X-ray absorption spectroscopy (XAS) measures the absorption characteristics of substances towards X-rays, revealing the local electronic states and geometric structure information of atoms. Its core consists of two parts: Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES). The physical mechanisms of both are derived from the scattering interference effect of photoelectron waves and neighboring atoms after X-ray excitation of inner layer electrons. However, the differences in energy range and scattering path result in different emphases of the structural information reflected by them.
The physical mechanism of EXAFS
The oscillation signal in the range of 30-1000eV observed by EXAFS on the high-energy side of the absorption edge is rooted in the single scattering effect of photoelectrons. When the X-ray energy exceeds the ionization energy of the inner layer electrons, atoms absorb photons and excite the inner layer electrons into photoelectrons, which propagate outward in the form of wave functions. If neighboring atoms are encountered during the propagation process, elastic scattering (backscattering) occurs, and the scattered wave interferes with the outgoing wave at the absorbing atom. Due to the wavelength variation of photoelectrons with energy, interference effects cause the absorption coefficient to exhibit periodic oscillations. The oscillation signal can be converted into a radial distribution function through Fourier transform, which directly obtains information such as the bond length (with an accuracy of 0.01 Å), coordination number, and disorder of the coordinating atoms around the absorbing atom. EXAFS is sensitive to short-range ordered structures and is suitable for analysis of crystalline, amorphous, and liquid samples.
The physical mechanism of XANES
XANES focuses on absorbing fine structures from the edge eV to about 50eV behind the edge, and its formation mechanism is the multiple scattering effect of photoelectrons and neighboring atoms. Low energy photoelectrons (kinetic energy<50eV) undergo multiple scattering during propagation, forming complex wave function superposition. This multiple scattering is highly sensitive to the local coordination environment around the absorbing atoms, such as atomic species and geometric arrangement, resulting in absorption coefficients exhibiting discrete peaks, shoulder peaks, and other characteristics in the near edge region. The XANES spectral features directly reflect the distribution of electron density of states of absorbing atoms. For example, the valence state changes of elements can be quantitatively analyzed by the offset of absorption edge positions, while the presence of edge front peaks reveals information about unoccupied molecular orbitals. Unlike EXAFS, XANES is more suitable for studying short-range ordered or disordered systems, such as catalyst surface active sites, biomacromolecule metal cofactors, etc.