X-ray emission spectroscopy

The X-ray emission spectroscopy (to English: X -Ray E mission S pectroscopy: XES ) is a röntgenspektroskopisches measuring method in which the light emitted from a material (emitted) X-ray spectrum is recorded.

One distinguishes between:

Fig. 1: and V2C ${\displaystyle K_{\beta }}$

• Non-resonant X-ray emission spectroscopy (XES)

1. ${\displaystyle K_{\beta }}$-Measurements
2. Valence-to-Core (VtC / V2C) measurements
3. ( ) Measurements ${\displaystyle K_{\alpha }}$

• Resonant X-ray Spectroscopy (RXES or RIXS)

1. XAS + XES 2D measurements
2. High resolution XAS
3. 2p3d RIXS
4. Mössbauer XES combination measurements

Electrons or X-ray photons of sufficient energy are usually used for excitation , but there are also X-ray sources that generate their characteristic X-ray radiation by decelerating protons and / or ions . During a measurement, only a certain energy range is usually selected according to the electron transitions to be observed. Transitions that correspond to the energies of the valence electrons (valence to core, VtC / V2C for short) are particularly meaningful. When the line is measured, these transitions can be found as a small hill on slightly higher energies than the main peak (Fig. 1). For this reason, the K lines are also the most frequently examined emission lines. The exact electron transitions can be seen in Figure 2. ${\displaystyle K_{\beta }}$ ${\displaystyle K_{\beta 1,3}}$

Figure 2: Energy level diagram of the K-line emissions

history

As the forefather of this process the British physicist applies Henry Moseley , the relationship between the wavelength - line in X-rays and atomic number ( Moseley's law discovered). Later it turned out that the characteristic energies of the emission lines of the elements can be used as direct fingerprints for the elemental analysis. ${\displaystyle K_{\alpha }}$

Since X-rays in most media have a refractive index of n ≈ 1 , special optics are required to diffract X-rays according to their energies or wavelengths. William Lawrence Bragg developed the Bragg equation for this . This describes the patterns that arise during the diffraction of neutrons and X-rays when they pass through a crystal lattice ( X-ray diffraction ).

The formula he established ,, states that an X-ray quantum with a certain energy bends at a precisely defined angle within a crystal. ${\displaystyle n\lambda =2d\,\sin(\theta )}$

(Further) development

Usually emission measurements with X-rays are / were only carried out on the beam paths of synchrotrons. These offer many advantages, but the "beam time" for scientists is usually very short, which is why independent devices with a lower resolution than that of a synchrotron are becoming more and more popular. The scientists can use this to examine the chemical environment of their samples before a “beam time”. Many full protection devices today are based on the principle of the Rowland Circle (Fig. 3). This describes a circle on which, at certain angles, all the photons of an energy emitted by a sample can be focused in one point.

Figure 3: Typical Rowland circle from XES and XAS

The X-ray emission spectrum is characteristic of the respective excited element, and in the soft X-ray range one can recognize the dependence on the chemical environment of the element from the fine structure. If X-ray photons are used for excitation, resonant effects can become evident in the X -ray emission spectra , when excited close to X -ray absorption edges , which are known as resonant inelastic X -ray scattering (RIXS). In contrast to the electron-excited X-ray emission spectra, strong excitation-energy-dependent structures can occur.

As an analytical method, X-ray emission spectroscopy has some advantages over electron spectroscopy:

• great depth of information, since the interaction of photons with matter is far less than that of electrons; so-called buried layers can be probed, and the use of thin X-ray windows allows measurements on gases and liquids.
• high selectivity of the information in the spectra with regard to orbital and spin symmetry, as well as the chemical environment.
• the resonant inelastic X-ray scattering does not suffer any broadening due to the lifetime of the trunk level excitation.

Individual evidence

1. ↑ S. DeBeer: Advanced X-Ray Spectroscopy . (PDF) June 2016, accessed on July 22, 2019.
2. ↑ ^{a b} Jeroen A. van Bokhoven, Carlo Lamberti: X-Ray Absorption and X-Ray Emission Spectroscopy Theory and Applications . John Wiley & Sons, 2016, ISBN 978-1-118-84423-6 , pp. 125 ( limited preview in Google Book Search).
3. ↑ J. Kowalska, S. DeBeer: The role of X-ray spectroscopy in understanding the geometric and electronic structure of nitrogenase . In: Biochimica et Biophysica Acta (BBA) - Molecular Cell Research , Vol. 1853 Issue 6, June 2015, accessed on July 22, 2019.
4. ^ R. Vanselow, Russell Howe: Chemistry and Physics of Solid Surfaces IV . Springer Science & Business Media, 2013, ISBN 978-3-642-47495-8 , pp. 22 ( limited preview in Google Book search).