ARGON-SENSITIZED RADIOLYSIS OF ETHYLENE
2409
The Argon-Sensitized Radiolysis of Ethylene in the Liquid Phase'
by Norman V. Klassen Radiation Research Laboratories, Mellon Institute, Pittsburgh, Pennsylvania
(Received September BS, 1966)
The Co60 y-ray radiolysis of ethylene and ethylene-argon solutions has been studied in the liquid phase a t - 161". Ethylene is efficiently decomposed even in solutions containing as low as 0.0005 mole fraction of ethylene. The production of acetylene, ethane, and n-butane is enhanced by dilution with argon, while that of the hexenes and butenes is reduced. Below l%, ethylene the G values of all observed products are essentially independent of ethylene concentration. The results are discussed in terms of energy transfer to ethylene from ionic and excited states of argon.
Introduction It is well known that the radiolysis of hydrocarbons may be sensitized by the presence of "inert gases" such as argon. This sensitization is usually attributed to charge-transfer reactions of inert gas ions and to energy transfer from electronically excited species. Lampe2 found that the presence of argon accelerated the radiolysis of gaseous ethylene with an efficiency which decreased as the partial pressure of argon was increased. Rleisels3 discussed the excited states of ethylene likely to be formed by the irradiation of such mixtures. The radiolysis of argon-ethylene liquid solutions has not been reported previously. However, Koob and Kevan4found that considerable energy transfer to propane occurred when a solution of 7% propane in liquid argon was irradiated. They concluded that sensitization was due to charge-transfer reactions. Recently, von Bunau5 has reviewed the inert gassensitized radiolysis and photolysis of hydrocarbons and other compounds. The present investigation differs from previous studies in that very dilute solutions of ethylene have been examined. This was done in hopes of distinguishing between sensitization by argon ions and sensitization by excited argon species. It is possible that such a distinction exists. However, any interpretation of the results must be speculative.
were calculated assuming that absorption of energy is proportional to the electron density of the sample. The argon was Matheson Prepurified grade. It was used after slow passage through a bed of glass beads a t -183". This argon is stated to normally contain about 1 ppm of nitrogen and of oxygen. For one experiment, an impurity, probably oxygen, was removed from the argon by preirradiating the argon in the presence of ethylene. The purified argon was obtained by passage of the irradiated mixture through a column of silica gel a t -78" which removed all hydrocarbons. The ethylene used was Phillips Research grade. Radioactive ethylene, 14CzH4, having a specific activity of 0.1 mcurie/mg, was obtained from h'ew England Nuclear Corp. In some experiments this radioactive material was diluted with inactive ethylene before use. All ethylene samples were purified by gas chromatography to reduce hydrocarbon impurities to levels a t which they were undetected by the analytical procedures used to measure radiolysis products. The solutions were irradiated in sealed Pyrex U tubes having a break-seal in each arm. Prior to filling, these tubes were sparked vigorously with a tesla coil while under high vacuum. This seemed to be an efficient way to degas the inner surface. Ethylene and then argon were frozen into the tubes which were then sealed off. The contents of the tubes were liquefied
Experimental Section Irradiations were carried out in a uniform field of Co60 y rays at a dose rate in liquid argon of 6 X 10'' ev/g min (lo4 rads/min). Dosimetry was done with a Fricke dosimeter. Dose rates for different materials
(1) Supported in part by the U. 5. Atomic Energy Commission (2) F. W. Lampe, Radiation Res., 10, 691 (1959). (3) G.G.Meisels, Nature, 206, 287 (1965). (4) R. Koob and L.Kevan, J. Phys. Chem., 70, 1336 (1966). ( 5 ) G. von Bunau, Fortschr. Chem. Forsch., 5 , 347 (1965).
Volume 71,Number 8 July 1067
NORMAN V. KLASSEN
2410
Table I: Radiolysis Products of Argon-Ethylene Solutions a t CZHI,' electron
Range of CzHb decomposition,"'b
5%
?&
Hz
1.07
lood
0.06-0.5
100 81
0.7 0.r
37
0.3-0.7
37 7 1.3
0.3-0.7 0.8-1.6 2.5
0.84 0.076 0.046' 0.046
0.9-6.0
7.5 0.5-9.0
I-r.5
- 161' a valuee"+
,
...
... ...
...
...
... ... ... ...
