Surface ordering at the air-nematic interface. Part 2. A spectroscopic ellipsometry study of orientational order

摘要

Spectroscopic ellipsometry has been used to measure enhanced orientational ordering at the nematic-air interface of 8CB as the smectic A phase was approached by cooling from the isotropic phase. The depth profile of the orientational order has been estimated by calculating the ellipsometric parameters for a homeotropic uniaxial surface film on a uniaxial sub-phase using the Abelès matrix method. This showed that the depth of the enhanced orientationally ordered region was 10 nm at 0.5°C above the nematic-smectic A transition. This is substantially less than the thickness of the region with surface enhanced smectic order as determined by neutron reflection and a model of the surface structure consistent with both sets of results is proposed.