Measurement method to measure the absorption of X-rays at energies near and above (typically several hundred
\(\pu{eV}\) greater) an absorption edge, over which fine structure (modulation of the X-ray absorption coefficient) can be detected.
Notes: - XAFS includes both extended X-ray absorption fine structure spectroscopy and X-ray absorption near-edge spectroscopy. It involves transitions from a core-level to an unoccupied orbital or band and mainly reflects the local atomic structure and bonding (SEXAFS) and the density of the unoccupied electronic states (XANES).
- XAFS measurements usually start some \(\pu{10 eV}\) before the core-level binding energy (the absorption edge) of the emitting atoms, because in many cases pre-edge features are used to identify chemical bonds [example: \(\uppi\!^*\) resonances (excitation into lowest unoccupied molecular orbitals) in C K-edge spectra of polymer samples].
- Usefully sharp absorption edges are commonly observed in X-ray absorption spectra, although broader increases can be observed for some inner-shell excitations with short lifetimes.
- XAFS spectra are best recorded when a highly intense beam of X-rays from a synchrotron is used along with a high resolution double crystal or curved crystal spectrometer. Detectors include ionization chambers, scintillation counters, and solid state detectors.
Source:
PAC, 2020, 92, 1781. 'Glossary of methods and terms used in surface chemical analysis (IUPAC Recommendations 2020)' on page 1794 (https://doi.org/10.1515/pac-2019-0404)