A measurement of the cross section for production of He + (2 S ) ions by electron impact excitation of ground state helium ions

Abstract

A crossed beams method has been used to measure the cross section for the production of He$^+$(2S) by electrons incident upon He$^+$(1S) in the energy range from threshold to 750 eV. The cross section was measured in arbitrary units with an accuracy of $\pm$5% and at the higher energies its energy dependence is in close agreement with that calculated by means of the plane-wave Born approximation. Consequently the cross section has been normalized to the plane-wave Born cross section at energies between 435 and 750 eV to obtain the absolute magnitude. An independent estimate of the absolute magnitude was made to within $\pm$30% using only the experimental parameters, and the absolute cross sections given by the two methods agree within the experimental uncertainties. The normalized cross section is compared with cross sections given by the close-coupling approximation and various Coulomb-Born approximations. At the lower energies the normalized cross section is considerably smaller than any of the theoretical values, but the measurements are consistent with the existence of a finite cross section at threshold if the energy spread of the electron beam is taken into account.