X-Ray Emission Spectroscopy
There are various experimental schemes for detecting X-rays that are emitted from a sample. We take the liberty within this wiki to define X-ray Emission Spectroscopy (XES) as measuring the X-rays with an energy lower than the excitation energy, e.g. inelastically scattered, and with an instrumental energy broadening that is on the order of the core hole lifetime broadening. This article only discusses hard X-ray XES. The 1s core hole lifetime broadening of 3d transition metals is around 1 eV and increases to several eV for the L lines of 5d elements.
XES is a secondary process. First, a core hole is created by, e.g., the absorption of an incident photon. The core hole decays after a time τ (the core hole lifetime) and the energy that is freed upon this decay is either transferred to an electron (Auger decay) or to a photon (XES). With a tunable incident energy we can distinguish between resonant and non-resonant XES. The latter is usually referred to as X-ray fluorescence. When the incident energy is tuned around the resonances of an absorption edge we can observe a variation of the XES spectral shape depending on the incident energy. Resonant XES can be described within the theoretical framework of resonant inelastic X-ray scattering (RIXS). XES, RXES and RIXS have been reviewed by several authors.[2-5]
The intermediate states in the image below are the excited states that give rise to an X-ray absorption spectrum.
XES with lifetime resolution is sensitive to the local electronic structure and coordination of the emitting atom. It provides information that is complementary to XAS in particular with respect to electronic structure.
XES in combination with XAS at a synchrotron radiation beamline is a photon-in photon-out technique. The hard X-ray probe makes all techniques suitable for in-situ (operando) studies and experiments under extreme conditions (e.g. high pressure). XAS-XES can be performed within the same experimental setup.
 J.C. Fuggle and J.E. Inglesfield, eds. Unoccupied Electronic States. Topics in Applied Physics. Vol. 69. 1992, Springer-Verlag: Berlin.
 F.M.F. de Groot, High-Resolution X-ray Emission and X-ray Absorption Spectroscopy. Chem. Rev., 2001. 101: p. 1779-1808.
 F.M.F. de Groot, Multiplet effects in X-ray spectroscopy. Coordination Chemistry Reviews, 2005. 249(1-2): p. 31-63.
 P. Glatzel and U. Bergmann, High resolution 1s core hole x-ray spectroscopy in 3d transition metal complexes - Electronic and structural information. Coordination Chemistry Reviews, 2005. 249: p. 65-95.
 F.M.F. de Groot and A. Kotani, Core Level Spectroscopy of Solids. Taylor and Francis, New York (2008)