Local Density Enhancement in Supercritical Carbon Dioxide Studied by Raman Spectroscopy

The polarized IVV and depolarized IVH Raman profiles of the Fermi dyad (1285 cm(-1) and 1388 cm(-1)) of supercritical (SC) CO2 have been measured along the isotherms 307, 309, 313, and 323 K in the reduced density range 0.04<rho*=rho/rhoC<2.04 (rhoC is the critical density). A band shape analysis of the dyad component shows that each one can be decomposed in two well-defined Lorentzian profiles in all of the temperature and density ranges investigated. These profiles have been assigned with the transitions of CO2 probing two kinds of environments. In each dyad peak, the Lorentzian profiles centered at higher frequency is associated with CO2 interacting through usual Van der Waals interactions with its nearest neighbors. The Lorentzian profiles centered at lower frequency in each dyad peak have been related to the transition of CO2 involved in a transient (CO2)2 dimer. The evolution with the density of the band center positions and bandwidths of the Lorentzian profiles shaping the lower and upper dyad components exhibits a nonlinear behavior along the near critical isotherm (307 K) for rho* ranging from 0.4 to 1.7. This behavior, although less pronounced, is still detected at higher temperatures. The deviation from linearity was interpreted as being due to an enhancement of the density that leads to a reduced local density excess Deltarho*=rho*loc-rho*bulk. Even if the spectroscopic observables involved probe the interactions in SC CO2 differently, we emphasize that the scaled spectral features are straightforwardly related to the Local Density Enhancement (LDE) phenomenon taking place in SC fluids (SCF). We show that the LDE effect can also be put in evidence from the band shape analysis of the weak satellite band situated at 1370 cm(-1) associated with the upper Fermi dyad transition of the 13C16O2 molecule (1% isotopic natural abundance). The asymmetric shape of the evolution with density of the Deltarho* found from our spectroscopic observables shows some similitude with that obtained in a recent MD simulation [Skarmoutsos, I.; Samios, J. J. Chem. Phys. 2007, 126, 44503.] and the possible inter-connection and difference with these calculations are discussed.