Interaction of arsine ith evaporated metal films

Abstract

The interaction of AsH3 with evaporated films of Fe, Ni, Pd, W, Ag, and Pb has been studied in the temperature range from -80 to ~200°. The extent of adsorption was expressed by X, representing the ratio of the volume of AsH3 adsorbed to the volume of krypton corresponding to a monolayer. Rapid dissociative chemisorption, which was accompanied by H2 evolution, occurred on the first three metals at -80°. The total AsH3 uptake was very much the same (X 1.7) for these three metals at this temperature irrespective of the different amounts of hydrogen which remained in the adsorbed phases. On W film, both dissociative and weak molecular adsorption took place at -80°while physical adsorption was the main feature of the AsH3 uptake on Ag and Pb films at this temperature. The structure of the adsorbed phase at -80°c orresponded mainly to AsH on Fe, AsH2 on Ml, and to a mixture of AsII2 and AsH3 on Pd and W films. The uptake of AsH3 on oxidized films of Fe, Ni, Pd, and W at -80°was more extensive than on the corresponding clean films. The adsorption on oxide sites gave rise probably to the formation of H3AsO" on the surface in which the hydrogen atoms were strongly bound to As atoms so that H2 evolution was not possible at such low temperature. The tarnishing of clean and oxidized films by AsH3 at temperatures h 30°occurred with constant energies of activation and the parameter X was regarded as a measure of the extent of arsenidation. The rate of arsenidation, at constant temperature and AsH3 pressure, became slower as X increased due to the decrease of the frequency factor (A) in the rate expression; the concentration of the reacting sites decreased with increasing thickness of the arsenide layer (X). The relationship between log A and X was quite linear in each case from which the rate of AsH» adsorption was determined.