Sticking probabilities, heats of adsorption and redistribution processes of nitrogen on tungsten films at 195 and 290 °K

Abstract

A kinetic investigation has been made of the interaction of nitrogen with evaporated films of tungsten prepared under ultra-high vacuum conditions. A technique is described for the accurate determination of the sticking probability of gases on metal films; the value for a clean film a t 290 °K (0·75) is higher than values reported for filaments or sheets, which have relatively smooth surfaces, and is attributed to multiple collisions at the film surface. At 195 and 290 °K, a redistribution process has been characterized, which, as in the hydrogen-molybdenum film system at 78 °K (Hayward, Taylor & Tompkins 1966), is due to the desorption of weakly held adsorbate from the outer, more accessible surface, and readsorption on to the inner porous structure of the film. Heats of adsorption for this weakly held state varied between 14 and 24 kcal/m ole. The redistribution process has been quantitatively treated utilizing an exponential variation of the activation energy for desorption with coverage. At high coverage an activated conversion of weakly bound to strongly bound adsorbate has been identified, this process being distinct from the redistribution between outer and inner surfaces of the film.