Generalized Oscillator Strengths and Absolute-Zero-Angle Cross Sections for Inelastic Electron—Helium Collisions

Abstract

Absolute-zero-angle cross sections for excitation of helium to the 21P level have been calculated with the Bethe theory, using Lassettre's generalized oscillator strengths (for 11S→21P) normalized to the optical oscillator strength of Schiff and Pekeris. Absolute-zero-angle cross sections (in the 25–1000 eV energy region) for transitions to several other helium states were obtained, using (a) the calculated cross section for excitation to the 21P level and (b) already measured cross-section ratios (obtained from energy-loss spectra) or known optical oscillator strengths. These cross sections, which are not very accurate for small incident electron energies due to application of the Bethe theory, have been used to determine generalized oscillator strengths for the transitions 11S→31P, 41P, 51P, 61P, 21S, and 31S. It is pointed out that in the calculation of generalized oscillator strengths possible errors in the zero-angle cross sections used may cancel out, which is illustrated for the transition 11S→31P. ``Quadrupole'' matrix elements have been derived for the 11S→21S, 31S transitions. In most cases the agreement with both experimental and theoretical results of other investigations is very good.