Effects of electron scavengers in the radiolysis of water ...

Effects of electron scavengers in the radiolysis of water vapor1

R. S. DIXONAND M. G. BAILEY

Atomic Energy of Cat~adaLimited, WhiteslzeN Nuclear Research Establishment, Pitza,va, Manitoba

Received July 31, 1967

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 50.116.19.84 on 06/29/16 Water vapor irradiated with X-rays in the presence of nitrous oxide gives mainly nitrogen and oxygen
For personal use only.
+ +together with small amounts of hydrogen and, possibly, hydrogen peroxide. The yield of nitrogen,

G(N,) = 3.0 0.3, is constant over a wide range of conditions and G(02) = 1.6 0.3 after an induc-
tion period. The hydrogen yield is constant at low doses with G(H2) = 0.45 f 0.1, but reaches a steady

+state at higher doses. The nitrogen yield is equated with the yield of scavengeable electrons in water

vapor, the value g(e) = 3.0 0.3 being in reasonable agreement with the theoretical value based on
W(H20) = 30 eV. Addition of other electron scavengers to water vapor containing nitrous oxide sup-
presses the nitrogen yield by competing efficiently for electrons. On the basis of their efficiency in sup-
pressing the nitrogen yield, limiting values for the relative rates of reaction of N 2 0 , SF6, CCI,, and HCI
with electrons in water vapor are obtained and comparisons are made with their known electron attach-
ment cross sections in the gas phase.

Canadian Journal of Cllemistry. 46, 1181 (1968)

Introduction Experimental

Observed radiation decomposition yields of Materials
water vapor in the presence of additives are few Water was successively distilled from acid dichromate,
and there is considerable disagreement amongst
the recent data (1-3). The value2 g(H) = g(0H) from alkaline permanganate, and through a quartz tube
= 11.7 quoted by Firestone (1) from the P-ray in- at 500 OC in a stream of oxygen. Nitrous oxide, sulfur
duced exchange of D 2 with H 2 0 is much higher hexafluoride, and hydrogen chloride (Matheson research
than the values obtained from water vapor con- grade), carbon tetrachloride and hydrogen peroxide
taining organic scaveilgers where g(H) = 7-8 and (Fisher certified), and propylene (Phillips research grade)
g(0H) = 8.5 (2) and from the H20-NH,-0, were used without further purification except for several
system where g(H) = 5.7 and g(0H) = 6.5 (3). freeze-pump-thaw cycles.

Recent investigations (4, 5) have shown that TecR~zique
the hydrogen yield is reduced on addition of The irradiation vessels were 450 cc cylindrical quartz
nitrous oxide or sulfur hexafluoride to H,O or
D 2 0 vapor containing organic additives. This cells equipped with a center thermocouple well and a
reduction is thought to be caused by electron break-seal for analysis. Before filling, the cells were
capture by N,O or SF,, thus preventing the washed with permanganic acid, rinsed with acidified
neutralization reaction hydrogen peroxide, rinsed several times with triply dis-
tilled water, heated in air at 500 "C overnight, cooled, and
The reduction in the hydrogen yield AG(H2) z3 evacuated to Torr at room temperature. A weighed
has therefore been equated with the yield of amount of water (- 0.25 g) was pipetted into a small
electrons in water vapor. flask attached to a grease-free preparation line and, after
degassing with several freeze-pump-thaw cycles, was
The importance of the yield and reactions of distilled into the irradiation vessel immersed in liquid
electrons in the radiation chemistry of water nitrogen. Gases were added by condensing a known
vapor has led us t o study irradiated water vapor volume of gas, at a known temperature and pressure, into
containing several additives, all of which are the vessel. The vessel and contents were finally evacuated
known t o react with electrons from studies in to Torr at -196 OC before sealing off. The vessel
other systems, and some of which are thought to was mounted in a furnace which was kept at 125 & 2 "C
be specific scavengers for electrons. during irradiation. The temperature was controlled by a

