The role of the potential saddle in He2++H impact ionization

We compute the full three-dimensional momentum space distribution of ejecte d electrons resulting from alpha particle impact ionization of hydrogen. At low impact velocities the transverse-momentum distributions are shown to e xhibit strong oscillations with energy. For the longitudinal component, the momentum distribution peaks near the target at high energies, shifts towar ds the projectile centre at intermediate energies and then back towards the target at low energies. The shift of the longitudinal momentum distributio n towards the target at low energies gives the experimental signature of th e importance of potential saddle for impact ionization at low energies.