Water monolayers on MgO(100): structural investigations by LEED experiments, tensor LEED dynamical analysis and potential calculations

Abstract

LEED experiments and tensor LEED dynamical analysis of diffracted intensities are combined with semi-empirical potential calculations to determine the detailed structure of water monolayers adsorbed on MgO(100) single-crystal surfaces. The densities of the two experimentally observed solid phases are evaluated from LEED kinetic isotherms. A numerical procedure based on energy minimization at 0 K gives the positions and the orientations of the water molecules in various commensurate phases. The calculations show that the perfect MgO surface accommodates very stable flat monolayers with the oxygen atoms of the water molecules adsorbed nearly above the cation sites. These structures, which are energetically very close, differ only by the mutual orientations of the molecules in a plane parallel to the surface. The most stable configuration corresponds to a (1×1) geometry, with an adsorption energy per molecule of −945 meV, a value in fair agreement with both experiments and ab initio calculations. The other geometries including the experimental p(3×2) phase correspond to slightly higher energy values. The results of the calculations on the positions of the oxygen atoms in the p(3×2) water monolayer are used as reference structure for a dynamical tensor LEED analysis and it is shown that the agreement is good.