ON THE NATURE OF DENSE CO ADLAYERS

We have performed Monte Carlo simulations in order to study the ordered structures formed by CO on Pt{111} at high coverage. The results are compared with LEED and infrared (IR) spectra. The calculations are based on a recently constructed potential energy surface for CO on Pt{111} and a CO-CO interaction potential deduced from the variation of the CO binding energy with coverage. Ordered adsorbate structures are obtained at θ = 0.5, 0.6, 0.67, and 0.71 in the simulations. The so-called compression structures (θ > 0.5) are stabilized by the energy lowering which results when CO molecules at the high density domain walls move away from the on-top sites because of the unbalanced repulsive CO-CO interactions. If this relaxation channel is blocked, disordered adsorbate structures occur. We present the resulting (θ, T) phase diagram and discuss its qualitative properties. The LEED data show ordered structures at θ = 0.5, 0.6, and 0.71, but, in contrast to previous results, or perhaps to the interpretation thereof, not at θ = 0.67. The IR data show that the compression structures still consist of CO molecules adsorbed on distinct surface sites. Finally, we discuss the changes in adsorbate structures which would result from variations in the CO-substrate potential energy surface and, in the light of these results, briefly look at the Cu{111}-CO, Ni{111}-CO, and Pd{111}-CO chemisorption systems. © 1990 American Institute of Physics.