Electron impact excitation of He(n1S), (n=4,5) at intermediate energies

The distorted wave polarized orbital approximation is applied to the excitation of 4 1 S and 5 1 S levels of helium. Results are presented for total cross sections over an electron impact energy range of 25 to 400 eV and also for small-angle differential cross sections at impact energies of 50, 60, 100 and 200 eV. These are compared with experiment and the accurate Born results of Bell et al. (1969). The introduction of explicit s-p coupling into the T matrix has a marked influence on the small-angle differential cross sections, producing a strong enhancement, and bringing them into close agreement with experiment at the highest energy considered.