NONMONOTONIC LATERAL INTERACTIONS IN CO PT(111)/

CO/Pt(111) is undoubtedly one of the best-studied chemisorption system s. However, there remain peculiarities in its coverage-dependent behav iors, e.g., formation of large adsorbate superstructures at relatively low coverage, which are not yet understood. We present a lattice gas transfer matrix analysis of existing adsorption isotherm data for this system. Despite a statistical correlation between the various lattice interaction energies, we find that the isotherms are inconsistent wit h any attractive interactions. Surprisingly, however, the repulsive la ttice interactions do not decay monotonically as a function of distanc e since the interaction in the second shell is consistently smaller in magnitude than that in the third. We discuss a qualitative relationsh ip between these observations and the phase behavior of this system. I n particular, this sort of lateral interaction easily explains the for mation of large superlattices at coverages well below one third of a m onolayer. We also compare these interactions to results from similar C O adsorption systems.