X-Ray Reflectivity of Free-Standing Smectic Films

The X-ray reflectivity has been measured of free-standing smectic liquid crystalline films, which provides information on the electron density perpendicular to the layers. An extraordinary sensitivity is reached due to the interference between the signal from the fringes arising from multiple reflections against the two parallel interfaces and the signal from the Bragg peak and its satellites. Experiments on two smectogenic compounds, 12CB and 8CB, provide examples of a commensurate and an incommensurate structure in which the film thickness is an integer/noninteger times the bulk periodicity of the smectic layer structure, respectively.