A measurement of cross sections for the 1S-2S excitation of He+ions by electron impact

Abstract

A crossed beam experiment has been performed to measure the function for 1S-2S excitation of He+ ions by electron impact. Absolute cross sections for direct excitations to the 2S state were deduced by fitting the measurements to Born's approximation at high energies after allowance had been made for the contribution from cascade. Just above threshold the cross section was 1.28*10-2 pi a02, which agrees closely with the previous measurement by Dance et al. (1966), but it is less than half the threshold cross section predicted by the close-coupling calculation of Burke et al. (1964). There is, however, much better agreement with the recent distorted wave polarized orbital calculation by McDowell et al. (1973). The present measurement was free from any evidence of error due to space charge interactions between the colliding beams and it was possible to examine structure in the excitation function associated with the formation of autoionizing states of helium.