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J. Phys. Chem. 1981, 85, 2694-2698
in accordance with the observations, whereas CzH4 formation from the oxidation of thermal CzH5 should increase as [M]/[OZ] increases, contrary to our observations. At very high [M]/[OZ], a(CZH4) = 43b,so that we can conclude that C2H4production from thermal C2H5 oxidation is negligible and that all of the excess CzH4formation in the presence of O2 comes from the oxidation of hot CzH5 radicals. If we assume a 5% uncertainty in a(C,H4), then Dinthe upper limiting value for &kl/k2 is -5 X gledy and Calvert5 report that k2 = 6.9 X cm3/s, and we find 43a= 0.31. This gives an upper limiting value for kl of 1.0 x cm3/s, The most surprising result of this study is that the sum of the primary processes, 4% f$3b, is only 0.32. It is not clear what becomes of the remaining 68% of the absorbed photons which do not lead to photodissociation. A highresolution, gas-phase absorption spectrum has not been obtained, but the liquid spectrum indicates that the absorption is into a continuum.16 If so, it is difficult to believe that the energy is dissipated by emission, internal conversion, or collisional quenching. Yet no other photodecomposition paths appear to be possible. There are no other reported measurements for the primary quantum yields in the gas phase. Three mea-
+
(16)D. Porret and C. F. Goodeve, Proc. R. SOC.London, Ser. A, 165, 31 (1938).
surements of the quantum yield of I, formation have been reported in the liquid-phase photolysis of CzH61. Iredalel’ reported c $ ~= 1.0 and 1.2, respectively,at 366 and 300 nm. West and P a u P found 43a= 0.30 at 313 nm in hexane solution. Nortonlg used improved techniques to get 43a = 0.315 at 313 nm in pure liquid CzH& The values of both West and Paul and of Norton agree exactly with our value in the gas phase.
Conclusion We estimate that kl < 1 X cm3/(molecule s) and is probably considerably less than this value. This result is consistent with earlier estimate^.'^^^^*^ Therefore, this reaction is unimportant in the oxidation of C2H6radicals at 22 OC, except possibly at very low total pressures (
