Ionization and metastable excitation in low-energy collisions of ground state argon atom

Amme, R. C.

Haugsjaa, P. O.

English

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I o n i z a t i o n  and Metastable  Exc i t a t ion  
i n  Low- Energy C o l l i s i o n s  of Ground 
S t a t e  Argon A t o m s  
Paul  0.  Haugsjaaj and Robert 6. Amme 
DEPARTMENT OF PHYSICS 
DENVER RESEARCH INSTITUTE 
JUNIVERSITY OF DENVER 
DENVER, COLORADO 80210 
https://ntrs.nasa.gov/search.jsp?R=19690030688 2020-03-12T04:27:59+00:00Z
I o n i z a t i o n  and 
i n  Low- Energy 
S t a t e  
Me t a  s t a b l e  Exc i t a  t i o n  
C o l l i s i o n s  of Ground 
Argon A t o m s  
* 
4 
P a u l  0. Haugsjaa’ and Robert C.  Amme 
1 ~ ~ ~ - u 6 - 0 0 + - ~ ~ 6  
* 
Supported i n  p a r t  by a grant  from t he  Nat iona l  Aeronautics and 
Space Adminis t ra t ion.  
%ow a t  General  Telephone and E l e c t r o n i c s  Labora tor ies  , Bayside, N.Y. 
1 
ABSTRACT 
A low- energy beam of argon atoms, formed by non- resonant 
charge t r a n s f e r  of Ar’ i n  H has  been used t o  explore  the near- 
threshold behavior f o r  i on iz ing  and e x c i t i n g  c o l l i s i o n s  between 
argon atoms. For excess  center-of-mass ene rg ie s  below 12 e V ,  the  
i o n i z a t i o n  da ta  a re  c o n s i s t e n t  with the empi r i ca l  r e l a t i o n s h i p  
CT- w1.8 x 
i n  e l e c t r o n - v o l t s .  
were used i n  an e f f o r t  t o  observe metastable  atoms formed i n  atom- 
atom c o l l i s i o n s  by Penning ion iza t ion .  I n  t h i s  ca se ,  beam ene rg ie s  
were r e s t r i c t e d  t o  va lues  less than 30 eV t o  avoid d i r e c t  i o n i z a t i o n  
of the ace ty l ene  (Penning) gas by the ground-state  atom beam. A t  
2’ 
( ~ - 1 5 . 8 ) ” ~ ,  where E i s  the  c e n t e r  of mass energy 
Target gas mixtures  of argon and ace ty l ene  
14.5 eV c.m. energy, t he  c r o s s  sec t ion  f o r  me tas t ab le  production 
is shown t o  be (3.6 _+ 2 . 4 ) ~ 1 0 - ~ ’  cm2, but  t h i s  value i s  dependent 
upon assumed values  of the Penning c r o s s  sec t ion  f o r  A r  
c o l l i s i o n s .  
* 
+ C2H2 
2 
INTRODUCTION 
1 I n  1957, Petschek and Byron proposed t h a t  the  i n i t i a l  
i o n i z a t i o n  of shock-heated argon proceeds v i a  a two- s t e p  process .  
The f i r s t  s t e p  i s  considered t o  be the  e x c i t a t i o n  of argon by 
atom-atom c o l l i s i o n s ,  A r  + A r  4 A r  + Ar*. The second s t e p  i s  
presumably the  c o l l i s i o n  between an exc i t ed  atom and another  atom, 
A r  + A r  -, A r  + Ar' + e- ,  which produces the  f i r s t  e l e c t r o n s  i n  
the  bulk i o n i z a t i o n  process .  
* 
An a c t i v a t i o n  energy f o r  the  e x c i t a t i o n  s t e p  has  been 
repor ted  by s e v e r a l  i n v e s t i g a t o r s  t o  bracke t  t he  f i r s t  four  
exc i t ed  s t a t e s  of the  argon atom. 2-4 
from the  3p 4s e l e c t r o n  c ~ n f i g u r a t i o n . ~  
a t  11.55 and 11 .72  eV, a r e  metas tab le  and the  o t h e r s ,  a t  11.62 and 
11.83 eV, a r e  n o t .  
