A TIME-RESOLVED RAMAN MICROPROBE FOR STUDYING THE SPLAY FREEDERICKSZ TRANSITION IN NEMATIC LIQUID-CRYSTALS

We describe in detail here a Raman microprobe that is capable of acquiring high quality Raman spectra from thin films of nematic liquid crystal, and of recording the transient response to applied electric fields, of the signal associated with particular Raman active bands. The apparatus has been used to acquire very low noise flat baseline spectra from a 14.85-mu-m thick sample of planar aligned nematic 4-pentyl-4'-cyanobipheynl (5CB). From the transient data, in the limit of small deformations, we have used the analysis of Pieranski et al to determine effective viscosity coefficients over a range of temperatures for the major Raman bands associated with the rigid core and flexible alkyl chain sections of the molecules. The effective viscosity coefficients for the signals associated with the rigid core are self-consistent within themselves, whilst those associated with the flexible alkyl chain sections are consistently some 50-60% lower. These derived viscosity coefficients bracket those otherwise determined by classical light scattering and permittivity techniques. We suggest that these data reflect a bending of the molecular director field even in the small distortions limit.