Ligand Effect in Hydrogen Adsorption on Epitaxial Nickel Films

The adsorption structure of hydrogen on ultrathin epitaxial nickel films in pseudomorphic arrangement on the unreconstructed lr(100) Surface is investigated by quantitative low-energy electron diffraction (LEED). This is highly interesting in the light of the fact that on Ni(100) adsorption is in hollow sites whilst on unreconstructed lr(100) it is exclusively in bridge sites. On Ni films thicker than I monolayer (ML) the adsorption is in 4-fold coordinated hollow sites as it is on Ni(100). This is in spite of the considerable tensile strain of the Ni film (+8.9%) and is interpreted by the dominance of ensemble effects in the hydrogen bonding. In contrast, on the I ML Ni film on lr(100), with iridium atoms being nearest neighbours to nickel atoms, there is occupation of both bridge and hollow sites of the nickel layer - a clear indication for the presence of ligand effects.