Double differential cross sections for electron impact ionisation of helium

Double differential cross sections for electron impact ionisation of helium have been measured in a crossed-beam experiment for primary energies E 0 of 100, 200, 300, 400, 500 and 600 eV. Angular dependences of the cross sections are presented for the scattered (primary) as well as for the ejected (secondary) electrons. The energies E B of the ejected electrons were chosen to be 2, 4, 10, 20 and 40 eV. The cross sections have been put on an absolute scale by extrapolating the generalised oscillator strength for the primary electrons to the optical limit. The authors also used the fact that the integrated cross section of a scattered electron of a given energy loss E O -E A corresponds to the integrated cross section of a secondary electron with kinetic energy E B =E O -E A -IP where IP is the first ionisation potential of He, when double excitations and ionisation are ignored. Cross sections for double excitations and ionisation of He by electron impact are negligible (<2%) compared with single excitation and ionisation cross sections.