Abstract
The adsorption of methane on MgO(100) is examined using extensive first-principles DFT methods combined with inelastic neutron scattering. The results provide evidence that the structure of the first adsorbed methane layer is dominated by surface-adsorbate interactions, although adsorbate-adsorbate interactions still play a role. More specifically, the former selects for a structure wherein partially positive hydrogen atoms are oriented towards lattice oxygen sites, whereas the latter effect dictates that each methane molecule is rotated by 90� with respect to its neighbor. The structure of a second added layer is consistent with that predicted solely based on interadsorbate repulsion, although, as this structure is also favored by the electrostatic character of the surface, the roles of each effect can not be entirely evaluated independently. At the third layer, and presumably all higher layers, methane is structures so that the number of close H-H contacts is minimized. Quantum molecular dynamics simulations lend further support to the highly influential role of repulsive interadsorbate interactions in determining structure. Methane rotation at each layer is also studied.
