Two-step mechanisms in ionization-excitation of He studied by binary(e,2e)experiments and second-Born-approximation calculations

An electron momentum spectroscopy study on ionization-excitation processes of He is reported. The symmetric noncoplanar $(e,2e)$ cross sections for transitions to excited ion states have been measured relative to that to the ground ion state at impact energies of 1240 and $4260\mathrm{eV}$. The experimental results exhibit a marked dependence on the impact energy. This provides strong evidence that higher-order effects are involved. Second-Born-approximation calculations are performed and their results are compared with the experimental ones. It is shown that the two-step mechanisms play crucial roles in the ionization-excitation processes.