POLARIZATION-FTIR-SPECTROSCOPIC STUDY OF THE 2D-GAS SOLID TRANSITION IN CO2 ON NACL(100) - REORIENTATION, CORRELATION FIELD BREAKDOWN, HEATS OF ADSORPTION AND 2D-CONDENSATION

Molecular reorientation and correlation field breakdown in the two-dimensional evaporation of CO2 on NaCl(100) were measured by polarization Fourier-transform infrared spectroscopy. In the ordered 2D-condensed phase with p(2 x 1) structure a correlation field exists, the CO2 molecules being tilted 34-degrees +/- 5-degrees from the surface. In the 2D-vapor the CO2 molecules lie flat on the surface, and no correlation field is detected. Both coexisting phases, clearly distinguished in the spectra, have constant density at constant temperature independent of coverage. From the variation of the 2D-vapor density with temperature the enthalpy of the 2D-evaporation was found to be DELTA-H(v)2D = 3.0 +/- 0.1 kJ/mol. The isosteric heats of adsorption of the 2D-gas and the 2D-solid were determined to 32.4 +/- 1.1 kJ/mol and 35.6 +/- 1.3 kJ/mol, respectively.