These exc i t ed  s t a t e s  a r i s e  
5 Two of the s t a t e s ,  those 
Since i t  was not  known t o  what e x t e n t  t he  e x c i t a t i o n  of 
metas tab le  l e v e l s  i s  respons ib le  f o r  the  r a t e - c o n t r o l l i n g  s t e p  
t o  an  exc i t ed  s t a t e ,  atomic beam measurements of the  c r o s s  sec t ion  
f o r  metas tab le  formation, Om, have been at tempted.  Also, measure- 
ments have been made of the  t o t a l  c ros s  sec t ion  f o r  i o n i z a t i o n ,  o-, 
dur ing  c o l l i s i o n s  between ground- state argon atoms f o r  ene rg ie s  
very nea r  the  i o n i z a t i o n  threshold .  This l a t t e r  c r o s s  sec t ion  
g ives  information about the  energy dependence of t he  c ros s  s e c t i o n  
f o r  the  second s t e p  i n  the  i n i t i a l  i o n i z a t i o n  process ,  02, s ince  
these  two c r o s s  s e c t i o n s ,  0- and 02, a r e  expected t o  have e s s e n t i a l l y  
the same threshold  behavior6 when compared i n  terms of the  excess  
energy Ee a v a i l a b l e  i n  the center-of-mass system: E =-! E - E*, 
where E* denotes  the  i o n i z a t i o n  p o t e n t i a l  o r  t he  e x c i t a t i o n  energy. 
e 2 l a b  
IONIZATION MEASUREMENTS 
The t o t a l  c r o s s  sec t ion  f o r  nega t ive  charge formation, CL, 
was measured i n  a manner previously descr ibed .  7-9 Measurements of 
0- f o r  argon-argon c o l l i s i o n s  have be'en repor ted  f o r  excess  c.m. 
ene rg ie s  above 10 eV using both argon and n i t rogen  a s  charge- t r a n s f e r  
3 
gases.  'OY1' However, i n  the present  work, hydrogen 
charge- t r a n s f e r  c e l l  by which argon i o n s ,  formed by 
and acce le ra t ed  e l e c t r o s t a t i c a l l y ,  were n e u t r a l i z e d  
beam." U s e  of hydrogen a s  the charge- t r a n s f e r  gas 
was used i n  the 
e l e c t r o n  impact 
t o  form an atomic 
e l imina te s  any 
e x c i t a t i o n  i n  the n e u t r a l  beam during n e u t r a l i z a t i o n  f o r  a l l  beam 
ene rg ie s  below 230 e V .  
I n  the  p re sen t  work, CT, was measured f o r  excess  c.m. ene rg ie s  
between 1.5 e V  and 500 e V ,  by n e u t r a l i z i n g  an ion beam having a f u l l  
width a t  half-maximum i n t e n s i t y  of less  than 1 e V  ( i . e . 7  less than 
0.5 e V  i n  the c.m. system). It was d e s i r a b l e  t o  i s o l a t e  the t a r g e t  
gas from the n e u t r a l i z a t i o n  region t o  e l imina te  any Ar' + A r  charge- 
t r a n s f e r r i n g  c o l l i s i o n s  and the accompanying metastable  formation. 
Thus, a sepa ra t e  t a r g e t  chamber was used which allowed the n e u t r a l  
beam t o  pass i n t o  i t  v i a  a different ia l ly-pumped a p e r t u r e .  The 
pressure i n  t h i s  chamber, about  2x10 Torr ,  was monitored continu- 
ously with an MKS Instruments Baratron capaci tance manometer. 
1 3  
- 4  
I n  the t a r g e t  chamber the beam passed through a p a r a l l e l - p l a t e  
8 i o n i z a t i o n  d e t e c t o r .  Any negat ive charge formed along a guarded 
c o l l e c t o r  length of 10 cm was measured on a Cary model 401 v i b r a t i n g  
reed electrometer .  A 0.15 m i l  tungsten wire g r i d  was used t o  suppress 
secondary e l e c t r o n s  from the back p l a t e  of the d e t e c t i o n  assembly. 
