Analysis of e-e angular correlations in near-threshold electron impact ionisation of helium

Abstract

Using a coincidence technique in a coplanar geometry, triple differential cross sections (TDCS) for electron impact ionisation of helium are measured in the 0.5-2 eV energy range above threshold. As a few (LS Pi ) states (0<or=L<or=2) of the two outgoing electrons are obviously involved in the process, their respective intensities appear as unknown parameters in the theoretical TDCS as deduced in the frame of the Wannier theory. We show that almost all these parameters can be determined through normalisation to the measured TDCS in two specific geometries: in the first one the two electrons are kept in opposite directions while in the second one they remain symmetrical with respect to the incident beam. A comparison with the complete set of data is then performed. The measured TDCS are in agreement with the Wannier theory for the lowest energies (0.5 and 1 eV). The angular correlation predicted by this theory is confirmed but a precise determination of the angular correlation width cannot be achieved. At 2 eV the overall agreement becomes poorer, although some predictions of the Wannier theory still apply: in particular the measured TDCS are almost independent of the energy sharing between the two electrons. Finally specific measurements at 8 eV clearly show from considerations of symmetry that the Wannier theory no longer applies at this energy.