THE INFLUENCE OF COADSORBATES ON THE OVERLAYER STRUCTURE OF PF3 ON PT(111)

We have used measurements of the infrared lineshape of the symmetric P-F stretch fundamental of PF3 chemisorbed on Pt(111) in the presence of coadsorbed Xe, CO and NH3 to probe the PF3 overlayer structure on Pt(111). In the absence of coadsorbates, PF3 forms two distinct phases on the surface at intermediate coverages as indicated by a splitting of the P-F stretch band into two components. The higher frequency component is attributed to molecules in dense two-dimensional islands and the lower frequency component to molecules in a lower density disordered phase. Coadsorbed Xe induces red shifts in the P-F stretch which depend on the number of Xe, atoms adsorbed at sites adjacent to the PF3 molecules. This enhances the sensitivity of the P-F stretch to the molecular environment and allows additional components of the P-F stretch to be resolved. Coadsorbed CO causes the higher frequency components of the P-F stretch to shift to higher frequencies and the lower frequency components to shift to lower frequencies. This is attributed to the coalescence of small PF3 islands into larger islands. Coadsorbed NH3 decreases the P-F stretch by as much as 27 cm-1 and affects the PF3 through a short-range interaction. Unlike Xe and CO, NH3 appears to break apart small islands of PF3 molecules.