On the angular distribution of Auger electrons

Volume 37A, number 1

ON THE

PHYSICS LETTERS

ANGULAR

DISTRIBUTION

OF

25 October 1971

AUGER

ELECTRONS

*

B. C L E F F and W. MEHLHORN

Institut ~ r Kernphysik, Universitttt M~inster, 44 Miinster, Germany Received 10 August 1971

For the first time experimental evidence for a nonisotropie angular distribution of Auger electrons in the case of L3M2,3M2,3(1S0) transitions of argon is given.

It has been p r e d i c t e d e a r l i e r [1], that the X r a d i a t i o n and the Auger e l e c t r o n s following an 1 i n n e r shell v a c a n c y with quantum n u m b e r j > ~, c a u s e d by impact ionization of a d i r e c t e d beam of e l e c t r o n s or p r o t o n s , may have a n o n i s o t r o p i c a n g u l a r d i s t r i b u t i o n r e s u l t i n g in a p a r t i a l p o l a r i zation of the X radiation. This is due to the diff e r e n t ionization p r o b a b i l i t i e s Q(j, I m]) of magnetic s u b l e v e l s m c a u s i n g an a l i g n m e n t of the ionized atoms r e l a t i v e to the axis of the p r i m a r y p a r t i c l e beam defining the quantization axis. Subsequently Hrd~ et al. [2] have m e a s u r e d a p a r t i a l p o l a r i z a t i o n of the L a 1 (L3Mh) c h a r a c t e r i s t i c X r a d i a t i o n of a m e r c u r y vapor t a r g e t excited by e l e c t r o n impact. The p u r p o s e of this l e t t e r is to give for the f i r s t time e x p e r i m e n t a l evidence for a n o n i s o t r o p i c a n g u l a r d i s t r i b u t i o n of Auger e l e c t r o n s following an L 3 v a c a n c y in argon caused by e l e c t r o n impact. Of all L3MM t r a n s i t i o n s of argon the L3M2,3M2,3(1S0 ) Auger e l e c t r o n s a r e best suited to i n v e s t i g a t e a n o n i s o t r o p i c d i s t r i b u t i o n , b e c a u s e here the Auger e l e c t r o n is e m i t t e d as a single P 3 / 2 - w a v e r e s u l t i n g in a s i m p l e a n g u l a r d i s t r i b u tion. A s s u m i n g the ionization c r o s s s e c t i o n s Q(j.l[m ½ ) of the i n i t i a l magnetic s u b l e v e l s to be Q(~z, ) and Q(3, i I[ ) and introducing the p a r a m eter a = {Q(~, l ~ l ) - Q(3, 1½1)}/Q(~, [½i)

sity ratio of L3M 2 3M2 3(1S0 ) and L2M2,3M2. 3 (3P0,1, 2) l i n e s (f@. 1), ' w h e r e the l a t t e r line has an isotropic d i s t r i b u t i o n . In addition both l i n e s a r e well s e p a r a t e d from all other l i n e s and could t h e r e f o r e be m e a s u r e d with high p r e c i s i o n . The a p p a r a t u s used c o n s i s t e d of an e l e c t r o n gun, a c o l l i s i o n c h a m b e r and an 127 ° c y l i n d r i c a l e l e c t r o s t a t i c a n a l y s e r . Auger e l e c t r o n s e m i t t e d at the angles 0 = 30, 60, 90, 120 and 150 ° r e l a tive to the incident beam could be m e a s u r e d . The e l e c t r o n detection and r e c o r d i n g was exactly the s a m e as has been d e s c r i b e d e a r l i e r [3]. The p r e s s u r e of the argon in the c o l l i s i o n c h a m b e r was about 10 -3 t o r r . Since the ionization c r o s s sections, and t h e r e f o r e also a, a r e e n e r g y dependent, we m e a s u r e d the i n t e n s i t y r a t i o s l r e l (0) ~3 = 1(0, L3M 2 3M2,3(1S0))/I(0, L2M2,3M2,3( P0,1 2)) for t h r e e different p r i m a r y e l e c t r o n e n e r g i e s ' Ep = 2E(L3), 4E(L 3) and 16E(L3) , where E(L 3) = = 248.5 eV is the binding e n e r g y of an L 3 electron. In fig. 2 the m e a s u r e d i n t e n s i t y r a t i o s Irel(0) a r e plotted for 0 = 30, 90 and 150 °. We have n o r m a l i z e d all v a l u e s in such a way that Irel(O= O) = 1.

5 Argon

(1)

the following d i s t r i b u t i o n of the L3M2,3M2,3(1S0) Auger e l e c t r o n s can be d e r i v e d

I(0) ~ [1 + }a(1 - cos20)] .

