LIQUID METAL ION SOURCE STUDY OF Bip+n IONS (n/p ≤ 14, p ≤ 4)

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LIQUID METAL ION SOURCE STUDY OF Bip+n IONS (n/p ≤ 14, p ≤ 4)

J. van de Walle, P. Joyes

To cite this version:

J. van de Walle, P. Joyes. LIQUID METAL ION SOURCE STUDY OF Bip+n IONS (n/p ≤14, p

≤4). Journal de Physique Colloques, 1986, 47 (C7), pp.C7-371-C7-374. �10.1051/jphyscol:1986763�.

�jpa-00225958�

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L I Q U I D METAL ION SOURCE STUDY OF Bi:+ I O N S (n/p X 14, p $ 4 )

J. VAN DE WALLE and P. JOYES

Laboratoire de Physique des Solides, Université Paris-Sud, F-91405 Orsay Cedex, France

1. INTRODUCTION

The L i q u i d Metal Ion Source Technique (LMIS) has a l r e a d y been used f o r s t u d y i n g noble metal aggregates x!* (X = AU ( 1 ) ; Cu, Ag ( 2 ) ) . F o r p = 1, one observes t h a t X: aggregates w i t h odd n values a r e more abundant. T h i s p r o p e r t y i s a l s o observed by u s i n g t h e Secondary Ion Mass Spectrometry Technique (SMIS) ( 3 ) . The e x p l a n a t i o n o f t h e preceding odd-even e f f e c t i s t h a t X i aggregates w i t h odd n v a l u e s have l a r g e r s t a b i l i t i e s because t h e y have an even number o f valence e l e c t r o n s ( n

-

1 ) and, t h e r e f o r e , a complete

l e v e l s t r u c t u r e . T h i s s i m p l e model a l s o appl i e s t o x:' n o b l e metal aggregates where t h e number o f valence e l e c t r o n s ( n

-

^{2 )} leads t o t h e p r e d i c t i o n o f

l a r g e r s t a b i l i t i e s f o r even n values. Indeed, t h l s phenomenon has been observed f o r Cun 2+ ions ( 2 ) .

I n t h i s paper we p r e s e n t t h e LMlS study o f 6 i P * aggregates. The aggregates o f t h i s element a r e i n t e r e s t i n g s l n c e t h e y have a l r e a d y been s t u d i e d by two o t h e r approaches. On t h e one hand, t h e y have been produced i n seeded supersonic beams (4,5,6) and mass s p e c t r a o r v e l o c i t y d i s t r i b u t i o n s o f

~ii

c l u s t e r s have been recorded. On t h e o t h e r hand, some ~ i : * c a t i o n s have been chemical l y prepa- r e d ( 7 , 8 ) f o r example by t h e r e a c t f o n between bismuth metal and d i l u t e s o l u t i o n s o f bismuth t r i c h l o r i d e i n l i q u i d AICI3-NaCI m i x t u r e ( 9 ) . We w i l l t u r n back t o t h e r e s u l t s i n t h e forthcoming d i s c u s s l o n ,

II. EXPERIMENTAL RESULTS AND DISCUSSION.

The mass spectrum and Our peak identification a r e g t v e n F i g . 1. I t can be n o t i c e d t h a t t h e mass spectrum e x h l b i t s a general c h a r a c t e r l s t i c which can be e s t a b l i s h e d from t h e examinatlon o f t h e peaks f o r which t h e v a l u e o f t h e charge

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986763

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C7-372 JOURNAL DE PHYSIQUE

p i s c e r t a i n : t h e peaks a r e narrow f o r p = 1 and 2 and a r e w i d e r f o r p = 3 and 4. L e t us f i r s t d i s c u s s t h e f i r s t k l n d o f peaks.

1 1 .l SLnslr-a!b-bo:bLr-char~ed-ag9ce9a1f s~~O!<:~~EE-:L~ZCE:~L~~~

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I n most o f t h e t h e o r e t i c a l rnodels (10,11,12) i f i s a d r n i t t e d t h a t t h e chernical bonding i s m a i n l y due t o t h e 5p v a l e n c e e l e c t r o n s ( f h r e e e l e c t r o n s p e r atom).

By a p p i y i n g a g a i n t h e s i m p l e model g i v e n b e f o r e i t can be p r e d l c t e d t h a t an odd- even a l t e r n a t i o n s h o u l d appear, w i t h odd-n Bi: i o n s ( w h i c h bave an even number o f e l e c t r o n (3n-1)) more abundant.

T h i s a l t e r n a t i o n e f f e c t i s observed when a g g r e g a t e s a r e produced by t h e seeded s u p e r s o n i c beam t e c h n i q u e ( 6 ) . We o b s e r v e i n Our spectrum a s i r n i l a r behaviour, e x c e p t f o r n/p = 4. One may e x p l a i n t h i s d i f f e r e n c e by i d e n t i f y i n g t h e n/p = 4 Peak as b e i n g B i g * and n o t Bi:. Two arguments t e n d t o r e i n f o r s e t h i s i n t e r p r e t a t i o n . F i r s t l y , t h e B i 8 a g g r e g a t e i s t h e o n l y d o u b l y charged b i s m u t h 2+

c l u s t e r which has been c h e m i c a l l y p r e p a r e d (7,9) ; secondly, i t i s n o t s u r p r i s i n g t h a t t h e same e f f e c t does n o t o c c u r i n seeded s u p e r s o n i c bearn e x p e r i m e n t s where t h e d o u b i y charged i o n s a r e h a r d l y produced.

