Insights from calculated phonon dispersion curves for an overlayer of H on Pt(111)

TL;DR

This study uses first-principles calculations to analyze the vibrational modes of hydrogen overlayers on Pt(111), revealing detailed phonon dispersion curves and the effects of adsorption sites on vibrational frequencies.

Contribution

It provides the first detailed phonon dispersion curves for hydrogen on Pt(111) using first-principles methods, including surface and subsurface adsorption effects.

Findings

Identified vibrational modes at specific energies for hydrogen on Pt(111)

Observed shifts in substrate surface modes due to hydrogen adsorption

Detected new vibrational modes associated with subsurface hydrogen sites

Abstract

We have calculated the dispersion curves of H vibrational modes on Pt(111), 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, H atoms are assumed to occupy the fcc hollow sites. At the Gamma point of the surface Brillouin zone, we find modes, respectively, polarized parallel and perpendicular to the surface at 73.5 meV and 142.6 meV. 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 substrate surface modes are also found to shift in frequencies from their calculated values for clean Pt(111). We discuss the details of the changes in surface force constant on H adsorption on Pt(111). 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 modes in the range of 98 . 106 meV (octahedral site) and 124 . 162 meV (tetrahedral site) is discussed in the context of experimental data.