'Issued as A.E.C.L. NO. 3049. -Honeywell controller/recorder system and was constant
2Primary yields are written as g(x) and experimental
yields as G(x), n~oleculesper 100 eV. to + 1 OC over the whole vessel. The total pressure during

irradiation was 750 Tom. Duplicate irradiations were
carried out with X-rays from a 1.5 MeV Van de Graaff
accelerator using a water-cooled stainless steel target.
After irradiation, the gases not condensable at -196 "C
were measured volurnetrically and nitrogen, oxygen, and
hydrogen were analyzed by gas chromatography using
helium (for N 2 and 0 2 ) or argon (for H Z ) carrier gas.
Hydrogen peroxide was estimated by its oxidation of
iodide ion.

1182 CANADIAN JOURNAL OF CHEMISTRY. VOL. 46, 1968

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 50.116.19.84 on 06/29/16 Dosimetry '''
For personal use only.
Thedoserateof3.0 x 1017eV g-' min-' in H 2 0(at an "'
electron beam current of 1 n1A) was estimated by the
nitrous oxide dosimeter irradiated at ambient temperature $
(- 25 OC) inside the furnace. A value G(N2) = 11.0 was
used (6). The yields of nitrogen from irradiated nitrous \

'oxide at 400 and 700 Torr were linear with dose up to the 2.0

maximum dose used, 1.4 x loZ0eV g- (2.8 x 1017 eV

ml-' NzO at s.t.p.), and were independent of pressure

within the range used. The relative dose rate was checked
periodically with the Fricke dosimeter. The energy ab-
sorbed by each additive was calculated by assuming it t o
be directly proportional to the electron density of the
additive.

Results

+Water Vapor N 2 0 H2fX5)

The yield-dose plots for nitrogen, oxygen, and I 60
2 0 4.0

hydrogen produced from irradiated water vapor DOSE (X I O - ~ ~eIv,p-l

-The slopes of the linear portions give G(N2)

+= 3.0 0.3 and G(0,) = 1.6 4 0.3, in the

latter case after an induction period. The nitro-

+gen yield was independent of dose from 0.15-5.0
+x lo2' eV g-l, of dose rate within the range 0.4-
contaking nitrous oxide are shown in ~ i 1.~ : FIG.1. Product yields from water vapor containing
nitrous oxide. Pressure 750 Torr and temp. 125 OC.
Dose rate 3 x 1017eV g-' m i x 1 . Nz yields: ( 0 )
(e)0.1 mole % N 2 0 ; (0)1 mole % NzO; (8)2.2 mole %

N 2 0 ; ( 0 )5.2 mole % NzO; ( 0 )10 mole % NzO;
% NzO m%oplreop%ylNenZeO. ;O(ZH)y~1emldosl:e(%0)
1 mole % NzO; 1 mole
1 mole ( 0 ) 5.2
NzO 1 mole % propylene. Hz yields: ( x ) 1 mole %
N 2 0 . Dose rate 4 x 1016eV g-I min-'. N2 yields: (6)
3.0 x 1017eV g-l min-l, and of nitrous oxide 1 mole % N 2 0 . 0, yields: (B)1 mole % N20.

concentration between 0.1-10 mole %,". The

intercept in the nitrogen plot at zero dose is the oxygen measured from this decomposition

outside experimental error and appears to indi- was usually less than 10% of that expected.

cate that G(N,) > 3.0 at low doses. However,
+ +this may be due, in part, to air contamination Water Vapor N 2 0 Additives

since blank experiments without irradiation Addition of sulfur hexafluoride (0.0016-1.0
always gave small quantities of nitrogen (- 0.1 mole %) to water vapor containing nitrous oxide
pmoles) and oxygen (- 0.01 pmoles). Thus it is (1-10 mole %) caused essentially complete sup-

not possible to estimate the magnitude of G(N,) pression of the nitrogen yield in each case. Small

at doses lower than 10' eV g-I except that it may -quantities of nitrogen (0.1-0.2 pmoles) were
be greater than the steady state yield of 3.0. As
produced in all the samples, which in terms of
shown in Fig. 1, addition of 1 mole % propylene "yields" amounted to G(N2) 0.4 compared
to water vapor containing 1 mole % nitrous with 3.0 in the absence of sulfur hexafluoride.

