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XAFS spectrometer

Figure1: Schematic view of the laboratory XAFS spectrometer with its main components: microfocus X-ray tube, cylindrical HAPG-optic and ccd camera.
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Figure 2: Comparison of spectra obtained with a synchrotron radiation facility-based spectrometer equipped with a Si (111) crystal (green), the laboratory spectrometer with the XANES-optimized optic (red) and with the EXAFS optimized optic (black). The sample was a pure copper foil with a thickness of 10 µm.
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A highly efficient laboratory setup for X-ray absorptions spectroscopy to measure and characterize diluted samples is located at the BLiX/TU Berlin [1]. The goal was and is to extend the group of users of X-ray absorption spectroscopy compared to the usual synchrotron radiation-based spectrometers by offering a more accessible, but also easy to operate, laboratory solution. Not only industrial processes, e.g. control by means of determination of mixture ratios of species, but also scientific research, e.g. the measurement of oxidation state and bonding distance in catalysis research, are the main motivation. Therefore, it neither does nor have to reach the same performance as high end, synchrotron radiation facility-based XAFS beamlines. Rather factors e.g. as accessibility, reliability and simplicity are of importance.

Based on the von Hamos geometry (Link zu Unterkategorie 3) and HAPG mosaic crystals (Link zu Unterkategorie 4) a setup for measurements in transmission mode was built, see figure 1. The main components are a 30 W microfocus X-ray tube with a Mo anode, one of two different HAPG optic with each a size of 5 cm times 5 cm and an Andor Newton ccd camera with a size of 1 inch times 0.25 inch. Linear and angular movement of optic and detector ensure that the requirements of the von Hamos geometry can be fulfilled for various photon energies. These can range from 5 keV up to 12 keV, mainly covering the K absorption edges of the 3d-transition metals. The spectrometer combines a high efficiency with high spectral resolving power of up to E/ΔE = 4000, that is constant over the range of the covered absorption edges. Tailored optics either deliver a higher resolving power for XANES investigations or a larger energy bandwidth for EXAFS measurements, see figure 2. Acquisition times for the whole spectrum are in the range of a few minutes up to hours. This mainly depends on the sample’s matrix and the analyte’s concentration, which can go down to a few weight percent.

Research in catalysis has proven to be one major field for current and hence future applications [2, 3, 4]. For that purpose, two spectrometers optimized for the determination of bonding distances (EXAFS investigations) will be constructed and built. One of these spectrometers will then be transferred to the research group of Prof. Serena DeBeer at the Max-Planck-Institute for chemical energy conversion (Link) while the second one will be used in the frame of the excellence cluster Unifying Systems in Catalysis (UniSysCat) (Link) for routine investigations. Both setups highlight the potential of and desire for the development of laboratory based XAFS spectrometers.

 

Point of contact:  Wolfgang Malzer, Christopher Schlesiger

Relevant publications:

[1] C. Schlesiger, Dissertationsschrift (in german) (2019). Link

[2] M. Dimitrakopoulou, X. Huang, J. Krohnert, D. Teschner, S. Praetz, C. Schlesiger, W. Malzer, C. Janke, E. Schwab, F. Rosowski, H. Kaiser, S. Schunk, R. Schlögl, A. Trunschke, Faraday Discuss. 208 (2018), 207–225. Link

[3] H. V. Le, S. Parishan, A. Sagaltchik, C. Goebel, C. Schlesiger, W. Malzer, A. Trunschke, R. Schomaecker, A. Thomas, ACS Catalysis 7 (2017), Nr. 2, 1403-1412. Link

[4] X. Zhao, P. Pachfule, S. Li, T. Langenhahn, M. Ye, C. Schlesiger, S. Praetz, J. Schmidt, A. Thomas, J. Am. Chem. Soc. 141 (16) (2019), 6623-6639. Link

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