The asymmetric (e, 2e) collisions at intermediate and high impact energy: success and limits of first-order models

Absolute triple differential cross sections for ionisation of helium are obtained in asymmetric coplanar conditions at three incident energies, 0.5, 1.5 and 8 keV, for an ejection energy of 100 eV and for a momentum transfer range extending from the dipolar to the impulsive region, with the objective of accurately testing first-order interaction models in each region. Extrapolation of the experimental generalised oscillator strength (GOS) to zero momentum transfer (K to 0) yields values that are in good agreement with the photoabsorption data. The first Born-Coulomb wave model is found to correctly describe the experiment only at 8 keV, while significant deviations are observed for the experiments at 1.5 and 0.5 keV. These deviations are attributed to Coulomb interactions in the final state.