oxide reduced the oxygen yield to zero but did However, since this "yield" did not change as

not affect the nitrogen yield. the N,0:SF6 ratio increased from 1:1 to 1250:l

The yield of hydrogen from water vapor con- and since the absolute amounts of nitrogen were
taining 1 mole % nitrous oxide appears t o build similar to those produced in blank runs without

up to a steady state value of about0.4-0.5 pmoles irradiation, these small quantities of nitrogen are

cm-3 H 2 0 ,though the yields tended to be some- probably due to air contamination. It is con-

what erratic. At low doses the hydrogen yield cluded that even at a N20:SF6 ratio of 1250:l

+was approximately constant a t G(H,) = 0.45 the sulfur hexafluoride causes complete suppres-
0.1. Generally nitrogen, oxygen, and hydro-
gen accounted for all the gaseous products -sion of the nitrogen produced by reaction of
collected. Small quantities of hydrogen peroxide,
nitrous oxide with intermediates produced from
the water. G(H,) was 0.4 a t doses less than 5
G(H20,) _< 0.15, were found in some samples. x lo1' eV g-l but decreased a t higher doses.
Blank experiments with water vapor containing Carbon tetrachloride (0.04-1.2 mole %) had a

added hydrogen peroxide (up to M) similar effect in suppressing the nitrogen yield

but without irradiation showed that greater than from water vapor containing nitrous oxide (1-6
90 % of the hydrogen peroxide decomposed, but mole %) at all N20:CCl, ratios between 1:1 and

DIXON AND BAILEY: EFFECTS OF ELECTRON SCAVENGERS 1183

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 50.116.19.84 on 06/29/16 110:l. Hydrogen was not measured in these may therefore be taken as a measure of the yield
For personal use only. samples. of scavengeable electrons in water vapor. The

In water vapor containing nitrous oxide (1 value G(N,) = 3.0 i 0.3 = g(e) found over a

mole %) and hydrogen chloride, G(N,) decreased wide range of dose rate and nitrous oxide con-
centration is in good agreement with the theoret-
from 2.9 to 0.3 as the hydrogen chloride con- ical value of 3.3 based on the average energy
required to form an ion pair in water vapor,
centration increased from 0.075 to 1 mole %. W(H,O) = 30 eV (14, 15) and with Baxendale
and Gilbert's value (4) of 3.0 -t 0.4 at low nitrous
G(H,) increased from 0.2 t o 3.5 over the same
concentration range. oxide concentration ( 5 0.7 mole %).

All the above yields were based on the energy Since nitrous oxide concentration has no effect
absorbed by water only. Corrections for nitrogen on G(N,) it appears that the reactions
and hydrogen produced by direct absorption of
energy by nitrous oxide and hydrogen chloride do not compete favorably with other reactions of
were made using G(N,),,, = 11.0 (6) and H and OH. This is confirmed by the lack of effect
G(H,),,, = 8.0 (7). These corrections were on the nitrogen yield of propylene, which is
made assuming that energy transfer from additive known to react rapidly with both H atoms (see
to water does not occur and that addition of for example ref. 16) and OH radicals (17).
sulfur hexafluoride, carbon tetrachloride, or
hydrogen chloride does not affect G(N,) from Similarly, Fig. 1clearly shows that the buildup
pure nitrous oxide. It is possible that neither of of oxygen does not affect G(N,), indicating that
these assumptions is strictly correct but we the quantities of oxygen produced in our experi-
estimate that any inaccuracies due t o these ments are insufficient for electron capture by
assumptions will not seriously affect the conclu- oxygen to compete with electron capture by
sions arrived at in the present study. nitrous oxide. Also, since G(N,) is in good
agreement with the expected value of g(e) from
Discussion the W-value in water vapor (14, 15), it appears
that the reaction
+Water Vapor N,O

