Collision-Induced Dissociation of NO+ and O2+ at Low Kinetic Energies: Effects of Internal Ionic Excitation

Abstract

The reactions resulting in dissociation of NO+ and O2+ upon impact with various atoms and molecules have been investigated in a double mass spectrometer of in-line geometry in the reactant ion kinetic energy range below 50 eV. Cross sections measured for these processes range from about 0.02 to 0.3 Å2. The dependence of the cross section upon kinetic energy is markedly affected by the energy of the ionizing electrons which produce the reactants. At the lowest electron energy, Ee = 13 eV, kinetic-energy thresholds are observed for the production of N+ from the NO+/NO reaction, and for O+ production from the O2+/Ar reaction, which are in excellent agreement with known adiabatic dissociation energies for the ground-state molecule ions. The production of O+ from the NO+/NO interaction is shown to involve a rearrangement process rather than simple collision-induced dissociation. At higher electron energies, the cross-section dependence plots exhibit structure which can be interpreted in terms of the participation of internally excited reactant ions. These results and related charge transfer experiments with NO+ indicate that the reactant ion beam formed by 20-eV electrons includes both vibrationally excited ground-state ions and NO+ ions in the δ3 electronic state. Isotopic experiments in which 15NO+ and N18O+, respectively, were impacted at low energies on normal NO demonstrate that the N+ and O+ products are derived in approximately equal amounts from the reactant ion and the reactant molecules. Products from the reaction of N18O+ with 15NO show that the major reaction process is simple charge transfer and that the extent of isotopic scrambling in this reaction is negligible. The implications of these observations for the mechanism of the dissociation reactions is discussed.