The p o t e n t i a l  of the g r i d  with r e s p e c t  t o  the c o l l e c t o r  was maintained 
a t  -300 v o l t s ,  while with r e s p e c t  t o  the back p l a t e  i t  was ke.pt a t  
-90 v o l t s .  Measured c o r r e c t i o n s  t o  the da t a  were made f o r  beam l o s s  
a t  the e x i t  of  the cha rge - t r ans fe r  c e l l  and a t  the entrance t o  the 
t a r g e t  chamber, a s  w e l l  as  f o r  incomplete c o l l e c t i o n  of slow ions  i n  
the charge- t r a n s f e r  c e l l  and incomplete c o l l e c t i o n  of negat ive charge 
i n  the t a r g e t  chamber. 
The r e s u l t s  of the 0- measurements are shown i n  Fig. I. N o  
c o r r e c t i o n s  have been app l i ed  t o  account f o r  beam energy spread. 
The v e r t i c a l  e r r o r  b a r s  a t  low ene rg ie s  are i n d i c a t i v e  of t he  un- 
c e r t a i n t y  pre  sen t  i n  the  measurement r e s u l t i n g  from u n c e r t a i n t i e s  
i n  the background and the  measured co'rrection t o  the da t a  f o r  
secondary e l e c t r o n s  due t o  the gr id .8  I n  a d d i t i o n  t o  the  e r r o r  
ba r s  shown, a systematic  unce r t a in ty  of 230% i s  assigned t o  the 
4 
abso lu te  c r o s s  s e c t i o n  measurements. The s t r a i g h t  l i n e  drawn through 
the da t a  below 12 e V  excess  c.m. energy suggests  a threshold power 
l a w :  
(cm2> 
- 21 1 . 3  - 15.8) @c .m 0, w 1 .8  x 10 
10,11 The s t r u c t u r e  p re sen t  between 50 and 150 e V  found i n  ea r l i e r  work 
i s  seen t o  p e r s i s t  f o r  a H n e u t r a l i z e d  beam. Therefore,  t h i s  s t ruc-  
t u r e  i n  the t o t a l  i o n i z a t i o n  c r o s s  sec t ion  must be a r e s u l t  of i n t e r -  
a c t i o n  between two ground s ta te  argon atoms. This w a s  f i r s t  concluded 
i n  earlier work with a n i t rogen  charge- t r a n s f e r  gas. 
v a r i a t i o n  i n  the c r o s s  s e c t i o n  f o r  ex t r eme-u l t r av io l e t  photon production 
has  a l so  been observed f o r  excess c.m. ene rg ie s  above 15 eV. 
2- 
A corresponding 
14 
METASTABLE EXCITATION MEA SUREHENT S 
A s  i n  t he  i o n i z a t i o n  measurement, a f a s t  beam of n e u t r a l ,  ground- 
s t a t e  argon atoms w a s  obtained by charge t r a n s f e r  i n  hydrogen. I n  the 
t a r g e t  chamber was placed a mixture of argon and ace ty l ene .  Any argon 
me tas t ab le s  formed have s u f f i c i e n t  energy of e x c i t a t i o n  t o  Penning- 
ion ize  ace ty l ene .  (Acetylene h a s  an i o n i z a t i o n  p o t e n t i a l  of 11.4 eV.) 
I n  t h i s  way, me tas t ab le  formation from A r - A r  c o l l i s i o n s  was de t ec t ed  
by measuring the nega t ive  charge produced i n  the mixture between the 
i o n i z a t i o n  p l a t e s .  
p re sen t  leading t o  s i g n i f i c a n t  nega t ive  charge production, r e s t r i c t i o n s  
were placed on the maximum energy a t  which the  process  was s t u d i e d .  
The lower energy l i m i t  i s  threshold f o r  the e x c i t a t i o n  process: 
Since i t  was necessary t h a t  no o t h e r  process  be 
* 
A r  + A r  4 A r  + A r ,  (2) 
a t  23.1 e V  l abora to ry  energy. 
i s  t h e  threshold f o r  the d i r e c t  i o n i z a t i o n  of ace ty l ene :  
The upper l i m i t  of t h e  energy "window" 
A r  + C2H2 4 A r  + C2H: + e- ( 3) 
a t  29.0 e V .  
