A MONTE-CARLO SIMULATION OF THE OVERLAYER STRUCTURE OF PF3 ON PT(111)

Infrared reflection-absorption spectroscopy of the P-F symmetric stretch region of PF3 chemisorbed on Pt(111) shows two absorption bands whose relative intensities change as a function of temperature and coverage. These changes in the infrared spectra are attributed to changes in the PF3 overlayer structure. Here we have carried out Monte Carlo simulations of this system based on the Ising model. We find that when we consider the interaction energy between a molecule and up its third-nearest neighbors, a constrained lattice gas model reproduces the experimental infrared spectroscopy results. One of the two bands arises from those molecules which condense into compact ordered clusters, and the other band from the rest of the molecules. Because the exact relationship between the P-F stretch frequency and the adsorbate environment is not known, the contribution of each molecule to a particular band is not straightforward and a statistical method for assigning molecules to each infrared band is developed. Though the model is very simple, the simulation results are in quantitative agreement with the experimental data.