Theoretical investigation of molecular water adsorption on the Al(111) surface

Abstract

Cluster calculations are used to theoretically investigate the interaction of water at low surface coverages with the Al (111 ) surface. Adsorption geometries, vibrational frequencies and binding energies are computed. Molecular water is found to adsorb at an Al on-top site and the computed vibrational frequencies agree well with data from HREELS experiments. However, in contrast to the experimental interpretation which assigns a 3720 cm−1 band to a hydroxyl stretch, the calculations indicate there is little water dissociation taking place at this stage and we reassign the 3720 cm−1 band to the symmetric OH stretch in water. We also propose refinements to the previous experimental assignments for some of the low frequency modes. A novel feature of the calculations presented is that they model adsorption induced surface relaxation effects on the Al substrate. Significant relaxation effects are found although, unlike for the adsorbate geometry, the substrate geometries do not converge rapidly with cluster size.