Benchmark differential cross sections for electron impact excitation of the n = 2 states in helium at near-ionization- threshold energies

In this paper, we present new normalized experimental differential cross sections for electron impact excitation of the n = 2 states in helium. The incident electrons have energies in the range 23.5-35 eV, while the scattered electrons are detected over the angular range 10-130deg. Corresponding theoretical results from our convergent close-coupling approach are also presented and in general are in very good accord with our measured data. Where possible, a comparison of the present experimental and theoretical results with those from previous measurements is also made. The case for the n = 2 levels in helium constituting a benchmark cross section data set is examined.