ADSORPTION OF POLAR-MOLECULES ON SUBSTRATES WITH STRONG ELECTRIC SURFACE FIELDS - FROM AGGREGATES TO MONOLAYERS

The adsorption of ammonia and water molecules on MgO(100) and NaCl(100) substrates, which generate strong electric surface fields, is studied for the first steps of aggregation (M)N., N = 2, ..., 5 and for the monolayer. Potential calculations show that water molecules form planar aggregates on both substrates and, at higher coverage, commensurate (2 x 2) monolayers on MgO or (4 x 2) bilayers on NaCl. In contrast, the competition between the electrostatic interactions due to the substrate and the lateral molecular interactions leads to a specific behavior of ammonia admolecules: on MgO they do not aggregate and can only arrange in incomplete monolayers, while on NaCl they form 3D aggregates with a tendency to 3D crystallization at higher coverage.