Saddle-point electrons in proton-impact ionization of H: A classical trajectory study

Ionization in H(1s)+H⁺ collisions is studied in the relative collision energy range 4.0--25.0 keV by classical trajectory simulations. Improved total ionization cross sections are reported for the collision energies 2.0--10.0 keV. A thorough study of the qualitative dynamics of ionization has been carried out, with emphasis placed on the distinction between ``direct-impact'' and ``saddle-point'' mechanisms. A precise criterion for classifying trajectories according to these mechanisms is given, and by analyzing the trajectories in a quasistatic molecular frame it is shown that the distinction is established early in the collision. The development of position and velocity-space correlations in ensembles of trajectories of each type is studied.