EXAFS
Definition and meaning of EXAFS in chemistry.
EXAFS (Extended X-ray Absorption Fine Structure) is the oscillatory variation in a material's X-ray absorption coefficient observed at energies extending from roughly 50 to 1000 eV above an element's absorption edge. These oscillations arise from interference between the outgoing photoelectron wave, ejected when an atom absorbs an X-ray photon, and the portion backscattered from neighboring atoms.
In more detail
Because the interference pattern depends on the distances, types, and number of surrounding atoms, Fourier analysis of the EXAFS oscillations yields precise bond lengths, coordination numbers, and a measure of structural (thermal or static) disorder around the absorbing element. Unlike X-ray diffraction, EXAFS does not require long-range crystalline order, so it can probe local structure in amorphous solids, liquids, glasses, catalysts, and metal sites in proteins. Measurements are almost always made using tunable, high-intensity synchrotron radiation. EXAFS is one part of the broader XAFS technique, complementing XANES, which probes the near-edge region for oxidation state and electronic structure.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Full name | Extended X-ray Absorption Fine Structure |
| Energy range probed | ~50–1000 eV above the absorption edge |
| Typical source | Synchrotron radiation, tunable X-ray beamline |
EXAFS spectroscopy at the iron K-edge can determine the Fe-N and Fe-O bond distances at the active site of a metalloenzyme in frozen solution, even though the protein itself is not crystalline.
Frequently asked questions
What structural information does EXAFS provide?
It gives interatomic distances (typically accurate to about 0.01 Å), coordination numbers, and the Debye-Waller-like disorder factor for atoms surrounding the absorbing element.
How does EXAFS differ from XANES?
XANES examines the near-edge region and is sensitive to oxidation state and electronic/geometric symmetry, while EXAFS examines the extended region further from the edge and is dominated by single-scattering interference that encodes local bonding geometry.