EFFECTS OF SUBSTRATE ANHARMONICITY ON THE VIBRATIONAL LINE-SHAPES OF ADSORBED MOLECULES

The vibrational line shape of a molecule adsorbed at a crystal surface is influenced by its interaction with phonons of the substrate. While the vibrational line shift and broadening induced by an anharmonic ad sorption potential have received considerable attention in the past, i n this paper we show that the anharmonicity of the interatomic bonds i n the substrate itself can also play a major role in determining the t emperature-dependent line-shape parameters of an adsorbed molecule. Wi th a nonperturbative treatment of the bilinear interaction between the admolecule and substrate, the anharmonicity of the crystal potential is shown to couple to the localized, molecular vibration. This is demo nstrated with a simple model calculation, in which only nearest-neighb or interactions between the admolecule and substrate atoms are include d. However, the formulas derived here may be readily extended to inclu de other physical features. Explicit calculations of the frequency shi ft and linewidth of the perpendicular, admolecule-surface vibrational mode are presented for the adsorption systems CO/Pt(111) and CO/Ni(100 ), respectively, and found to compare favorably with experimental data in the literature.