CORRELATION FIELD, 2-DIMENSIONAL 1ST ORDER PHASE-TRANSITION AND HEATS OF ADSORPTION IN CO2 ON THE NACL(100) CLEAVAGE PLANE

Infrared spectra of 13C16O2 adsorbed on the NaCl(100) cleavage plane from zero to monolayer coverage θ and the corresponding adsorption isotherms have been measured. At 0.3 θm ≲ θ ≲ θm, θm being the CO2 monolayer coverage, the ν3 vibrational band of adsorbed CO2 shows a doublet structure due to a correlation field, whereas at θ ≲ 0.3 θm a single band is observed. - The coverage and polarization dependence of the spectra as well as the shape of the isotherms between 80 and 95 K adsorbate temperature reveal a two-dimensional (2D) first order phase transition from a 2D low density phase into a 2D island-like solid. In the 2D low density phase a parallel orientation of the CO2 molecules with respect to the surface and a site symmetry C2v has been found.