Anharmonic effects on monolayer phonons

The anharmonic frequency shifts of the phonons in monolayer solids of Ar/Pt (lll) and commensurate Xe/Pt(lll) are calculated with quasiharmonic perturb ation theory and with self-consistent-phonon theory. Results from the two m ethods are in good agreement for Ar/Pt(lll). Uncertainties in the frequency shifts for the Ar/Pt(lll) modes are small compared to present uncertaintie s in the interaction model and in the experimental data. Anharmonic effects in the commensurate Xe/Pt(lll) solid at 80 K are large, both because of th e small adatom-adatom force constants in this dilated lattice and because o f the large degree of thermal excitation. A version of the improved self-co nsistent-phonon approximation is used for comparison to experimental data, and it is found that the anharmonic terms cause changes on the scale of adj ustments 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 t o be negligible for Xe/Pt(111).