Adsorption kinetics of CO on Cr/Ru surfaces

We report on sticking coefficient measurements for CO adsorption at 90 K on differently prepared Cr layers on Ru(0 0 0 1). Using a supersonic molecular beam we study initial and coverage-dependent sticking coefficients, S0 and S(θ), respectively, as function of the kinetic energy of the CO molecules. For a pseudomorphic Cr monolayer (prepared at 700 K), S0 decreases from 0.93 for low kinetic energies (0.09 eV) to 0.82 at 0.4 eV, and stays unchanged for higher kinetic energies up to 2.15 eV. In contrast, on a Cr/Ru surface alloy (for Cr deposition at 1000 K) S0 closely follows the behavior of clean Ru(0 0 0 1), decreasing from 0.93 to about 0.6 for 2.15 eV. The coverage-dependent sticking coefficients at low kinetic energies show pronounced precursor-mediated behavior, while for higher kinetic energies a Langmuir-like adsorption kinetics is found, which is most prominent for the Cr monolayer.