TEMPERATURE-DEPENDENCE OF THE LAMBDA-3 ABSORPTION OF THE MONOLAYER CO2 ADSORBED ON NACL(100) - CONTINUOUS CHANGE IN ADSORBATE GEOMETRY AND VIBRATIONAL DEPHASING

CO2 on NaCl(100) at monolayer coverage forms an ordered overlayer with p(2 x 1) structure and pg symmetry, having two molecules per adsorbate unit cell. This results in a doublet nu(3) absorption due to the correlation field. The splitting is not observed for diluted isotopes, e.g. (CO2)-C-13-O-16 in the monolayer of (CO2)-C-12-O-16 at natural abundance (approximately 1%). The polarized (CO2)-C-12-O-16 doublet and the polarized (CO2)-C-13-O-16 singlet have been recorded simultaneously in the temperature range between 5 and 80 K with a high-resolution FTIR spectrometer. The main and the diluted isotope show different behaviour with respect to the temperature dependence of the lineshapes. Because of the strong dynamic dipole-dipole coupling the doublet peak maxima are first of all determined by slight changes of the tilt angle curly-theta (approximately 1.2-degrees) and of the intermolecular angle PHI (approximately 0.6-degrees) whereas the dephasing process dominates the temperature dependence of the singlet of the diluted (CO2)-C-13-O-16. According to the exchange model for vibrational phase relaxation the experimental data have been analyzed, considering the thermal excitation of external modes at 29, 49 and 61 cm-1 (measured by inelastic He scattering [15]), and the damping according to the elastic continuum model with an optimized set of anharmonic coupling constants. Crude agreement between measured and calculated data is achieved only.