Vacuum ultraviolet studies of electron impact of helium: Excitation of He n1P0 Rydberg series and ionization-excitation of He+nl Rydberg series

Laboratory measurements of the electron excitation cross sections of emission in the He Rydberg series (1s21S-1snp1P0) for n = 2,3,4 have been obtained. The cross sections were estimated by two methods: (1) analysis of calibrated laboratory spectra placed on an absolute scale using the H Lyα dissociative excitation standard and (2) analysis of relative cross section data using a modified Born approximation. A new method has been developed for the application of the Bethe-Born approximation using experimental relative excitation functions that does not require extrapolation in a Fano plot in the determination of the absolute cross section. The two methods agree to within 3% for the 58.4 nm line when allowance is made for cascade transitions. We find the direct excitation cross sections at 200 eV for the n = 2,3,4 members of the published electron impact experimental measurements of the two electron process of ionization-excitation impact are generally in agreement, especially on the energy dependence of excitation functions, but show significant differences with theoretical calculations