Dissociation of Adsorbed CO by Slow Electrons

We have investigated ions emitted from molybdenum and tungsten surfaces in CO when the surfaces are bombarded by electrons. The surfaces, in the form of ribbons, can be cleaned at will by heating, and bombarded by an electron stream in which current and energy are controlled separately. The product ions are observed in a mass spectrometer whose envelope contains the experimental filament. Electron bombardment liberates only the O+ ion from adsorbed CO with any abundance; it may be 50–100 times more abundant from the surface than from space. No CO+ nor C+ ions are detected nor any negative ions of CO or its fragments. Carbon atoms remain on the surface. Significant amounts of F+ and Cl+ are also liberated from new filaments; these diffuse from the interior and are more tightly bound than CO, and disappear only after prolonged heating. Their surface abundance is so slight that they do not interfere with the O+ process. The threshold electron energy for liberation of O+ ions and the dependence of O+ ion current on electron current and energy are given. The method might be useful for studying kinetics of complex adsorption phenomena, although precautions are necessary to avoid perturbing the adsorbed film by the incoming electrons. The following phenomena are readily observed and probably understood: (a) The growth of a monomolecular film of CO from the ambient following a flash of the filament. (b) Competition for available sites by chlorine, fluorine, carbon, and CO. (c) Poisoning of the surface, for adsorption of CO, by carbon atoms freed by departure of O+ ions. A simple theory reproduces the experimental kinetics of adsorption and poisoning semiquantitatively.