Physisorption energy of successive layers of H₂ molecules with a (1 0 0) surface of copper

Many theoretical studies already dealt with the interaction of one H₂ molecule with a metallic surface, but so far none aimed at the description of the interaction of additional layers of molecules, which is the goal of the present work. A model system consisting of a cluster of 13 Cu atoms and two/three H₂ molecules has been used to calculate with highly correlated wavefunctions the interaction of successive layers of H₂ with a surface of copper already covered by chemisorbed, dissociated H₂ molecules. A second H₂ molecule, approaching the Cu surface with different orientations, on top of a first one, is not dissociated and its physisorption interaction with the surface is calculated to be in the range 600-1000 cm^-1, approximately twice that of the first H₂ molecule. The physisorption interaction of H₂ molecules forming a third layer is estimated to be in the range of 60-100 cm^-1, which is much smaller than the interaction of the second layer with the surface but still larger than the H₂/H₂ interaction between a pair of isolated molecules. It is shown that this effect mainly results from the interactions with the ions H^- and Cu⁺ formed on the surface.