Auger Electron Emission from Clean and Carbon-Contaminated Mo

Measurements have been made of the Auger electron yield from polycrystalline Mo targets bombarded by mass-selected beams of ${\mathrm{Ar}}^{+}$ and ${\mathrm{He}}^{+}$ ions at normal incidence. The yields were measured for incident-ion kinetic energies of 50-400 eV. The experiments were conducted in ultrahigh vacuum to insure well-defined surface conditions. It was found that atomically clean Mo surfaces could not be produced by heating the targets to 2300°K, but that carbon impurities diffusing from the bulk of the sample accumulated at the surface and were not removed by heating. The carbon-contaminated surfaces gave reduced electron yields for both ions. This effect is thought to explain discrepancies among experimental results previously reported in the literature. The carbon contamination was removed by heating the target in ${\mathrm{O}}_2$, and it is argued that the resulting surface is atomically clean. The measured thermionic work function of the carbon-covered surface was 0.4 eV greater than the clean-surface value.