSSION
The p r e s e n c e of adatoms i n t h e D-part of t h e t i p h a s a l r e a d y been observed
/ 1 3 , 1 4 / .
The v a l i d i t y o f t h e i n t e r p r e t a t i o n by t h e Van d e r Waals i n t e r a c t i o n h a s been c a r e f u l l y d i s c u s s e d/ 7 /
i n a p r e v i o u s p a p e r on t a n t a l u m . S i n c e , we a l s o have r o u g h l y c a l c u l a t e d t h e e f f e c t s of p a t c h f i e l d s/ I S / ,
s t e p d i p o l e i n t e r a c t i o n s/ 1 6 /
and r e c o i l e f f e c t s/ 1 7 / .
These e f f e c t s c o u l d h a r d l y i n v o l v e a n i n t e r a c t i o n e n e r g y g r e a t e r t h a n 3% of t h e p o l a r i s a t i o n and Van d e r Waals e n e r g i e s .A g r a p h i c a l s i m u l a t i o n o$ t h e t r a j e c t o r i e s showed t h a t t h e Van d e r Waals i n t e r - a c t i o n
i s
s h o r t ranged ( % 30 A ) . Consequently we d i d n o t checked a C / Z ~ r e l a t i o n ; t h e r e t a r i a t i o n e f f e c t s b e i n g o n l y e x p e c t e d t o have n o t i c e a b l e e f f e c t s f o r z g r e a t e r t h a n 800A.
A s f o r t a n t a l u m and f o r t h e same r e a s o n s , i ti s
beyond t h e scope of t h i s p a p e r t o c a l c u l a t e a t h e o r e t i c a l v a l u e of C f o r comparison w i t h o u r e x p e r i m e n t a l r e s u l t . However t h e o r d e r of magnitude of C i s r e a s o n a b l e and t h e v a l u e o b t a i n e d i s s m a l l e r t h a n f o r t a n t a l u m . We c o u l d e x p e c t t h i s r e s u l t b e c a u s e t h e v a l u e of t h e s t a t i c p o l a r i s a b i l i t yi s
p r o b a b l y a l s o s m a l l e r f o rI r
t h a n f o r Ta a s t h e o r e t i c a l l y c a l c u l a t e d /1/ and e x p e r i m e n t a l l y confirmed by o u r p r e l i m i n a r y r e s u l t s on Ta.It i s d i f f i c u l t t o compare t h e v a l u e of C w i t h t h o s e o b t a i n e d by d i f f e r e n t methods ; i n d e e d , C was o n l y measured f o r n o b l e g a s i n t e r a c t i o n w i t h a l k a l i h a l i d e s s u r f a c e s
/ 1 8 , 1 9 /
and a l k a l i m e t a l s w i t h m e t a l l i c s G r f a c e s/ 2 0 , 2 1 / .
The v a l u e o b i a i n e d f o r t h e p o l a r i s a b i l i t y i s i n good agreement w i t h t h e theo- r e t i c a l r e s u l t
7 ~3
g i v e n by M i l l e r and Bederson/ I / .
The c u r v e s of F i g . 2 p e r m i t t o e s t i m a t e an e r r o r of a b o u t 1 2 % . Thus a = 8 i 1 eV8 3 .
We stress t h e f a c t t h a t t h e d e t e r m i n a t i o n of t h e Van d e r Waals c o n s t a n t C and t h e v a l u e of t h e p o l a r i s a b i l i l y a do n o t i n v o l v e a d j u s t a b l e p a r a m e t e r s . The know- l e d g e o f C i s mandatory t o o b t a i n t h e v a l u e o f
a ,
b u t t h e d e t e r m i n a t i o n of C i s done i n d e p e n d a n t l y by t h e measurements w i t h o u t e l e c t r i c f i e l d .CONCLUSION
We have, f o r t h e f i r s t t i m e t o o u r knowledge, measured t h e Van d e r Waals cons- t a n t of i n t e r a c t i o n between I r i d i u m atoms and a m e t a l l i c s u r f a c e . We a l s o have measured f o r t h e f i r s t t i m e , w i t h a good a c c u r a c y , t h e v a l u e o f t h e p o l a r i s a b i l i t y of
I r
atoms. The agreement between e x p e r i m e n t a l and t h e o r e t i c a l r e s u l t s i s good.The u s e of t h e sum of d i s t a n c e s t o t h e L l i n e r e d u c e s t h e number of e x p e r i m e n t a l r u n s r e q u i r e d t o o b t a i n a n a c c u r a t e v a l u e o f t h e p o l a r i s a b i l i t y . I n i t s p r e s e n t form, t h e method
i s
l i m i t e d t o t h o s e adatoms t h a t a r e r e a d i l y imaged by FIM. I t c o u l d b e extended t o o t h e r atoms by u s i n g a F i e l d D e s o r p t i o n Microscope.ACKNOWLEDGMENTS
The a u t h o r s v e r y much a p p r e c i a t e d h e l p f u l d i s c u s s i o n s w i t h Dr. R. Morin and D r . J. Bermond. Throughout t h e i n v e s t i g a t i o n , t h e y a l s o t h a n k P r . M. D r e c h s l e r f o r h i s c o n s t a n t i n t e r e s t i n t h e development of t h i s work.
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