Energy and temperature dependent sticking coefficients of CO on ultrathin copper layers on Ru(001)

The chemisorption of CO on ultrathin copper layers on Ru(001) has been investigated using a supersonic molecular beam and temperature programmed desorption. We are comparing the adsorption kinetics of CO on clean Ru(001) with those on a pseudomorphic Cu monolayer and a fully relaxed Cu(111) multilayer (>40 layers) on Ru(001). For all three surfaces, a decrease of the initial sticking coefficient, S-o, with increasing beam energy was observed. For beam energies below 0.3 eV S-o is highest on the Cu monolayer, even though the binding energy of CO is significantly lower than on Ru(001). For higher energies S-o scales with the CO binding energy on the different substrates. The dependence of S-o of a low energy CO beam (0.09 eV) on the copper coverage is consistent with the Stranski-Krastanov growth mode of Cu on Ru(001) that is known from STM measurements. (C) 1999 Elsevier Science B.V. All rights reserved.