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Secondary electron energy spectra from foils under light-ion bombardment
Volume 33A, number 2
SECONDARY
PHYSICS
LETTEIIS
5 October t97!)
ELECTRON ENERGY SPECTRA UNDER LIGHT-ION BOMBARDMENT
FROM
FOILS
K. G. HARRISON and M. Wo L U C A S Sch(~n/ q/Mc~lh('~amtical a~zd t ) h \ s i c a [ Scictlcc,n
Ut*D,'J.sil 3 q t ' S a s s c ~ ,
Falim',
lT;'iA,h/,m H:VI 9 ( V t .
I 1(
[leeeived 26 August t970 A strong peak has been observed in the energy s p e c t r u m of forward s c a t t e r e d secondary electrons. This is i n t e r p r e t e d as an effect of charge exchange into a continuum state, a special case of ionisation.
M e a s u r e m e n t s of the e n e r g y s p e c t r u m of e l e c i r o n s k i n e t i c a l l y e j e c t e d f r o m ion b o m b a r d e d s o l i d s a r e r e v i e w e d by K a m i n s k v [1]. In t h e s e e x p e r i m e n t s the i o n s w e r e i n c i d e n t on a t h i c k t a r g e t a n d the e l e c t r o n s r e c e i v e d by the d e t e c t i o n s y s t e m w e r e t h o s e e j e c t e d f r o m the f r o n t s u r f a c e of the t a r g e t . M o s t of t h e s e e l e c t r o n s h a d s u f f e r e d m u l t i p l e s c a t t e r i n g in the t a r g e t s o that it w a s d i f f i c u l t to i d e n t i f y the n a t u r e of the p r i m a r y c o l l i s i o n r e s p o n s i b l e f o r the k i n e t i c e j e c t i o n p r o c e s s . P t o c h [2] p r o p o s e d an i o n i s a t i o n m e c h a n i s m by w h i c h S t e r n g l a s s [31 s u c c e s s f u l l y c a l c u l a t e d the d e p e n d e n c e of the t o t a l y i e l d on ion e n e r g y . In an a t t e m p t to e x a m i n e t h i s m o d e l m o r e c r i t i c a l l y e n e r g y s p e c t r a of e l e c t r o n s e m i t t e d f r o m the b a c k s u r f a c e of c a r b o n and g o l d f o i l s d u r i n g p r o t o n (H+). and h y d r o g e n - i o n ( H ~ ) b o m b a r d l n e n t h a v e b e e n m e a s u r e d . A m a g n e t i c 180 ° s p e c t r o m e t e r w a s o r i e n t e d to d e t e c t e l e c t r o n s t r a v e l l i n g f o r w a r d w i t h i n a c o n e of 3 . 5 ° h a l f - a n g l e on the a x i s of the c o l l i m a t e d (0.03 ° ) ion b e a m . T h e c a r b o n foil w a s 8 6 0 ± 60 A t h i c k , the gold 350:L 4 0 , ~ a n d the v a c u u m 1 , 10 - 7 t o r r . S p e c t r a w e r e t a k e n at five i o n e n e r g i e s b e t w e e n 110 and 320 k e y and all s h o w the s a m e g e n e r a l s h a p e (fig. 1). T h e e n e r g y of the p e a k m a x i n m m i s not w h e r e one w o u l d e x p e c t to find 6 - r a y s ( S t e r n g l a s s [3]) u s i n g the g e o m e t r y d e s c r i b e d a b o v e . I n s t e a d it c o r r e s p o n d s q u i t e well to e l e c t r o n s of the s a m e v e l o c i t y a s the i o n s in the i n c i d e n t b e a m . T h e b a s e w i d t h of the p e a k (with b a c k g r o u n d s u b t r a c t e d ) i s s u b s t a n t i a l l y c o n s t a n t in v e l o c i t y s p ~ c e b e i n ~ about two B o h r v e l o c i t i e s (4.4 - 10 e m se~'). W i t h i n the s m a l l c o n e a c c e p t e d by the d e t e c t i o n s y s t e m the y i e l d in the p e a k is a p p r o x i m a t e l y 0.002 e l e c t r o n ion a n d b e g i n s to f a l l in m a g n i t u d e f o r p r o t o n e n e r g i e s a b o v e 2 0 0 k e V . T h e r e i s l i t t l e doubt t h a t t h i s s t r o n g p e a k i s a s s o c i a t e d with a s p e c i a l i o n i s a t i o n c o l l i s i o n c a l l e d by M a c e k [41 " c h a r g e e x c h a n g e to a c o n t i n u u m 142