...
CzHn
1.5
1.6 1.65
CzHa
0.8 0.8
n-CdHlo c-CdHs
c-CtHa
0.6
0.12'
0.023
0.04
0.04
0.004
0.7
0.8
0.07
...
...
...
0.7
0.05
... ...
...
...
...
... ...
...
...
...
...
0.04
... ...
0.07
0.004
...
0.36 0.36
0.06
...
0.6 0.r
...
0.11 0.05
...
0.1
...
...
... .,.
0.4 0.22
0.10 0.10
0.063 0.022
... ...
0.05
0.07
0.02
0.002
0.23
0.20
0.05
0.06
...
...
...
...
... ...
... ... ...
0.22 0.82
0.20
0.05
0.10
0.022
+
0.44
...
CHI
CaHs
...
...
0.8
0.6
0.97 1.06 0.98
0.89 0.82 0.80
2.3 9.06
0.96
0.90
...
...
...
1.0
0.95
0.005
0.03
9.0
...
...
...
0.15
0.05
...
2.5 9.9
2.86 2.02
...
...
CiHa
c-CiHs
C4Hs
CsHiz
CaHu
... ...
...
Estimated by assuming G(-C2H4) = 15 (see Table 111). ' Based on the a Numbers in italics refer to experiments using l4CzH4. total energy absorbed by the solution. Two producb, tentatively identified as a C7 and a Cs compound, had G values of about 0.1 and 0.15, respectively. * This value taken from ref 6. See Table 11.
'
and shaken vigorously. Visual observations and the efficient energy transfer which occurred during radiolysis verified that the solutions were homogeneous. The solutions were irradiated, as liquids, in a bath of liquid methane a t - 161'. Following an irradiation, the arms of the U tube were sealed to an inlet line, the break-seals broken, and the entire sample was flushed into the gas chromatograph which employed a hydrogen flame detector. The chromatograph column most commonly employed contained a 3 : 1 (by weight) mixture of silica gel and alumina. Erratic results were obtained when attempts were made to analyze for less than 1 pmole of acetylene using this and other columns. The irreproducibility appeared to be due to incomplete elution of the acetylene. Hence, acetylene was analyzed in the following manner. The solution to be irradiated was made up with radioactive ethylene. A "carrier" mixture, containing large quantities of all the radiolysis products, was added to the irradiated material prior to chromatography. The eluted hydrocarbons were identified using a thermal conductivity detector and the G value of each product was calculated from its radioactivity, measured as the gas flowed through a proportional counter. With this method the loss of a trace of acetylene on the column packing could not affect the results significantly and reproducible acetylene yields were obtained. The irradiation tubes were always of such a size that only a small correction in the dose calculations was necessitated by the presence of argon vapor a t 7 atm and ethylene vapor a t less than 3 torr above the liquid. At each concentration of ethylene several irradiations of different duration were done to check The Journal of Physical Chemistry
for dose dependencies. Only a t the lowest ethylene concentration, 0.046 electron %, did a significant dose effect arise. Calculations of G values (molecules of product per 100 ev of absorbed radiation) are based on the dose absorbed by the entire solution.
Results The results of the radiolysis of ethylene and ethylene-argon solutions in the liquid phase a t -161" are tabulated in Table I. A direct comparison of these results with published values is only possible for 100% ethylene which was studied in detail by Holroyd and Fessenden.'j The agreement is quite satisfactory. Ethane and n-Butane. The G values of ethane and n-butane exhibited a dose dependency only a t 0.046 electron % ethylene, the lowest concentration examined. Ethane and n-butane appear to be produced by free-radical reactions so that this dose effect, detailed in Table 11, could be caused by a few parts per million of oxygen in the argon. Such a conclusion is strengthened by the virtual removal of the dose effect by the use of argon which had been purified by preirradiation with ethylene. Cyclobutane and nbutane were separated from each other only at 0.046 electron % ethylene. The value of G(cyc1obutane) was found to be essentially the same as in pure ethylene.6 Hence, the yield of n-butane was calculated by subtracting the small interpolated yield of cyclobutane from the total yield of saturated C4 products. Isobutane was not detected as a product. As shown in Figure 1, G(C2Hs)and G(n-C4Hlo)increase slightly as (6) R. A. Holroyd and R. W. Fessenden, J. Phys. Chem., 67, 2743 (1963).
