The theory of phonon-induced infrared vibrational line shapes at surfaces.

Infrared radiation may be absorbed by a vibrational mode of an adsorption system at resonance. The homogeneous IR absorption line shape of a vibrational mode of an adsorbate is determined by its anharmonic interactions with other localized and delocalized modes of the adsorbate and substrate. These anharmonic interactions include both the adsorption potential between an adsorbed molecule or atom and the surface atoms, and the anharmonic couplings among the substrate atoms themselves at the adsorption site. Temperature dependent effects of substrate phonons and local mode self interactions on the line broadening and frequency shift are studied in a finite temperature field theory for both the internal stretching mode and admolecule-substrate stretching mode of a diatomic admolecule at an ontop site, and adatom-substrate stretching mode of an adatom absorbed at a bridge site. Modified phonon emission and elastic phonon scattering processes are described in a unified formalism. For the adsorption of an admolecule or adatom at an ontop site, a reduction factor is found which reduces all substrate phonon contributions induced by the anharmonicity of the adsorption potential on the line shift and line width of the low frequency admolecule-substrate stretching or adatom-substrate stretching mode. For the adsorption of an admolecule or adatom at a bridge site, reduction factors are found which reduce only some of the substrate phonon contributions induced by the same anharmonicity on the line shift and line width of the low frequency adatom-substrate stretching mode. A bulk Debye model and a semi-infinite elastic continuum model for the substrate are employed to describe the harmonic substrate behavior. The theory is applied to the extensively studied adsorption systems CO/Pt(111) and O/Cu(110). Qualitative agreement with experimental data is found in considering the partial contributions and simplicity of the model