Cross sections and product kinetic energy analysis of H2O+–H2O collisions at suprathermal energies

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Abstract
The reaction of H2O+ with H2O is studied using a longitudinal geometry double mass spectrometer in the collision energy range Ec.m.=0.5–25 eV. Cross sections are reported for oxonium ion (H3O+) production and the symmetric charge exchange. Isotopic substitution is used to discern the product branches, including the separation of the two channels for oxonium ion production: (i) proton transfer to the target molecule; and (ii) atom pickup by the primary ion. The largest branching ratio is observed for the charge exchange channel, where no isotope effect is detected in the investigated energy range. Proton transfer exhibits the second largest branching ratio and accounts for more than 90% of the oxonium ion production throughout the measured energy range. The proton transfer cross section is dependent on isotopic substitution, while the atom pickup channel is too weak to make a distinct statement on its isotopic behavior. Product ion energies, determined by time-of-flight measurements, are also reported for each of the three channels. These measurements show that most (>95%) of the oxonium ions are formed via a direct, spectator stripping type mechanism while a small amount of reaction products exhibit considerable internal excitation. The charge exchange secondary ions are primarily formed at near-thermal energies in the laboratory frame. Small amounts of high laboratory energy product ions are also observed which at least partly originate through the dissociation of excited oxonium ions.