X-ray absorption fine structure

Following early experimental and theoretical works in the thirties,[1] in the sixties using synchrotron radiation at the National Bureau of Standards it was established that the broad asymmetric absorption peaks are due to Fano resonances above the atomic ionization potential where the final states are many body quasi-bound states (i.e., a doubly excited atom) degenerate with the continuum.

Multi-electron excitations and configuration interaction between many body final states dominate the edge region in strongly correlated metals and insulators.

In this regime the photoelectron has a strong scattering amplitude by neighboring atoms in molecules and condensed matter, its wavelength is larger than interatomic distances, its mean free path could be smaller than one nanometer and finally the lifetime of the excited state is in the order of femtoseconds.

This energy region has been called later (in 1982) also near-edge X-ray absorption fine structure (NEXAFS), which is synonymous with XANES.

Information on the geometry of the local structure is provided by the analysis of the multiple scattering peaks in the XANES spectra.