Electron-energy-loss spectroscopy of H adsorbed on Rh(100): Interpretation of overtone spectra as two-phonon bound states.

Abstract

The vibrational properties of H adsorbed in the fourfold-hollow site on Rh(100) have been studied with high-resolution electron-energy-loss spectroscopy. At saturation coverage (one H per Rh atom), proper selection of the incident-electron energy and collection angle of the scattered electrons allow the detection of the symmetric (perpendicular) and asymmetric (degenerate parallel) fundamental transitions and five overtone transitions. The fundamental transitions exhibit significant frequency shifts as the H coverage is changed, establishing that H-H interactions are important in determining the local curvature of the adsorption potential-energy surface. Spectra of isotopically mixed layers (H and D) establish that significant dynamic coupling exists in both the parallel and perpendicular states at 1.0 monolayer coverage. The phonon bandwidths estimated from the isotope dilution experiments are 12--15 meV. The dynamic coupling is comparable to the local anharmonicity of the vibrational potential, and the overtones correspond to two-phonon bound-state excitations. A one-dimensional model of coupled, anharmonic oscillators is used to guide the interpretation of the overtone spectra. A qualitative interpretation of the spectra requires a proper accounting of the dispersion of both the fundamental and overtone excitations and indicates that the predominantly parallel polarized vibrations are extremely anharmonic, and the predominantly vertical polarized vibrations are only mildly anharmonic. Quantitative determination of the local anharmonicity requires a more detailed description of the H-H coupling than is currently available as the dispersion and anharmonicity are comparable.