Synchrotron Radiation is the electromagnetic radiation emitted by charged particles moving along curved orbits in a magnetic field at velocities close to the speed of light (v≈c), and is also known as synchrotron radiation or synchrotron radiation because it was first observed at synchrotrons. Synchrotron radiation. Synchrotron radiation is a pulsed light source with excellent properties such as wide range, high intensity, high collimation, high purity, and precisely controllable characteristics, and it can be used to carry out many cutting-edge scientific and technological researches that cannot be realized by other light sources.

Synchrotron radiation

Characteristics of synchrotron radiation

1. high brightness: the third generation of synchrotron radiation light source X-ray brightness is hundreds of millions of times that of an X-ray machine.
2. wide band: covering the far-infrared, visible, ultraviolet and X-ray bands, is currently the only light source that can cover such a wide spectral range and get high brightness.
3. high collimation: synchrotron radiation emission degree is very small, concentrated in the direction of electron motion as the center of a very narrow cone, collimation can be comparable with laser.
4. narrow pulse: synchrotron radiation is a pulsed light source, the width is adjustable between $10^{-11}$ ~ $10^{-8}$ seconds, the interval between pulses for tens of nanoseconds to microseconds.
5. high intensity: in the vacuum ultraviolet and X-ray band, can provide than conventional X-ray tube intensity 103 to 106 times higher than the light source, equivalent to a few square millimeters on the area of 100 kilowatts of energy flow.
6. high polarization: synchrotron radiation in the electron orbital plane is fully polarized light, polarization of 100%; in the orbit plane above and below the elliptical polarization; in all radiation, the horizontal polarization accounts for 75%.
7. high purity: as the synchrotron radiation is generated in ultra-high vacuum ($10^{-7}$ ~ $10^{-9}$ Pa) or high vacuum ($10^{-4}$ ~ $10^{-6}$ Pa) conditions, there is no ordinary light source in the electrode sputtering and other interference, is a very clean light source.
8. Other: High stability, high flux, micro beam diameter, quasi-coherent, etc..

Areas of application

1. Modern biology. For example, the determination of the structure of proteins and protein molecular structure, through the X-ray small-angle scattering can be studied protein physiological activity processes and neural processes, such as dynamic changes, through the X-ray fluorescence analysis can be determined in biological samples of the type and content of atoms, the sensitivity of up to $10^{-9}$ grams / gram.
2. Medicine. Can be used for the diagnosis and treatment of tumors, such as determining the content of some elements in the blood, angiography, diagnosis of various tumors in the body and microsurgery to remove some abnormal molecules in special parts of the human body.
3. Structural chemistry. Can be used to determine the coordination structure of atoms, chemical bonding parameters between macromolecules, such as catalysts, metalloenzymes structure determination.
4. solid state physics. Can be used to study the electronic state of solids, the structure of solids, excited state lifetime and crystal growth and solid damage and other dynamic processes.
5. surface physics and surface chemistry. Can be used to study the surface properties of solids, such as semiconductor and metal surface optical properties; material oxidation, catalysis, corrosion and other processes such as surface electronic structure and changes.
6. materials science. The use of synchrotron light, can clearly reveal the precise structure of the atoms in the material and valuable electromagnetic structure parameters and other information, they are both the understanding of the material properties of the "key", but also the design of novel materials principles and sources.
7. Photolithography. Due to the diffraction effect, the minimum line width of the commonly used ultraviolet lithography is about 2 microns, while synchrotron light is similar to a parallel beam, used for photolithography, its line width can be reduced to 20 Å, so that the resolution is improved by several orders of magnitude; this is of great significance for computers, automated controls, and optical communications technology.

Synchrotron Radiation Absorption Spectroscopy (XAFS)

X-ray Absorption Fine Structure (XAFS), also known as X-ray Absorption Spectroscopy (XAS), is a powerful tool based on synchrotron radiation light source to study the local atomic or electronic structure of materials, which is widely used in the popular fields of catalysis, energy, nano and so on.

Benefits of XAFS

1. does not depend on the long-range ordered structure, can be used for the study of amorphous materials;
2. It is not affected by the interference of other elements, and can be used to study different elements in the same material separately;
3. not affected by the state of the sample, can measure solid (crystal, powder), liquid (solution, molten state) and gas, etc.;
4. No damage to the sample, in-situ testing is possible;
5. can obtain high precision coordination atomic species, coordination number and atomic spacing and other structural parameters, generally believed that the accuracy of atomic spacing up to 0.01 Å.

Principle of XAFS

When X-rays pass through a material, the intensity is attenuated due to absorption, and according to Beer's law, this attenuation can be expressed by the absorption coefficient:

where $I_0$ and $I_t$ are the intensity of incident X-rays and transmitted X-rays, respectively. $x$ is the thickness of the sample and $\mu(E)$ is the absorption coefficient dependent on the energy of the photon.

xAFS is an energy-dependent fine structure based on the absorption coefficients of X-rays. The relationship between the absorption coefficient and the incident X-ray energy is affected by the underlying chemical state and the local environment, which can be categorized into side front, side back and extended side regions. When the incident X-ray energy is lower than the electron binding energy, the electrons are not excited into high-energy unoccupied orbitals or the vacuum. At this time, the weak interaction between X-rays and electrons leads to a flat region in the μ(E)-E spectrum; some lower-energy leaps conforming to the lepton rule can also be manifested as edge-front peaks in this region. When the X-ray energy is high enough, the core electrons are excited to high-energy unoccupied orbitals, μ(E) increases significantly, and the edge-front and near-edge regions are sensitive to the oxidation state and electron level of the element being detected, and are therefore together referred to as the XANES. The XANES contain a wealth of structural information that can be characterized in terms of coordination chemistry, molecular orbitals, energy band structure, and multiple scattering. By further increasing the X-ray energy, the wavefunctions of the ejected and scattered electrons excited to the continuum state interact with those of the absorbing atoms, and EXAFS emerges; a least-squares fit to EXAFS yields local coordination information such as bond lengths and coordination numbers of the absorbing atoms.

References

1. Shijie Ren, Sicong Qiao, Chongjing Liu, Wenhua Zhang, Li Song. Progress of synchrotron X-ray absorption spectroscopy of platinum monoatomic catalysts[J]. Journal of Chemistry in Higher Education,2022,43(09):107-120.