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

The absolute electron yield γᵐ of helium and neon metastable atoms incident on an atomically clean polycrystalline tungsten surface has been determined. The value of γᵐ for helium excited by 28-volt electrons is 0.306±{}0.025, where ¹S₀ and ³S₁ yields are equal within an experimental error of 20%; the value of γᵐ for neon is 0.215±{}0.020. The contribution of each of the two neon metastable states to γᵐ 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 ±{}30%. They are: 9×{}10^-16 cm² for both ¹S₀ and ³S₀ helium metastable atoms, and 11×{}10^-16 cm² for an unresolved mixture of ³P₀ and ³P₂ neon metastable atoms.