Development of Totally Reflected Forced Bayleigh Scattering System and its Application to Molecular Diffusion near Liguid Crystal Substrate Surface

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FRS is a kind of microscopic tracer method based on transient grating (interference fringe ) technique using writing Ar^+ laser beams and a probing He-Ne laser beam. We can determine thediffusion constant of tracer molecules in host liquid crystals accurately and quickly. For the determination of diffusion constants near surface area, i.e., TFRS,we used a total reflection prism as one of a cell surface and the probing beam under a condition of total internal reflection.A preliminary experiment was made for a nematic liquid crystal, 4'-n-pentyl-4-cyanobiphenyl ( 5CB ) . We found that the bulk diffusion shows a nearly single exponential decay. On the other hand, the surface diffusion includes a faster decay component in addition to a decayas same as that of the bulk diffusion. Thus, the surface diffusion is rather faster than the bulk one. Possible explanation is as follows : Surface roughness introduces disorder of molecular alignment and various kinds of defects, which accelerate the diffusion motion. This kind of phenomenon is well known in self-diffusion and impurity diffusion in metals.