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Reciprocal Space Mapping for Dummies

Grazing Incidence X-ray Diffraction (GIXD) is a surface sensitive X-ray investigation technique (or geometry configuration) that can reveal the structural properties of a film deposited on a flat substrate. The term grazing indicates that the angle between the incident beam and the film is small (typically below 0.5 degrees). This essential technique has been employed on liquid crystals, nanoparticles and colloids, nanostructures, corrosion processes, polymers, bio-materials, interfaces, materials for solar cells, photodiodes, and transistors, etc. Diffraction patterns in GIXD geometry are typically captured with a 2D detector, which outputs images in pixel coordinates. A step required to perform analyses such as grain size estimation, disorder, preferred orientation, quantitative phase analysis of the probed film surface, etc., consists in converting the diffraction image from pixel coordinates to the momentum transfer or scattering vector in sample coordinates (the reciprocal space mapping). This momentum transfer embeds information on the crystal or polycrystal and its intrinsic rotation with respect to the substrate. In this work we derive, in a rigorous way, the reciprocal space mapping equations for a 3D+1S diffractometer in a way that is understandable to anyone with basic notions of linear algebra, geometry, and X-ray diffraction.