The major reactions which produce H and OH
in irradiated water vapor are generally assumed
to be

HzO --M->HzO*, H,O+, e -

Reaction [I] is extremely fast (k = 1.26 x 10- proposed in the propane-nitrous oxide system
cm3 molecule-I s-l) (8) thus precluding other (1 1) is not occurring in water vapor under our
reactions of H,Oi at the pressure of water vapor
used. experimental conditions even at 10 mole %

The high efficiency of nitrous oxide as an nitrous oxide. The fate of 0- is presumably
electron scavenger has been postulated in various neutralization,
aqueous solutions (9), organic liquids (lo), and
gaseous systems (11). Its efficiency in gaseous or reaction with water,
systems is based on electron impact experiments
which have shown it to dissociatively capture +[9I 0- H z 0 -> OH -1 OH-,
electrons close to zero electron energy (12, 13).
It therefore seems probable that nitrous oxide is followed by neutralization
acting as an electron scavenger in water vapor,
+[lo] H 3 0 + OH- -> 2HzO.
thus preventing the neutralization reaction [2].
If no other reactions produce nitrogen, its yield Since reaction [8]probably occurs at a rate com-
parable to the rate of reaction of 0- with the
positive ion formed in propane radiolysis (ll),
where reaction [7] does occur, and since the dose
rates are similar in the two studies, the fate of 0-
is probably disappearance by reaction [9]. Since
the nitrogen yield was unaffected by nitrous oxide

concentration up to 10 mole % this indicates k,

1184 CANADIAN JOURNAL OF CHEMISTRY. VOL. 46, 1968

2 Ic,, assuming an uncertainty of 10% in G(N,). sition of the hydrogen peroxide, i.e. oxygen is
being removed by some process. Since the ex-
For electrons to disappear via [4] in competition tent of this removal is of the same magnitude as
the intercept in Fig. 1, we suggest that oxygen is
with [2] at the dose rate used (-- 3 x 10'' eV removed during radiolysis either by reaction with
impurities or by loss to the walls of the vessel.
cm-3 s-l) and at a minimum concentration of
Oxygen is probably produced by the following
0.1 mole % nitrous oxide, a steady state treat- reactions.

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 50.116.19.84 on 06/29/16 >ment of reactions [I], [2], [4], [8], [9], and [lo] + +[I41 013 H 2 0 2+ H 0 2 H,O
For personal use only.
indicates k4/k,1'2 1.5 x 10-lo, assuming Ic, -.+ +[I51 OH $. OH M H20, M
= k, = klO and a 10% uncertainty in G(N,).
Similarly for at least 90 % of 0- to disappear via --. +[I61 OH + HO, H 2 0 0,

