Structure of ethane monolayers adsorbed on MgO(100): Experiments and calculations

The structure of C₂{H}₆$ monolayers adsorbed on MgO(100) single-crystal surfaces has been explored by low-energy electron diffraction experiments. Two commensurate structures ${S}₁$ and ${S}₂$ are determiend. In the first geometry, the unit cell is oblique with one molecule and the coincidence cell contains eight molecules; the higher-density structure ${S}₂$ is rectangular (2 {}2 ×{} {}2 )R45^∘{} with two molecules per unit cell. Interaction potential calculations are performed to determine the adsorption sites and the equilibrium geometries of the molecules. In the S₂ monolayer structure, the ethane molecules have their C-C axis almost parallel to the surface with a herringbone geometry. Other stable geometries are also found at higher energy by about 30 meV where the molecules, alternately, stand upright and parallel to the surface. For the S₁ structure, the molecular axes of the eight molecules in the coincidence cell exhibit disorder due to the competition between molecule-surface interactions on nonequivalent sites and molecule-molecule interactions.