Ionization cross section for production of electrons by collisions between neutral argon atom

Amme, R. C.

Hayden, H. C.

English

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b" L O W  ENERGY IONIZATION OF ARGON A'IOMS BY ARGON ATOMS* ,.j 
Howard C .  Hayden 
and 
Robert C .  Amme 
Department of  Physics 
Unive r s i ty  of Denver 
Denver, Colo.  80210 
*Supported by NASA Gr-an; N s G -  392 
https://ntrs.nasa.gov/search.jsp?R=19660004969 2020-03-16T21:21:40+00:00Z
MW ENERGY IONIZATION OF ARGON ATOMS BY ARGON ATOMS* 
Howard C. Hayden and Robert C .  Arne 
Department of Physics ,  Un ive r s i ty  of Denver 
Denver, Colo. 80210 
r 
S t u d i e s  of i o n i z a t i o n  c ros s  s e c t i o n s  f o r  n e u t r a l  heavy-pa r t i c l e  
c o l l i s i o n s  may provide infonnat ion  on both  e x c i t a t i o n  and i o n i z a t i o n  
processes .  I n  t h i s  l e t t e r  we r e p o r t  t he  t o t a l  c r o s s  s e c t i o n  f o r  
product ion of e l e c t r o n s ,  < _ ,  by c o l l i s i o n s  between n e u t r a l  argon 
atoms. Marked s t r u c t u r e  observed a t  low e n e r g i e s  sugges t s  a ser ies  
of competing p rocesses ,  whlch could be e x c i t a t i o n  of  me tas t ab le  s t a t e s  
i n  e i t h e r  t he  atom o r  i o n ,  o r  possibly both.  An a l t e r n a t i v e  explana- 
t i o n  could be t h a t  of energy resonances i n  the  product ion of  autoion-  
i z i n g  s t a t e s .  
The method employs the use of  a n e u t r a l  argon beam formed by * 
symmetric charge t i a n s f e r .  The measurements were c a r r i e d  o u t  over  the  
l abora to ry  energy range of 30 eV ( j u s t  below the  i o n i z a t i o n  th re sho ld  
f o r  e l e c t r o n  product ion i n  ground- s t a t e  argon atom c o l l i s i o n s )  to 
2900 e V .  
* Supported by NASA Grant NsG 392. 
IN. C.  Uttei-back and C .  H .  M i l l e r ,  Phys. Rev. 1 2 4 .  1477(1961). 
2N. G .  Ut te rback ,  P h y s .  R e v .  L e t t e r s  l.2, 2 9 5 ( 1 9 6 4 ) .  
2 
An electron-impact  ion source was used t o  produce the ions  . 
3 
which were then e l e c t r o s t a t i c a l l y  a c c e l e r a t e d  and focused. The 
i o n i z i n g  e l e c t r o n  energy was 20 eV, although no e f f e c t  on the ion- 
i z a t i o n  c r o s s - s e c t i o n s  was observable up to  45 eV. The ions pro- 
duced were the 2P and *PI, s t a t e s ,  separated by 0 .17  e V .  
a n a l y s i s  of  the A r 3  beam showed a p u r i t y  g r e a t e r  than 99%. 
. . 
Mass 
3/2 2 
Neut ra l  beam i n t e n s i t i e s  of the o rde r  o f  5x10’ sec’l were 
ob ta ined  i n  the charge t r a n s f e r  c e l l .  Because of the magnitude of 
the 10/01 charge t r a n s f e r  c r o s s   section^,^ the n e u t r a l  beam was 
probably composed most ly  of ground-state  argon atoms. S i m i l a r l y ,  
i m p u r i t i e s  were p a r t i a l l y  e l iminated because of t h e i r  sma l l e r  charge 
t r a n s f e r  cross s e c t i o n s  i n  argon. The n e u t r a l  beam i n t e n s i t y  was 
determined a s  p rev ious ly  from the slow ion c u r r e n t  a r i s i n g  i n  the 
charge t r a n s f e r  c e l l ,  c o r r e c t e d  f o r  s c a t t e r e d  ions .  
3 
A p a r a l l e l - p l a t e  i o n i z a t i o n  chamber was used i n  the measure- 
ment of the e l e c t r o n  c u r r e n t  a r i s i n g  from i o n i z a t i o n .  The chamber 
i s  equipped with a g r i d  f o r  suppressing secondary e l e c t r o n s ,  and was 
employed i n  the same manner a s  descr ibed i n  Ref. 1. The e l e c t r o n  
c u r r e n t  was reed from a Cary v ib ra t ing - reed  e l e c t r o m e t e r .  Secondary 