The Penning i o n i z a t i o n  c u r r e n t  f o r  any p a r t i c u l a r  t a r g e t  mixture 
i s  i n  a c e r t a i n  proport ion t o  the c r o s s  s e c t i o n  f o r  e x c i t a t i o n  of  
5 
metastable  argon, i f  t h i s  c u r r e n t  has  been normalized t o  u n i t  argon 
pressure and u n i t  i n c i d e n t  beam i n t e n s i t y .  I n  Fig. 2 the normalized 
c u r r e n t  i s  p l o t t e d  a s  a func t ion  of  i nc reas ing  t a r g e t  pressure.  I n  
t h i s  f i g u r e ,  a one-to-one correspondence i s  assumed between each 
e x c i t a t i o n  event  and a Penning i o n i z a t i o n  even t ,  i . e . ,  t he  Penning 
d e t e c t i o n  e f f i c i e n c y  i s  taken t o  be u n i t y .  The o rd ina te  i s  there- 
f o r e  termed the apparent  e x c i t a t i o n  c r o s s  s e c t i o n .  The argon 
pressure i n  the t a r g e t  was kep t  a t  about 2 x 10 Torr throughout 
the measurements a t  24.4 e V  and 27.4 e V  l abora to ry  energy. 
the ace ty l ene  concen t r a t ion  was increased u n t i l  t h e  t o t a l  pressure 
i n  the t a r g e t  was 1 x 10 Torr.  Severe beam a t t e n u a t i o n  sets i n  
-4 
However, 
- 3  
- 2  above a t o t a l  pressure of 4 x 10 Torr.  The t a r g e t  mixture f o r  
optimum d e t e c t i o n  e f f i c i e n c y  was about  4 x 10 Torr of ace ty l ene  
with 2 x 10 Torr of argon. With the use of t h i s  mixture ,  the 
da t a  shown i n  Fig.  3 were obtained.  Here, the apparent  c r o s s  sec t ion  
i s  p l o t t e d  a s  a funct ion of center-of-mass energy between c o l l i d i n g  
argon atoms. I n  both Fig.  2 and Fig.  3, the e r r o r  ba r s  were deter-  
mined by cons ide ra t ion  of u n c e r t a i n t i e s  i n  the  i o n i z a t i o n  c u r r e n t  
and the background c u r r e n t .  Occasional ly ,  because of random f luctua-  
t i o n s ,  the background c u r r e n t  w a s  l a r g e r  than the i o n i z a t i o n  c u r r e n t ,  
g iv ing  a nega t ive  apparent  c r o s s  sec t ion .  These nega t ive  values  have 
s t a t i s t i c a l  meaning only.  I n  Fig.  3 ,  measurements were extended be- 
yond the upper l i m i t  of the energy window and c o r r e c t i o n s  f o r  A r  - 
d i r e c t  i o n i z a t i o n  had t o  be made, r e s u l t i n g  i n  g r e a t e r  u n c e r t a i n t y  
f o r  t hese  po in t s .  
-4 
-4 
C2H2 
By making the assumption t h a t  exc i t ed  atoms have small ang le s  
of d e f l e c t i o n  from the  beam a x i s  (a v a l i d  assumption nea r  t h re sho ld ) ,  
a simple c a l c u l a t i o n  can be made of t he  number of exc i t ed  atoms de- 
t e c t e d  (See Appendix). Torr ,  
a co l l ec t con  length of 10 cm, and a Penning i o n i z a t i o n  c r o s s  s e c t i o n  
of 542 (a value obtained from ex t r apo la t ed  measurements of deactiva- 
t i o n  c r o s s  s e c t i o n s  f o r  A r  + C2H2),15 it was found t h a t  about 50% 
of those me tas t ab le s  formed are detec-ted. 
s ec t ion  p l o t t e d  i n  Fig. 3 may be m u l t i p l i e d  by a f a c t o r  of 2 t o  
-4 
For a n  ace ty l ene  pressure of 4 x 10 
2 
* 
Thus, t he  apparent  c r o s s  
6 
ob ta in  an  estimate of the  a c t u a l  c r o s s  sec t ion  f o r  metas tab le  for- 
mation i n  A r - A r  c o l l i s i o n s ,  Om. A t  14.5 eV center-of-mass energy, 
We emphasize t h a t  t h i s  value depends 
Om 
d i r e c t l y  on the  ca l cu la t ed  d e t e c t i o n  e f f i c i e n c y  and hence upon the  
value used f o r  the  Penning ion iza t ion  c r o s s  s e c t i o n .  
