Producing optical excitations by low energy ion atom and ion molecule collision

Dworetsky, S.

Novick, R.

Smith, W. W.

Tolk, N.

English

NASA Technical Reports Server

F a i l u r e  of t h e  Adiabatic C r i t e r i o n  j Structure and Cohelcence 
i n  t h e  Low Energy E x c i t a t i o n  of H e l i u m  Atoms by Helium I o n s  
S. m o r e t s k y ,  R. Novick, W.W. Smith, and N o  Tolk 
Columbia Radiat ion Laboratory, 
Columbia U n i v e r s i t y ,  New York, N. Y .  
I N - 2 7 4 4  
(ACCESSION NUMBER) 
Submitted to  
F i f t h  I n t e r n a t i o n a l  Conference on t h e  P h y s i c s  
of E l e c t r o n i c  and A t o m i c  C o l l i s i o n s  
Leningrad, USSR, 17-23 July 1967 
Hard copy (HC), 
ff 653 July 65 
https://ntrs.nasa.gov/search.jsp?R=19680017972 2020-03-12T10:43:21+00:00Z
F a i l u r e  of t h e  Adiabatic C r i t e r i o n ;  S t r u c t u r e  and Coherence 
i n  t h e  Low Energy E x c i t a t i o n  of Helium A t o m s  by H e l i u m  fons* 
S. Dworetsky, R. Novick?, W 3 ,  Smith,  and No Tolk 
_1__ C o l u m b i a  Rad ia t ion  Laboratory 
Columbia U n i v e r s i t y ,  New York, N. Y, 
Low-energy ion-atom and ion-molecule c o l l i s i o n s  have 
r e c e n t l y  been demonstrated t o  be e f f e c t i v e  i n  producing opti- 
cal e x c i t a t i o n s .  ( 
known adiabatic c r i t e r i o n  ( 2 )  s i n c e  t h e y  show t h a t  e x c i t a t i o n s  
These  obse rva t ions  c o n t r a d i c t  t h e  w e l l -  
readily occur  i n  t h e  low-energy (adiabatic) r eg ion .  I n  t h i s  
paper w e  p r e s e n t  f u r t h e r  ev idence  for t h i s  conclus ion  i n  t h e  
impor tan t  and simple case of t h e  optical  e x c i t a t i o n  of h e l i -  
um atoms by helium ions. It is clear t h a t  curve  c r o s s i n g s  
of t h e  t y p e  first cons ide red  by Landau and Zener play a n  es- 
s e n t i a l  role i n  these collisions. ( 3 )  
l i s i o n s  wh ich  invo lve  on ly  "S"  states i n  t h e  i n i t i a l  and  
f i n a l  channel,  w e  f i n d  t h a t  t h e  e x c i t a t i o n  f u n c t i o n  depends 
i n  a s t r o n g l y  o s c i l l a t o r y  manner on t h e  bombarding energy of 
I n  t h e  case of col- 
-his work w a s  suppor ted  i n  par t  by t h e  Na t iona l  Aero- 
n a u t i c s  and Space Adminis t ra t ion  under Grant  NsG-360 and i n  
part by t h e  J o i n t  Services E l e c t r o n i c s  Program (U. S. Army, 
U. S. N a v y ,  and U. S ,  A i r  Force) under Con t rac t  DA-28-043 
AMC-O0099(E). 
? A l f r e d  P. Sloan  Research Fellow. 
2 
the incident ion. These observations indicate the importance 
of treating the collision as a coherent process in which the 
relative phase of different components of the wave function 
is maintained. 
It has been customary to discuss ion-atom and atom- 
atom collisions in terms of the adiabatic criterion.(2) Ac- 
cording to this principle, the cross section for a collision 
which changes the internal (electronic) energy of the col- 
liding partners by an amount AE is a maximum when the col- 
lision velocity is given by 
v 2 AE a/$ (. m 
Here 2 is a characteristic length of the order of atomic 
dimensions. 
and the cross section decreases exponentially with decreasing 
bombarding energy, becoming very small as the velocity is low- 
ered below v Many examples in the literature give interpre- 
tations of maxima in excitation cross sections in the 
10-100 keV range by using the adiabatic criterion.(4) 
have studied some of these published cases at lower energy 
and find that the observed high-energy maximum is only one 
of many such maxima, and that the interpretation in terms 
