Double Electron Capture and Loss by Helium Ions Traversing Gases

Abstract

A beam of pure ${\mathrm{He}}^{++}$ ions was passed through a cell in which the gas pressure could be varied, and the emergent beam was examined for its ${\mathrm{He}}^{0}$ and ${\mathrm{He}}^{+}$ content. With helium and air in the cell, it was observed that ${\mathrm{He}}^{0}$ persists to pressures so low that it could not have been formed except by double electron capture by ${\mathrm{He}}^{++}$ in a single collision. In hydrogen the double capture event is much less probable.The analogous experiment was performed with a helium atomic beam, and at 250 kev and 450 kev in air ${\mathrm{He}}^{++}$, formed by double electron loss, was detected. In hydrogen and helium only upper limits for double electron loss could be established.These results, with values of the sums (${\ensuremath{\sigma}}_{20}+{\ensuremath{\sigma}}_{21}$) and (${\ensuremath{\sigma}}_{01}+{\ensuremath{\sigma}}_{02}$) measured in previous researches, made it possible to compute values of ${\ensuremath{\sigma}}_{20}$ and ${\ensuremath{\sigma}}_{02}$, as in the following table. Cross sections in units of ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ per atom A table of the six charge-changing collision cross sections for helium ions is presented. The equations needed to compute the change in the charge composition of a helium ion beam due to passage through a gas film are given.