>[9] in competition with [8], the steady state +1171 HO, HO, + H,O, ;0,

treatment shows kg/k,1'2 1.5 X 10-13. The +[I81 H + 0 2 M - > H O , + M.
recombination coefficients k, and k, are not
accurately known but probably lie in the range Since reaction [4] essentially converts e- to OH,
10-6-10-s cm3 molecule-' s-l (18). Hence it the presence of nitrous oxide would cause OH to
be in excess of H by about 6 G units. Production
>follows that k, 1.5 x 10-l4 cm3 molecule-l of oxygen from these "excess" OH radicals
>s-' and k g 1.5 x 10-l7 cm3 molecule-' s-l, would yield G(0,) = 1.5, which is in good
agreement with the measured value of G(0,)
though the true values could be much higher than = 1.6 f 0.3. The complete suppression of the
the minimum values. The former is at least an
order of magnitude higher than the speciiic rate oxygen yield found on addition of 1 mole %
found in propane radiolysis at room temperature propylene to water vapor containing 1 mole %
(ll), assuming the same rates of electron -
positive ion recombination in water vapor and nitrous oxide is predicted by the above scheme,
propane. since propylene will rapidly react with all the
available H and OH radicals (16, 17).
The value of G(N,) at very low doses is not
known but may be greater than 3.0. However, As shown in Fig. 1, hydrogen builds up to a
since the small quantities of nitrogen produced in steady state concentration which is presumably
samples containing sulfur hexafluonde, which limited by the back reaction
captures all electrons, were similar to the nitro-
gen intercept in Fig. 1, then if G(N,) is greater -Although the hydrogen yields are small and
than 3.0 at low doses, the extra nitrogen prob-
ably does not originate from electrons. This is somewhat irreproducible, a steady state value of
supported by the lack of effect of nitrous oxide 0.4-0.5 pmoles cm-3 H,O is reached at a
concentration on G(N,) over the whole dose dose of about 5 x lo1' eV g-l, which is similar
range studied. Thus there seems little doubt that
nitrogen is formed by reaction [4] only and that to the steady state yield found in pure water
G(N,) is a measure of the yield of scavengeable
electrons in water vapor. vapor (3). At doses lower than this, the yield was

Oxygen is produced with G(0,) = 1.6 f 0.3 approximately linear with dose with G(H,)
after a dose of about 5 x 10' eV g-I and extra-
polation of the yield-dose plot for oxygen to zero = 0.45 f 0.1. This is in reasonable agreement
dose gives a negative intercept of 0.65 pmoles.
This suggests that oxygen is either removed by with the molecular hydrogen yield in water vapor
impurities at low doses or is formed by secondary
reactions of some product of radiolysis. Our found in several independent studies (19-21).
analysis for hydrogen peroxide showed little or
no yield, which is in accordance with our observa- When the steady state coilcentration of hydro-
tion that hydrogen peroxide is decomposed
thermally under our experimental conditions. gen is reached the net chemical change will be
However, in blank experiments with water vapor
containing added hydrogen peroxide (without given by +N2O + N2 + 0 2
irradiation) we found less than 10% of the
oxygen expected (0.5-1 pmole) from decompo- assuming that no other products are formed. The

DlXON AND BAILEY: EFFECTS OF ELECTRON SCAVENGERS 1185

TABLE I

Comparison between the relative rates of reaction of some molecules with electrons in irradiated water vapor and
their known electron attachment cross sections in the gas phase1

--

Relative cross section at: Relative integrated cross Estimated relative
section over electron
Electron 0.5 eV 1.0 eV energy range: rate of reaction
energy at 1st electron electron
maximum energy energy (M.5 eV &I eV with electrons
1st maxi-
Mole- nun1 (eV) from present
cule Reference work

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 50.116.19.84 on 06/29/16 SF6 0.0 15&13000 2&80 0.5 30&3000 4M00 23-26 >lo4
For personal use only. CC14 0.02 3&65 70 30 330 75 23
HCI 0.6 1-2 3 0.5 3 2 23 2 10,
N20 4.6 1 1 1 1 1 -10
12, 13, 22
Hz0 6.4 1-2 1
23

'All values relative to N 2 0 = 1.

results substantiate this since, within the limits of The effect of hydrogen chloride in irradiated
water vapor containing 1 mole % nitrous oxide
experimental error, the relationship

appears t o be somewhat more complex. G(N2)

falls from 2.9, which is approximately the valuein

is seen t o hold over the whole range of nitrous the absence of hydrogen chloride, t o 0.3 as the

