Secondary Positive Ion Emission from Metal Surfaces

Abstract

Secondary positive ions ejected from surfaces of Mo, Ta, and Pt under bombardment by inert gas ions of low energy (<1000 ev) have been studied in high vacuum (≤10−8 mm of Hg of background gases) using a 60° sector field mass spectrometer with a 6-in. radius of curvature, the object being to explore this as a technique for investigating solid surfaces in general. Ion species characteristic of the base metal, of surface compounds, and of a few bulk impurities were found. Quantitative analyses based on abundance ratios of the ions are unreliable, however owing to the enormous and essentially unknown differences in ionization efficiencies for the various species. Application of the technique to the investigation of surface kinetics seems highly promising, and as an illustration certain observations on the formation of PtO3 on a Pt surface and its desorption at high temperatures are presented. Another possible application is to the study of sputtering, although some of the properties of secondary ions are shown to be quite different from those of sputtered particles in general. The secondary ions characteristic of the base metal comprise only a small fraction (0.01% for Pt) of the total sputtering yield, are principally singly charged and monatomic, have a low average energy (4–5 ev) and energy spread, and may result either from physical sputtering of the base metal or from the dissociation of surface compounds or both.