A molecular beam study of the adsorption dynamics of CO on Ru(0001), Cu(111) and a pseudomorphic Cu monolayer on Ru(0001)

We have investigated the sticking coefficient of CO on Ru(0001), a pseudomorphic Cu monolayer on Ru(0001), and a fully relaxed Cu(111) multilayer as function of kinetic energy, surface coverage, and surface temperature. At a low kinetic energy of 0.09 eV, the initial sticking coefficients, S₀, on these surfaces are determined to be 0.92, 0.96 and 0.87, respectively. In all cases, a decrease of S₀ with increasing beam energy was observed, yielding values of 0.58, 0.14 and 0.07, respectively, at a kinetic energy of 2.0 eV. For all three surfaces the coverage dependent sticking coefficients, S(Θ), display very characteristic behavior at low kinetic energies: S(Θ) remains more or less constant up to coverages close to saturation, indicative of precursor adsorption kinetics. However, characteristic minima at intermediate coverages are observed, which are correlated to the formation of well ordered adsorbate phases. For high kinetic energies we observe a transition towards a linear decrease of S(Θ) for Ru(0001). In contrast, for the pseudomorphic Cu monolayer and for Cu(111) we find an increase in the sticking coefficients at low coverages, followed by a decrease close to saturation. This behavior is attributed to adsorbate assisted sticking, that is, to a higher sticking coefficient on adsorbate covered regions than on the bare surface. The comparison between the pseudomorphic monolayer and Cu(111) reveals that the CO bond strength to the former is larger by 40%. The initial sticking coefficients for both surfaces are very similar at low kinetic energies; at high kinetic energies, S₀ for the pseudomorphic Cu monolayer is, however, larger by a factor of two.