oxide concentration. HCl:N20 ratio increases from 0.076:l t o 1:I (a

Water Vapor + N 2 0 + Additives factor of 13) and G(H2) increases from 0.2 to 3.5
over the same range. Assuming an uncertainty of
Although nitrous oxide captures electrons about 10% in these G-values, the decrease in
close to zero electron energy, its capture cross G(N2) with increase in hydrogen chloride con-
section for such electrons is low (12, 13, 22). The centration is consistent with a simple competition
electron capture cross sections of sulfur hexa- between hydrogen chloride and nitrous oxide for
fluoride and carbon tetrachloride are much electrons,
higher than that of nitrous oxide and their first
maxima both occur close to zero electron energy, +[41 e- N 2 0 -> N 2 -t 0-
whereas that of nitrous oxide is at a much higher
electron energy (see Table I). Thus the suppres- - + +P I 1 e- HCI -> H C1-,
sion of the nitrogen yield from water vapor -
nitrous oxide mixtures is probably due to electron with k, ,/k, 10. However, the hydrogen yields
capture by SFGand CCI,,
appear to be too low since, although H atoms
+1191 e- SF6 -.SF6- from reaction [21.]will probably react with HC1,
+ +[201 e- CCI, -.CCI, C1-,
+ +[22] H HCI H, C1,
competing with reaction [4],
we would also expect H atoms produced in
+ +141 e- N 2 0 -.N 2 0-, reaction [3] to react with HCl via reaction [22].
I t is possible that secondary products are inter-
fering with the reactions of H atoms but in the ab-

and it appears that even at the lowest S F G : N 2 0 sence of a more detailed study, which is outside
and CC1,:N20 ratios used, reaction [4] is com- the scope of this work, we can only deduce that
pletely suppressed. Since the lowest SFG: N 2 0 hydrogen chloride probably captures electrons
and CC1,:N20 ratios were 1:I250 and 1:I10 about 10 times more efficiently than nitrous ox-
respectively, this indicates that the electron cap- ide. This is shown in the last column of Table I.

ture efficienciesof sulfur hexafluoride and carbon Comparison between the Cross Sections for
Electron Attachment andtlze Relative Rates
tetrachloride in water vapor are at least 10, and of Reaction with Electrons in Water Vapor

lo3 times greater than that of nitrous oxide. The probability of electron capture by an
additive will be given by an integral of the form
These values liamr eitisnhgowrenlatiinvethraetelaks(tec- ol+umSnF,)o/f
Table I. The N*Sf(E)o(E)dE,
+k(e- N,O) found in the present work is in

good agreement with that found by Johnson and

Warman in gaseous propane (1I). where NA is the concentration of the additive,

1186 CANADIAN JOURNAL OF CHEMISTRY. VOL. 46, 1968

f (E) is the flux of electrons at energy E, and o(E) electron energies, probably less than 0.5 eV, but
is the electron capture cross section of the addi- not necessarily completely at thermal electron
energy. However, more information is needed on
tive at energy E. It is interesting to speculate on both electron capture cross sections at low
the probability of electron capture in irradiated electron energies and the rates of reaction of
water vapor using available data on electron electrons in irradiated gases before a more
detailed comparison may be made.
capture cross sections compared with our values
of the relative rates of reaction of electrons with

the various additives.

Our experiments show that the majority of Acknowledgments

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 50.116.19.84 on 06/29/16 electrons are not captured by water even though It is a pleasure to acknowledge the critical
For personal use only. ~ ~ ~ ~ mbye Dnrst . sP. J. Dyne and A. W. Boyd
its concentration is higher than that of the On forms of the manuscript.

additive. This is evident from the lack of corn-

petition with nitrous oxide for electrons in water

vapor containing as little as 0.1 mole % nitrous

oxide. The appearance potential of H - from the 1- R. F. FIRESTONE.J+Am. C h m . Sot.79,5593 (1957).
2. JF.arHa.daByASXoEc.N3D6,A1La8nE6d(1G9.63P). GLLBERT.DiscussionS
reaction

+123I e- H ~ O-> H- -b OH 3. A. R. ANDERSONB., KNIGHTa, nd J. A. WINTER.

