NONMONOTONIC LATERAL INTERACTIONS IN CO/PT(111)

CO/Pt(111) is undoubtedly one of the best-studied chemisorption systems. However, there remain peculiarities in its coverage-dependent behaviors, 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 with any attractive interactions. Surprisingly, however, the repulsive lattice interactions do not decay monotonically as a function of distance since the interaction in the second shell is consistently smaller in magnitude than that in the third. We discuss a qualitative relationship between these observations and the phase behavior of this system. In particular, this sort of lateral interaction easily explains the formation of large superlattices at coverages well below one third of a monolayer. We also compare these interactions to results from similar CO adsorption systems.