Transient dimer formation in supercritical carbon dioxide as seen from Raman scattering.

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/ρ_C<2.04 (ρ_C∼467.6 kg m^-3, ρ_C 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. [ABSTRACT FROM AUTHOR]