Electron Energy Distributions from Ionizing Collisions of Helium and Neon Ions with Helium

Abstract

The electron-energy distributions arising from ionizing collisions of He and Ne ions with a He target have been examined under conditions such that the instrumental resolutions does not affect the distribution. Incident ions were in the range of 1-4 keV. The distributions consist of a sharp maximum near the zero of the electron energy with broader peaks occurring in the 20-40-eV region. Two broad peaks are detectable in the ${\mathrm{He}}^{+}$-He spectrum and six are found in the ${\mathrm{Ne}}^{+}$-He spectrum. The energy of the maxima of the prominent broad peaks have been found to fit a linear dependence on the inverse ion velocity over all or part of the ion range investigated. This dependence, when extrapolated to infinite ion energy, has yielded peak energies which correspond to previously identified auto-ionizing states. Where measurement is possible, these same peaks show a decrease in width at half-maximum also linear with the inverse ion velocity. A model is presented which attributes the observed behavior to a collision-induced excitation of auto-ionizing states followed by autoionization transitions which occur in the field of the ion.