Molecular charge transfer: Experimental and theoretical investigation of the role of incident-ion vibrational states in N2+–N2 and CO+–CO collisions

Abstract

The effect of reactant-ion vibrational energy on total charge-transfer cross sections has been examined for 0.03–2.20 keV N2+–N2 and CO+–CO collisions. A multistate impact parameter treatment was applied to the examination of charge transfer between the above ions occupying various vibrational levels and ground-state neutral molecules. The resulting first-order coupled equations were solved numerically, and convergence of the calculated cross sections was achieved by systematic introduction of additional states to the wavefunction expansion for the total system. The calculated cross sections were weighted according to the vibrational distribution present in the laboratory reactant-ion beam formed by electron impact and the results compared with experiment in which we employed time-of-flight techniques to measure the forward-scattered neutral N2 and CO products. Reactant-ion vibrational state distributions were varied by changing the ionizing electron beam energy in a controlled electron-impact mass spectrometer ion source. The vibrational state population of the reactant-ion beam was estimated from absolute excitation cross sections spectroscopically measured for electron-impact ionization of the above molecules, the squares of the overlap integrals of the respective ground and excited ionized states (where needed), and total electron-impact cross sections for molecular ion formation. The major contribution to the charge-transfer cross sections arises from those reaction channels with large vibrational overlaps and small energy defects with respect to the initial channel. Multistate cross sections for those channels involving near-reasonant states with favorable vibrational overlaps are found to be closely approximated by the low-velocity limit proposed by Bates and Reid. Measured cross sections and their dependence on reactant-ion vibrational state distributions are correctly predicted by the multistate model below approximately 1.5 keV ion kinetic energy. There are, however, indications that competitive, inelastic, electron-transfer processes tend to occur at higher kinetic energies.