Dissociative ionization of methyl chloride and methyl bromide by collision with metastable neon atoms

Total and partial cross sections for Penning ionization of methyl chloride and methyl bromide by metastable neon atoms have been measured as a function of collision energy in the 0.040-0.15 eV range. The partial cross sections for the formation of CH3X+, CH3+, and CH2X+ (X = Cl, Br) show a decreasing trend, with different slopes, when the collision energy increases. The branching ratios indicate that the production of fragment ions is favored at higher energies. Based on new correlation rules that allow to estimate pure van der Waals but also charge-transfer contributions to intermolecular potentials, the anisotropy of the Ne*-CH3Cl interaction has been semiempirically estimated. Within the electron exchange model of Penning ionization, it is shown that the anisotropy of interaction, together with the anisotropy of electron distribution of the orbitals involved in ionization, is correlated with the behavior of the ionization cross sections and branching ratios as a function of collision energy. In particular, the presence of the attractive interaction at the two ends of the molecule is responsible for the decreasing energy dependence of the total ionization cross sections, while a softer repulsive wall around the methyl group is responsible for the increase of fragment ions when collision energy increases.