STRUCTURE OF ETHANE MONOLAYERS ADSORBED ON MGO(100) - EXPERIMENTS AND CALCULATIONS

The structure Of C2H6 monolayers adsorbed on MgO(100) single-crystal surfaces has been explored by low-energy electron diffraction experiments. Two commensurate structures S1 and S2 are determined. In the first geometry, the unit cell is oblique with one molecule and the coincidence cell contains eight molecules; the higher-density structure S2 is rectangular (2 square-root 2 x square-root 2) R45-degrees 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 nouequivalent sites and molecule-molecule interactions.