Interaction of H2 with (100)W. II. Condensation

The sticking coefficient of hydrogen on (100)W has been studied as a function of substrate temperature, hydrogen mass, and coverage. A detailed knowledge of the structure and kinetics of this system permits a quantitative determination of the sticking coefficients in the individual states. It is shown that for the more tightly bound atomic or β2 state, the sticking coefficient s is initially independent of coverage, indicating a precurser intermediate; in the molecular β1 state s follows a site occupation model, s = s0(1 − θ1). In both states s is almost independent of substrate temperature in the interval 78 Ts 300°K. Considerations of the mass and coverage dependence of s appear to indicate that condensation is limited by chemical bond formation of a precurser state rather than thermal accommodation. The precurser is postulated to be a molecular state singly bonded to a protruding tungsten atom. It is also observed that desorption of one isotope from the surface can be stimulated by the presence of the other isotope in the gas phase at temperatures where ordinary thermal evaporation of these states is negligible. This is shown to occur via a weakly bound intermediate state rather than a direct gas-phase replacement.