Abstract
${\mathrm{He}}^{\mathrm{\ensuremath{-}}}$ is observed in the exit charge state distributions of He transmitted through a thin carbon foil over an energy range of 8--80 keV. The observed exit fraction of ${\mathrm{He}}^{\mathrm{\ensuremath{-}}}$ must be formed in the bulk foil or within several Angstroms of the exit surface and reaches a maximum of $2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ at ${0.7v}_{0}$ where ${v}_{0}$ is the Bohr velocity. The probability of forming the observed ${\mathrm{He}}^{\mathrm{\ensuremath{-}}}$ from its source population of ${\mathrm{He}}^{+}$ by two sequential electron-capture events decreases exponentially with increasing velocity. This behavior is similar to that of He transiting a Cs vapor, for which this dependence is the result of individual cross sections that collectively drive the charge-state distribution toward higher positive-charge states with increasing projectile velocity. No isotope effect in the exit charge state distributions of ${}^{3}\mathrm{He}$ and ${}^{4}\mathrm{He}$ is observed within experimental error.
Published Version
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