Fast molecular reorientations in liquid crystals

We report investigations on fast individual molecular reorientations in isotropic, nematic (N), and smectic-A (Sm-A) phases of chiral, achiral, and racemic liquid crystals. These motions, in the ${10}^{\mathrm{-}11}$-${10}^{\mathrm{-}10}$ s range, are probed by the degenerate four-waves-mixing technique and compared to results obtained by other techniques. In homeotropic N and Sm-A phases of achiral compounds and racemic mixture, the individual reorientation time about the long molecular axis is found to be unchanged down to the Sm-A→Sm-C transition, but increases by a factor of about 3 with a chiral compound. A quantitative comparison between theoretical prediction and experiment is presented. This result is in agreement with our previous work [Phys. Rev. Lett. 62, 3049 (1989)] and does not support the recent theoretical work by Brand et al. [Phys. Rev. Lett. 64, 1309 (1990)] as well as the results of the dielectric spectroscopy experiments of Kremer et al. [Phys. Rev. A 42, 3667 (1990)] performed at inappropriately low frequencies.