Abstract
The anharmonic frequency shifts of the phonons in monolayer solids of Ar/Pt(111) and commensurate Xe/Pt(111) are calculated with quasiharmonic perturbation theory and with self-consistent-phonon theory. Results from the two methods are in good agreement for Ar/Pt(111). Uncertainties in the frequency shifts for the Ar/Pt(111) modes are small compared to present uncertainties in the interaction model and in the experimental data. Anharmonic effects in the commensurate Xe/Pt(111) solid at 80 K are large, both because of the small adatom-adatom force constants in this dilated lattice and because of the large degree of thermal excitation. A version of the improved self-consistent-phonon approximation is used for comparison to experimental data, and it is found that the anharmonic terms cause changes on the scale of adjustments in Xe-Pt forces previously found necessary to achieve fits of the harmonic theory to the data. The contribution of static adsorption-induced dipole moments to the monolayer lattice dynamics is analyzed and is found to be negligible for Xe/Pt(111).
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