Interactions between coadsorbed species on the Rh(111) surface as a function of the separation distance between them

The interaction energies as a function of the separation distance between two molecules of CO and between CO and H or O coadsorbed on the Rh(111) surface have been calculated by the Extended Hückel Method. Weak interactions of the order of a few hundredths of eV occur between the adsorbates via the metal lattice. The interaction energy is found to change its signs as the distance between the adsorbates increases. For first neighbor occupation sites the interaction energies are positive. Second and third neighbor interaction energies between CO and CO are, however, negative, the latter being stronger than the former. The interaction energies between CO and H or O are positive for first and second order neighbors and, in general, negative for third order neighbors. The computed interaction energies allow one to explain the observed LEED patterns of adsorbed CO at low temperatures. The compression of the p(2 × 2) LEED structure of CO to the (√3 × √3)R30° structure, induced by the subsequent exposure to hydrogen or oxygen, is explained on the basis of the more favorable interaction energies when CO and H, or CO and 0 are segregated.