Interaction of CO molecules adsorbed on metal surfaces

The adsorbate-adsorbate interaction of strongly chemisorbed molecules can be mediated by the adsorbate-induced distortion of the substrate lattice and by the perturbation of substrate electrons. We consider CO molecules adsorbed on closely packed transition metal surfaces, using a model in which the perturbation upon the substrate lattice is described by a small number of parameters. We find that the perturbation leads to an induced surface stress and to the adsorbate-adsorbate interaction. The pairwise interaction is almost always repulsive, and has a non-monotonic distance dependence, which is different from the oscillatory dependence of the interaction mediated by substrate electrons. We conclude that the lattice mechanism gives good qualitative agreement with experiment, in particular for the most thoroughly studied system, CO on Pt(1 1 1). (C) 2001 Elsevier Science Ltd. All rights reserved.