Unexplained features of capture and ionization for ion–aligned-Rydberg-atom collisions

Observed but unexplained features, namely, oscillations in the capture cross sections and an unexpected increase in the ionization cross sections at lower velocities, are discussed using classical trajectory Monte Carlo simulated results for ion–aligned-Rydberg-atom collisions. The initial alignment (m=0, 1, and 2) dependence of the cross sections shows evidence of “capture through quasimolecular ion formation,” identified as the most likely cause for the oscillations. Spatial overlap, in addition to the velocity matching mechanism, is shown to play an important role in the collision process. The unexpected rise in the ionization cross section toward lower reduced velocities is explained qualitatively in terms of the multiple encounter model [Perumal and Tripathi, Nucl. Instrum. Methods B 143, 429 (1998)].