X-ray photon correlation spectroscopy in a reflection geometry: coherence and resolution

In X-ray photon correlation spectroscopy (XPCS) the degree of coherence of the X-ray beam determines the contrast of the observed intensity correlation function. In this article, we present XPCS measurements of smectic liquid crystal membranes in a reflectivity geometry showing that both coherence and resolution can influence the time dependence of the correlation function. Variation of the pre-detector slits as well as of the projected coherence length on the membrane induce a time dependence of the intensity correlation function. We also treat several practical aspects and limitations we encountered during our XPCS studies. Finally the conditions for heterodyne detection at the specular ridge and homodyne detection at off-specular conditions are discussed.