Electron-Impact Desorption of Carbon Monoxide from Tungsten

Abstract

The low-energy electron impact desorption of CO adsorbed on W has been investigated by means of a field emission technique. It was found that the previously postulated binding modes of CO could be distinguished and confirmed by their differing desorption cross sections which are σvirgin=3×10—19 cm2, σvirgin to β≥10—19 cm2, σβ∼3×10—21 cm2, σα=3×10—18 cm2 for 80-V electrons. Evidence for thermal conversion of virgin states to both β and α states was obtained; electrons also seem to convert virgin to β states. In addition to desorption, dissociation with carbon formation occurred. Although quantitative estimates are difficult, the ratio of dissociation to desorption was estimated to be between 0.5 and 0.1, and seems to be least for α and greatest for β desorption. The results on carbon formation show that (at least) a large fraction of CO in the α and virgin modes is adsorbed with the C end of the molecule only. It is pointed out that electron desorption is a sensitive tool for probing chemisorption states and is able to reveal details like interconversion which are not readily detectable by other means.