On the equilibrium vibrational renewal process: Dynamics of liquid crystal media by spectral profile analysis of probe oscillators

Abstract

We discuss here the distribution of the rapid time-dependent dynamics of the intermolecular potential in the liquid of 4-n-nonyl-4′-cyanobiphenyl (9-CB) in the temperature range 328–473 K through numerical Fourier transformation of the isotropic Raman profile of the CN stretching fundamental (Linear Response Theory). We consider the stochastic fluctuations of the intermolecular potential, coupling to the oscillator coordinate, to represent an equilibrium vibrational renewal process in terms of many independent, parallel relaxation pathways in the amorphous medium. The results predict persistence of glassy characteristics even to relatively high temperatures in the macroscopically isotropic liquid: Typical ranges of the relaxation times of the potential fluctuations of the liquid crystal medium stretch over 1–11 ps at 328 K but narrow to 0.50–2 ps at 473 K, reflecting thereby the persistence of dynamical molecular cluster formation at the clarification point and their gradual dissolution at kinetic energies but far above. The values of the most probable relaxation rates in the medium correspond to frequencies of 8 (328 K) and 33 (473 K) cm −1, which puts them into the domain of hindered inter- and intramolecular segmental-skeletal motions.