. * c c % ' * c COLLISIONAL-DISSOCIATIVE RECOMBINATION OF ELECTRONS WITH MOLECULAR I O N S C. B. Collins Southwest Center for Advanced Studies Dallas, Texas Submitted to The Physical Review i March, 1965 '. GPO PRICE $ This was by CFSTl PRICE(S) $ NASA Grant NsG-@69 Hard copy (HC) Microfiche ( M F) ecy ff 653 July 65 https://ntrs.nasa.gov/search.jsp?R=19660012922 2020-07-13T12:22:44+00:00Z
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. * c
c % ' * c
COLLISIONAL-DISSOCIATIVE RECOMBINATION OF
ELECTRONS WITH MOLECULAR I O N S
C . B. Collins S o u t h w e s t C e n t e r f o r A d v a n c e d S t u d i e s
Since, in addition the excited states have approximately 9
- 20 -
approximated by t h e idea l i zed molecule of t h i s s tudy 1 0 . P.lthough no r epu l s ive
states are known t o e x i s t f o r molecular helium, o the r than t h e ground state
which would not e f f e c t i v e l y increase t h e e l ec t ron removal rate, conversely
n o t a l l poss ib le states have been spec t roscopica l ly located.
Recent spectroscopic measurements" of t h e 10,8308 helium l i n e emit ted
by t h e 2 P + 2 S t r a n s i t i o n i n a t a n i c helium i n a recombining helium plasma
have revealed t h a t a majority of t h e 2 P atoms observed a t t h e h igher pressures
+ Absolute i n t e n s i t y measure- ( ~ 3 0 mm Hg.) r e s u l t from a recombination of H e
ments of t h e 10,8301 emission w e r e used t o e s t a b l i s h thil t t h e recombination
c o e f f i c i e n t c h a r a c t e r i s t i c of t h e process by which t h e rnolccular ions were
recombining and r e s u l t i n g i n t h e subsequent Droduction of n e u t r a i atoms w a s
3 3
3
2 .
3 5 2x10'~'- cm /sec aD
a t 1800OK and 2x1Ol2 electrons/cm 3 . Whereas t h i s value is q u i t e small t o be in t e rp re t ed by d i s s o c i a t i v e
recombination, comparison with Figure 3d shows t h a t co l l i s iona l -d i s soc ia t ive
recombination through a l e v e l near q = 4 could be expected t o given an e l ec t ron
removal rate of
-11 3 aD = 10 cm /sec a t 2000OK.
Consequently a d i s s o c i a t i v e s ta te having an energy a t t h e common equi l ibr ium
in t e rnuc lea r separat ion of t h e bound molecular s ta tes comparable t o or below
t h a t of t h e four th hydrogenic l eve l would y i e l d a co l l i s iona l -d i s soc ia t ive re-
combination rate i n agreement w i t h t h a t experimental ly determined. Since t h e
states with one of t h e poss ib le A-values a r i s i n g from t h e combination He(1 1 S ) + 3 H(2 PI, t h e 4pC, has not been experimentally located, such a r epu l s ive state w i t h
- 21 -
the desired energy could exist.
recombination rates, collisional-dissociative recombination offers a promising
alternative to dissociative recombination in the explanation of the dissociation
observed in decaying helium plasmas.
Consequently at least on the basis of
- 22 -
ACKNOWLEDGMENT
This research has been supported by the National Aeronautics and
Space Administration under grant No. NsG-269-62.
- 23 -
REFERENCES
1. D. R. Bates, Phys. Rev. 77, 718 (1950); 78 492 (1950); 82 103
(1951).
-' -9
2. These l a r g e rates are found only i n plasmas known t o form an abundance
of molecular ions; see L. B. Loeb: Handbuch d e r Physics, ,edited by S. Flugge
{S?*.ipSer-Verlag, 1956), p e 490.
3. D. R. Bates, A. E. Kingston, and R. W. P. McWhirter, Proc. Roy. SOC.
A267 297 (1962).
4.
(1957 .
-9
L. C. Green, P. P. Rush, C. D. Chandler, Astrophys. J. Suppl. 2, 37
5. D. R. Bates and A. Dalgarno, i n A t o m i c and Molecular Processes e d i t e d by
D. R. Bates (Academic Press Inc., New York, 19621, p. 249.
6. As discussed by Kingston: A. E. Kingston, Phys. Pev. 35, A1529 (1964)
t h e r e is a degree of a r b i t r a r i n e s s i n t h e choice of simplifying assumptions
used t o obta in numeric values. Those used here can be expressed using
Gryzinski 's o r i g i n a l terminology as follows:
where I'j and f (E2) is t h e Maxwell-BoltPnann d i s t r i b u t i o n func t ion for the
energ ies , E2, of t h e free e lec t rons , u
l e v e l s - i and j , u2 is t h e energy d i f f e rence between l e v e l s j and j i l , and v
is t h e v e l o c i t y of t h e electron having energy E2. The d i s t r i b u t i o n o f bound
ene rg ie s was assumed t o be a d e l t a func t ion g iv ing t h e exc i t ed bound e l e c t r o n
an energy exac t ly E
See: M. Gryzinski, Phys. Rev. 115 374 (1949). Frank-Condon f a c t o r s are
assumed t o be unity.
is t h e energy d i f f e rence between 1
At
being equal t o t h e ion iza t ion p o t e n t i a l of t h e j"' state. 1
-'
- 24 -
7.
