Vibrational and orientational relaxations in complex liquids: The case of ME6N liquid crystal in the isotropic phase

In the context of the dynamical characterisation of liquid systems, in this paper we critically compare methodological approaches formulated for the determination of the dynamics in complex molecular liquids. We will show that the experimental vibrational relaxation of the ν(CN) mode of the ME6N liquid crystal in the isotropic phase is successfully reproduced, at long times (t > 3 ps), by the non-Markovian modelling of the perturbation modulation proposed by Rothschild et al.. The non-hydrodynamic behavior of the orientational relaxation times obtained for the same normal mode confirms the picture of pseudo-nematic domains persisting in the isotropic phase of this liquid crystal well above the nematic-isotropic (N-I) transition. Finally, the comparison of the time and frequency domain methods for the determination of dynamical parameters indicates their qualitative equivalence.