Heavy-Particle Energy-Loss Spectrometry: Inelastic Cross Sections for Protons Incident upon Helium

A method is described whereby absolute cross sections are obtained from inelastic energy-loss spectra induced by ions incident upon gaseous targets at energies of 25-125 keV. Inelastic energy-loss spectra of helium for proton impact are presented. Apparent differential cross sections and absolute total cross sections are obtained from the spectra. Total cross sections for the sum of the $1{}_{}{}^1{}_{}{}^{}S\to 2{}_{}{}^1{}_{}{}^{}S$ and $1{}_{}{}^1{}_{}{}^{}S\to 2{}_{}{}^1{}_{}{}^{}P$ excitations are reported, along with estimates of the relative contributions of each. Total ionization cross sections, total cross sections for inelastic scattering, apparent energy distributions of ejected electrons, and partial ionic stopping powers are also reported. The excitation cross sections show excellent agreement with recent calculations on the coupled-state approximation. The ionization cross sections show good agreement with results obtained by other methods, except at the lower impact energies. The apparent energy distributions of ejected electrons fall below the results of direct measurements and show poor agreement with the available theory.