CPA theory with correlative effects: application to the νs(PF 3) stretch of PF 3/Pt(111)

We present an approach based on the coherent potential approximation (CPA) with allowance for small to moderate correlations among sites and apply it to a vibrational Hamiltonian for less than full coverage. The CPA averaged Hamiltonian is employed to interpret the IRAS spectra of the ν_s(PF ₃) stretch of PF ₃/Pt(111). Island formation is modeled by introducing a correlation between occupied and unoccupied sites (admolecules and "holes"), which results in an additional effective potential term in the averaged Hamiltonian. The correlative effect is thought to originate from the band formation of the PF ₃ orbitals at intermediate coverages; which perturbs the vibrational frequency of oscillators in near proximity. The model suggests a different interpretation of the split band structure of the ν_s(PF ₃) lineshape at intermediate coverages than has been previously purported. The parameters invoked in the model are either readily determined from, or consistent with those inferred from, the experimental system.