Low-frequency Raman-scattering study of the liquid-glass transition in aqueous lithium chloride solutions

Raman spectra of 15 and 30 mol % aqueous LiCl solutions were studied in the frequency range 3350 cm-1, from room temperature to 78 K. At high temperatures, the spectra are very similar to the Raman spectrum of pure water. As the temperature is lowered through the glass transition, the broad central peak that is typical of liquids evolves continuously into a narrow central peak plus a broad band centered near 60 cm-1, both with a depolarization ratio of 0.8. This evolution suggests a common origin for these two features, which are typical of glasses. Comparison of our results with the disorder-induced-scattering model of Martin and Brenig [Phys. Status Solidi B 64, 163 (1974)], which is often used for glasses, produced poor agreement for both the intensity and depolarization ratio. In an alternative approach, we combined Stephen's [Phys. Rev. 187, 279 (1969)] second-order Raman-scattering theory for fluids with generalized hydrodynamics and mode-coupling concepts. This approach gave predictions for the depolarization ratio in excellent agreement with our experimental results. It also produced a qualitatively correct description of the low-frequency Raman spectral shape in both the glass and liquid phases.