X-ray absorption fine structure (XAFS) is a specific structure observed in X-ray absorption spectroscopy (XAS). By analyzing the XAFS, information can be acquired on the molecular structure and the electronic state.

X-ray absorption spectroscopy shows a steep rise at the core-level X-ray energy of X-ray-absorbing atoms and attenuates gradually with the X-ray energy. An observed fine structure near the absorption edges was first explained by a theory of Walther Kossel and for many years was referred to as the “Kossel structure”. In contrast, an oscillatory structure extending for hundreds of electron volts past the edges was called the “Kronig structure” after the scientist, Ralph Kronig, who explained it theoretically. The former is now called X-ray absorption near-edge structure (XANES) and the latter is extended X-ray absorption fine structure (EXAFS). The XANES of organic molecules is sometimes called near-edge X-ray absorption fine structure (NEXAFS) for historical reasons, but NEXAFS is synonymous with XANES.

XANES spectroscopy corresponds to the transition from a core-level to an unoccupied orbital or band and mainly reflects the electronic state. In contrast, the oscillatory structure of EXAFS results from the interference effect between an emitted electron from an X-ray-absorbing atom and scattered electrons by surrounding atoms and provides information on the local structures around an X-ray absorbing atom.