# X-ray absorption fine structure (XAFS)

## 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

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

where $\mu(E)$ is the linear absorption coefficient measured or calculated and $\mu_0(E)$ 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

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

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