Structures and adsorption energies of commensurate rare-gas monolayers on MgO(100)

Abstract

Low-energy electron-diffraction (LEED) measurements of rare-gas monolayers adsorbed on a square-symmetry MgO(100) substrate allow the structure and the isosteric adsorption heat to be determined. The argon monolayer exhibits a typical frustration phenomenon induced by the troughs along the [110] Mg ion rows on the MgO surface. This frustration leads to a 2×n overlayer structure which evolves from n=3 to n=4 with increasing coverage. For Kr, the coexistence of the 2×n and hexagonal solid phases is observed. Finally, for Xe, LEED shows a hexagonal structure only, with a unidirectional disorder that appears to be induced by the square symmetry of the substrate. Potential calculations are performed to interpret these results by determining the more stable structures and evaluating the isosteric heat of adsorption within the framework of a simplified dynamical treatment of the monolayer atoms. The calculated stable structures are in good agreement with the experimental results for the three rare-gas species. The calculations give a better understanding of the experimental observations, especially by explaining the evolution of the 2×n Ar monolayer structures and the relative stabilities of the different structures of the Kr and Xe monolayers, due to the nonplanar nature of the adsorbate layer.