DYNAMICS OF A SMECTIC LIQUID-CRYSTAL - DIRECTOR FLUCTUATIONS AND REORIENTATION OF BIAXIAL MOLECULES

We report on the measurements of quadrupolar splittings and spectral d ensities in the mesophases of a partially deuterated smectogen -butoxy benzylidene-4'-n-octylaniline-2,3,5,6,-d(4) at two different Larmor fr equencies. A broad-band multiple-pulse sequence was used to simultaneo usly measure the Zeeman and quadrupolar spin-lattice relaxation times. Using a model that combines molecular rotational diffusion of an asym metric rotor and order director fluctuations, the measured spectral de nsities for the ring and C-alpha deuterons in the nematic and smectic A phases were accounted quantitatively. The contributions from directo r fluctuations were taken to second order for both J(1)(omega) and J(2 )(2 omega). A global target approach was used to derive the motional p arameters and a molecular prefactor for director fluctuations. It was found that the activation energy for spinning motions is lower than th at for tumbling motions of the liquid crystal molecule.