Anomalous adsorption kinetics. γ-Nitrogen on the {110} plane of tungsten

Abstract

This study extends previous work on nitrogen adsorption on tungsten single crystal planes on the [001] zone to the {110} plane at low temperatures. The functional dependence of the sticking probability s on coverage at crystal temperatures between 90 and 120 K shows anomalous behaviour, in that (a) the initial sticking probability is low (0.22) by comparison with the expectation from other planes on the [001] zone and (b)s increases with coverage to a maximum at around half the saturation coverage. Desorption from this plane indicates a γ(molecular) state desorption peak temperature of ≈ 190 K. The results are compared with kinetic data from other planes on the [001] zone and a simple kinetic model is proposed, which accounts for the major features of the sticking probability profiles. On a second {110} crystal evidence was found for significant β(dissociative) state formation, attributable either to the presence of impurities or steps on this crystal. Changes of secondary electron yield with coverage in the γ state are found to be complex, in contrast to work function changes.