Surface Processes on Evaporated Metal Films

Thin films of nickel, copper, platinum, palladium, and iron have been deposited on to a Pyrex glass substrate at various temperatures in a low pressure of krypton, It has been shown that when the substrate is maintained at 77 K during the evaporation process, quantities of krypton are incorporated in to the growing film. The highest value observed was for nickel; in one case, 5.4% of all the atoms in the complete film were krypton atoms. The process of krypton incorporation is believed to involve krypton atoms adsorbed at the growing surface of the film. The kinetics of the incorporation process in the region below 40% surface coverage have been interpreted in terms of the Langmuir theory of adsorption. Evidence has been obtained to show that the adsorbed state of krypton on the surface of a growing nickel film differs significantly from the state of classical physical adsorption. Release of the krypton held by metal films has been conveniently followed by using, in their preparation, krypton containing a proportion of the radioactive isotope 85Kr. On warming the films, it was found that most of the gas held by them at 77 K was lost in the temperature range 100K to 150 K, but that appreciable amounts were retained up to the highest temperatures studied (800 K). In terms of the thermal release of krypton, the films were found to behave in a manner similar to that observed for solid kryptonates. At each temperature, a stable concentration of krypton was attained, and no more krypton was lost until the temperature was increased. The quantity of krypton retained by a series of films prepared under widely different conditions of deposition, after the temperature had been allowed to rise to 295 K, has been shown to be fairly constant for each metal excepting iron, where insufficient data were obtained. The mean value at 295 K ranged from 0.25 atomic of the film for nickel, to 0.70 atomic of the film for palladium. The characteristics of krypton release during dissolution of nickel films in acid, and during oxidation of copper films at 443 K, have been interpreted to show that the retained gas is homogeneously distributed throughout the bulk of the films. The results which have been obtained for the oxidation of copper show that films produced by this technique are suitable for use as homogeneous kryptonates in chemical analyses. Films of nickel, palladium and platinum have been used in an investigation of the release of krypton during the processes of adsorption and catalysis. Some krypton was lost in all experiments, showing that the surface must be subject to considerable rearrangement during these processes. While thermal sintering owing to the heat of adsorption and catalysis has been eliminated as a possible explanation of the surface changes observed, the limitations of the data obtained have prevented the formulation of an alternative explanation