X-Ray Reflectivity and Crystal Truncation Rods

X-ray reflectivity is the direct analogue of x-ray crystallography for interfacial systems.  The x-ray scattering is in the form of “crystal-truncation rods” (CTRs) that are continuous rods of intensity connecting Bragg peaks along the surface normal direction. This shape derives from the loss of translational invariance along the surface normal direction and the crystalline order within the surface plane (resulting in "streaks" of intensity along the surface normal direction that are otherwise sharp within the surface plane). The variation of scattering intensity vs. Q along these CTRs is directly sensitive to the interfacial structure. The ability of x-rays to readily penetrate fluid layers makes it possible to directly define the interfacial structure through in-situ measurements.

Reference: Fenter, P., "X-ray Reflectivity as a Probe of Mineral-Fluid Interfaces:  A User Guide.  Reviews in Mineralogy and Geochemistry 49, 149-220 (2002).