Broadening peculiarities of vibrational bands in the spectrum of carbon dioxide close to the critical temperature

Carbon dioxide Fermi doublet 1388/1285 cm⁻¹ Q-band broadenings and shifts measured using coherent anti-Stokes Raman spectroscopy are presented. Measurements were performed over a wide density range (0.1ρ _c < ρ < 1.9ρ _c ) during compression in the gaseous and condensed states at temperatures close to critical (the reduced temperature values were T _r = 0.995, 1.000, and 1.006). At densities above the ρ _c critical value, the width of Q-bands did not increase as the density grew, and the low-frequency Q band considerably narrowed up to the density value 1.7ρ _c . The main reason for this anomalous behavior was progressing narrowing of the spectral contribution caused by the special features of rotational exchange in the condensed state and not related directly to the closeness to the critical point. The refined critical broadening value was about 10% of the width for the high-frequency Q-band and 15% of the width for the low-frequency Q-band.