Basic operating principles of the Bruker D8 Advance X-Ray Reflectometer
The simplest experiment this instrument allows is to obtain the reflectivity of a surface as a function of incident angle. This reflectivity (in specular scattering geometry, i.e. with incident and exit angle being equal) can be used to obtain information on the electron density along the surface normal. The reflectivity curve can be separated into 2 parts:
1) Most materials have a refractive index for X-rays that is smaller than unity. This means that the effect of total external reflection is observed for X-rays. This effect is analogous to the effect of total internal reflection, which is known from visible light. The difference is that for X-rays this effect is only observed at very low incident angles, usually below 1o. Before this angle is reached the reflectivity is unity.
2) Beyond the critical angle the reflectivity of a single sharp interface falls rapidly this is known as the ‘Fresnel reflectivity’. If the interface is not sharply defined but layers are present an interference pattern (‘Kiessig fringes’) is observed from the scattering from different interfaces present.
The reflectivity curve allows information about surface and interface roughness, film thickness, and density to be obtained via simulation and modeling.
The diffuse part of the reflectivity (typically measured with the exit angle differing from the incident angle) offers information about lateral correlations of the roughness, which provides insight into, e.g., the growth process of a film or the capillary wave spectrum of a liquid surface.
The technique of X-ray reflection is used extensively by Dr. Frank Schreiber’s research group, so for further details see his explanation of the technique, he has also written a brief tutorial on the reflectivity of thin organic films.
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