PHONON-INDUCED DEPHASING OF THE INTERNAL VIBRATIONAL-MODE OF A DIATOMIC ADSORBED MOLECULE

The line shape of an internal vibrational mode of an adsorbed molecule can be determined in part by dephasing processes due to anharmonic in teractions with lower-frequency vibrational modes of the molecule and substrate. Perturbative and exact treatments of the bilinear coupling between the molecule and the surface atoms are compared, for the case in which the lower-frequency molecular vibration is degenerate with th e phonon continuum. Line broadening and frequency shifts are calculate d explicitly in a model in which the admolecule is constrained to move perpendicularly to the surface. It is found that the magnitude and si gn of the temperature-dependent frequency shift depend sensitively on the shape of the adsorption potential, and that the exact treatment of the bilinear interaction leads to substantial improvements, compared to the purely perturbative treatment. Limitations on the applicability of the exchange model for dephasing are discussed.