Triple differential cross sections for molecular hydrogen, both under Bethe ridge conditions and in the dipolar regime. Experiments and theory

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Abstract
High-accuracy absolute triple differential cross sections for ionisation of molecular hydrogen by approximately 4 keV electron impact have been measured in a coplanar asymmetric arrangement. Ejected electron energies were Eb=20 and 100 eV, and momentum transfer values ranged from K=0.1 au (dipolar region) up to approximately 4 au (impulsive region). They are compared with calculations performed within the framework of the first Born approximation and the impulse approximation. A perturbation treatment developed up to second order where the distortion of the ejected electron trajectory is taken into account is in reasonable agreement with the experiments only under impulsive conditions (large K and Eb values), due to the slow convergence of the series development. The factorised first Born approximation where the ejected electron is described by an orthogonalised Coulomb wave is in general good agreement with the experiments. For this model, the length form is found to be better than the velocity form.