A time-resolved Raman microprobe for studying the splay Freedericksz transition in nematic liquid crystals

The authors describe in detail 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 '-cyanobiphenyl (5CB). From the transient data in the limit of small deformations, the authors have used the analysis of Pieranski et al (1973) 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 derived viscosity coefficients bracket those otherwise determined by classical light scattering and permittivity techniques. It is suggested that these data reflect a bending of the molecular director field even in the small distortions limit.