Trans. Faraday Soc. 62, 359 (1966).
4- &:5.:?~;ALE and G. P. GILBERT. science, 147,
is about 5 eV (23) and the capture cross section

for H 2 0 in the 5-7 eV energy range (where the 5. G. R. A. JOHNSOaNnd-M. SIMIC. J. ~ h y sC. hem. 71,
first maximum for H,O occurs) is higher than
1118 (1967).
6. J. A. HEARNaEnd-R. W. HUMMEL.Radiation Res.
those of the additives. Thus the value of f(E) in 254 (1961).

the 5-7 eV range must be small and the rate of 7. R. A. LEE,R. S. DAVIDOWa,nd D. A. ARMSTRONG.
loss of energy for electrons must be reasonably Can. J. Chem. 42,1906 (1964).

high in this energy range. 8. F. W. LAMPEF, . H. FIELDa, nd J. L. FRANKLIN.J.
Some arbitrary comparisons of the capture Am. Chem. Soc. 79, 6132 (1957).

sections of the molecules studied at low 9. F. S. DAINTOaNnd D. B. PETERSON.Proc. ROYS.OC.
electron energies are shown in Table I. The London, Ser. A, 267, 443 (1962).
qualitative correlation between the resonance
10. G. SCHOLEaSnd M. SIMC. NMat.urwe 2A02~8d95A(T1Nr9a6n4.s)..
capture cross sections and our relative reaction 11. G. R. A. JOHNSONand J.
kgfi$g:rNFaraday Soc. 61, 1709 (1965).
Phys. 347
12. and R. E. Fox. J.

rates shows that electrons with energies in the 13. G. J. SCHULTZ.J. Chem. P ~ Y S34. , 1778 (1969.
range 0-0.6 eV (the position of the first maximum 14. C. WINGATEW, . GROSSa,nd G. FAILLA. Radiation

for N2° and HC1) are important and f(E) Res. 8, 411 (1958).
have significant values in this range. HCl has 15. J. Booz and H. G. EBERT. Z. Angew. Phys. 13, 376

effectively zero cross section at 0.35 eV (the (1961).

appearance potential C1-) suggesting that 16. R. J. CVETANOVICA. dvan. Photochem. 1, 115
f(E) is significant at values of E greater than this. (1963).

general the comparisons show that while the 17. E. R. BELL,W. E. VAUGHANan, d F. F. RUSK. J.
Am. Chem. Soc. 79, 3997 (1957).
capture cross sections at low electron energies give
the correct sequence, SF6 and CCI, capture 18. A. VON ENGEL. Ionized gases. Clarendon Press,
electrons more efficiently, with respect to N,O, Oxford. 1965. p. 155.

than the cross sections would suggest. This dis- 19. A. R. ANDERSONB,. KNIGHTa, nd J. A. WINTER.
parity increases if we compare capture cross Nature, 201, 1026 (1964).
sections at energies above 1 eV, but a more quan-
20. J. H. BAXENDAaLnEd G. P. GILBERT.J. An?. Chen~.
titative agreement is obtained if we compare Sot, 86, 516 (1964).
integrated cross sections at energies below 0.5 eV,
21. J. Y. YANGand I. MARCUS. J. Am. Chem. SOC. 88,
though the available data in this region are 1625 (1966).

sparse. 22. D. RAPPand D. D. BRIGLIA. J. Chem. Phys. 43,
In general the above observations lead to the 1480 (1965).

conclusion that electron capture occurs a t low 23. I. S. BUCHEL'NIKOVAZ.h. Eksperim. i Teor. Fiz. 35,

J.1119 (1958)

21. W. M. HIc;AM and R. E. Fox. Chem. Phys. 25,
642 (1956).

25. R. K. ASUNDaI nd J. D. CKAGGS.Proc. Phys. Soc.
London, 83, 611 (1964).

26. B. H. MAHANand C. H. YOUNG. J. Chem. Phys. 44,
2192 (1966).