= ( 3 . 6  f 2.4) x cm2. 
4 From shock tube measurements , McLaren and Hobson est imated 
a s lope  f o r  the  l i n e a r  th reshold  e x c i t a t i o n  law which they assumed. 
This s lope ind ica ted  an e x c i t a t i o n  c r o s s  sec t ion  f o r  argon co l1 , i s ions  
a t  14.5 eV c.m. energy of (7.8 f 0.8) x cm , E a r l i e r  work by 
o t h e r   investigator^^'^ had implied e x c i t a t i o n  c r o s s  sec t ions  a s  l a r g e  
a s  2 x 10 cm . It i s  probable,  however, t h a t  e x c i t a t i o n  t o  meta- 
s t a b l e  s t a t e s  i s  n o t  the  only  mechanism i n  the  f i r s t  s t e p  of the  
bulk i o n i z a t i o n  process .  Exc i t a t ion  t o  l e v e l s  which decay by 
resonance r a d i a t i o n  and subsequent r a d i a t i o n  t rapping  i s  s t i l l  a 
l i k e l y  source of the  exc i ted-  s t a t e  populat ion.  Beam measurements 
of e x c i t a t i o n  t o  these  r a d i a t i n g  l e v e l s  a r e  descr ibed  i n  a separa te  
paper. 
2 
-18 2 
14 
ACKNOWLEDGMENTS 
One of u s  (P.O.H.) g r a t e f u l l y  acknowledges a graduate fe l lowship  
from the  Boet tcher  Foundation. W e  a r e  indebted t o  C .  F. Hansen and 
N .  G. Utterback f o r  h e l p f u l  d i scuss ions  and comments. 
7 
Re f erenc e s 
1. H. E .  Petschek and S. Byron, Ann. Phys. (N.Y.) 1, 270 (1957) .  - 
2 .  
3. A. J. Ke l ly ,  J. Chem. Phys. 4 5 ,  1723 (1966) .  
K. E. H a r w e l l  and R. G.  Jahn,  Phys. F l u i d s J ,  214 (1964) .  
4 .  T. I. McLaren and R. M. Hobson, Phys. F lu ids  11, 2162 (1968) .  - 
5. C. Moore, A t o m i c  Energy Levels ,  V o l  I, National  Bureau of 
Standards C i r c u l a r  4 6 7 ,  U. S. Government P r i n t i n g  O f f i c e ,  
Washington, D.C. (1949) .  
6 .  C. I?. Hansen, S ix th  I n t e r n a t i o n a l  Symposium on Rarefied Gas 
D y n a m i c s ,  J u l y  23-26, 1968 ,  Massachusetts I n s t i t u t e  of 
Techno logy, Cambridge , Massachuse t t s 
7 .  N .  G. Utterback and G.  H. Miller, Rev. Sci.  I n s t .  3, 1101 (1961) .  
8. N. G. Utterback and 6. H. Miller,  Phys. Rev. - 124 ,  1477 (1961) .  
9 .  R. C. Amme and P. 0. Haugsjaa, Phys. Rev. - 177 ,  230 (1969) .  
10. H. C. Hayden and R. C. h e ,  Phys. Rev. - 141, 30 ( 1 9 6 6 ) .  
1 2 .  R. C. Amme and H. C. Hayden, J. Chem. Phys. 44, 2828 (1966) .  - 
12 .  R. C. Amme and J. F. McIlwain, J. Chem. Phys. 4 5 ,  1224 (1966) .  
1 3 .  P. 0 ,  Haugsjaa, R. C. Amme and N .  G. Utterback, Phys. Rev. 
Letters 2 2 ,  322 (1969) .  