( s ee Ref. 3 ) .
This i s equivalent t o t h e c o l l i s i o n a l r a d i a t i v e recombination rate of Bates e t a l e
The values obtained here agree only t o wi th inabout a f a c t o r of 2
f o r a l l temperatures and d e n s i t i e s examined owing t o t h e p a r t i c u l a r choice
of approximations used i n evaluating t h e Q.. (see Fef . 6 ) . Those employed
here w e r e chosed t o give the smallest Q. . i n an effor t t o avoid overestimat-
ing t h e effects of t h e inc lus ion of a d i s soc ia t ing l eve l .
8. E. E. Ferguson, F. C. FehTenfeld, and A. L. S c h e l t e k o p f , Bull . Am.
Phys. SOC. l0, 187 (1965).
9. G. Herzburg, Elolecular Spectra and Molecular S t ruc ture (D. Van Nostrand
Co., Inc., Princeton, New Jersey, 1950), p. 535-536.
10.
4n2 r a t h e r than t h e hydrogenic 2n .
1 3
17
The e l ec t ron ic degeneracy of the nth state of molecular helium is
2 This of course w i l l have no e f f e c t on
t h e inelast ic and s u p e r e l a s t i c c o l l i s i o n rates between bound l e v e l s and
s ince t h e i o n i c degeneracy is 2 r a t h e r than the hydrogenic un i ty t h e rates
between bound and free s t a t e s a r e s imi l a r ly unaffected.
arises with t h e d i s soc ia t ive l e v e l i n t h a t t h e discussion of t h e s i n g l e d is -
The only variance
s o c i a t i v e state must be considered t o apply l i t e r a l l y t o a doubly degenerate
s t a t e instead. With t h i s understanding t h e t h e o r e t i c a l r e s u l t s could be
expected t o apply t o molecular helium.
11.
See: C. B. Col l ins and W. W. Robertson, Bu l l . Am. Phys. SOC. 10 189 (1965).
This w a s reported a t t h e 17th Caseous Elec t ronics Conference (1965).
-'
e
- 25 -
CAPTIONS
FIGURE 1 L e f t :
showing t h e hydrogenic spacing i n energy of t h e v ib ra t ion le s s ground state
The hypothet ical po ten t i a l curves used i n these ca l cu la t ions
of each e l e c t r o n i c s t a t e .
Right: The equivalent energy l e v e l diagram. I n t h i s case the
tL:-a lllbu .=qul.alent --..... hydzgen ic l s v e l hts been assu.ed tn he dissociative,
FIGURE 2 The co l l i s iona l - r ad ia t ive recombination rate i n t h e absence of a
d i s soc ia t ing l e v e l as a funct ion of t he number of bound quantum l e v e l s con-
s idered i n t h e ca l cu la t ions for an e lec t ron temperature of 250°K and a dens i ty
of l o l o electrons/cm 3
FIGURE 3
.-collisional-dissociative rate, aD, for var ious values of q, t h e p r i n c i p a l
quantum number of t h e d i s soc ia t ing l eve l ; --- co l l i s iona l - r ad ia t ive rate,
aCR, i n t he absence of a d issoc ia t ing l e v e l ,
( a ) 250OK; (b) 500OK; ( c ) 1000OK; ( d ) 2000OK.
Calculated recombination rates as func t ions of electron densi ty:
Electron temperatures are:
FIGURE 4
-total e l ec t ron removal rate, Q T ~ ~ ~ ~ ~ f o r var ious values of q, t h e p r i n c i p a l
quantum number of t h e d i s soc ia t ing l eve l ; --- co l l i s iona l - r ad ia t ive rate, aCR,
i n t h e absence of a d i s soc ia t ing level .
( b ) 500OK; ( c ) 1000OK; ( d ) 2000OK.
