Measurements of large angle inelastic scattering cross sections for electrons on helium

The ratios of the inelastic electron scattering cross sections for the n=2 levels of helium to the corresponding elastic scattering cross sections have been measured in a crossed beam apparatus over the 30 degrees to 150 degrees range for incident energies of 82 and 200 eV. In both cases, at angles greater than 40 degrees the ratio of the sum of the inelastic cross sections to the elastic cross section was constant at a few percent in qualitative agreement with a similar experiment in atomic hydrogen by Williams. The four n=2 states were observed to have markedly different angular dependencies at 82eV incident energies. Only the ¹S and ¹P states were important at 200eV; the ratio ¹S/¹P was very nearly 2 at all angles larger than 60 degrees . Within experimental error, over the range of variables studied, the observed cross sections agreed with those predicted by the first born approximation only for ¹P at angles less than 45 degrees at 82eV incident energy and for ¹S and ¹P at 30 degrees and 200eV incident energy.