Dynamics of monolayer xenon adsorbed on Pt(111)

Effects of substrate dynamics for xenon adsorbed on the ( 111) face of platinum are calculated using realistic xenon interatomic potentials, force constants for the platinum that reproduce the bulk lattice dynamics, and the Barker-Rettner semiempirical Xe-Pt potential. There is only a minor change in the platinum density of states when the coupling of harmonic normal modes of the xenon monolayer solid to the platinum substrate is included. Effects of the dynamic coupling on the xenon are small, apart from the previously known Brillouin-zone-center mixing, both for the simply commensurate root3R30degrees lattice and for a possible denser root19-commensurate unit cell with seven Xe atoms. The frequency spectra are calculated for four other hypothetical higher-order commensurate xenon lattices with average lattice spacings close to those for experimentally observed triangular incommensurate Xe/Pt(111) lattices, but assuming a static substrate. Compared to the root3 Xe lattice, the spectrum omega(perpendicular to)(q) of perpendicular Xe vibrations of the other lattices is shifted to lower frequencies and is greatly broadened. This significant calculated dispersion of omega(perpendicular to)(q) is counter to experiment for incommensurate monolayer xenon. Also, the coefficient of Brownian friction for a single Xe adatom on Pt(111) is evaluated.