Multi-adatom Interactions on Metal Surfaces

The determination of interactions in adsorbed layers has attracted much attention in recent years due to their importance for the crystal growth, interface formation and heterogeneous catalysis. Experimental information about two-, threeand many body interactions can be extracted, at least in a qualitative way, from adsorbate superstructures on single crystals observed by low energy electron diffraction (LEED) [l], the coverage dependence of the heat of adsorption in thermal desorption spectroscopy (TDS) [2], and most directly from (pair-. trio-) distribution functions and lifetimes for dissociation observed directly in field ion microscopy (FIM) [3]. So far, however, only few quantitative data have been reported [3,4]. The magnitude of adsorbate interactions can also be estimated from the comparison of order-disorder phase diagrams calculated in lattice gas models [5] and observed by LEED. Since the choice of interaction energies which explain a phase diagram need not be unique, the only reliable way to determine and understand such interactions is from total energy calculations. Theoretical calculations of adsorption systems have achieved a high level of sophistication in the treatment of single impurities [6] or completely ordered overlayers 171. Due to the low symmetry of the problem, mainly model calculations have been performed to determine the interaction between few