Observation of a quasimolecular ionization window in low-to-intermediate impact velocity collisions ofHe+ions withH2and He

Abstract

The ionization of ${\mathrm{H}}_{2},$ leading to ${\mathrm{H}}_{2}^{+}$ recoil ions in nondissociative states, and the ionization of He by incident ${\mathrm{He}}^{+}$ ions were investigated in the 0.25--1.23-a.u. impact velocity range employing electron and target recoil-ion momentum-imaging techniques. Similarities as well as differences were observed in the electron velocity distributions from ${\mathrm{H}}_{2}$ and He targets. In both cases the data strongly suggest of the promotion of molecular orbitals formed between target and projectile within a rather well-defined projectile velocity window. In particular, the data support the promotion of the $2p\ensuremath{\pi}$ molecular orbital populated via rotational coupling. Outside this molecular promotion window, in particular at lower velocities, mechanisms of a different kind appear to dominate the ionization process, and electron momentum distributions are very dissimilar for He and ${\mathrm{H}}_{2}.$ Reduced projectile scattering cross sections, derived from measured target recoil-ion transverse momentum distributions, support these conclusions and point to the coexistence, at certain impact velocities, of different ionization mechanisms.