Then, i f we a d m i t t h a t t h e n/p = 4 peak i s r n a i n l y due t o B i g a g g r e g a t e s and 2+

i f we suppose t h a t f o r t h e o t h e r peaks w i t h i n t e g e r n/p v a l u e s o n l y ~ ic l u s t e r s i a r e pr,esent one say t h a t Our mass spectrum, as s u p e r s o n i c seeded beam s p e c t r a , e x h i b i t s an odd-even a l t e r n a t i o n b e h a v i o u r o f t h e B i f i n t e n s i t i e s .

11.2 Bi:* ... and ~i:*, m d u l o - 8 p e r i o d i c i t y .

One o f t h e advantages o f t h e LMIS t e c h n i q u e i s t h a t a g g r e g a t e s i n h i g h l y charged s t a t e s a r e produced. We w i l l d i s t i n g u i s h two n/p ranges. F o r n/p 2 5, t h e e x i s t e n c e o f t h e p = 4 charge i s v e r y u n l i k e l y because o f t h e c o u l o m b i c r e p u l s i o n ( 1 3 ) . We w i I 1 t h u s i d e n t i f y t h e "wide" peaks as B i :+ a g g r e g a t e s and we t h u s o b t a i n :

3+ 3+ 3+ 3+ 3 +

B i 4 , B i s , B i o , B i 8 , Bilg. F o r n / p S 5 t h e e x i s t e n c e o f p = 4 a g g r q a t e s i s no l o n g e r f o r b i d d e n and we g i v e t w o p o s s i b l e i d e n t i f i c a t i o n s f o r each peak. As we w i l l see i n t h e f c r t h c o m i n g d i s c u s s i o n one o f ?hem ( w h i c h i s u n d e r l i n e d ) i s more

3. 4+ 3+ 4+ 3+ 4 + 3+

l i k e l y B i l g o r B i 2 7 ; B i z 7 o r B i 3 6 ; B i z 3 o r B i 3 1 ( s m a l l i n t e n s i t y ) ; B i 3 5 o r

-

^- -

4 + 3 + 4 + 3 + 4+ 3+ 4 +

B i 4 7 ; B i 3 6 o r B i 4 8 ; B i 4 2 o r B i 5 6 ; B i 4 3 o r B i 5 7 (sometimes t h e two i d e n t i f i c a - t i o n s c o r r e s p o n d

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t w o d i f f e r e n t n / p x u e s compati b 1 e w i t h t h e peak w i d t h )

.

I f one o n l y c o n s i d e r s t h e u n d e r l i n e d v a l u e s i t appears a modulo-8 p e r i o d i c i t y f o r t h e B T . The main argument which s u p p o r t s t h e c h o i c e o f t h e u n d e r l i n e d v a l u e s i s t h a t such p e r i o d i c a l phenomena have a i r e a d y been observed f o r

SA

(14), 0; ( 1 5 )

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I n t h e s e l a s t s t u d i e s t h e p e r i o d i c i t i e s a r e due t o s u c c e s s i v e a d d i t i o n s o f n e u t r a l b u i l d i n g b l o c k s w i t h r e s p e c t i v e l y 8, 3 and 4 atoms i n t h e f i e l d o f a c e n t r a i p o s i t i v e charge. l n o u r case, t h e r e p e t i t i v e b u i l d i n g b l o c k would be

3 ⁺ B i 8 and t h e observed p e r i o d i c i t y o n l y s t a r t s f r o m Bilg.

L e t US now compare Our r e s u l t s on sma l l B i :+ t o t h e chemica l r e s u 1 t s . The 3 +

o n l y t r i p l y charged b i s m u t h a g g r e g a t e which have been p r e p a r e d i s B i 5 whose s t r u c t u r e ( 1 0 ) i s a t r i a n g u l a r b i p y r a m i d (D3h symmetry). T h i s a g g r e g a t e i s

i s o e l e c ~ r o n i c t o t h e B H'- borane a n i o n and has t h e same s t r u c t u r e . 5 5

I n Our measurements we do n o t o b s e r v e a r e m a r k a b l y l a r g e i n t e n s i t y f o r B i 3+

5 which i s s i m i l a r t o t h e i n t e n s i t y o f Bi:+, Bi:+ o r si:*. The d i t fe r e n c e between chemical and LMIS r e s u l t s can be e x p l a i n e d by t h e d i f f e r e n c e between t h e t w o p r o d u c t i o n modes. I n p a r t i c u l a r , ~ h e a n i o n s which a r e p r e s e n t i n t h e c h e m i c a l p r o c e s s seem t o p r e f e r e n t i a i l y s t a b i l i r e some Bi:+ as ~i:* o r Bi:* ( w h i c h i s n o t observed i n LMIS e x p e r i m e n t s ) .

L e t us a i s o g i v e t h e c r i t i c a l s i z e s n ( ~ ) ( B i ) above which Bi:+ i o n s b e g i n t o c r i

be observed :

n ( " ( B i ) c r i = 3 , nL;i(Bi) = 4.

We n o t e t h a t t h e f i r s t v a l u e i s i n agreement w i t h t h e v a l u e measured by J e n t s c h e t a l . ( 1 6 ) .

F i g . 1 - Mass spectrum o f b i s m u t h

R E F E R E N C E S

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