Key words: critical point, carbon dioxide, vibrational spectroscopy, CARS-spectroscopy, spectral shift and broadening
The results of measurements of spectral shifts and broadening for carbon dioxide Fermi-dyad 1388/1285 cm-1 Q-branches are presented. They are obtained in a wide range of densities (0.1ρс < ρ < 1.9ρс) that was achieved at compression in dense gaseous and condensed states at temperatures near the critical point (reduced temperature Tr = 0.995; 1.000; 1.006). At densities higher than the critical value ρс, Q-branch line widths do not increase with density. Moreover, low-frequency Q-branch demonstrates considerable narrowing at density increasing up to 1.7ρс. Such anomalous behavior is due to a progressive narrowing of the spectral contribution caused by the features of the rotational energy transfer in condensed phase. It is not directly linked with an approaching to the critical point. Revised estimate of the critical broadening ranges from about 10 % of line width for the high-frequency Q-branch and 15 % for the low-frequency one.