e l e c t r o n s  a r i s i n g  from the g r i d  were shown t o  be  of  minor s i g n i f i c a n c e  
i n  the measurement, except  very n e a r  t h re sho ld .  (See Re€. 5 ) .  The 
i o n i z a t i o n  c u r r e n t  was determined by e x t r a p o l a t i n g  p res su re  sa tu ra -  
t i o n  c u r v e s  t o  the zero- i n t e r c e p t .  Complete vol tage s a t u r a t i o n  was 
achieved with the g r i d  a t  -1500 v o l t s .  
N .  G .  Utterback and G. H .  Miller, Rev. S c i .  Ins!r- .  3 2 ,  l lOl(1961) .  3 
4 R .  C .  Anme and H .  C .  Hayden, J .  Chcm. Pt iys .  - 4 2 ,  2L!11(1365). 
- 
3 
Cross s e c t i o n s  were obtained using the equa t ion :  
k i  
PB 0- = - (cm2) , 
i n  which i s  the e l e c t r o n  (negative) c u r r e n t  to  the c o l l e c t o r ,  P is 
the t a r g e t  gas p re s su re  and B i s  the n e u t r a l  beam i n t e n s i t y .  The 
c o n s t a n t  k depends upon the c o l l e c t o r  l eng th  (10 cm.) and i s  equa l  
numerical ly  to  3.05 x i f  t he  p re s su re  P i s  expressed i n  u n i t s  
of 
s e c t i o n  c j -  i s  the sum of the t a r g e t  p a r t i c l e  i o n i z a t i o n  c r o s s  s e c t i o n  
and the s t r i p p i n g ,  o r  r e i o n i z a t i o n ,  c r o s s  s e c t i o n ,  which f o r  t h i s  
ca se  a r e  both equa l  t o  ~ - 1 2 .  
t o r r ,  wi th  B and i i n  t he  same c u r r e n t  u n i t s .  The c r o s s  
The observed c r o s s  s e c t i o n s  up t o  1000 e V  ( l a b )  a r e  p l o t t e d  
i n  F i g .  1 as  a f u n c t i o n  o f  one-half the beam energy less the  ioniza-  
t i o n  p o t e n t i a l  of 15.8 eV. The absc i s sa  thus corresponds t o  the ex- 
c e s s  energy a v a i l a b l e  i n  the center-of-mass system over  t h a t  r equ i r ed  
f o r  s i n g l e - e l e c  t ron  product ion.  
I o n i z a t i o n  measurements5 t o  1000 eV fo r  H e  on He and He on N 2  
were found t o  e x t r a p o l a t e  smoothly t o  the h ighe r  energy resu l t s  of 
So lov ' ev ,  I l ' i n ,  Oparin and Fedorenko. However, the p r e s e n t  resu l t s  
d i d  n o t  seem t o  a l i g n  wi th  those o f  Afrosimov and these  au tho r s .  To 
i n v e s t i g a t e  the in t e rmed ia t e  energy r e g i o n ,  t he  measurements were 
performed a t  e n e r g i e s  up t o  2900 e V .  These e n e r g i e s  ove r l ap  the 
6 
5H. C .  Hayden and N. G.  Utterback, Phys. Rev. 135, A 1 5 7 5 ( 1 9 6 4 ) .  
6V. V.  Afrosimov, R. N. I l ' i n ,  V .  A .  O , a r i n ,  E .  S .  Solov 'ev and 
N. V .  Fedorenko, Sov ie t  Phys. JET€' l4, 7 4 7 ( 1 9 6 2 ) .  
4 
o l d e r  work of Berry7 and approach those of S l u y t e r s ,  de Haas and 
8 
Kistemaker. A comparison i s  made i n  Fig.  2 with the measurements 
of Rostagni' and with Rosen s c a l c u l a t i o n  from tLme dependent per- 
t u r b a t i o n  theory." A reduct ion of our c r o s s  s e c t i o n s  by about  30% 
t o  50% would be r equ i r ed  t o  f i t  them o n t o  the resu l t s  of S l u y t e r s ,  
e t  a l .  Even so,  i t  appea r s  t h a t  a d d i t i o n a l  broad s t r u c t u r e ,  with 
poss ib ly  a minimum n e a r  8 keV l a b ,  would be r equ i r ed .  This t rend 
i s  suggested i n  t h e i r  d a t a .  The d i s t a n c e  of c l o s e s t  approach for 
8 keV head-on c o l l i s i o n s  i s  on t h e  o r d e r  of 1 a . u . ,  a t  which a number 
of M-shell p o t e n t i a l  c r o s s i n g s  become p o s s i b l e .  
11 
As a check on ou r  n e u t r a l  beam i n t e n s i t y ,  secondary e l e c t r o n  
