Heat of adsorption of Cu and Pb on hydroxyl-covered MgO(1 0 0)

The heat of adsorption and sticking probability are measured versus coverage for Cu and Pb adsorption on the MgO(1 0 0) surface pre-decorated with surface hydroxyl groups, and compared to prior results on OH-free MgO(1 0 0). The presence of OH increases the low-coverage heats of adsorption of both Pb and Cu, but causes these heats to increase more slowly with metal coverage as the metal particle size increases. Measurements by Auger electron spectroscopy show that the Pb island density increases and island size decreases at a given Pb coverage with OH present. This explains the slower rise in heat of adsorption with coverage when OH is present, since smaller islands of metal bind the next metal atom more weakly. The initial sticking probability of Pb is lower on the OH-dosed surface, which we attribute to a lower adsorption energy for the precursor state (isolated Pb adatoms). This contrasts with Pb atoms in tiny clusters, which bind more strongly to MgO(1 0 0) when OH is present.