Diffuse x-ray scattering in specular direction: Analysis of a wetting film.

X-ray diffraction measurements within the region of total external reflection from a thin wetting film of liquid CC₁₄on a Si/SiO₂Substrate are presented. The in-plane structure of the liquid surface yields a peak in the diffuse Scattering at the specular position. Since there is no way to separate specular and off-specular contributions, the data were analysed simultaneously, which leads to a consistent set of interface parameters. Although the obtained power spectral density function of the liquid surface is very similar to that used in the case of free capillary waves, a modified cut-off has to be introduced. The difference results from the constrained thin-fllm geometry and is therefore caused by the interaction with the underlying Substrate.