Grazing incidence diffraction
Grazing incidence X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

 and neutron
Neutron diffraction
Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material: A sample to be examined is placed in a beam of thermal or cold neutrons to obtain a diffraction pattern that provides information of...

(GID, GIXD, GIND), typically from a crystalline structure uses small incident angles for the incoming X-ray or neutron beam, so that diffraction can be made surface sensitive. It is used to study surfaces and layers because wave penetration is limited. Distances are in the order of nanometres. Below (typically 80%) of the critical angle of the surface material studied an evanescent wave
Evanescent wave
An evanescent wave is a nearfield standing wave with an intensity that exhibits exponential decay with distance from the boundary at which the wave was formed. Evanescent waves are a general property of wave-equations, and can in principle occur in any context to which a wave-equation applies...

 is established for a short distance and is exponentially damped. Therefore Bragg
Bragg may refer to:*Bragg, Texas, a US ghost town*Electoral district of Bragg, a state electoral district in South Australia*Bragg , a crater on the Moon*Bragg Communications, a Canadian cable television provider...

 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. A disadvantage is the limited in-plane spatial resolution (beam footprint).

When employed under very small scattering angles,
the technique is called grazing-incidence small-angle scattering (GISAS, GISAXS, GISANS), and requires quite special methodology.

Further reading

Jens Als-Nielsen, Elements of Modern X-ray Physics ISBN 0 471 498572
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