ARGON-SENSITIZED RADIOLYSIS OF ETHYLENE
3.0
-
2.5
-
H2 2.0
-
m
0
2411
I
10.4
... -f 1.5 a
.
0
1.0
0.5
0.2
-
0.0I
0
U
ll
1
0.I
I
CYCIO-CH,
x----------------------x 0
8
1
I
1
1
IO
100
1
1
1
I
0.1
I
IO
IO0
Ethylene [Reclron Percent)
Figure 2. Yield of butenes and hexenes from the radiolysis of liquid argon-ethylene solutions a t 161'.
-
Ethylene (Eleclron Percenll
Figure 1. Yield of acetylene, ethane, and butane from the radiolysis of liquid argon-ethylene solutions a t - 161
'.
the ethylene concentration is reduced until, below 1% ethylene, they remain constant. Acetylene. The acetylene yields were determined using l4C2H4and appeared to be unaffected by impurities in the argon. In a 0.4% ethylene solution, G (C2H2) was essentially independent of dose up to 20% decomposition of the ethylene. This observation is noteworthy because acetylene is not a prominent product of the radiolysis of any of the products. As shown in Figure 1, G(C2H2) has its maximum value in solutions of about 20% ethylene and is constant below 1%ethylene.
centration. Butadiene was never identified and it is believed to be a very minor product. Ethvlene. G values for ethylene disappearance were - _ determined in 0.43% ethylene solutions and are given in Table 111. G(-C2HJ moved in the remarkably small range of 11.2-16.4 for experiments in which 3.4-8Q% of the ethylene was consumed.
Table 111:
Determination of a G Value for the Decomposition
of Ethylene in 0.43 Electron % Ethylene Solutions Decomposition of ethylene, %
C( - CZHI)
3.4 19.6 48.0 80.5
11.2 16.4 14.6 13.1
Table 11:
Dose Dependency of Ethane and Butane in 0.046 Electron % Ethylene Solutions Dose, ev/K
x
10-17
2.6 7.8 18 47 a
Approximate decomposition of ethylene,a %
0.5 1.5b 3.6
9.0
Q(CiHs) 0.19 0.78 0.77 0.96
+
G~Z-C~HIO c-C~HE) 0.33 0.93 0.76 0.92
Estimated by assuming G(-C2H4) = 15 (see Table 111).
' Prepurified argon (see text) was used in this experiment.
Other Products. The butenes and the hexenes were not resolved into separate isomers. Figure 2 shows that their G values decrease with decreasing ethylene concentration until they reach constant values below 1% ethylene. The yields of the separate butene and hexene isomers in liquid ethylene have been reported previously.6 The G value of hexanes was always less than 0.1 and it did not vary much with ethylene con-
Gaseous Argon-Ethylene at -161'. During the radiolysis of liquid solutions an insignificant amount of product resulted from radiolysis of the vapor above the liquid. This was shown by the radiolysis, at - 161°, of a wholly gaseous sample which closely approximated the vapor pressures of ethylene and argon at -161". The results in Table IV indicate that, a t this temperature, G values in the gas phase closely resemble those in the liquid phase. The value of 0.33 for G(C2H2) must be considered as a lower limit since the l4C2H4 technique was not used in this experiment. Solid Argon-Ethylene at -196". The single experiment given in Table IV indicates that energy transfer from argon to ethylene is much less efficient in the solid than in the liquid phase.
Discussion Reactive Species in Argon. Energy initially absorbed by argon may be transmitted to ethylene by the transfer of charge from argon ions and by the transfer of energy Volume 71, Number 8 July 1967
NORMAN V. KLASSEN
2412
~
_ _ _ _ _ _ _ _ ~ ~
~~~
~~
Table 1V : Radiolysis Products of Argon-Ethylene Mixtures CZH4,
CZH4
electron
decomposed,a
%
%
r -
- CZH4
CIH,~
CHI
CzHs
-____
G values n-C4H1oC
c4H8
0.042
Gas Phase at -16l0, Argon Pressure = 4 . 9 atm, Ethylene Pressure = 1 . 6 torr 7.25 8.3 0.33 1.19 0.00 0.67 0.07
0,039
0.48
a Experimentally determined. which may have been produced.
