Role of Work Function in Electron Ejection by Metastable Atoms: Helium and Argon on (111) and (110) Tungsten

Abstract

A tungsten single crystal, cut to expose the (111) and (110) planes, was employed to examine the influence of the work function on electron ejection by beams of helium or argon metastable atoms. The (111) and (110) planes have work functions which differ by approximately 1.5 eV. The electron yield of the (111) surface, with incident helium metastable atoms, is about 1.5% higher than that of the (110) surface; with argon this difference is about 4%. The ejected-electron energy distributions are essentially the same for both crystal planes, but different distributions are obtained with helium and argon. The work-function values inferred from these measurements are the same for both crystal planes, but different values are obtained depending on which metastable-atom probe is used. These results differ from what has been previously predicted, and they are qualitatively accounted for by recognizing that the resonance ionized metastable atom strongly perturbs the surface electron cloud, causing the local vacuum potential to be shifted. The shift in vacuum potential is calculated for a one-dimensional model using a pseudopotential approximation for the resonance ionized metastable atom.