The polarized and depolarized Raman profiles of supercritical CO2 have been measured in the region of the ν2 bending mode (forbidden transition at about 668 cm−1) and for the Fermi dyad (1285 and 1388 cm−1) along the isotherms 307, 309, 313, and 323 K in a reduced density domain 0.04<ρ* = ρ/ρC<2.04 (ρC ∼ 467.6 kg m−3, ρC is the critical density). The spectral features associated with the ν2 mode (degeneracy removal of the mode and Raman intensity activation) are found to be due to the formation of transient complexes. This is supported by the spectral signatures predicted for parallel slipped dimer and trimers (cyclic and noncyclic) from ab initio calculations taking into account the frequency anharmonicity. The band-shape analysis of the Fermi doublet (observed in the spectral range of 1260–1400 cm−1) shows that on the subpicosecond time scale of the Raman spectroscopy, a tagged CO2 molecule probed two kinds of environment in its first shell of neighbors independent of local density enhancement phenomenon. The first one involves interactions of CO2 with surrounding molecules in the first shell whereas the latter is associated with a transient dimer formation. Finally, a broad band observed between the Fermi dyad (at about 1335 cm−1) is assessed from symmetry considerations and from its depolarization ratio as a further evidence of transient complex formation in supercritical CO2.
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