0.43
0.00
Solid Phase a t - 196’ 0.08 0.00
+ 2Ar +Ar2+ + Ar -4r** + Ar +Ar2+ + e
Ar+
(1) (2)
where Ar** is an excited atom capable of forming Ar2+. The irradiation of gaseous argon produces a variety of electronically excited states of argon, both atomic, Ar*, and molecular, Ar2*. Only those excited species with long lifetimes will transfer energy to ethylene at low ethylene concentrations. The lowest metastable states of argon are the 3P0(11.72 ev) and the 3P2 (11.54 ev) triplets.1*~’2 Long-lived excited argon atoms with energies close to the ionization potential have been 0 b ~ e r v e d . l ~It has been suggested7!l4that metastable argon molecules with energies of 14.5 and 11.5 ev and lower may be formed in three-body collisions involving long-lived excited atoms. Three-
+ 2Ar +Ar2* + Ar
body collisions are favored by the density of liquid argon. Hurst, Bortner, and GlicF have stressed the importance of resonance states to energy transfer a t pressures which are sufficiently high to permit trapping of the resonance photon. The argon resonance levels 3P1(11.62 ev), ‘P1 (11.83 ev), 3P1(14.09 ev), and ‘P1 (14.26 ev) have been 0b~erved.l~ Liquid Ethylene Radiolysis. Previous examinat i o n ~ ~of~the ‘ ~radiolysis ~’~ of liquid ethylene have shown the predominant radical species to be ethyl, vinyl, and 3-butenyl radicals and hydrogen atoms. It has been s h o ~ n ~ ~that ’ ~ ~ethane ’’ and n-butane are mainly produced by the reactions of ethyl radicals. About half
CsHii
0.07
0.06
0.004
0.008
G(n-C4Hlo) also includes any c-CaHs
C2H4
+
The Journal of Physical Chemistry
0.02
* The acetylene yields may be low as discussed in the text.
from electronically excited argon. In the a irradiation of pure argon, both gaseous’ and liquid,8 the value of G(Ar+ Ar2+) is 3.8. Ar+ has an appearance potential of 15.76 ev in the gas phase. Kebarle, et al.,9 suggest a recombination energy of 13.9 ev for Ar2+. This molecular ion is probably formed quite readily in liquid argonlo by
Ar*
0.02
CeHiz
+H
2C2Hs -+
CzH5
C2H6
+ C2H4
(3)
(4) (5)
+n-CdHlo
of the butene yield and three-quarters of the hexene yield arise directly or indirectly (through 3-butenyl radicals) from vinyl radicals. Acetylene production is not affected by small amounts of oxygen.6 This proved to be true in argonethylene solutions also. Acetylene may be formed by ionic and molecular p r o c e s s e ~ ~ ~ ~ ~ ~ ~ ~ CzH3+
+ C2H4
+ C2Hz
C2H4* +C2H2 C2H2
+ C&+
(6)
+ 2H
+ Hz
where CzH4* represents an electronically excited state of ethylene. Meisels20concluded that acetylene is not (7) 42, (8) (9)
G. S. Hurst, T. E. Bortner, and R. E. Glick, J . Chem. Phys., 713 (1965). D. W. Swan, Proc. Phys. SOC.(London), 85, 1297 (1965).
P. Kebarle, R. M. Haynes, and S. Searles, Advances in Chemistry Series, No. 58, American Chemical Society, Washington, D. C., 1966, p 210. (10) H. T. Davis, S. A. Rice, and L. Meyer, J . Chem. Phys., 37, 947 (1962). (11) “Argon, Helium and the Rare Gases,” Vol. I, G. A. Cook’ Ed., Interscience Publishers, Inc., New York, N. Y . , 1961. (12) A. H. Futch and F. A. Grant, Phys. Rev., 104, 356 (1956). (13) V. Cermak and Z. Herman, Collection Czech. Chem. Commun., 29, 953 (1964). (14) T. D. Strickler and E. T.Arakawa, J . Chem. Phys., 41, 1783 (1964). (15) H. Boersch, J. Geiger, and H. Hellwig, Phys. Letters, 3, 64 (1962). (16) R. W. Fessenden and R. H. Schuler, J . Chem. Phys., 39,2147 (1963). (17) R. A. Holroyd and G. W. Klein, J . Appl. Radiation Isotopes, 15, 633 (1964). (18) M. C. Sauer and L. M. Dorfman, J . Phys. Chem., 66, 322 (1962). (19) H. Okabe and J. R. McNesby, J . Chem. Phys., 36, 601 (1962). (20) G. G. Meisels, ibid., 42, 2328 (1965).
