Electron loss and deexcitation for fast, metastable helium atoms in gases

Total cross sections in the energy range 25-450 keV for destruction of metastable helium atoms (i.e., electron loss and deexcitation) penetrating ${\mathrm{H}}_2$, He, ${\mathrm{N}}_2$, Ne, or Ar gas targets have been measured by the beam-attenuation technique. The fast, metastable helium atoms were formed by electron detachment from ${\mathrm{He}}^{-}$ ions in either a strong electric field or under single-collision conditions in ${\mathrm{H}}_2$ or He gas. Apparent metastable fractions of the beams, which have also been extracted from the attenuation curves, depend strongly on the attenuation gas used. This shows that not only electron loss but also deexcitation is an important destruction mechanism for metastable helium atoms in these gases. The measured total destruction cross sections and apparent metastable fractions define upper as well as lower limits for both the electron loss and the deexcitation cross sections for metastable helium atoms. Generally, these limits lie close to each other. The experimental electron-loss cross sections for metastable helium atoms are in good agreement with semiempirical cross sections calculated in the classical impulse approximation of Bates and Walker. The experimental upper limit for the deexcitation cross section in a given gas is close to the electron-capture cross section for ${\mathrm{He}}^{+}$ in the same gas.