Observation of a quasimolecular ionization window in low-to-intermediate impact velocity collisions of He+ ions with H-2 and He - art. no. 012711

The ionization of H-2, leading to H-2(+) recoil ions in nondissociative sta les, and the ionization of He by incident 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 o bserved in the electron velocity distributions from 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 projecti le velocity window. In particular, the data support the promotion of the 2p pi molecular orbital populated via rotational coupling. Outside this molec ular promotion window, in particular at lower velocities, mechanisms of a d ifferent kind appear to dominate the ionization process, and electron momen tum distributions are very dissimilar for He and H-2. Reduced projectile sc attering cross sections, derived from measured target recoil-ion transverse momentum distributions, support these conclusions and paint to the coexist ence, at certain impact velocities, of different ionization mechanisms.