Variation of sticking probabilities with temperature and coverage, and desorption spectra for nitrogen on tungsten films

Abstract

From 78 to 150°K, and at coverages < 8 x 10 14 molecules/cm 2 , the sticking probability s of nitrogen on tungsten films (= 0·9) is independent of both temperature and coverage, whereas at temperatures above 150°K it is a function of both these variables. These results are interpreted in terms of a physically adsorbed precursor state with a heat of adsorption of ca . 3 kcal/mole. It is concluded that only a fraction of molecules colliding with the surface enter this state and that it is this fraction which determines the initial value of s at low temperatures. The decrease of s with increasing temperature above 150°K is a consequence of the inactivity of some planes, such as the (110), at the higher temperatures. Desorption spectra were obtained by warming films from 78°K to room temperature and recording the subsequent pressure changes as a function of time. From these data the distribution of site energies for the weakly held adsorbate (the αγ state) was evaluated, indicating a continuous distribution with heats of adsorption varying between 6 and 20 kcal/mole.