State selected ion–molecule reactions by a TESICO technique. III. H2+(v)+Ar→ArH++H, Ar++H2: Observation of enhanced charge-transfer cross sections at near resonance

Abstract

Reactions of the H2+ ions with Ar, i.e., H2++Ar→ArH++H [proton transfer, Reaction (1)] and H2++Ar→Ar++H2 [charge transfer, Reaction (2)] have been studied for the individual vibrational states v (v = 0–4) of the reactant ion at collision energies of 0.48, 0.77, and 1.24 eV, utilizing the threshold electron–secondary ion coincidence (TESICO) technique. At 0.77 and 1.24 eV, the cross sections for Reaction (1) are almost independent of the vibrational energy or slightly increase with increasing vibrational energy, wheras those for Reaction (2) show a resonance-like enhancement at v = 2 . At the lowest collision energy studied (0.48 eV), however, the enhancement of the charge transfer cross section at v = 2 is accompanied by a concurrent decrease in the proton transfer cross section, indicating the competitive nature of the two channels at this collision energy. These results are consistent with the trajectory–surface hopping model of the ArH2+ system by Chapman and Preston. Reaction (2) has also been studied at a higher collision energy of 19 eV, where the endoergic reaction with v = 1 is found to be considerably enhanced and has a cross section close to that for the near-resonant reaction with v = 2.