Emission of Positive Ions Formed at Metal Surfaces (Surface Ionization)

Abstract

When atoms or molecules from a molecular beam or from a vapor impinge on an incandescent metal surface, they may evaporate partly as neutral atoms and partly as positive or negative ions after a mean residence time τ _a or τ _i respectively. Langmuir and Kingdon [420] first observed the formation of positive cesium ions on an incandescent wolfram surface. Subsequently, numerous studies of positive surface ionization have been conducted, since this effect opens interesting possibilities for ion production as well as for the detection of molecular and atomic beams. The developments in recent years have followed two main directions. On the one hand there have been attempts to extend the classes of atoms and metal surfaces for which surface ionization occurs and the Saha-Langmuir equation is valid. On the other hand, and to a much smaller extent, there has been an attempt to study the mechanism of surface ionization itself, as for example by direct determinations of desorption probabilities for ions and atoms, and for the charge-transfer probabilities