Absolute cross sections by excitation of the 43S, 33P, and 43D levels of helium by electron impact: measurements at very low target-gas pressures

Abstract

From spectral line-intensity measurements cross sections are determined as a function of electron-impact energy E_el (100-1000 eV) in the helium pressure region from 3*10^-5 to 3*10^-3 Torr. In the evaluation of the excitation cross sections polarization of the emitted radiation and cascade is taken into account. Special attention is paid to cascade from directly excited nF-levels which do not have well defined multiplicities. Pulse-counting techniques and automatic data-handling were necessary in order to measure the very low light-intensities with reasonable statistics. Only cross sections measured at the lowest target-gas pressures were used to evaluate excitation cross sections. The resulting excitation functions which may be attributed to exchange excitation show the following energy dependences for E_el>100 eV without cascade corrections: 4³S, E_el^-.250; 3³P, E_el^-2.85; 4³D, E_el^-2.80.