Transfer ionization and total electron emission for 25 keV amu-1 He2+ colliding on He and H2

We have investigated electron emission for transfer ionization (TI) and total electron emission, for He and H-2 targets. Double differential cross sections in angle and energy of the ejected electrons have been obtained covering the angular range from 0degrees to 175degrees and energies from 2 to 140 eV. Pure ionization differential cross sections, derived by subtraction, show an increasing contribution to the electron emission at low energies. The main observed structure, a cusp in the forward direction for electrons emitted with velocities close to the incident projectile velocity, presents a symmetric shape for TI. This fact, together with the similar shape of the cusp for both He and H-2 targets, suggest the presence of a two-electron intermediate state centred at the projectile leading to cusp formation in TI. The cusp for pure ionization, in contrast, shows a strong asymmetry, and a different shape for each target. The differential electron emission measurements allow us to estimate the cross sections for capture to doubly excited states resulting in autoionization of the projectile, which contributes to the TI channel. We obtained a much larger cross section value for H-2 than for He targets, in contrast with known cross sections for true double capture having the same order of magnitude for both targets.