Differences and links between XAS and XAFS

X-ray Absorption Spectroscopy (XAS) and Synchrotron Absorption Spectroscopy (XAFS) are two analytical techniques based on the principle of X-ray spectroscopy, which have a wide range of applications in the research of material science, chemistry, physics and related fields; although both techniques use the spectral information generated by the interaction of X-rays with substances to analyse the nature and structure of substances, they have a number of differences in terms of the technical principle, experimental devices, Although both techniques use the spectral information generated by the interaction of X-rays with substances to analyse the properties and structures of substances, there are some differences and connections between them in terms of technical principles, experimental devices, data processing and application fields.

Figure 1: Nat. Commun.(2020)11:418: Ti02 photocatalysis by Ti K edge XAFS

X-ray Absorption Spectroscopy (XAS)

X-ray absorption spectroscopy (XAS) is a technique used to study the absorption properties of materials on X-rays. When X-rays pass through a material, due to the interaction with the atoms in the material, the intensity of X-rays will change, and this change is related to the composition of the material, the electron arrangement of atoms, and the energy of the X-rays; XAS is usually divided into two parts: the absorption coefficient smooth decline area and the absorption coefficient sudden change area; in the Smoothly decreasing region, the absorption coefficient decreases gradually with the increase of X-ray energy, which is mainly related to the absorption of the K-shell layer of atoms; whereas in the mutation region, when the X-ray energy reaches a specific value, it will lead to the jump of the inner electrons, and the absorption coefficient will increase drastically, forming the absorption edge.

Synchrotron Radiation Absorption Spectrum (XAFS)

Synchrotron absorption spectroscopy (XAFS) is an X-ray absorption technique based on synchrotron radiation, which is a type of radiation produced in particle accelerators that is highly monochromatic and has continuously tunable energy; in XAFS experiments, synchrotron X-rays are irradiated onto a sample, and the absorption spectrum of the sample is obtained by detecting transmitted or scattered X-rays; XAFS can provide information about the sample's local structure XAFS can provide information about the local structure and electronic state of the sample, and is particularly effective for structural analyses on the atomic scale.

Difference and Connection

Difference

1. Energy range: XAS usually uses X-rays at lower energies (tens of keV), whereas XAFS can use X-rays at higher energies (hundreds of keV), which is useful for analysing electron transitions at different energies.
2. Experimental setup: XAS experiments are usually performed on ordinary X-ray sources, whereas XAFS requires the use of specialised synchrotron radiation sources, such as third-generation synchrotron radiation sources.
3. Data processing: XAS data processing is relatively simple, mainly involving the measurement of absorption coefficients and the identification of absorption edges; XAFS data processing is more complex, requiring the use of multi-body models and complex algorithms to fit and interpret spectral data.
4. Application areas: XAS is more suitable for elemental analysis, valence studies and certain types of structural analyses; XAFS is used for structural analyses on the atomic scale in fields such as materials science, condensed matter physics and chemistry.

Connection

1. Basic principle: Both XAS and XAFS are based on the principle that X-rays interact with matter, i.e. X-rays are absorbed or scattered by matter, resulting in spectral information.
2. Structural analysis: Both techniques can be used to analyse the structure of materials, and in particular XAFS has a unique advantage in structural analysis on the atomic scale.
3. Sample requirements: XAS and XAFS have similar sample requirements and can be applied to solid, liquid and gaseous samples.
4. Complementarity: In some cases, XAS and XAFS can complement each other, e.g. XAS can provide information on the overall absorption behaviour of a material, while XAFS can provide more detailed information on the local structure.