Extended X-ray absorption fine structure

Oscillatory absorption above a core-ionization edge caused by interference between outgoing and neighbor-scattered photoelectron waves. It probes local coordination, distances and disorder. The high-energy shell expansion is distinct from the more complicated near-edge regime; phase corrections are required before interpreting Fourier-transform peaks as bond lengths.

The bond-direction variance of relative atomic displacement. A Gaussian distance distribution damps the coherent round-trip phase by $e^{-2k^2{\sigma }^2}$. Correlated atomic motion and static bond disorder contribute, so this is not generally a single atom's mean-square displacement about its lattice site.