A study of the energy loss spectra induced by impact of 23-125 keV protons on helium atoms

Results of a study of the inelastic energy loss spectra induced by the impact of protons on gaseous helium at energies of 25 to 125 keV are reported. The spectra were obtained by sending an accelerated beam of protons through a chamber containing the helium target gas, mass-analyzing the forward-scattered beam, then decelerating the beam to a low, well-defined energy for energy analysis and detection. The accelerating potential was varied to produce the spectra. An energy resolution of 2 eV was obtained. Apparent differential cross sections and absolute total cross sections were obtained from the spectra. Total cross sections for the sum of the 1¹S-2¹P excitations in the helium atom 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, but show little or no agreement with calculations on the distortion and first Born approximations. No other experimental data on these excitation cross sections are available for comparison. The ionization cross sections show good agreement with results obtained by other methods and show fair agreement with the first Born approximation at high impact energies. The apparent energy distributions of ejected electrons fall below the results of direct measurements and show poor agreement with the available Born and classical theories. The total cross sections for inelastic scattering show agreement with theory similar to that found in the case of ionization. No other experimental data on total inelastic cross sections are reported in the literature. The partial ionic stopping powers fall below the total stopping powers, the differences increasing at lower impact energies, whee charge exchange is more probable. This behavior is consistent with the difference in definition of the two quantities --Abstract, pages ii-iii