CARS spectroscopy of carbon dioxide in the critical point vicinity

The transformation of the Q-band of the low-frequency 1285-cm^-1 component of the 2v₂/v₁ Fermi doublet of a CO₂ molecule is studied in the critical point vicinity (T_c = 31.03 °C, P_c = 72.8 atm) by the CARS method. CARS spectra were recorded by changing pressure isothermically from 48 to 120 atm at several temperatures in the range between 25 and 36°C. At the temperature above 29°C, the pressure dependences of the Q-band width pass through the maximum, which exceeds by 40% –50% the typical Q-band width in the liquid phase. The position of the maximum shifts to higher pressures with increasing temperature. The inhomogeneous broadening of the Q-band is interpreted based on the cluster microstructure of a supercritical fluid.