Insights from calculated phonon dispersion curves for an overlayer of H on Pt(1Â 1Â 1)

We have calculated the dispersion curves of H vibrational modes on Pt(1 1 1), using first-principles, total energy calculations based on a mixed-basis set and norm-conserving pseudopotentials. Linear response theory and the harmonic approximation are invoked. For 1 ML coverage with H atoms in the fcc hollow sites, we find modes at 73.5 meV and 142.6 meV, polarized, respectively, parallel and perpendicular to the surface, at the Γ point of the surface Brillouin zone. The degeneracy of the parallel mode is lifted at the zone boundaries, yielding modes at 69.6 meV and 86.3 meV, at the M point, and at 79.4 meV and 80.8 meV, at the K point. The changes in Pt surface force constant on H adsorption are found to shift the frequencies of the substrate surface modes from their calculated values for clean Pt(1 1 1). We also consider the case of subsurface adsorption for 2 ML of H and present vibrational frequencies of H atoms adsorbed in several subsurface sites. The appearance of new vertically polarized H modes in the range 98-106 meV (octahedral site occupancy) and 124-162 meV (tetrahedral site occupancy) is discussed in the context of experimental data.