Resonant Charge Exchange in Atomic Collisions. II. Further Applications and Extension to the Quasi-Resonant Case

Abstract

Further applications are made of the author's previous discussion of charge exchange in wide-angle (small-impact-parameter) collisions between simple atoms. Oscillatory charge exchange is predicted for H-${\mathrm{H}}^{\ensuremath{-}}$ and resonant double exchange for He-${\mathrm{He}}^{++}$ collisions. The case of near-resonant collisions is treated as a quasi-resonant process with damping, which arises from the lack of exact resonance. As in the resonant case, the frequency of oscillation can be predicted from the molecular model, and a qualitative estimate of damping is possible. A quality factor $Q$ is introduced as a convenient parameter for classifying quasi-resonances in charge-exchange collisions. A discussion is given to account for the presence of oscillatory charge exchange in proton-helium collisions and the lack of oscillations in ${\mathrm{He}}^{+}$-H encounters. Pronounced oscillatory behavior is predicted for the He-${\mathrm{Li}}^{++}$ system, but not for H-${\mathrm{He}}^{+}$ or H-${\mathrm{He}}^{++}$. A discussion is given of ionization by collisions between neutral atoms, such as H-H or H-He. The role of level crossing in resonant charge exchange also is discussed. The failure of the Landau-Zener theory to predict resonant charge exchange is shown to result from omission of quantum-mechanical phase factors. When a properly modified Landau-Zener model is used, oscillatory energy dependence is predicted in certain cases, in agreement with experiment and the present discussion. In general, oscillatory charge exchange is expected in high-$Q$ collisions.