emission c o e f f i c i e n t s  (y' and yo r e s p e c t i v e l y )  were measured f o r  
both the At-+ and n e u t r a l  A r  beams i n c i d e n t  on a gold s u r f a c e .  
energy was va r i ed  from 250 t o  2500 e V .  Although the s u r f a c e  was 
n o t  mic roscop ica l ly  c l e a n ,  one expects  t h a t  a t  high e n e r g i e s  the 
The 
two s p e c i e s  should e j e c t  e l e c t r o n s  wi th  n e a r l y  the same e f f i c i e n c i e s .  
This  assumption was made, per 2, by S l u y t e r s ,  e t  a l .  i n  o b t a i n i n g  
t h e i r  i o n i z a t i o n  c r o s s  s e c t i o n s .  A s  i n  Ref. 3 ,  i t  was found t h a t  
t h e  y+ measurement showed a marked dependence upon the time a f t e r  
t h e  ion beam was allowed t o  s t r i k e  the su r face .  A t  high e n e r g i e s ,  
'H. W. Be r ry ,  Phys. Rev. 61, 63(1942). For a d i s c u s s i o n  of some e a r l i e r  
measurements see H .  Massey and E .  Burhop, "E lec t ron ic  and I o n i c  Impact 
Phenomena" (Oxford Univ. P r e s s ,  London, 1952) p .  532. 
8Th. J .  M .  S l u y t e r s ,  E .  DeHaas, and J. Kistemaker,  Physica - 25,  1376(1959). 
9A. Rostagni ,  Nuovo Cimento 11, 621(1934). 
lop .  Rosen, Phys. Rev, 109, 351(1958). 
l l U .  Fano and W. L i c h t e n ,  Phys. Rev, L e t t e r s  - 14,  627(1965). 
A f a c t o r  of 2 must  be used 
t o  conve r t  those resu l t s  t o  the t o t a l  c r o s s  s e c t i o n ,  0.. 
These r e s u l t s  have been doubled a l s o .  
5 
I 
.' 
. the measured y+ dropped approximately 15% wi th in  the f i r s t  f i f t e e n  
seconds. This e f f e c t  i s  a t t r i b u t e d  to  the bui ldup of space charge 
nea r  t he  s u r f a c e ,  causing the r e t u r n  of  secondaries .  O u r  secondary 
emission c o e f f i c i e n t s  a r e  i n  good agreement above 350 e V  u s i n g  the  
maximum y+ and a 20X c o r r e c t i o n  t o  the n e u t r a l  beam i n t e n s i t y  (see 
Ref. 3) t o  account f o r  incomplete c o l l e c t i o n  of slow charge- t r ans -  
f e r r e d  ions .  E r r o r s  in  the c r o s s  s e c t i o n s  from a l l  u n c e r t a i n t i e s  
a r e  e s t ima ted  t o  be 5 201. 
A s  a f u r t h e r  t e s t  of procedure, measurements of A r - N 2  ioniza-  
t i o n  c r o s s  s e c t i o n s  (argon beam) have been compared a t  the same cen te r -  
of-mass e n e r g i e s  with N 2 - A r  i o n i z a t i o n  c r o s s  s e c t i o n s  (N2 beam). 
r e s u l t s  a r e  i n  good agreement at excess  CM e n e r g i e s  above 35 eV. A t  
lower e n e r g i e s ,  t h e  Ar-N2  resu l t s  appear  c o n s i s t e n t l y  h i g h e r ,  which 
sugges t s  t h a t  me tas t ab le  argon atoms may a r i s e  du r ing  charge t r a n s f e r .  
(Me tas t ab le  helium atoms were found t o  a r i s e  i n  t he  same manner by 
U t t e rback ,2  with a concen t r a t ion  of less than l%.) Hence the s t r u c t u r e  
i n  t he  A r - A r  i o n i z a t i o n  c r o s s  s e c t i o n  a t  beam e n e r g i e s  less than 100 e V  
may be a t t r i b u t a b l e  i n  p a r t  t o  a n e u t r a l  me tas t ab le  component. The high 
background c u r r e n t  observed i n  the neighborhood of t h re sho ld  tends t o  
support  t h i s  p o s s i b i l i t y .  
The 
The a u t h o r s  a r e  indebted t o  D r .  N. G .  Utterback f o r  h i s  con- 
t i nued  i n t e r e s t ,  and t o  Mr. John McIlwain f o r  l abora to ry  a s s i s t a n c e .  
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Low energy ionization of argon atoms by argon atoms

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