Vibrationally Inelastic Low-Energy CO+- Ar Collisions

Relative differential cross sections for inelastic scattering of C${\mathrm{O}}^{+}$ by Ar have been examined with a high-resolution ion-beam apparatus in which a C${\mathrm{O}}^{+}$ beam interacts with a neutral-Ar beam and the energy, mass, and angular distribution of scattered ions are measured. Maxima in the inelastic energy-loss spectra occur at energies corresponding to C${\mathrm{O}}^{+}$ spectroscopic vibrational spacings. The probabilities of multiquantum vibrational transitions are found to increase with both reactant-ion kinetic energy and scattering angle. Cross sections for vibrational excitation at small scattering angles are adequately described by a semiclassical oriented-nonlinear-encounter model in which an impact-parameter treatment is used to estimate collisional energy transfer for a forced oscillator, and time-dependent wave functions employed to evaluate vibrational transition probabilities. The energy width of the inelastic peaks indicated that simultaneous vibrational-rotational excitations occur. Weakly inelastic processes are observed below the threshold for vibrational energy loss, corresponding to rotational excitation with the relative importance of rotational transitions increasing with decreasing energy and scattering angle.