Low frequency collective vibration modes of the CO 2 monolayer adsorbed on the NaCl(100) surface: interpretation of dispersion curves and mode polarizations

Abstract

The low frequency vibrational modes of the (2 × 1) commensurate CO 2 monolayer adsorbed on NaCl( 100) , obtained from recent inelastic helium atom scattering experiments, are interpreted on the basis of semi-empirical interaction potential calculations. The identification of the various dispersion branches is done in terms of coupled translational and librational modes of the admolecules by solving the dynamical matrix of the adlayer unit cell, for two perpendicular orientations of the transfer wavevector. Indeed this cell contains two translationally inequivalent molecules which can be arranged according to two perpendicular directions defining two domains. It is shown that hybridization of the branches is relatively strong, indicating no pure modes due to the significant influence of the lateral interactions. The hybridization between parallel and perpendicular modes is responsible for the observation of most of the branches below 10 meV using a helium beam probe.