A synchrotron is the light source; XAS is the measurement
Synchrotron radiation (SR) and X-ray absorption spectroscopy (XAS) are often mentioned together, but they are not the same object. A synchrotron is a light source. XAS is an experiment that uses X-rays interacting with matter to analyse a sample.
The synchrotron: a source of tunable X-rays
Synchrotron radiation is the electromagnetic radiation emitted along the tangent when a fast charged particle is deflected in a magnetic field. In 1947, researchers saw the intense radiation produced when high-speed electrons in an accelerator passed through a magnetic field. A modern source accelerates electrons close to the speed of light, then uses magnets and a storage ring to produce radiation over a wide energy range.
The light is bright, polarised, broadband, and pulsed. Bending magnets, wigglers, or undulators shape the beam and send it into a beamline. Slits, mirrors, and a monochromator select the energy and the spot size.
XAS: absorption versus energy
XAS is a spectroscopy based on X-ray absorption. The incident energy is scanned and the change in absorption is recorded. Once the energy is high enough, the X-rays can excite core electrons, for example from the K or L shell. The spectrum can be used to discuss elemental valence, local coordination, and electronic structure.
| Concept | What it is | What it provides | Typical use |
|---|---|---|---|
| Synchrotron | a bright, tunable light source | the X-ray beam the experiment needs | XAS, X-ray diffraction (XRD), imaging, and other spectroscopies |
| XAS | a measurement of absorption versus X-ray energy | element-specific valence, local structure, and electronic states | catalysts, battery materials, nanomaterials |
XANES and EXAFS
XAS is commonly split by energy region around the absorption edge:
- X-ray absorption near-edge structure (XANES) looks at the shape near the edge and is used for valence and chemical environment.
- Extended X-ray absorption fine structure (EXAFS) looks at the oscillations above the edge and is used for the local coordination and interatomic distances around the absorbing atom.
How the two fit together
High brightness and a tunable energy range help produce the signal and the scan range that XAS needs, especially for weak samples. A synchrotron also supports diffraction, imaging, and X-ray emission spectroscopy (XES). XAS itself stays on the local chemical environment and electronic structure of the absorbing element. The first answers “which light source”. The second answers “which measurement”.