Two-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 pt(T) &([a-z]+); 1 × 10−10 mbar in the temperature range 45 ≤ T/K ≤ 75 the 2D solid and the ideal 2D lattice gas coexist in a 2D thermodynamic equilibrium. The temperature dependence of the spreading pressure π(T) of the coexisting 2D lattice gas yields the enthalpy of the 2D sublimation [Formula: see text] In the two-dimensional sublimation reorientation of the CO2 molecules takes place; both the polar and the azimuthal molecular angles change.