X-Ray Standing Waves

The Bragg x-ray standing wave approach measures the modulation of x-ray fluorescence as the crystal is rotated through a substrate Bragg reflection. Due to the coherent superposition of the incident and reflected Bragg beams, an x-ray standing wave (XSW) is formed both above and below the crystal surface. The modulation of x-ray fluorescence by the XSW field is directly sensitive the location of the fluorescing atoms and provides a powerful probe of interfacial ion site distributions. With a known substrate lattice, the fluorescent yield, Y(q), can be described by two parameters:  coherent fraction, FH, and coherent position, PH, where H = (hkl) indicates the Bragg reflection. Because the fluorescing atom is located in the near field, the XSW measurement does not suffer from the well-known "phase problem" of x-ray crystallography.

Reference: Bedzyk M. J. and Cheng L., "X-ray Standing Wave Studies of Minerals
and Mineral Surfaces: Principles and Applications", Reviews in Mineralogy &
Geochemistry 49, 221-266 (2002).