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

FRS是一种基于瞬态光栅（干涉条纹）技术的显微示踪方法，使用写入Ar^+激光束和探测He-Ne激光束。我们可以准确、快速地测定示踪分子在主体液晶中的扩散常数。为了测定表面积附近的扩散常数，即，在全内反射条件下，采用全反射棱镜作为池面和探测光之一，对双折射液晶4 '-正戊基-4-氰基联苯（5CB）进行了初步实验。我们发现，体扩散表现出近单指数衰减。另一方面，除了与体扩散相同的衰减之外，表面扩散还包括更快的衰减分量。因此，表面扩散比体扩散快得多。可能的解释如下：表面粗糙度引入了分子排列的无序和各种缺陷，这些缺陷加速了扩散运动。这种现象在金属的自扩散和杂质扩散中是众所周知的。

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.