X-ray absorption spectroscopy (XAS) is an advanced analytical technique based on synchrotron radiation sources, which measures the absorption characteristics of substances towards X-rays and reveals information about the local electronic states and geometric structures of atoms. The core principle can be decomposed into two dimensions: physical processes and energy partitioning:
Physical Process: Electronic Transition and Scattering Interference
When the X-ray energy reaches the ionization energy of the inner layer electrons (such as K and L layers) of the atom, the electrons are excited into photoelectrons, forming a sudden jump in the absorption edge. Photoelectrons propagate outward in the form of wave functions. If they encounter neighboring atoms, elastic scattering (backscattering) occurs, and the scattered wave interferes with the outgoing wave at the absorbing atom, causing the absorption coefficient to oscillate periodically with energy. This process can be quantitatively described by Lambert Beer's law: the relationship between the absorption coefficient μ (E) and the sample thickness d, incident intensity I ₀, and transmission intensity I is I=I0 ⋅ e − μ (E) d.
Energy partitioning: collaborative analysis of XANES and EXAFS
X-ray Absorption Near Edge Structure (XANES)
Focusing on the strong oscillation region from the absorption edge of 10 eV to the back of 50 eV, reflecting the multiple scattering effects between photoelectrons and neighboring atoms. The spectral features (such as front edge peaks and shoulder peaks) are directly related to the electronic density distribution of the absorbed atoms. For example, by shifting the absorption edge position, the valence state changes of elements can be quantitatively analyzed (such as distinguishing Fe ² ⁺ from Fe ³ ⁺), and the presence of front edge peaks reveals unoccupied molecular orbital information.
Extended X-ray Absorption Fine Structure (EXAFS)
The weak oscillation region of 50-1000eV behind the absorption edge is due to the single scattering effect of photoelectrons. By converting the oscillation signal into a radial distribution function through Fourier transform, accurate information such as coordination atom bond length (with an accuracy of 0.01 Å), coordination number, and disorder can be obtained. For example, in the research of lithium-ion battery materials, EXAFS can reveal the coordination environment evolution of transition metals (such as Ni, Co) during charge and discharge processes.
Experimental method: Multi mode adaptation and in-situ characterization
transmission mode
Suitable for high concentration samples (such as powders and films), the absorption coefficient can be calculated by measuring the ratio of incident and transmitted X-ray intensity. The thickness of the sample needs to be controlled to avoid self absorption effects, which is commonly used for static analysis of crystalline, amorphous, and liquid samples.
fluorescence mode
By using the fluorescence signal intensity emitted by the excited target atom to infer the absorption amount, it is suitable for low concentration or single atom systems (such as catalyst surface active sites). For example, in the study of Pt catalysts for fuel cells, fluorescence mode can accurately locate the coordination state of surface Pt atoms.
In situ/openando technology
Real time tracking of the structural dynamics of materials during the reaction process by combining high voltage, high and low temperature, or electrochemical environments. For example, in the study of electrocatalytic CO ₂ reduction, in situ XAS can reveal the valence state changes and coordination reconstruction mechanisms of catalyst active sites.
Technical advantages and typical applications
XAS has no strict requirements for sample morphology (powder, liquid, gas are all acceptable), and does not damage the sample. It is widely used in materials science, energy storage, environmental monitoring and other fields. For example, in the study of rare earth doped semiconductor materials, XAS can simultaneously analyze local structural distortions and electronic state distributions; In the biomedical field, XAS can characterize the coordination environment of metal ions in metalloproteins (such as hemoglobin), providing structural basis for drug design.
Through the collaborative analysis of XANES and EXAFS, combined with transmission, fluorescence, and in-situ experimental modes, XAS has become a key tool for revealing the atomic scale structure performance relationship of materials, promoting a leapfrog development from basic research to industrial applications.