Adsorbate-induced substrate relaxation and the adsorbate–adsorbate interaction

We formulate the theory of the relaxation of the substrate lattice induced by a chemisorbed molecule, using the quadratic expansion of the potential energy of a free surface. We assume that the adsorbate introduces local perturbations around the adsorption site and calculate the relaxation of the surrounding substrate atoms, the induced surface stress and the relaxation energy. When two adsorbates occupy nearby sites, the relaxation is incomplete, leading to an effective adsorbate–adsorbate interaction. We show that the interaction energy has a non-monotonic dependence upon the adsorbate–adsorbate separation, characteristic of each low-index surface. Using simple models of the substrate lattice potential, it is possible to deduce the strength of the perturbation. Good agreement is obtained with experimental data on the induced stress and the adsorbate–adsorbate interaction of CO chemisorbed on several closely packed metal surfaces.