Ultrafast dynamics of nematic liquid crystals in the isotropic phase

Measurements of orientational relaxation over 6 decades in time have been made on the liquid crystal Methoxy Benzylidene butyl aniline (MBBA) using a Transient Grating Optical Kerr effect experiment (TG-OKE). The Slower dynamics have been shown to fit to Landau-de Gennes modified Debye Stoke Einstein Hydrodynamic equation. The faster dynamics show a power law behavior that is temperature independent for 43 degree(s) above the nematic-isotropic phase transition. The slower dynamics deviate from Landau-de Gennes behavior at the same temperature that the faster dynamics become temperature dependent. This is attributed to the domain size, the factor controlling the slow decay, becoming small enough that the local structure is disturbed. Two possible sets of processes are proposed for the power law dependence of the faster dynamics. A parallel process, the Forster direct transfer model, where there is a distribution of potential surfaces for the system to propagate along and the serial process (or Hierarchically constrained dynamics model) where some degrees of freedom are suppressed unless other degrees of freedom are in particular states. These results are compared to earlier work on pentylcyanobiphenyl(5CB). The same behavior is seen in both 5CB and MBBA.