(2)

In o r d e r to be independent of the effective size of the c o l l i s i o n region and of the shifts of the appar a t u s and the e l e c t r o n i c s we m e a s u r e d the i n t e n * This work has been financially supported by the Deutsche For sehungsgemeinschaft.

.2 •.

..',.. .. ... " , ~ i

200.87 eV

.6 .,

: .:

.

:=

I 20~OleV

Fig. 1. Selected L2 3M2 3 M2 3 transitions of argon to 1 measure the angular distribution of L3M2,3M2.3 (SO) Auger electrons.

Volume 37A, number 1

PHYSICS a=-

""~ 0,9

x

E o _-2 E ( L 3 )

,~

Ep = 4 E ( L 3]

o

Ep =16E(L3 ) I

0°

I

30 °

0.02-* 0 0 3

"'--+t

-" "~ ~

I

60 o

I

90 °

I

120 °

&,

150 o

I

180 o

Fig. 2. Intensity ratios lre[(~) : I(e, L3M2 3M2 3(1S0))/ / I ( O , L2M 2 3M2 3(3P0 1 2)) for three different primary energies Ep. Th~ valu~S'Irel(0) are normalized to I r e l ( e = 0) = 1. F r o m fig. 2 it can r e a d i l y be s e e n that for E_ = = 2 E(L3) and 4 E ( L 3) we m e a s u r e d n o n i s o t r o ~ i c d i s t r i b u t i o n s with a < 0. F o r E p = 1 6 E ( L 3) the d i s t r i b u t i o n is i s o t r o p i c w i t h i n e x p e r i m e n t a l error. Only r e c e n t l y the i o n i z a t i o n c r o s s s e c t i o n s Q(2p, m l = O) = QO and Q ( 2 p , m l = ± 1) = Q1 h a v e b e e n c a l c u l a t e d by M c F a r l a n e [4, 5] f o r a q u a n t i z a t i o n a x i s w h i c h c o i n c i d e s with the i n c i d e n t e l e c t r o n b e a m u s i n g B o r n a p p r o x i m a t i o n . An e a r l i e r c a l c u l a t i o n [1] of Q 0 and Q 1 w a s i n c o r r e c t due to the e r r o n e o u s a s s u m p t i o n that the d i r e c t i o n of i n c i d e n t e l e c t r o n s w a s t a k e n a s q u a n t i z a t i o n a x i s in the d e r i v a t i o n of the d i f f e r e n t i a l c r o s s s e c t i o n s of the 2p s u b s h e l l s by B u r h o p [6]. The p a r a m e t e r a of eq. (1) is g i v e n in t e r m s of Q0 and Q1 by

LETTERs

a = 2(Q1 - Q O ) / ( Q 1 + 2Q0).

25 October 1971

(3)

F o r a the f o l l o w i n g r e s u l t s can be d e d u c e d f r o m t h e v a l u e s Q0 and Q1 c a l c u l a t e d by M c F a r l a n e [5]: a) E p = 2 E ( L 3) and 4 E ( L 3) the p a r a m e t e r a h a s the v a l u e s - 0 . 0 3 and - 0 . 0 7 , r e s p e c t i v e l y . b) F o r E p about 16E(L 3) both c r o s s s e c t i o n s Q0 and Q1 a r e e q u a l , thus g i v i n g a = 0 and an i s o t r o p i c d i s t r i b u t i o n , c) F o r the high e n e r g y l i m i t a is found to be +0.15 and e n e r g y i n d e p e n d e n t . T h e f i r s t two t h e o r e t i c a l r e s u l t s a g r e e q u i t e w e l l w i t h o u r m e a s u r e m e n t s , the l a t t e r r e s u l t w i l l be c h e c k e d in a f o r t h c o m i n g e x p e r i m e n t . Volz and Rudd [7] h a v e a l s o m e a s u r e d a n g u l a r d i s t r i b u t i o n s of the i n t e n s i t y of s e v e r a l L 3 M M t r a n s i t i o n s of a r g o n e x c i t e d by 300 keV p r o t o n s , but t h e y did not find any s i g n i f i c a n t d e v i a t i o n s from isotropy.

References

[1] W.Mehlhorn, Phys. Letters 26A (1968) 166. [2] J. Hrd~, A. Henins and J. A. Bearden. Phys. Rev. A2 (1970) 1708. [3] D. Staiherm, B. Cleff. H. Hiilig and W. Mehlhorn, Z. Naturforsch. 24a (1968) 1728. [4] S. C. McFariane, Abstracts VIIth intern. Conf. on Physics of electronic and atomic collisions. Amsterdam. July 1971, voI. 1. p. 407. [5] S. C. McFariane, J.Phys. B, to be published. [6] E. H. S. Burhop, Proc. Cambridge Phi|. Soc. 36 (1940) 43. [7] D.J. Volz and M.E. Rudd, Phys. Rev. A2 (1970) 1395.