LATTICE GAS MODELS FOR MULTILAYER ADSORPTION - VARIATION OF PHASE-DIAGRAMS WITH THE STRENGTH OF THE SUBSTRATE POTENTIAL

The simple cubic lattice gas model with nearest-neighbor attractive interaction is considered for the case where the potential V(z), that an adatom at a distance z from the surface experiences due to the substrate, is V(z) = - A/z3. Exact ground state phase diagrams are obtained for different A, while the behavior at nonzero temperatures is studied both by Monte Carlo simulations and the molecular field approximation. We show that the detailed sequence of the layering transitions in the first few layers depends very strongly on the strength of the substrate potential: for strong potentials individual first-order layering transitions in layers 1, 2, 3, ..., while for intermediate potential strength the transition of layers 1 and 2 coincide whereas the remaining transitions are still distinct. For yet weaker substrate potentials the separate layering of the third layer occurs only for temperatures exceeding a triple point which disappears for still weaker substrate potentials where the layering of the first three layers coincide. These different patterns of behavior lead to remarkedly different adsorption isotherms. We discuss these findings in the context of recent theoretical predictions. © 1990.