X-ray absorption spectroscopy (XAS) is a widely used technique for determining the local geometric and/or electronic structure of matter.[1] The experiment is usually performed at synchrotron radiation facilities, which provide intense and tunable X-ray beams. Samples can be in the gas phase, solutions, or solids.[2]

  • The X-ray Absorption Near-Edge Structure (XANES) introduced in 1980 and later in 1983, also called NEXAFS (Near-edge X-ray Absorption Fine Structure) which are dominated by core transitions to quasi bound states (multiple scattering resonances) for photoelectrons with kinetic energy in the range from 10 to 150 eV above the chemical potential, called "shape resonances" in molecular spectra since they are due to final states of short life-time degenerate with the continuum with the Fano line-shape. In this range multi-ele

    There are three main regions found on a spectrum generated by XAS data which are then thought of as separate spectroscopic techniques (Figure 2):

    XAS is a type of absorption spectroscopy from a core initial state with a well defined symmetry; therefore, the quantum mechanical selection rules select the symmetry of the final states in the continuum, which are usually a mixture of multiple components. The most intense features are due to electric-dipole allowed transitions (i.e. Δℓ = ± 1) to unoccupied final states. For example, the most intense features of a K-edge are due to core transitions from 1s → p-like final states, while the most intense features of the L3-edge are due to 2p → d-like final states.

    XAS methodology can be broadly divided into four experimental categories that can give complementary results to each other: metal K-edge, metal L-edge, ligand K-edge, and EXAFS.

    The most obvious means of mapping heterogeneous samples beyond x‐ray absorption contrast is through elemental analysis by x‐ray fluorescence, akin to EDX methods in electron microscopy.[5]


    XAS is a technique used in different scientific fields including molecular and condensed matter physics, materials science and engineering, chemistry, earth science, and biology. In particular, its unique sensitivity to the local structure, as compared to x-ray diffraction, have been exploited for studying: