Metastable fractions of fast He atoms produced by electron detachment from He− ions

The metastable fraction of He atoms produced by electron detachment from ${\mathrm{He}}^{\ensuremath{-}}$ ions in single collisions with ${\mathrm{H}}_{2}$, He, ${\mathrm{N}}_{2}$, Ne, and Ar has been measured in the energy range 25-400 keV by the beam-attenuation technique, using ${\mathrm{H}}_{2}$ as attenuation gas. These data support earlier results, pointing to the existence of a bound, long-lived doublet state of ${\mathrm{He}}^{\ensuremath{-}}$. Further, the electron loss and deexcitation cross sections for metastable He atoms in He, ${\mathrm{N}}_{2}$, Ne, and Ar have been measured individually at 50 keV, and it is shown that the deexcitation cross section is comparable to the electron-loss cross section.