ARGON-SENSITIZED RADIOLYSIS OF ETHYLENE
2413
produced bv reaction 6 in the gas-phase radiolysis of ethylene above 50 torr. In Table I it can be seen that products containing six carbon atoms or less account for a G(-CzH4) of 8. In the gas phase G(-CzH4) has been reported as 20.21 If G(-C2H4) has a similar value in the liquid phase, the ethylene unaccounted for was probably converted into high molecular weight products. Wagnerz2 found such products in the radiolysis of solid ethylene and suggested that they were formed by successive condensation reactions initiated by CzH4+ or possibly C2H4*. Liquid Argon-Ethylene Radiolysis. The abundance of excited and ionic species produced by the irradiation of argon suggests the possibility that numerous energy-transfer processes may be occurring simultaneously in liquid argon-ethylene solutions. Energy transfer from ethylene to argon is not likely to be a very important feature of the radiolysis since the ionization potential of ethylene (10.5 ev) is lower than the ionization potentials and the important electronic levels of argon. Iiebarle, Haynes, and Searles9 have examined the charge transfer and ionic condensation reactions taking place in gaseous ethylene-argon mixtures with a high-pressure mass spectrometer. MeiselsZ3 has recently reviewed the gas-phase radiolysis of ethylene and his own excellent series of papers on the subject. Of necessity, gas-phase results form the basis for much of the following discussion even though the danger of extrapolation to the liquid phase at -161” is obvious. It has been shomn6s16 that the butene and hexene yields in the radiolysis of liquid ethylene arise largely from the reactions of vinyl radicals. The very marked decrease in butenes and hexenes with decrease in ethylene concentration (Figure 2 ) indicates that energy transfer from argon to ethylene does not produce vinyl radicals efficiently. Nor is hydrogen abstraction by Ar+ important. Ar+
+ CZH4
+ArH+
+ CZH3
The Ar+ ion is certainly formed by the radiolysis of liquid argon. At high ethylene concentrations charge transfer will occur. Of the energetically possible charge-exchange reactions Ar+
+ CZH4 +Ar + C2H4+ + CzH3++ H +Ar + CzH2++ Hz ----f
Ar
(7)
it has been shown that reaction 7 predominates in the g a s b h a ~ e . ~ J +Production ~~ of a vinyl ion in reaction 7 followed by reaction 6 might account for the rapid in-
crease in acetylene yield as the ethylene concentration is decreased. The hydrogen atoms produced by reaction 7 would form ethane and butane via reactions 3-5. It is seen that this reaction sequence does not involve vinyl radicals. Another possible explanation of the increased yield of acetylene in the argon-sensitized radiolysis involves the formation of the molecular ion Arz+ by reactions 1 and 2. Both reactions 1 and 2 are favored by increased argon concentration. Kebarle, et U Z . , ~ place the recombination energy of Arz+ at 13.9 ev. In the gas phase, the dominant charge-exchange reaction is Arz+
+ C2H4
2Ar
+ C2Hz++ HZ
(8)
The most likely reaction of C2Hz+is charge exchange with ethylene to produce molecular a~etylene.~’
+
+
CZHZ+ CZH4 +CZHZ CZH4’
(9)
A number of objections can be raised to the “ionic mechanisms” suggested above for the enhanced production of acetylene in solutions containing more than 1% ethylene. Koob and Kevan4have suggested that added stabilization in the liquid phase causes the chargeexchange fragmentation pattern for argon ions with liquid propane to be more like the gas-phase pattern for krypton or xenon ions with propane than the gas-phase pattern for argon ions with propane. An alternative explanation of their results is that the effective ion was not Ar+ but Arz+ with its lower recombination energy. The most important objection to the “ionic mechanisms” is the long lifetime observed for intermediate ion complexes in the gas p h a ~ e . 9This ~ ~ ~lifetime ~ ~ permits condensation reactions to occur in preference to fragmentation. Thus, [C4H7+] appears to have a lifetime before decomposition CzH3+
+ CzH4 + [C4H,+] +CzHs+ + CzHz
of about 10-7 sec which, a t high pressures, permits condensation reactions [C4H7+] [CsHn+]
+ CzH4
+ CzH4
4[CBHU+] 4[Cs&s+l
(21) G . G. Meisels and T. J. Sworski, J. Phys. Chem., 69,815 (1965). (22) C. D. Wagner, ibid., 66, 1158 (1962). (23) G. G . Meisels, Advances in Chemistry Series, No. 58, American Chemical Society, Washington, D. C., 1966, p 243. (24) E. Lindholm, Advances in Chemistry Series, No. 58, American Chemical Society, Washington, D. C., 1966, p 1. (25) J. L. Franklin and F. H. Field, J. A m . Chem. SOC.,83, 3555 (1961),
(26) v. Germ& and 2. Herman, NucLeonk8, 19, 106 (1961). (27) F. H. Field, J. Am. Chem. SOC., 83, 1523 (1961).
