Resonant elastic x-ray scattering (REXS) occurs when the energy of the incident x-ray photon is tuned close to some absorption edge of an element in the target sample under study. Thus, by definition, in order to perform resonant scattering, a tuneable energy x-ray source is required. Conventional x-ray sources rely on the deceleration of electrons on collision with an anode and produce x-rays predominantly at energies characteristic of the anode material (usually tungsten, molybdenum or copper). In addition, lower intensity x-rays are produced over a wide spectrum where the high energy cut off is determined by the accelerating voltage. Although early determinations of REXS scattering factors were derived from such sources, the advent of synchrotron radiation providing sources of tuneable x-rays energy marked an explosion in the field of REXS research.
In both x-ray absorption spectroscopy (XAS) and REXS, a core electron is excited from an initial state to some energy above the Fermi level. Specifically, where in XAS the final state involves a core hole, in REXS the initial and final states are the same and intermediate states are probed with virtual core electron transitions.
Today, REXS provides a unique tool in the study of solid state systems providing spatial information from diffraction combined with electronic information through the absorption process. This combination of diffraction and absorption allows the probing of magnetic, charge, spin and orbital degrees of spatial order together with electronic structure.