Reactions of State-Selected Atomic Oxygen Ions O+(4S, 2D, 2P) with Methane

An experimental study has been carried out on the reactions of state selected O+(4S, 2D, 2P) ions with methane with the aims of characterizing the effects of both the parent ion internal energy and collision energy on the reaction dynamics and determining the fate of oxygen species in complex media, in particular the Titan ionosphere. Absolute cross sections and product velocity distributions have been determined for the reactions of 16O+ or 18O+ ions with CH4 or CD4 from thermal to 5 eV collision energies by using the guided ion beam (GIB) technique. Dissociative photoionization of O2 with vacuum ultraviolet (VUV) synchrotron radiation delivered by the DESIRS beamline at the SOLEIL storage ring and the threshold photoion photoelectron coincidence (TPEPICO) technique are used for the preparation of purely state-selected O+(4S, 2D, 2P) ions. A complete inversion of the product branching ratio between CH4+ and CH3+ ions in favor of the latter is observed for excitation of O+ ions from the 4S ground state to...