ENERGY SPREAD OF CLUSTER IONS FROM A MOLTEN METAL

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Submitted on 1 Jan 1979

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ENERGY SPREAD OF CLUSTER IONS FROM A MOLTEN METAL

A. Dixon, A. von Engel

To cite this version:

A. Dixon, A. von Engel. ENERGY SPREAD OF CLUSTER IONS FROM A MOLTEN METAL.

Journal de Physique Colloques, 1979, 40 (C7), pp.C7-441-C7-441. �10.1051/jphyscol:19797214�. �jpa-

00219195�

JOURNAL DE PHYSIQUE Colloque C 7 , supplement au n07, Tome 40, JuiZZet 1979, Page C7- 441

ENERGY SPREAD OF CLUSTER IONS FROM A MOLTEN METAL

A. J. Dixon and A. von Engel.

Engineering Science Dept., Oxford University.

I n a r e c e n t paper (Dixon 1979) a mechanism was have t o be exchanged) and s u g g e s t s t h a t t h e energy proposed by which t h e i o n s drawn from t h e i r molten broadening of t h e c l u s t e r s t o o was p r i m a r i l y caused m e t a l by a s t r o n g e l e c t r i c f i e l d might have a wide by charge exchange. However, i n t h i s c a s e t h e pro-

spread i n energy. I t was concluded t h a t t h e pre- c e s s

sence of a dense cloud of m e t a l vapour c l o s e t o A u + ( f a s t ) n + Aun(slow) + A u n ( f a s t ) + ~u:(slow) t h e r e g i o n from which t h e i o n s a r e e m i t t e d would h a s a low p r o b a b i l i t y because t h e number of n e u t r a l broaden t h e energy spread by r e s o n a n t charge ex- c l u s t e r s i n t h e vapour i s t o o low. Furthermore, change. Moreover, i t was shown t h a t t h i s p r o c e s s such charge exchange would g i v e a s i n g l e peak w i t h w i l l have a f a r g r e a t e r e f f e c t on t h e energy spread a long t a i l r a t h e r t h a n 2 s e o a r a t e peaks a s obser- of s i n g l y charged than of doubly charged i o n s ved. Another energy-broadening r e a c t i o n i s because t h e c r o s s s e c t i o n f o r charge exchange b e t - AU+ + Aun + AU*

n+ l

ween a doubly charged i o n and a n e u t r a l atom i s b u t i t can be r u l e d o u t because i t r e q u i r e s thermal r e l a t i v e l y small.

Experimental evidence f o r t h i s h y p o t h e s i s was provided by t h e work of Sudraud e t a l . (1979) on t h e energy spread of gold i o n s e m i t t e d from a n e e d l e of ^% 1 0 ~ t i p r a d i u s wetted w i t h molten gold by c a p i l l a r y flow. They observed t h a t t h e i o n s AU+

had a h a l f width (F.W.H.M.) i n energy of 65 eV ( i n s t r u m e n t a l width unknown) and a v e r y long t a i l of slower i o n s extending t o s e v e r a l hundred eV below t h e a p p l i e d v o l t a g e of 8 kV. The i o n s Au

++ ,

on t h e o t h e r hand, had a h a l f width i n energy of

r e l a t i v e v e l o c i t i e s t o be e f f i c i e n t . Of t h e many processesJwe favour AU+ + Au ⁺ Au + AU'.

n n

Though non-resonant, because Au h a s a h i g h e r i o n i - z a t i o n p o t e n t i a l t h a n Au,, i t may have a l a r g e enough c r o s s s e c t i o n i f AU+ i s e x c i t e d .

n

The s u r p r i s i n g o b s e r v a t i o n i s t h a t t h e r e a r e 2 peaks whose r e l a t i v e s i z e v a r i e s g r e a t l y w i t h n.

This can be understood by assuming t h e p r e s e n c e of two independent s o u r c e s of AU+. one n ' i s t h e m e l t , t h e o t h e r t h e vapour. The m e l t may be a s s o c i a t e d 45 eV and no t a i l of slow i o n s . w i t h t h e f i r s t peak. The p r o c e s s i n t h e vapour i s

Sudraud e t a l . a l s o measured t h e energy spread thought t o r e p r e s e n t t h e second peak. That t h e

+ + + + + +

of t h e c l u s t e r i o n s Au2, Aug, Au4, Au5, Au3

.

t r i c f i e l d (up t o 10 V/m) which can broaden t h e peak. It became more prominent (compared with t h e energy l e v e l s s o t h a t t h e c r o s s s e c t i o n of t h e f i r s t peak) a s t h e s i z e of t h e c l u s t e r s i n c r e a s e d . r e a c t i o n favoured above i n c r e a s e s i f only i n a nar- The energy spread of t h e i o n s Au3 ++

,

A. J - Dixon, 1979, J.Phys.D.: t o b e published.

played no second peak. T h i s simple f a c t i s analo-

++

3.Physique L e t t s . 1979, submitted.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797214