Effect of Monolayer Adsorption on the Ejection of Electrons from Metals by Ions

Abstract

Electron ejection from metals by ions is shown in this work to be a surface-sensitive phenomenon which is profoundly affected by the adsorption of a monolayer of foreign gas on an atomically clean metal surface. Monolayer adsorption is shown to decrease the total electron yield, ${\ensuremath{\gamma}}_{i}$, primarily at the expense of the faster electrons ejected from the metal. Measurements of ${\ensuremath{\gamma}}_{i}$ show it to decrease steadily as the monolayer forms and to level off at a value characteristic of the covered surface when the monolayer is completed. Measurements have been made for the adsorption of ${\mathrm{N}}_{2}$, ${\mathrm{H}}_{2}$, and CO on tungsten. Electron ejection by all the singly-charged ions of the noble gases has been studied. It was possible to clean the tungsten surface in the presence of ${\mathrm{N}}_{2}$ and CO but not in ${\mathrm{H}}_{2}$. In ${\mathrm{H}}_{2}$ the tungsten surface, as judged from ${\ensuremath{\gamma}}_{i}$ measurement, was found to be covered with about 75% of a monolayer immediately upon cooling from 2000\ifmmode^\circ\else\textdegree\fi{}K. It is shown that the effect of monolayer adsorption cannot possibly be simply the result of change in work function. There is also evidence that electrons are ejected in non-Auger processes at higher ion energies when the surface is covered.