Comparison of classical, quasi-classical and quantal approximations for the ejected electron spectrum in 1.5 MeV/u F9+ + He collisions

The study of the ejected electron spectrum in intermediate energy ion - atom collisions has recently lead to a fuller understanding of these interactions and to a strenuous test of theoretical approaches. In particular, the details of the formation and resulting asymmetry of the electron-loss or -capture cusp, the magnitude, shape and position of the binary peak and the important role played by low energy, saddle point electrons, have been elucidated. We present here new theoretical results utilizing an extensive classical trajectory Monte Carlo simulation, an approach which makes use of the time evolution of a quasi-classical ensemble representing the wavefunction. These results are compared with the experimental measurement of the doubly differential ionization cross section performed at Kansas State University for zero degree ejection and with other classical (binary encounter and on-shell impulse approximations) and quantal perturbative approaches (PWBA, DWSPB, CDW-EIS and CDW-CDW). Close examination is made of the spectrum of electrons ejected at low energies, near the cusp and near the binary peak. In addition, comparison is made for ejection angles larger than zero degrees, a regime not yet explored experimentally for this system