Absolute Differential Elastic- and Inelastic-Scattering Cross Sections in 25-140 keVHe++ He Collisions

Abstract

Data obtained as energy-loss spectra at angles from 0 to 7.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ rad (c.m.) with typical energy-loss resolution of 0.7 eV and angular resolution of 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ rad (c.m.) were used to calculate average differential cross sections ${〈\frac{d\ensuremath{\sigma}}{d\ensuremath{\Omega}}〉}_{\mathrm{av}}$ for elastic scattering, direct excitation of the individual $\mathrm{He}(n=2)$ states, and direct excitation of the ${\mathrm{He}}^{+}(n=2)$ states. The total cross section for excitation of the $\mathrm{He}(n=2)$ states obtained by integrating the differential cross sections was found to be 20\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$ at 25 keV (lab) decreasing to 7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$ at 140 keV. At incident lab energies below 100 keV the $\mathrm{He}(2^{2}S)$ and $\mathrm{He}(2^{3}P)$ states dominate the $\mathrm{He}(n=2)$ structure at angles close to zero, while the $\mathrm{He}(2^{1}P)$ state dominates at larger angles. The contribution of the $\mathrm{He}(2^{1}S)$ state remains below 25% at all angles and energies.