XSW is used to study the surfaces and interfaces of materials. If the light hitting the sample obeys the Bragg condition, light is reflected. Both the incident and reflected waves are inherently coherent and thus they interfere making a standing wave. The periodicity of the standing wave is the same as the periodicity of the crystal. By scanning phase 0 to π, the positions of the antinodes (of the standing wave) with respect to the scattering planes of the sample change. There is a dramatic change in the photon intensity of a standing wave as a function fo the position between the scatter planes and this gives the coherent faction and the coherent postion. The former signifies the level of order (0→1) and the latter give sthe mean position of an atom with respect to the crystal.
The probe in XSW is the crystallinity of the structure (inherently global) and the detcetor is the atom (inherently local) thus, giving information on the entire system. It determines the locality of what you are looking at with respect to the whole crystal. XSW gives the same information as PhD but from different standpoints.
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