Grazing incidence diffraction

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Grazing incidence X-ray and neutron diffraction (abbreviation GIXD or GID) uses small incident angles for the incoming X-ray or neutron beam, so that diffraction can be made surface sensitive. If one stays below (typically 80% of) the critical angle of the surface material studied, the radiation does not penetrate into the material, but only an evanescent wave is established for a short distance (typically tens of Angstroms) into the material, but exponentially damped. Therefore Bragg reflections are only coming from the surface structure.

An advantage of GIXD is that the electric field at the critical angle is amplified locally by a factor of four, therefore the GIXD signal is stronger.

Further reading: Jens Als-Nielsen, Elements of Modern X-ray Physics