Electron Ejection from an Atomically Clean Tungsten Surface by Helium and Neon Metastable Atoms

Abstract

The absolute electron yield ${\ensuremath{\gamma}}^{m}$ of helium and neon metastable atoms incident on an atomically clean polycrystalline tungsten surface has been determined. The value of ${\ensuremath{\gamma}}^{m}$ for helium excited by 28-volt electrons is 0.306\ifmmode\pm\else\textpm\fi{}0.025, where $^{1}S_{0}$ and $^{3}S_{1}$ yields are equal within an experimental error of 20%; the value of ${\ensuremath{\gamma}}^{m}$ for neon is 0.215\ifmmode\pm\else\textpm\fi{}0.020. The contribution of each of the two neon metastable states to ${\ensuremath{\gamma}}^{m}$ has not been resolved in this determination. These results are consistent with present theory, which predicts that the absolute electron yield of slow rare-gas metastable atoms and ions incident on an atomically clean tungsten surface should be the same. Nitrogen and air contamination of the tungsten surface has been observed to reduce the electron yield of helium metastable atoms. This reduction is approximately 45% for nitrogen and 80% for air. Penning cross sections for the ionization of argon by helium and neon metastable atoms have been obtained from the yield data to an estimated accuracy of \ifmmode\pm\else\textpm\fi{}30%. They are: 9\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ for both $^{1}S_{0}$ and $^{3}S_{0}$ helium metastable atoms, and 11\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ for an unresolved mixture of $^{3}P_{0}$ and $^{3}P_{2}$ neon metastable atoms.