Grazing-incidence scattering of coherent X-rays from a liquid surface.

The scattering image produced by coherent X-rays appears grainy and is denoted a speckle pattern. An analysis of the static and time-dependent properties of a speckle pattern generated by scattering of a partially coherent synchrotron beam from a liquid surface is given here. Unique surface sensitivity is achieved by applying the X-rays under a grazing angle of incidence. The observed contrast of the speckle pattern depends on the momentum transfer parallel to the surface, unlike the case of transmission small-angle X-ray scattering (SAXS), where essentially no q-dependence of the contrast has been observed. The appearance of the speckles and the contrast of the image can be understood qualitatively by use of geometrical arguments and by the fact that liquid surfaces are extremely flat.