14. P. 0 ,  Haugsjaa and R. C. Amme, Phys. Rev. L e t t e r s  2 3 ,  Sept.22, 1969.  
15. M ,  H o l l s t e i n ,  D. C. Lorents ,  J. R. Peterson,  and R. A. Young, 
Research on Metastable  Species i n  Atomic and Molecular Beams -
Produced by Charge Trans fe r ,  Stanford Research I n s t i t u t e  
Semi-Annual Report, 29 Dec. ( 1 9 6 7 ) ,  Menlo Park,  C a l i f o r n i a .  
8 
APPENDIX 
For impact ene rg ie s  near  th reshold ,  it may be assumed t h a t ,  
a f t e r  e x c i t a t i o n ,  the argon metas tab le  w i l l  move forward i n  the 
l abora to ry  system. Thus, one may consider  t he  f l u x  of metas tab le  
atoms, M ,  as  i n t e r a c t i n g  wi th  the  ace ty l ene  t a r g e t  of d e n s i t y  nH 
over the  same i n t e r v a l ,  d a  , a s  the  argon beam i n t e r a c t s  with the  
argon t a r g e t  of dens i ty  n We may write A' 
dM = Q n B a & -  op n H M d t  , m A  
where Om and CT 
formation and d e s t r u c t i o n  (assumed t o  be the  Penning ion iza t ion  c r o s s  
s e c t i o n ) .  
so lu t ion  t o  t h i s  f i r s t  o rder  l i n e a r  d i f f e r e n t i a l  equat ion i s  
are r e s p e c t i v e l y  the  c r o s s  s e c t i o n s  f o r  metas tab le  P 
I f  B i s  n e a r l y  cons tan t  over  t he  d e t e c t i o n  l eng th  ,f , the  
M = (om nA B/up nH)[l - e x p ( - g  nH$ 1 . 
For ene rg ie s  wi th in  the  "window", the only poss ib l e  source of ioniza-  
t i o n  c u r r e n t  I i s  the Penning process .  Thus, d I  = oP nH M d j  , o r ,  
d I  = om nA B E 1  - exp(-op n H R  ) ] d l  . 
A' I n t e g r a t i o n  over t he  c o l l e c t i o n  length  and s u b s t i t u t i o n  f o r  % and n 
the  number d e n s i t i e s  corresponding t o  the  t a r g e t  gas pa r t i a l  pressures  
u t i l i z e d ,  g ives  I as a func t ion  of the  Penning c r o s s  sec t ion .  Extrapo- 
l a t i o n  of the  deac t iva t ion  c r o s s  sec t ions  f o r  Ar*  + C H 
Ref. 15 t o  6 eV g ives  a value Q M 54g2. 
o I I / [ ( l O  cm) B n 1, one then f i n d s  0~ 0.50 am, o r  
presented i n  2 2  
For the measured c ross  sec t ion ,  P 
A 
0 m22a . m 
9 
Figure Captions 
Fig. 1. Tota l  i o n i z a t i o n  c r o s s  sec t ion  f o r  argon-argon c o l l i s i o n s  
with H2 employed a s  beam-neutralizing gas .  
Fig. 2. Apparent c r o s s  sec t ion  f o r  argon metastable-atom production 
from A r  4- A r  c o l l i s i o n s  as a func t ion  of t o t a l  t a rge t -gas  
pressure .  The argon pressure  was maintained a t  0 . 2  mTorr 
and the  ace ty l ene  gas  pressure  was va r i ed .  Resul t s  are 
shown f o r  two d i f f e r e n t  l abora to ry  beam energ ies .  
Fig.  3 .  Apparent c r o s s  sec t ion  f o r  argon metas tab le  production vs. 
center-of-mass energy. E r ro r  ba r s  include u n c e r t a i n t i e s  
i n  background c u r r e n t  and i n  e lec t rometer  measurements. 
The a c t u a l  c r o s s  sec t ion  i s  es t imated  t o  be 20, but  depends 
d i r e c t l y  upon the  Penning c r o s s  sec t ion  f o r  Ar*  + C H 2 2 '  
I IO IO0 IO00 
( E  - 15.8 I EV CENTER-OF-MASS SYSTEM 
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Ionization and metastable excitation in low- energy collisions of ground state argon atoms

Abstract

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