Energy transfer in adsorption. I. Experiments

The angle-dependent sticking coefficients $s\left(\phi \right)$ of ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, ${\mathrm{N}}_2$, CO, and C${\mathrm{O}}_2$ on W(100) and W(110) have been measured as a function of the angle of incidence $\phi$ using a simple molecular-beam apparatus. In general, $s\left(\phi \right)$ increases slightly towards grazing incidence, as predicted by classical models based on condensation via momentum transfer to the substrate. However, for ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ on W(110), $s\left(\phi \right)$ decreases markedly towards grazing incidence. For these systems, measurements of $s\left(\phi \right)$ and the total sticking coefficient $s$ as functions of gas and surface temperature have also been carried out. The experimental results are discussed briefly in terms of various mechanisms of energy transfer from the gas to the substrate.