THEORY OF CO ADSORPTION ON MGO(100) - THE INFLUENCE OF INTERMOLECULARINTERACTIONS ON THE CO ORIENTATION

From periodic Hartree-Fock calculations, the mode of CO chemisorption on MgO(100) is found to vary with the coverage. At low coverage, the b est adsorption mode is predicted to be perpendicular to the surface. A t higher coverages, the interaction between the adsorbed molecules inf luences the ordering of the surface. At coverage theta = 1/2 monolayer , lateral effects dominate and CO is adsorbed parallel to the surface, bridging pairs of nearest-neighbor Mg atoms, At theta = 3/4, for the c(4 X 2) unit cell, the COs are adsorbed differently: whereas one thir d of the COs remains perpendicular to the surface, the two other third s are bent on the surface. In this case, we find two geometrical arran gements that are equally favorable from an energetic standpoint. In a first arrangement, the non-perpendicular COs differ: one of them is ti lted off-normal while the other one is parallel to the surface and bri dges Mg atoms as at theta = 1/2. This arrangement therefore distinguis hes three kinds of COs. It is very close to results obtained by potent ial energy calculations. In another model, the two non-perpendicular C Os are symmetrically positioned relative to the perpendicular one. Thi s model distinguishes only two kinds of COs in a 2:1 ratio and is very close to results derived from spectroscopies at low temperature.