Near threshold ionisation of helium by electron impact

An experimental study of near threshold ionisation of helium has been performed with an electron-impact spectrometer. The energy distribution of the two outgoing electrons was measured for excess energies of up to 6 eV, together with their angular distribution. These experimental results were compared to theoretical calculations and good agreement was found with the theory of Wannier (Phys. Rev.; 90:817 (1958)) but in a limited energy range: the predicted isotropy is only verified up to 0.8 eV, whereas the uniformity of the energy distribution is still valid to within 20% at 3.6 eV above threshold. (author)