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 trian gular lattice of adsorbed molecules are studied with regard to their effect on the dephasing of these vibrations caused by coupling with a low-frequen cy resonance mode. Ferroelectric molecular structures with an arbitrary inc lination angle theta to the surface-normal direction are considered. As sho wn for the case when the collective vibration band width Delta Ohm is small compared with the resonance width of the low-frequency mode eta, the spect ral line shift and width are described well by the approximate formula of E rley and Persson [Surf. Sci. 218 (1989) 494] which makes no allowance for s pecific features of dipole-dipole interactions. In the opposite case, at De lta Ohm much greater than eta, the spectral characteristics in question man ifest a radically different behavior dominated by the state density for the excitations concerned and substantially dependent on the angle theta. For instance, the spectral line width is proportional to eta ln(1/eta) at theta = 0, takes non-zero values at eta = 0 and 47 degrees < theta < 90 degrees, and is proportional to eta(1/4) at theta = 90 degrees. The dependences of the spectral characteristics on the ratio Delta Ohm/(eta + Delta Ohm) calcu lated for various values theta may be applicable in determination of struct ural parameters for adsorbed molecular ensembles based on spectroscopic evi dence. This inference is exemplified by the (2 x 1) phase of the CO/NaCl(10 0) system. (C) 2000 Published by Elsevier Science B.V. All rights reserved.