Volume 71, Number 8
J u l y 1967
NORMAN v. KLASSEN
2414
to occur. However, Kebarle, et U Z . , ~ found that, in ethylene-xenon gas mixtures, the ratio of decomposition to stabilization for [CnH2,+] ions decreased with increased xenon pressure but increased with increased ethylene concentration. Moreover, they9 observed that reaction 6, in which CzH3+produces C2H2,may in part not involve a deactivatable complex. The C,H2n+chain, begun by CZH4+, grows above Clo quite readily. The CnH2,-2+, CnHtn-1+, and CnH2n+1+, begun by C2H2+, C2H3+, and C2H6+, respectively, usually ended at C4 or Cg. The occurrence of chains other than C,H2,+ seems to be minor a t low ethylene concentrations in liquid argon since the butene, hexene, and hexane yields are quite small. I n view of the uncertainty of the "ionic mechanisms,'' it is worthwhile to examine the possibility that the products observed in the liquid argon-ethylene radiolysis are largely due to exciton transfer from Ar* and that ions lead primarily to unanalyzed, high molecular weight products. A large variety of species of Ar* and Arz* are known to exist, many of which are longlived. Exciton transfer will lead to an excited ethylene molecule which may decompose. The product disAr*(Ar2*)
+ C2H4 +Ar (2Ar) + CzH4* CzH4* +C2H2
+ €32
(10)
+ C2H2
+ 2H
(11)
+C2H3
+H
(12)
tribution observed, in the present work could arise if reaction 10 resulted from an excited argon species which had a lifetime only long enough to effect energy transfer above 1% ethylene, if reaction 11 arose from very longlived excited argon species, and if reaction 12 were small. Unfortunately, a very important value, G(H2), is not known a t present. A rough approximation shows that the efficiency with which energy initially absorbed by argon is utilized to produce acetylene, GAr(C2H2), is 2.9 a t 7% ethylene and 3.5 at 20% ethylene if Figure 1 is correct. It could be much larger a t 20% ethylene if the interpolated maximum in the acetylene curve (Figure 1) has been underestimated. It seems unlikely that a GAr(C2H2)of this magnitude can be attributed entirely to Ar*. It is interesting to speculate further that the decrease in acetylene yields between 10 and 1% ethylene may be due to neutralization of argon ions by electrons before reactions 7 or 8 can take place. I n the liquid phase, the lifetime of those ion pairs which normally undergo geminate recombination is estimated to be lO-'O*' sec.28 It is possible to calculate a t what ethylene concentration the time for diffusion of an argon The Journal of Physical Chemiatry
ion to an ethylene molecule will be 10-lo sec. The irradiation of pure, liquid argon leads to the production of Ar2+ by reactions 1 and 2. Let t be the average time between encounters of Arz+ and C2H4 in a homogeneous solution. The diffusion coefficient D A ~a ~t +
112°K is about 1.2 X cm2 sec-1.10,29 Dc~H, was cm2sec-I. A value of 2.3 X estimated to be 5.3 X lo-* cm was estimated for the collision radius r A r z + and a value of 1.9 X cm for rCzH,. Using these values, t equals 10-lo sec in 1% ethylene. If so, below 1% ethylene, Ar+ and Arz+ ought to be neutralized before charge transfer to ethylene can occur. However, in liquid argon the cross section for collision is very low for free electrons of less than about 12 ev.30,31 It is not evident to what extent this will influence recombinati,on lifetimes. Between 1 and 0.046% ethylene, the product yields shown in Figures 1 and 2 were constant. These constant yields seem to preclude argon ions as precursors since geminate recombination of argon ions and electrons should compete with charge transfer a t these concentrations. It is believed that in this concentration range energy transfer is due almost solely to excited argon species, be they metastable or resonance states, atomic or molecular. Considerable data have accumulated concerning the