Surface Strain versus Substrate Interaction in Heteroepitaxial Metal Layers: Pt on Ru(0001)

By studying the adsorption of CO on up to 30 layers of Pt deposited on Ru(0001) the influence of surface strain on the adsorption energy has been disentangled from the residual chemical interaction with the substrate. While the electronic influence of the substrate has largely vanished for three Pt layers, the effect of surface strain due to the 2.5% lattice mismatch of Pt and Ru remains initially intact and is only gradually released for $n\ge 5$ Pt layers. Electronic structure calculations confirm the experimental observations, in particular, the dramatic decrease of the CO adsorption energy on a single Pt layer which is caused by the strong Pt-Ru interlayer coupling.