Observations of the diffraction of evanescent x rays at a crystal surface

Diffraction of x rays from a crystal during total external reflection imposes structure on the x-ray wave field in three dimensions. Standing-wave interference modulates the x-ray intensity parallel to the surface, while boundary effects damp the intensity with distance normal to the surface both within the crystal and in the region above the surface. Experiments on carefully prepared Ge single crystals show that the x-ray wave field can be manipulated to provide model-independent information on the structure of surfaces.