of the adiabatic criterion is incorrect as usually applied, 
Accordingly the collision becomes adiabatic, 
m' 
We 
3 
The energy  defect AE is o f t e n  e v a l u a t e d  from t h e  known energy 
levels of t h e  isolated particles, and no al lowance is made 
for t h e  change i n  t h e  e l e c t r o n i c  s t r u c t u r e  du r ing  t h e  col- 
l i s i o n .  I n  t h e  case of t h e  helium l i n e s  of i n t e r e s t  here ,  
t h i s  energy d e f e c t  is  about  22 eV,  and t h e  corresponding 
l a b o r a t o r y  energy  for maximum cross s e c t i o n  is  about  100 keV 
accord ing  t o  t h e  adiabatic c r i t e r i o n .  
Figure 1 shows, as a n  example, t h e  exci ta t ion func- 
t i o n  for one of the H e - I  m u l t i p l e t s  o r i g i n a t i n g  from t h e  
3 P levels,  when helium gas is  bombarded w i t h  helium ions .  
The l a b o r a t o r y  energy  i n  these s t u d i e s  ranged  f r o m  a20 eV 
t o  =5 keV,  t h e  high energy  l i m i t  w a s  chosen t o  o v e r l a p  w i t h  
t h e  l o w e r  l i m i t  employed i n  t h e  work of de H e e r  and co- 
workers., ‘5) 
does n o t  decrease monotonica l ly  a t  e n e r g i e s  below several 
k i l o v o l t s ,  and, i n  fact, i n  many cases t h e  cross s e c t i o n  
a t t a i n s  a large f r a c t i o n  of i ts  maximum v a l u e  w i t h i n  a f e w  
e l e c t r o n  v o l t s  of t h r e s h o l d .  These  o b s e r v a t i o n s  are grossly 
i n c o n s i s t e n t  w i t h  t h e  p r e d i c a t i o n s  based on t h e  adiabatic 
3 
It is  immediately e v i d e n t  t h a t  t h e  cross s e c t i o n  
c r i t e r i o n .  
levels of t h e  He2 molecule formed dur ing  t h e  c o l l i s i o n  are 
T h i s  d i sc repancy  c e r t a i n l y  arises because t h e  
+ 
g r e a t l y  modi f ied  from those of t h e  f r e e  particles. I n  
4 
p a r t i c u l a r ,  pseudo-crossings are known t o  occur between t h e  
i n i t i a l  state ( l a  ) (lo ) 2C and t h e  f i n a l  states l ead ing  
I n  t h i s  case t o  a l l  of t h e  excited states of t h e  a t o m .  
low-energy e x c i t a t i o n s  are t o  be expected s i n c e  t h e  energy 
defect is g r e a t l y  reduced a t  t h e  pseudo-crossings.  
I t  i s  noteworthy t h a t  t h e  apparent  thresholds f o r  t h e  
2 
U 53 4 
(6) 
v a r i o u s  levels  occur  a t  somewhat h igher  e n e r g i e s  t h a n  t h a t  
r e q u i r e d  by simple energy conse rva t ion .  T h i s  i n d i c a t e s  
t h a t  t h e  r e l e v a n t  pseudo-crossing between t h e  molecular  
levels occur s  above t h e  energy corresponding t o  t h e  f i n a l  
s ta te  of t h e  isolated atoms. I n  p a r t i c u l a r ,  t h e  c r o s s i n g s  
for t h e  n = 3 and n = 4 levels both appear t o  be F; 7 e V  
h igher  t h a n  t h e  l e v e l s  of t h e  f r e e  atom, i n  q u a l i t a t i v e  
agreement w i t h  Michels ' c a l c u l a t i o n s .  (7) 
Several of t h e  e x c i t a t i o n  f u n c t i o n s  show an extra- 
o r d i n a r i l y  r i c h  and complex o s c i l l a t o r y  behavior i n  t h e  ener -  
gy range  s t u d i e d .  The s t r u c t u r e  i n  t h e  e x c i t a t i o n  f u n c t i o n  
depends markedly on t h e  p a r t i c u l a r  s tate i n  H e  which i s  
excited, becoming less pronounced as w e  go from S t o  P t o  D 
states of t h e  s a m e  p r i n c i p a l  quantum number .) The 4 S state  
(see Fig.  2)  s h o w s  a bigger oscil latory effect than  t h e  3s 
states. Most of t h e  s t r u c t u r e  cannot  be exp la ined  by 
3 
5 
