| Abstract
| - The short-range structure of the neat fluid of fluoroform (CHF3) is investigated around the gas−liquid criticalpoint by measurements of spontaneous Raman spectra of the C−F symmetric stretching (ν2) and the C−F3symmetric deforming (ν3) modes. The spectra are obtained at reduced temperatures 0.96 ≤ Tr= T/Tc ≤ 1.06,the conditions of which permit isothermal studies in the gas, liquid, and supercritical states as functions ofpressure and/or density. As the density increases, the spectral peaks shift toward the lower energy side andspectral widths become broader. In the supercritical region, the amount of shifting shows nonlinear densitydependence, while the width becomes anomalously large. We analyze these density dependences alongvibrational coordinates by the perturbed hard-sphere model. The amount of shifting is decomposed into attractiveand repulsive components, and the changes of attractive and repulsive energies are evaluated as functions ofdensity and packing fraction, both of which are continuously varied by a factor of 50. For both vibrationalmodes, the spectral shift consists principally of the attractive component at all densities and temperatures.Here we evaluate local density enhancement as a function of bulk density by the use of the values of attractiveshift and by the use of dielectric analysis. Local density enhancement is highest at the bulk density, wherethe spectral width becomes anomolously broad. We analyze the density dependences of widths by comparingthe experimental values with calculated homogeneous and inhomogeneous widths. The experimental resultsagree with the theoretical calculations. It is elucidated that the main contribution to width is densityinhomogeneity. By comparing short-range structure studied in the present study with long-range structurepreviously studied by small-angle X-ray scattering, we found that the vibrational motion becomes significantlyaffected by the dielectric structure as the correlation length of the density fluctuation grows longer than thesize of a first solvent shell of CHF3. Under that situation, the ν2 mode is more sensitive than the ν3 mode tothe dielectric structure in the vicinity of a vibrating molecule.
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