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X-ray absorption spectroscopy (XAS) is nowadays a widely used technique at synchrotron radiation sources to study the local geometric and electronic structure or the chemical state of matter under various conditions of application.

XAS refers to the details of how x-rays are absorbed by an atom at energies near or above the cole-level binding energies of that atom. X-ray absorption spectra are especially sensitive to the formal oxidation state, coordination chemistry and the distances, coordination number and species of the atoms immediately surrounding the selected element. Since XAS is an atomic probe, it places few constraints on the samples that can be studied.

For example crystallinity is not required for XAS measurements, making it one of the few structural probes available for noncrystalline and highly disordered materials, including solutions and gas phases.

The x-ray absorption spectrum is typically divided into two regimes:

1. The x-ray absorption near-edge structure (XANES), also called near-edge x-ray absorption fine structure
(NEXAFS) extending about 50 - 100 eV around the absorption edge.

2. The extended x-ray absorption fine structure (EXAFS) extending a few hundred eV above the absorption edge.

Both have the same physical origin, but their distinction is convenient for the interpretation. XANES (or NEXAFS) is strongly sensitive to formal oxidation state and coordination chemistry of the absorbing atom, while EXAFS is used to determine the distances, coordination number and species of the neighbours of the absorbing atom.