Local order effect on molecular orientational dynamics: Time resolved non-linear spectroscopy

We have investigated the orientational relaxation in the isotropic phase of a liquid crystal (ZLI-1167) and in the liquid phase of the p-terphenyl (pTP) and p-quater-phenyl (pQP) molecules. The experimental results show new aspects of the liquid molecular orientational dynamics common to all three systems, independently from their chemical nature and state of aggregation. They are characterised by slow collective dynamics in good agreement with the Landau-de Gennes theory of phase transition and by a fast dynamics in fair agreement with the Debye diffusive theory. These new aspects are shown to be related to local order effects.