Investigation of a disordered adsorption system by electron reflection: H/Ru(001) at intermediate coverages

For dissociatively adsorbed H on Ru(001) at 170 K, a (1×1) LEED pattern only is found at all coverages. Below 60 K and around half a monolayer, a (2×2/2×1) structure becomes visible. Whereas the (1×1) LEED pattern corresponds to an ordered H layer at saturation (equal to H: Ru = 1:1) which, as shown previously, occupies fcc threefold sites only with a H-Ru interplane distance of 1.10 Å, the adsorbate is disordered below saturation at 170 K. We have applied elastic electron reflection at low energies (VLEED) to determine the geometry of these disordered layers as a function of coverage. Experimental VLEED curves were compared to electron reflectivities calculated with three methods for substitutionally disordered alloys (amplitude averaging (AAM), average t-matrix (ATA) and virtual crystal approximation (VCA)), and for ordered structures which would imply the importance of short range order even at 170 K. It is found that H adsorbs on threefold fee sites only throughout the whole coverage range, and that there is a clear tendency for the H-Ru interlayer distance to increase as a function of coverage by 0.2–0.3 Å to 1.10 Å at saturation. Comparing the approximations tested for disordered layers, ATA gives the best agreement at low coverages, while around half saturation best agreement with calculations for a (1×2) layer implies the possible prevalence of short range order even at the comparatively high measuring temperature. At very high and very low coverages the differences between the various approximations become too small for safe discrimination.