GEOMETRY OF (NH3)N AND (H2O)N AGGREGATES ADSORBED ON WELL CHARACTERIZED MGO(100) AND SI(111)-(1X1)H SUBSTRATES

On the basis of semi-empirical potentials used to determine the structure of small gas phase clusters, we calculate the geometry of molecular aggregates (M)n(n = 2-5) physisorbed on ideal and clean corrugated surfaces. The competition between intra-adsorbate and adsorbate-substrate interactions is studied for the cases of ammonia and water aggregates adsorbed on H-passivated Si(111) and MgO(100) surfaces. It is shown that the adsorbate-substrate interactions are dominant for ammonia on Si(111) and for water on MgO(100) leading to planar aggregate geometries. However, the lateral interactions tend to prevent NH3 aggregation on the ionic substrate as they are repulsive and to favor compact H2O aggregates on the semiconductor. These features are in qualitative agreement with recent experimental results.