•Observed products of the reaction of O+ + CH3X (X = Cl, Br, I) are CH3X+ and CH3+.•CBr and CI bond cleavage to form Br+ and I+, respectively, are also observed.•CH3X+ and X+ are formed by direct charge transfer and do not require a change in multiplicity.•CH3+ formation proceeds through spin-allowed dissociative charge transfer or halide abstraction via a spin flip.•CH3+ formation by halide abstraction is strongly enhanced as the halogen is varied from Cl to Br to I.Velocity map images have been recorded for the charge transfer reactions of O+(4S) with CH3Cl, CH3Br, and CH3I at collision energies near 4 eV. The production of methyl halide cations occurs for all systems, with relative abundances ranging from 10 to 15% of the total reaction yield. In addition, small yields of the dissociative charge transfer products Br+ and I+ are observed. The velocity space images show that charge transfer is energy resonant, and occurs by long range electron transfer. The predominant product in all three systems is CH3+, which may form by dissociation of the nascent CH3X+ product on a quartet potential energy surface, or by halide ion abstraction from CH3X by O+ following a spin-changing transition from the initial quartet surface to a doublet surface. The kinetic energy distributions for CH3+ formation show both a sharp low energy feature that appears to describe the formation of CH3+ + O(3P) + X(2P), in addition to a higher energy component that describes the more exoergic channel forming CH3+ + XO(2Π). The relative intensity of this high energy feature increases as the halogen atom X changes from Cl to Br to I, a progression that correlates with the strength of the halogen atom spin–orbit splitting. This observation supports the conclusion that the formation of the CH3+ products via the most exoergic channels, i.e., CH3+ + XO(2Π), does occur through spin-changing collisions from quartet to doublet state surfaces that are mediated by spin–orbit coupling.

The D+ transfer reaction between O− (2P) and D2 to form OD and D− was studied using the crossed molecular beam technique at collision energies of 1.55 and 1.95 eV. The reaction appears to proceed by a direct mechanism through large impact parameters. At both collision energies, more that 70% of the excess energy is partitioned into product translation. At the lower collision energy, the OD products are formed in the ground vibrational state with a bimodal rotational energy distribution. At the higher collision energy, both v′ = 0 and 1 products are formed; ground vibrational state products have a mean rotational energy of 0.05 eV, corresponding to J′ ≈ 6. In contrast, OD products formed in v′ = 1 are formed with significant rotational excitation, with the most probable J′ ≈ 15. The bimodal rotational distribution is rationalized in terms of trajectories that sample two potential surfaces coupled by a conical intersection in the vicinity of the [O···DD]− intermediate that correlate to (OD−,D) or (OD,D−) products.