Abstract
Triple differential cross sections for electron impact ionisation of helium by 600 eV electrons have been measured and normalised to an absolute scale by extrapolating the generalised oscillator strength to zero momentum transfer. In this optical limit the triple differential cross sections can be expressed by the dipole transition matrix elements which are accurately known from photoionisation. The same cross sections have been calculated in the first Born approximation using quantum defect phaseshifts and standing-wave solutions for the ejected electron within a simple static potential approximation. Rather good agreement between this first-order theory and experiment is obtained for momentum transfer up to approximately 0.7 au, which contains a large fraction of all collisions leading to single ionisation. On the other hand, the experiments demonstrate the existence of higher-order effects for q>0.5 au, but a quantitative estimate of their importance for the calculation of triple differential cross sections is not yet possible.