iI l
100
200 300 Energy lei}
_i
/,O0
l.'ig, t. T l : a n s m i s s i o n s e e o n d a r \ " electroJ<} e n e r g y s p e e t r t n n for 275 keV' p r o t o n s i n c i d e n t on an ~61) A eg/l%on [oil,
s t a t e " ; The i n c i d e n t ion p i c k s up a bound e l e c t r o n f r o m the s o l i d a n d c a r r i e s it a l o n g in a c o n t i n u u m s t a t e at the i o n i s a t i o n t h r e s h o l d . W h i l e t h i s can h a p p e n f o r any p o t e n t i a l , it s h o w s up a s a p e a k h e r e b e c a u s e of the high d e n s i t y of s t a t e s at the i o n i s a t i o n t h r e s h o l d r e s u l t i n g f r o m the C o u l o m b p o t e n t i a i b e t w e e n t h e ion and e l e c t r o n . R u c l d e t al. I51 have a l s o o b s e r v e d s t r o n g f o r w a r d s c a t t e r i n g of s e c o n d a r y e l e c t r o n s in g a s c o l l i s i o n e x p e r i m e n t s We s h o u l d like it. l h a n k K. I ) e t l n i a n n [tJr pl'
[1 t M.Kaminsky. Atomic ~lnd ionic impact phc, l~onien:t or. metal s u r f a c e s (Sprh~gec-Verla~, [9fi5). [2j W. Ploch, Z. Physik lao (t,,)si) 174. [3] 1,i..[,Sternglass. Phvs_ I{ev. II)'q (1957) 1, [4 l.l.Macel<, Ph3s. Ilev. 1 ([9701 2:~5, 151 hi. I:].I{udd, C, A. Stnutter :/nd 1'. I,. l~aiIey. I'hs'~. t{c,, . t51 (1.~)GO) 2U.
SECONDARY
PHYSICS
LETTEIIS
5 October t97!)
ELECTRON ENERGY SPECTRA UNDER LIGHT-ION BOMBARDMENT
FROM
FOILS
K. G. HARRISON and M. Wo L U C A S Sch(~n/ q/Mc~lh('~amtical a~zd t ) h \ s i c a [ Scictlcc,n
Ut*D,'J.sil 3 q t ' S a s s c ~ ,
Falim',
lT;'iA,h/,m H:VI 9 ( V t .
I 1(
[leeeived 26 August t970 A strong peak has been observed in the energy s p e c t r u m of forward s c a t t e r e d secondary electrons. This is i n t e r p r e t e d as an effect of charge exchange into a continuum state, a special case of ionisation.
M e a s u r e m e n t s of the e n e r g y s p e c t r u m of e l e c i r o n s k i n e t i c a l l y e j e c t e d f r o m ion b o m b a r d e d s o l i d s a r e r e v i e w e d by K a m i n s k v [1]. In t h e s e e x p e r i m e n t s the i o n s w e r e i n c i d e n t on a t h i c k t a r g e t a n d the e l e c t r o n s r e c e i v e d by the d e t e c t i o n s y s t e m w e r e t h o s e e j e c t e d f r o m the f r o n t s u r f a c e of the t a r g e t . M o s t of t h e s e e l e c t r o n s h a d s u f f e r e d m u l t i p l e s c a t t e r i n g in the t a r g e t s o that it w a s d i f f i c u l t to i d e n t i f y the n a t u r e of the p r i m a r y c o l l i s i o n r e s p o n s i b l e f o r the k i n e t i c e j e c t i o n p r o c e s s . P t o c h [2] p r o p o s e d an i o n i s a t i o n m e c h a n i s m by w h i c h S t e r n g l a s s [31 s u c c e s s f u l l y c a l c u l a t e d the d e p e n d e n c e of the t o t a l y i e l d on ion e n e r g y . In an a t t e m p t to e x a m i n e t h i s m o d e l m o r e c r i t i c a l l y e n e r g y s p e c t r a of e l e c t r o n s e m i t t e d f r o m the b a c k s u r f a c e of c a r b o n and g o l d f o i l s d u r i n g p r o t o n (H+). and h y d r o g e n - i o n ( H ~ ) b o m b a r d l n e n t h a v e b e e n m e a s u r e d . A m a g n e t i c 180 ° s p e c t r o m e t e r w a s o r i e n t e d to d e t e c t e l e c t r o n s t r a v e l l i n g f o r w a r d w i t h i n a c o n e of 3 . 