EXAFS spectroscopy

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The EXAFS (from English extended X-ray absorption fine structure , EXAFS ) is a method of X-ray absorption spectroscopy for analyzing the near-edge fine structure of an X-ray spectrum. This method can be used to determine the type, number and distance of neighboring atoms ( ligands ) of a certain chemical element in molecules , a liquid or a solid . The method is also known as SEXAFS spectroscopy (from English surface extended X-ray absorption fine structure) Designated when eng for the study of molecules on surfaces (. Surface ) is used.

The method is closely related to the investigation of X-ray absorption directly at the absorption edge, X-ray near-edge absorption spectroscopy (NEXAFS or XANES). It can be used to investigate unoccupied electron states (orbitals).

Basics

X-ray absorption spectrum in the area of ​​an absorption edge (schematic). The edge is marked with an arrow and the energy range examined by EXAFS is highlighted in light blue.

The method is based on that an atom in the absorption of X-ray - quants ionized is. So an electron is released, the kinetic energy of which depends on the energy of the X-rays. The released electron spreads as a matter wave and is scattered by the neighboring atoms . Depending on the wavelength of the electron, there is constructive or destructive interference between the outgoing wave and the backscattered waves. This interference affects the probability that an X-ray quantum will be absorbed.

If you vary the energy of the X-ray radiation, the energy of the released electrons also changes, and thus the associated wavelength of the electrons. There is therefore alternating constructive and destructive interference, and thus a change in the X-ray absorption as a function of the energy. These changes in absorption can be measured in the energy range from just above the absorption edge to a few hundred electron volts above; thus in the upper area of ​​the absorption edge (hence the word " extended " in the English name). Since the change in absorption is relatively small (maximum approx. 10 percent, for energies well above the absorption edge less than one percent), these changes are referred to as the fine structure in the absorption spectrum.

From the form and strength of the changes in the absorption, it can be concluded at what distance from the ionized atom and how strongly it is scattered, so a so-called radial distribution function is obtained . From this it is possible to roughly estimate at what distance which or (if the atom types of the ligands are known) how many atoms can be there. By comparing with simulation calculations of the scattering of electrons on and between neighboring atoms, it can be determined whether an assumed structure fits the experimental data; if this is the case, the atomic distances (bond lengths) can be determined with high accuracy.

Since each element has a different energy of the absorption edge, the surroundings of the atoms of each chemical element can be examined separately. However, it is not possible to differentiate between several atoms of the same element, so EXAFS spectroscopy is normally only used for elements of which only one occurs in each molecule. In contrast to methods based on diffraction , no crystals are required for SEXAFS spectroscopy ; spectra of spatially completely disordered molecules of the same type can also be examined.

For EXAFS measurements, monochromatized synchrotron radiation is normally used. The experimental aspects are covered in the article X-ray absorption spectroscopy .

Applications

  • The main application of EXAFS is the investigation of organic molecules, whereby the environment of the carbon atoms cannot be investigated, as these exist in too many different configurations. Because of this, only the analysis of "rare" atoms, e.g. B. metal atoms, sulfur or phosphorus atoms useful. Metal atoms are often the active centers of enzymes ; the structure of these centers and the changes occurring there can therefore be investigated in a targeted manner (“BioXAS spectroscopy”).
  • EXAFS spectroscopy can also be used to determine how many or which atoms are in clusters , especially on the surface. The surface atoms have fewer neighbors, which results in a weaker fine structure of the absorption spectrum than with volume atoms.

Related and Complementary Techniques

  • Other techniques for determining bond distances are X-ray diffraction , LEED (low energy electron diffraction), and photoelectron diffraction (XPD). In contrast to EXAFS spectroscopy, the diffraction methods require crystalline samples.
  • The X-ray absorption directly at the absorption edge is examined in X-ray near-edge absorption spectroscopy (NEXAFS or XANES spectroscopy). This allows unoccupied electronic states (orbitals) to be examined.
  • Very similar results can also be achieved with electrons (e.g. in a transmission electron microscope (TEM)) instead of X-ray photons. The method is then called "extended electron energy-loss fine structure", EXELFS.

See also

literature

Web links

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