2-DIMENSIONAL SUBLIMATION OF CO2 ON NACL(100) WITH MOLECULAR-REORIENTATION AS REVEALED BY POLARIZATION INFRARED-SPECTROSCOPY

Two-dimensional sublimation of CO2 on NaCl(100), prepared by cleaving a single crystal in situ under ultrahigh vacuum, was observed by polarization Fourier transform infrared spectroscopy. At the transition pressure p(t)(T) < 1 x 10(-10) mbar in the temperature range 45 less than or equal to T/K less than or equal to 75 the 2D solid and the ideal 2D lattice gas coexist in a 2D thermodynamic equilibrium The temperature dependence of the spreading pressure pi(T) of the coexisting 2D lattice gas yields the enthalpy of the 2D sublimation Delta H-S(2D) = (3.0+/-0.1) kJ/mol. In the two-dimensional sublimation reorientation of the CO2 molecules takes place; both the polar and the azimuthal molecular angles change.