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 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.
|Physical Review A - Atomic, Molecular, and Optical Physics
|Published - 2007 May 2