# X-ray absorption fine structure (XAFS)

### From Online Dictionary of Crystallography

##### Revision as of 15:38, 13 April 2016 by BrianMcMahon (talk | contribs)

## Definition

**X-ray absorption fine structure** (XAFS) is the modulation of the absorption coefficient (*q.v.*) at and above an absorption edge (*q.v.*) of an element due to its chemical state and structure of its immediate surroundings. XAFS is commonly divided into the 'near edge’ region (XANES or NEXAFS) which extends to ~50 eV above the absorption edge and the 'extended’ region (EXAFS) that displays oscillations in the absorption coefficient extending from ~50 eV above the absorption edge.

The distinction in usage for **XANES** (X-ray absorption near edge spectroscopy) and **NEXAFS** (near-edge X-ray absorption fine structure) is historical. Generally, XANES is used for hard X-ray edges (~1 keV and above), while NEXAFS is used for soft X-ray edges. Spectral features that occur before the rise of the main absorption edge are referred to as 'pre-edge’ features, associated with transitions to bound states.

The extended X-ray absorption fine structure (EXAFS) (*q.v.*) region contains modulation of the absorption coefficient that can be interpreted in terms of photo-electron scattering.

In relation to the (linear) absorption coefficient, the XAFS is defined as

[math]\chi(E) = {[\mu(E) - \mu_0(E)]\over\mu_0(E)}[/math]

where [math]\mu(E)[/math] is the linear absorption coefficient measured or calculated and [math]\mu_0(E)[/math] a smooth background as calculated in the absence of scattering or fit (*e.g.* a spline fit) *etc.*

The XAFS is often practically defined as

[math]\chi(E) = {[\mu(E) - \mu_0(E)]\over\Delta\mu(E)}[/math]

where [math]\mu(E)[/math] is either measured or calculated and [math]\Delta\mu(E)[/math] is either the jump in absorption at the edge or a reference background that simulates the edge and a smooth atomic-like background.