Spectroscopic studies of supercritical fluids

Vibrational relaxation is a sensitive probe of local stmcture and dynamics [5]. Vibrational lifetimes and absorption spectra of the asymmetric CO stretching mode (-1990 cm ) of W(CO)6 in siq)ercritical CO2 are reported as functions of solvent density and temperature [6]. Close to the critical temperature, the observables are density-independent over a 2-fold range of density. A cluster model can explain the data if small fixed-size solute-solvent clusters are formed in the range of densities around the critical density. If the size, and therefore the properties, e.g., local density and spectrum of fluctuations, are density-independent then the observables also become density-independent. Such a stmcture may form if there is a liquid-like 

An important point to note here is the separation of timescales between vibrational relaxation and density relaxation. Vibrational relaxation is expected to be faster than density relaxation [6]. Therefore, vibrational relaxation is a good probe of the density inhomogeneity in supercritical water that is present on a short timescale - short compared to density relaxation. The latter could occur in nanosecond timescales when close to the critical temperature. 

Simulation studies of solvation dynamics (SD) in SCW were reported for the first time by Rey andLaria [7]. Their studies indicated a biphasic decay of solvation energy, with an ultrafast decay, ratiier similar to the one observed for bulk water. This is rather surprising because here density is low and the extended HB network is non-existent, thereby eliminating the contributions fi om the libration and intermolecular vibration modes. Their results were subsequently corroborated by theoiy, which shows that the ultrafast component arises here from the fast rotational motion of small water molecules [8]. 

Recent simulation studies find that the SD in SCF, CHF3, and CO2 is also biphasic in nature. The fast component of the total solvation energy here decays with a time constant of about a picosecond. The other component relaxes at a rate with time constant in the tens of picosecond regime. A set of recent experimental studies employing a time-correlated single-photon counting technique has, however, indicated that the slow component has a time constant of about 50-70 ps, which is much slower than that observed in the above simulation, and probably was missed in later studies [9]. 

Some of the anomalies of supercritical fluids can be understood by using the idea of the Widom line. One can then relate, for example, file width of a Raman tine to the temperature- and density-dependent correlation length of the fluid. As we cross the Widom line at constant density, we would expect a sharp rise in the width of the Raman 

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JP Blitz, CR Yonker, RD Smith. Infrared spectroscopic studies of supercritical fluid solutions. J Phys Chem 93 6661, 1989. 

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