decomposition of ethylene by direct photolysis, sensitized photolysis, electron swarms, and radiolysis. Much of this has been summarized by AiIeisels and S w o r ~ k i . ~This * evidence suggests that, for highly excited ethylene, process 11 is favored over 10 and 12. They estimated that in direct, high-energy radiolysis in the gas phase, reaction 11 accounts for about 87% of the dissociations of excited ethylene molecules. In the present work, the production of C2H2,C2He, and C4HI0 in solutions containing less that 1% ethylene is believed to result largely from excitation of ethylene by Ar* or Ar2* Ar* (Ar2*)
+ C2H4+Ar (2Ar) + C2H4* +Ar (2Ar) + C2H4++ e C2H4+ + e + C2H4*
followed by reactions 11, 3, 4, and 5 ,
This mech-
(28) G. R. Freeman and J. M. Fayadh, J . Chem. Phys., 43, 86
(1965). (29) J. Naghizadeh and 5. A. Rice, ibid., 36, 2710 (1962). (30) R. L. Williams, Can. J . Phys., 35, 134 (1957). (31) F. D.Stacey, Australian J . Phys., 11, 158 (1958). (32) G. G. Meisels and T. J. Sworaki, J. Phys. Chem., 69, 2867 (1965).
ARGON-SENSITIZED RADIOLYSIS OF ETHYLENE
2415
+
anism requires that G(C2H2) = G(CzH6) G(nproducts can be formed by ionic or freeradical chain reactions. Such condensation reactions have been C4HIO) as is found experimentally. It is difficult to estimate what fraction of the excited argon is produced postulated for C&4+ ions in solid ethylene.22 The rate constants for reactions such as 13 and 14 by neutralization of argon ions. It would be interesting to know how low an ethylene concentration is required H CzH4 +CzH5 (13) to affect the mode of energy transfer operative below 1% ethylene. Such information would be valuable in H CZH5 + CZH6 (14) determining the lifetime of excited argon atoms and the at low temperatures are sufficiently well k n ~ w n ' ~ J ~ mechanism of exciton transfer. However, proceeding to establish that reaction 13 predominates under to lower concentrations raises many experimental present conditions at 1% ethylene. It is suggested problems, an important one being solvent purity. above that, in solutions of less than 1% ethylene, the In liquid argon the cross section for collision is very ethane and n-butane result mainly from the disproporlow for free electrons of less than about 12 e v . 5 0 ~ ~ ~ tionation and combination reactions of ethyl radicals. It is quite conceivable that interaction of these electrons If this is true, the ratio of disproportionation to comwith ethylene bination under these conditions is 1.2. This value is much higher than the ratio determined by Dixon, eCZH4 eC2H4* Stefani, and Szwarca4for ethyl radicals at these temrepresents a significant fraction of the energy transfer peratures but corresponds more closely to the results in dilute ethylene solutions. Meisels and Sworskis2 of Watkins and Moser36 for ethyl radicals in solid have discussed the excitation of gaseous ethylene by an ethylene a t 63'K. electron swarm. In a condensed medium, decomposition of ethylene will only occur from the higher Acknowledgments. The author is indebted to many electronic levels and these will favor decomposition via colleagues for helpful discussions and to Mr. H. Ostfield, reaction 11. For this reason it is not possible in the who enthusiastically assisted in the experimental work. present study to distinguish between excitation of ethylene by Ar* and excitation by free electrons. (33) M.Sswarc, J . Phg8. Chem., 68, 385 (1964). In 0.43% ethylene, unanalyzed products (those (34) P. 8.Dixon, A. P. Stefani, and M. Sswarc, J . Am. Chem. Soc., containing seven or more carbon atoms) represent half 85, 2551 (1963). of the reacted ethylene if the results in Table I11 are (35) K. W. Watkins and H. C. Moser, J . Phy8. Chem., 69, 1040 valid a t lower conversions. Higher molecular weight (1965).
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Volume 71 Number 8 July 1967 I