Calculated recombination rates as func t ions of e l ec t ron densi ty:
E lec t ron temperatures are: (a ) 250OK;
FIGURE 5
recombination a t e lec t ron temperatures of 2000OK and 1013 electrons/cm3
(a ) --- populations expected i n systems including a d i s soc ia t ing l e v e l a t q = 4;
-populations expected i n t h e absence of d i s soc ia t ing levels; ( b ) --- popula-
t i o n s expected i n systems including a d i s soc ia t ing l e v e l a t q = 6; -populations
expected i n t h e absence of d i s soc ia t ing l eve l s .
Calculated populations of exc i ted s t a t e s produced by electron-ion
- 26 - 1 ,
FIGURE 6 Calculated co l l i s iona l -d i s soc ia t ive recombination rate as a func t ion
of AD, t h e r a t e c o e f f i c i e n t for spontaneous d i s soc ia t ion of t h e qth l e v e l , ' and paramet r ica l ly a s a funct ion of q.
FIGURE 7 Calculated t o t a l e lec t ron removal rate as a funct ion of t h e e f f e c t i v e
quantum number a t which d issoc ia t ion is assumed t o occur for an e l ec t ron
temperature of 2000OK and 1013 electrons/cm . d i s soc ia t ing state with continuously varying e f f e c t i v e quantum number; -.- r e s u l t s for a d i s soc ia t ing l e v e l w i t h hydrogenic degeneracy and i n t e g r a l quantum
numbers.
3 ,-A r e s u l t s f o r a non-degenerate
15
IO
ENERGY ( in. e.v.)
5
0
MOLECULAR POTENTIALS
- LI =
EQUIVALENT ENERGY LEVELS
'7 ION
5 4
3
2
I
INTERNUCLEAR SEPARATION
FIGURE 1
- 28 =-
- 0
h) 0
W 0
P , 0
'0
z 0 D r
2 1
P * z + c iz
z c L tD m I)
0 n
c 0 W m ;II z 0 UJ -I
r m < m r
0
4 6 cb
0
0
0
0
0
0
0
0
0
0
0
0
$ 0 ' w m
h) - O- v, \o 0
NG
ELECTRON DENSITY (cm-3) FIGURE 3a
. DISSOCIATING - 30 =
le = 500°K LEVEL IO+ t
€
loo8
QD
(cm3/s e c)
I I
q = 14
q * 12
q = IO
q = 8
q = 5
q r 4
%R
-- IO'O IO" lo'2 I O ' ~
ELECTRON DENSITY (cG3) FIGURE 3b
i .
- 31 DISSOCIATING
Te = IOOOOK LEVEL
€
10-8
IO-^
a, ( cm3/sec
1 6 ' O
Io-"
Io-'* IOIO IO" IOi2
q =I2
' q=lO
q =8
q = 6 r a C R q =5
q =4
ELECTRON DENSITY (cm3) FIGURE 3c
.
LEVEL
m=lZ m i l 0
IO'*
IO-^
I o"0
10-12
= 32 0
Te = 20000K DIS
I I
SOCl ATlNG
IO" 1Ol2
rn=8
m = 6
rn =5
aCR rn = 4
ELECTRON DENSITY (ern") FIGURE 3d
10-4
10-5
10'6
a TOTAL
cm3/sec
I 0-7
10-8
10-9
i 33 i
Te = 250°K D IS S OC I AT I NG
IO"
.EVEL
16
14
12 -10 - 8 '%R
ELECTRON DENSITY (cG3) FIGURE 4a
,
- 34 - DISSOCIATING
Te = 5 W K LEVEL
14 13 c /
I 0-7
10-8
TOTAL cm3/sec
10-9
I o-'*
12 II IO 9
-6 \ E
%R
IO'" J I I
IO'O IO" IOi2 loi3 ELECTRON DENSITY (cfi3 )
FIGURE 4b ,
-
Te = 35 - 1000° K DISSOCIATING
10-8
TOTAL cm3/sec
10-9
I I
IO'O IO" IO'* loi3 ELECTRON DENSITY ( ~ 6 ~ )
FIGURE 4c
~
LEVEL
14 is 12
II
IO
9
8
7
6
5 aCR
c . - 36 - DISSOCIATING
Te = 2000° LEVEL
Io-*
10-9
TOTAL cm%c
I o"0
12
I I
IO
9
8
7
6
5
4
'QCR
IO" l0l2 ELECTRON DENSITY (criT3)
FIGURE 4d
. e - e
0 c D z + c z r m < m r
* 37 - POPULATION
N2
9
0 Q,
I I
FIGURE Sa
,* . .
5-
0 c D z + c z r m < m r
(0
a 4
Q,
P
w
- 38 - POP U LATlO N
0
0 P
N2
0
0 UI
- 0
m
\ \ \ I I
1 FIGURE 5b
t I I I I I I I I I I i I I I
b' .? 0
- 39 -
T = 2 0 0 0 O K Ne = 10'3/cm3 DISSOCIATING L E V E L