Dipolar dispersion laws and their contribution to dephasing of high-frequency collective vibrations in surface molecular ensembles

Dipolar dispersion laws for high-frequency collective vibrations in a triangular lattice of adsorbed molecules are studied with regard to their effect on the dephasing of these vibrations caused by coupling with a low-frequency resonance mode. Ferroelectric molecular structures with an arbitrary inclination angle θ to the surface-normal direction are considered. As shown for the case when the collective vibration band width ΔΩ is small compared with the resonance width of the low-frequency mode η, the spectral line shift and width are described well by the approximate formula of Erley and Persson [Surf. Sci. 218 (1989) 494] which makes no allowance for specific features of dipole–dipole interactions. In the opposite case, at ΔΩ≫η, the spectral characteristics in question manifest a radically different behavior dominated by the state density for the excitations concerned and substantially dependent on the angle θ. For instance, the spectral line width is proportional to η ln(1/η) at θ=0, takes non-zero values at η=0 and 47°<θ<90°, and is proportional to η1/4 at θ=90°. The dependences of the spectral characteristics on the ratio ΔΩ/(η+ΔΩ) calculated for various values θ may be applicable in determination of structural parameters for adsorbed molecular ensembles based on spectroscopic evidence. This inference is exemplified by the (2×1) phase of the CO/NaCl(100) system.