Rayleigh–Brillouin scattering studies of liquid and supercooled liquid o-terphenyl

Abstract

A comprehensive study of the Rayleigh–Brillouin scattering spectra of o‐terphenyl in the liquid and supercooled liquid state has been carried out. Anomalies in the Brillouin frequency and linewidth data are observed at the glass transition temperature (T_g) and at a temperature (T_K) about 36 K higher than the melting point. The anomaly at T_g is due to the absence of free volume associated with the freezing‐in of the density fluctuation. The longitudinal hypersonic frequency kink as well as the linewidth maximum and the dimunition of the transverse sound frequency at T_K are interpreted as due to the relaxation of translational order which disappears above T_K. The result of the depolarized Rayleigh scattering spectral study shows that cluster formation as previously proposed is unlikely. Hydrodynamic slip boundary conditions for molecular reorientation are found to be appropriate in liquid o‐terphenyl. Rotation about the short and long axes lying in the plane of the parent phenyl ring is found to be consistent with the experimental data, whereas rotation about the axis perpendicular to the plane of the parent phenyl ring gives too high a hydrodynamic volume. Also observed is quasidiscontinuity at T_g in the integrated intensity versus temperature plot for the central Rayleigh component of the V_V scattering spectral density. The Brillouin intensity is found to follow the temperature dependence of the configuration entropy, suggesting the occurence of the adiabatic configurational changes. The configuration changes are found frozen in below T_g, leading to a large residual entropy.