K-shell X-ray emission cross sections and ionization cross sections for light atoms and molecules by H +, H +2 and He + impact at 30–145 keV

Abstract

The cross sections for K-shell X-ray emission have been measured for 30–145 keV H +, H + 2 and He + ions incident on the gases CH 4, CO, N 2, Ne and Ar. Using values for the fluorescence yields derived from similar electron impact experiments, in which the same gas targets were used, the ionization cross sections for these K-shells by proton impact have been deduced. The values thus obtained show reasonable agreement with the Auger-electron measurments of Stolterfoht for the gases CH 4 and N 2 in the energy range of overlap, 50–145 keV. For the lightest elements, C and N (in CH 4 and N 2), the K-shell ionization cross sections agree reasonably with the Born theory for ionization at the highest energies used. For Ne and Ar, with larger K-shell binding energies, and at low energies for C and N, the experimental cross sections are much smaller than the ones calculated by the Born approximation. These results are better described by the ionization theory of Brandt including Coulomb repulsion, binding and polarization effects, and the binary encounter theory of Garcia, which includes Coulomb repulsion effects. The contribution of electron-capture processes to the K-shell ionization cross sections is discussed.