Energy Transfer in Ion—Molecule Reactions in the Methane System

Abstract

Cross sections for the reactions of CH4+ with CH4 have been measured by determining the pressure-dependent loss of CH4+ in the mass spectrum of methane produced by electron impact in the mass-spectrometer ion source. CH4+ loss occurs by reaction of low-velocity ions in the source and via collision-induced dissociation of CH4+ in the mass-spectrometer analyzer tube. The velocity dependence of source reactions permits a quantitative separation of these two processes. Experimentally determined cross sections have the magnitude and velocity dependence predicted by the Langevin cross section. This is in sharp contrast to results obtained by earlier workers who studied the reactionCH4++CH4→CH5++CH3.Here values of the phenomenological cross section usually fall below theoretical values and a reciprocal kinetic-energy dependence rather than velocity dependence had been proposed. Some of this earlier work has been repeated and confirmed in this paper. The difficulty in interpretation of the earlier work is shown to come from the assumption that CH5+ production is the only channel of reaction available in collisions of CH4++CH4. This assumption was based on the fact that CH5+ production appears to be the only reaction exothermic with ground-state CH4+ ions. We have shown that both internal excitation and energy transfer from kinetic to internal energy contributes to the production of excited CH5+ ions which can decompose into CH3+ and H2. This reaction makes up a major fraction of the ``missing'' cross section in the CH4+–CH4 reactions and the kinetic-energy transfer process involved in producing CH3+ accounts for the peculiar velocity dependence observed in the CH5+ production reaction. Thus it is demonstrated that the Gioumousis—Stevenson assumption of unit collision efficiency taken with the classical Langevin cross section provides an accurate description of these ion—molecule reactions.