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Total and differential cross sections for simultaneous excitation of H(2s), H(3s) and He(ls2s) by electron impact in the presence of a photon field have been calculated in second-order perturbation theory. The characteristics of the results are interpreted in terms of their physical origin. The off-shell excitation cross section for the He(ls2s) state is found to be one order of magnitude smaller than that for excitation of the H(2s) state. This is consistent with our expection that atoms with higher dipole-polarisability should favour larger off-shell cross sections for transitions between s-states.

Investigating the relation between the asymptotic condition and the dynamic Coulomb correlation for single and multiple ionization the authors discuss a complete set of spatially separable N-electron final-state wavefunctions, satisfying multiple ionization boundary conditions. They apply these results to electron and positron impact ionization of atomic hydrogen in the energy range 54.4 and 250 eV on the basis of a parameter-free model formulated within the scope of the multiple scattering approach. A comparison between their results and available experimental data and alternative theoretical calculations are made and discussed. The role of exchange is also investigated. The contribution of exchange in the triple differential cross section is found to be virtually negligible whereas the asymmetry parameter A is significantly affected.