ON THE LOCALLY ANISOTROPIC UNIAXIAL STOCHASTIC REORIENTATIONS OF MOLECULAR CORES IN SOME MESOMORPHIC PHASES

Based upon the model of random walk of a particle on an arc of the cir cular segment, it is argued that the measurements of aniline ring spec tral densities of motion, J1(omega0) and J2(2omega0) of 50.7-d, sample [Dong, R. Y., Phys. Rev. A42, 858 (1990)] represent the case of a str ong biasing of the ring uniaxial reorientation about its para-axis in the nematic and for the first time also in the Sm A phase. The para-ax es themselves seem to be subjected to the slower reorientational motio n about the direction of the macroscopic uniaxial symmetry. Both motio ns are thermally activated and seem to be additionally slowed down in the smectic A phase, accompanied, however, by an increase of the segme nt apex angle at the nematic-Sm A transition temperature. It is sugges ted, that the two spectral densities of motion are likely to undergo a discontinuity at the nematic-Sm A phase transition, yet only J1(omega 0) might be amenable to an experimental verification.