cascad ing  from ne ighbor ing  states. I t  is impor tan t  t o  n o t e  
t h a t  i n  t h e  case of t h e  3888-! l i n e  (3  P-2 S) our  r e s u l t s  
connec t  smoothly un to  those of de H e e r  a t  5 k e V ;  w e  have 
used t h i s  p o i n t  t o  normalize t h e  a b s o l u t e  cross s e c t i o n .  
It is  sugges ted  t h a t  t h e  o s c i l l a t o r y  behavior r e s u l t s  from 
an  i n t e r f e r e n c e  e f f e c t  between t h e  incoming and outgoing 
passage through t h e  c r o s s i n g  p o i n t .  The "S" states can  
on ly  be excited through a c r o s s i n g  of t w o  C molecular  
states, t h e  P states through C and JJ states, and t h e  D states 
through C, Il, and b. The greater m u l t i p l i c i t y  of channels  
f o r  t h e  P and D states may account  for t h e  apparent  absence 
of s t r u c t u r e .  
3 3  
6 
References 
1. M. L i p e l e s ,  R .  Novick and  N. Tolk ,  Phys. R e v .  
L e t t e r s  15, 815 (1965) ;  N.G. Utterback and HOP. Broida, Phys. 
R e v ,  L e t t e r s  608 ( 1 9 6 5 ) 3  S.H. N e f f  a n d  N+P. C a r l e t o n ,  
As t rophys .  J. 140, 348 ( 1 9 6 6 ) ;  D.C* L o r e n t s ,  W. Aber th ,  and 
V.W. Hesterman, Phys.  R e v ,  Letters 17, 849 (1966) .  
2 .  H.S.W. Massey a n d  E.H.S. Burhop, E l e c t r o n i c  
and I o n i c  Impact Phenomena (Oxford, Clarendon P r e s s ,  1952) 
pp. 513-514, 
3. L. Landau, Phys ik .  2. Sowje t .  2, 46 (1932) ;  
C. Zener, P roc .  Roy. SOC. (London), A137, 696 ( 1 9 3 2 ) ;  for 
more r e c e n t  work see, f o r  example, V. K. Bykhovski i  a n d  E.E. 
N i k i t i n ,  Zh. Eksperim, i Teor .  F i z ,  48, 1499 (1965) [ E n g l i s h  
t r a n s l .  : Soviet Phys.  - JETP 21, 1003 (1965) 1 ,  and D - R -  B a t e s ,  
P roc ,  Royo SOC. (London) A257, 22 (1960). 
4. The adiabatic c r i t e r i o n  is  stated very 
c a u t i o u s l y  i n ,  for example, Massey and Burhop, reference 2 ,  
It h a s  o f t e n  b e e n  i n c a u t i o u s l y  applied, assuming AE t o  be 
c o n s t a n t  d u r i n g  a co l l i s ion  i n  order to  o b t a i n  t h e  e n e r g y  
for a maximum i n  t h e  cross s e c t i o n ,  See,  f o r  a r e c e n t  ex- 
ample, F.J. de H e e r  i n  Advances i n  A t o m i c  and Molecular 
P h y s i c s ,  ( A c a d e m i c  P r e s s ,  N.Y., 1966), V o l .  2, pp. 349. 
7 
5. F . J .  de Heer, L. Wolterbeek-Muller, and R. 
Geballe, Physica 2, 1T45 (1965). 
6, W, Lichten, Phys. Rev. 131, 229 (1963). 
7. H.H. Michels, B u l l .  Am. Phys. SOC. 11, 456 
(1966), and private communication. 
8 
Figure  Capt ions  
F i s u r e  1. E x c i t a t i o n  f u n c t i o n  f o r  t h e  vis ible  H e - I  
m u l t i p l e t  a t  3888 A o r i g i n a t i n g  from t h e  3 P state when H e  
is bombarded by H e  i o n s .  The  cross s e c t i o n  i s  normalized 
a t  5 keV t o  t h e  a b s o l u t e  data of de H e e r ,  Wolterbeek-Muller, 
and Geballe ( r e f e r e n c e  5 ) .  T h e i r  va lue  of  t h e  cross s e c t i o n  
rises t o  w 7.7 x 10 cm2 a t  90 keV. 
have been applied. The o r d i n a t e  va lue  below t h e  marked 
threshold shows t h e  background n o i s e  l e v e l .  
3 
f 
-19 N o  cascade c o r r e c t i o n s  
F i q u r e  2,  E x c i t a t i o n  f u n c t i o n  €or t h e  4713-P l i n e  
3 + 
of H e  (which o r g i n a t e s  on t h e  4 S state) w i t h  H e  bombard- 
ment, Beam energy spread R 1 eV.  ./ 
cu 
E u 
Q, 
8 
0 
- 
0 0 0 0 0 
0 0 0 0 
33?/ SlNllQ?? 
- 53 d- 


Failure of the adiabatic criterion, structure and coherence in the low energy excitation of helium atoms by helium ions

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