Pulse- and gamma ray-radiolysis of cyclohexane: Ion recombination mechanisms

Abstract

The products formed in the γ-radiolysis and pulse-radiolysis of gaseous cyclohexane have been interpreted in terms of the ion fragmentation, ion-molecule reaction, and ion recombination mechanisms. It is shown that the fragmentation of the parent ion is partly quenched at a pressure of 55 torr. The products resulting from homogeneous neutralization of the major unreactive ions, c- C 6 H + 12, c- C 6 H + 11, and c- C 6 H + 10, are deduced by comparing product yields at high dose rates, where the ions undergo homogenous neutralization, with yields in the gamma radiolysis, where the unreactive ions disappear mainly by reaction with impurities or neutralization on the wall. Ethylene and 1,3-butadiene are the major products resulting from electron neutralization of these ions. Fragmentation is strongly reduced when the neutralization process involves an atomic- or polyatomic- anion rather than an electron. For instance, addition of CCl 4 to cyclohexane results in a sharp drop of the yield of 1,3-butadiene, and a concurrent rise in the yield of 2-C 4H 8. In the gas phase, a value of 0.16±0.08 is suggested for the ratio of neutral excited molecule formation to ionization. In the liquid phase, it is seen that the relative importances of processes observed in the radiolysis are very different from the importances of these same processes in the far ultraviolet photolysis (8-11.6eV). In the radiolysis, where the mean energy taken up by the cyclohexane molecule may be higher than 11.6eV, solvent assisted fragmentation of the parent ion to give c-C 6H + 11, and the formation of triplet excited molecules in ion recombination processes are considered as explanations for the discrepancies. In the radiolysis, geminate neutralization of the vibrationally-relaxed parent ion to produce a singlet excited state of cyclohexane accounts for no more than about 25% of the cations.