5 ° h a l f - a n g l e on the a x i s of the c o l l i m a t e d (0.03 ° ) ion b e a m . T h e c a r b o n foil w a s 8 6 0 ± 60 A t h i c k , the gold 350:L 4 0 , ~ a n d the v a c u u m 1 , 10 - 7 t o r r . S p e c t r a w e r e t a k e n at five i o n e n e r g i e s b e t w e e n 110 and 320 k e y and all s h o w the s a m e g e n e r a l s h a p e (fig. 1). T h e e n e r g y of the p e a k m a x i n m m i s not w h e r e one w o u l d e x p e c t to find 6 - r a y s ( S t e r n g l a s s [3]) u s i n g the g e o m e t r y d e s c r i b e d a b o v e . I n s t e a d it c o r r e s p o n d s q u i t e well to e l e c t r o n s of the s a m e v e l o c i t y a s the i o n s in the i n c i d e n t b e a m . T h e b a s e w i d t h of the p e a k (with b a c k g r o u n d s u b t r a c t e d ) i s s u b s t a n t i a l l y c o n s t a n t in v e l o c i t y s p ~ c e b e i n ~ about two B o h r v e l o c i t i e s (4.4 - 10 e m se~'). W i t h i n the s m a l l c o n e a c c e p t e d by the d e t e c t i o n s y s t e m the y i e l d in the p e a k is a p p r o x i m a t e l y 0.002 e l e c t r o n ion a n d b e g i n s to f a l l in m a g n i t u d e f o r p r o t o n e n e r g i e s a b o v e 2 0 0 k e V . T h e r e i s l i t t l e doubt t h a t t h i s s t r o n g p e a k i s a s s o c i a t e d with a s p e c i a l i o n i s a t i o n c o l l i s i o n c a l l e d by M a c e k [41 " c h a r g e e x c h a n g e to a c o n t i n u u m 142
iI l
100
200 300 Energy lei}
_i
/,O0
l.'ig, t. T l : a n s m i s s i o n s e e o n d a r \ " electroJ<} e n e r g y s p e e t r t n n for 275 keV' p r o t o n s i n c i d e n t on an ~61) A eg/l%on [oil,
s t a t e " ; The i n c i d e n t ion p i c k s up a bound e l e c t r o n f r o m the s o l i d a n d c a r r i e s it a l o n g in a c o n t i n u u m s t a t e at the i o n i s a t i o n t h r e s h o l d . W h i l e t h i s can h a p p e n f o r any p o t e n t i a l , it s h o w s up a s a p e a k h e r e b e c a u s e of the high d e n s i t y of s t a t e s at the i o n i s a t i o n t h r e s h o l d r e s u l t i n g f r o m the C o u l o m b p o t e n t i a i b e t w e e n t h e ion and e l e c t r o n . R u c l d e t al. I51 have a l s o o b s e r v e d s t r o n g f o r w a r d s c a t t e r i n g of s e c o n d a r y e l e c t r o n s in g a s c o l l i s i o n e x p e r i m e n t s We s h o u l d like it. l h a n k K. I ) e t l n i a n n [tJr pl'
[1 t M.Kaminsky. Atomic ~lnd ionic impact phc, l~onien:t or. metal s u r f a c e s (Sprh~gec-Verla~, [9fi5). [2j W. Ploch, Z. Physik lao (t,,)si) 174. [3] 1,i..[,Sternglass. Phvs_ I{ev. II)'q (1957) 1, [4 l.l.Macel<, Ph3s. Ilev. 1 ([9701 2:~5, 151 hi. I:].I{udd, C, A. Stnutter :/nd 1'. I,. l~aiIey. I'hs'~. t{c,, . t51 (1.~)GO) 2U.