STUDIES OF THIN SMECTIC C-ASTERISK FILMS BY X-RAY REFLECTIVITY

The smectic order in thin and ultra thin films (150-600 angstrom) of the chiral ferroelectric liquid crystal mixture ZLI-3654 is studied using the X-ray reflectivity technique. The spin cast films on various substrates (float glass, Si wafer, polymer coated glass, etc.) order spontaneously with smectic layering parallel to the substrate surface. A simple model which assumes a sinusoidal density modulation can describe well the experimental reflectivity profiles. The X-ray reflectivity provides a method to evaluate the phases of the structure factor. We demonstrate, for the first time, that it is possible to extract the molecular tilt angle, alpha, in ferroelectric liquid crystals from X-ray reflectivity measurements of ultra thin films. The temperature dependence of the tilt angle in the smectic C* phase are almost independent of the film thickness (down to approximately 200 angstrom) and are similar to those in the bulk.