Triple differential cross sections for the electron impact ionisation of helium at 35 eV collision energy

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Abstract
In a coincidence experiment with coplanar geometry, angular correlations of the two outgoing electrons have been studied for the ionisation of helium atoms by electrons with an impact energy 10.5 eV above the first ionisation threshold. The measured angular dependencies of the triple differential cross section are compared with calculated results of the Coulomb-projected Born approximation including exchange. Neither the measured data nor the calculated ones show the features that have been found at medium and high impact energies. The momentum transfer K0a loses its importance as a means of classification. For these low-energy collisions the amplitudes of direct scattering, exchange scattering and electron capture are of similar size and only the direct scattering amplitude has the momentum transfer vector K0a as a symmetry axis (in Born approximation). Therefore the angular dependence of the triple differential cross section are no longer symmetrical with respect to the momentum transfer. Rapid variations of the angular distributions for different scattering angles theta a of one of the two outgoing ionisation electrons indicate interference of the different scattering amplitudes. Moreover the measurements demonstrated that the angular correlations for fixed scattering angle theta a are independent of the ratio of the energies of the two emerging electrons.