A classical model of singly charged ion collisions with Rydberg atoms in linear Stark states directed toward the oncoming ion beam predicts a bimodal distribution of final states resulting from electron capture. At a reduced velocity $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{v}{=v}_{\mathrm{ion}}{/v}_{\mathrm{Bohr}}=0.45,$ the calculated energy distribution of captured electrons peaks near the target binding energy
Ei, but has a second peak at energies roughly
Ei/4. Analysis of the impact-parameter dependence of the capture probabilities indicates that the second peak arises from three-swap capture events. The decrease in binding energy of the captured electrons is attributed to excitation via united-atom rotational coupling.
The classical trajectory Monte Carlo method has been used to simulate fully stripped ion-circular Rydberg atom collisions. We report that the mechanisms that govern the capture process for ions colliding with aligned Rydberg targets, namely direct and indirect velocity matching, alternate as to which is dominant. Previous experimental and theoretical results for singly charged ion-Rydberg atom collisions demonstrate that direct capture processes dominate at intermediate collision speeds. We have found that for highly charged ion-Rydberg atom collisions electron capture via indirect velocity matching is dominant at intermediate collision speeds and contributes up to 90% of the total capture cross-section.