Analytical X-Ray Physics

Chemical Speciation with hard X-rays: XES and XAS

Especially in the field of geology and catalysis there is a need for the investigation of chemical species of different kinds of samples. In this case, the chemical species means the chemical state, such as different oxidations states of the same atom (for example Fe, FeO, Fe2O3), or the orientation within a molecule.

Both properties are accessible with X-ray Emission (XES) and Absorption (XAS) spectroscopy. Due to the small changes in corresponding energy levels in the range of a few eV or less, the needed spectral resolving power is very high besides high requirements for the stability of the energy axis. This is the reason why these measurements are usually carried out at synchrotron radiation (SR) sources, which combine a high flux with a very good spectral resolving power up to E/delta E = 10000. That means you can resolve 0.8 eV at the energy of the copper Kα emission line duplet.

To transfer this method to the laboratory wavelength dispersive spectrometers based on the von Hamos geometry were designed and built up, for XES [1, 5] as well as XAS [6].

To ensure the necessarily high efficiency of the spectrometer due to the low brilliant laboratory sources, Highly Annealed Pyrolytic Graphite (HAPG)[3, 7] mosaic crystals are used as the dispersive element. These crystals have the highest integral reflectivity among all known crystals and provide a spectral resolving power of up to E/delta E = 4000 in first order of reflection. Also, they promise to be usable in ultrafast x-ray absorption investigations [8].

With these spectrometers, it is now possible to perform high resolution XES  and XAS measurements on a laboratory scale for various applications, especially as shown in catalysis research [2, 4, 9].

Relevant Publications

[1] L. Anklamm et al., Rev. Sci. Instrum. 85 (2014), 053110. Link

[2] M. Dimitrakopoulou et al., Faraday Discuss. 208 (2018), 207–225. Link

[3] M. Gerlach et al., J. Appl. Cryst. 48 (2015), 1381-1390. Link

[4] H. V. Le et al., ACS Catal. 7 (2017), Nr. 2, 1403-1412. Link

[5] W. Malzer et al., Rev. Sci. Instrum. 89 (2018), 113111. Link

[6] C. Schlesiger et al., J. Anal. At. Spectrom. 30 (2015), 1080-1085. Link

[7] C. Schlesiger et al., J. Appl. Cryst. 50 (2017), 1490-1497. Link

[8] H. Stiel et al., Proc. SPIE 8849 (2013), 88490H. Link

[9] X. Zhao et al., J. Am. Chem. Soc. 141 (2019), 16, 6623-6639. Link