Spectroscopic Probe of Free Volume Changes at Freezing: Raman Scattering from CS₂ in Cyclohexane

We study the reorientational dynamics of CS₂ molecules dissolved in cyclohexane, which undergoes a temperature-induced liquid to plastic crystal transition. Rotational correlation functions of the CS₂ are obtained from Fourier inversion of polarized Raman scattering line shapes. Just below the plastic crystalline transition, the short-time reorientation rate increases with decreasing temperature. The same measurements on CS₂ in n-propanol (which vitrifies at low temperatures) show a monotonic slowdown of short-time dynamics across the glass transition temperature. These results may provide the first direct evidence of the increase in local free volume accompanying the freezing of simple liquids.