DEPHASING OF LOCAL VIBRATIONS IN A PLANAR LATTICE OF ADSORBED MOLECULES

We investigate anharmonically coupled high- and low-frequency excitati ons in a planar lattice of adsorbed molecules interacting with phonons of a crystal. Dephasing of high-frequency local vibrations by low-fre quency resonance modes is described in terms of the temperature Green' s function. The equations obtained are solved, first, with a small rat io of the anharmonic-coupling coefficient for high-and low-frequency m odes to the resonance width, and second, in the low-temperature limit. High-frequency spectral-line positions and widths depend on dispersio n laws and resonance-mode lifetimes. It is shown that lateral interact ions of low-frequency modes of adsorbed molecules can lead to a signif icant narrowing of high-frequency spectral lines, which is consistent with experimental data.