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XES spectrometer

Figure 1: Schematic view of the laboratory XES spectrometer at the MPI CEC with its main components: glove box, excillum Metal-Jet source for sample excitation, full cylinder HAPG optic and ccd camera.
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Figure 2: Comparison of measured (left) and calculated (right) spectra for various Ca compounds [3]. The comparison makes it possible to understand the electronic structure of the compound as emission lines are attributed to the difference of energy levels.
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n X-ray emission spectroscopy (XES), X-ray line spectra are measured with a spectral resolution sufficient to analyze the impact of the chemical environment on the X-ray line energy and on branching ratios. Especially the so-called satellite lines are affected by different bonding partners and geometries.

While synchrotron radiation experiments provide a unique sensitivity and flexibility, the access to it is limited. The motivation is to overcome this limitation and to better exploit the potential XES offers for research in chemistry. As an X-ray tube is used for excitation, which is much less brilliant than synchrotron radiation, the analyzer part of the spectrometer must be extremely efficient to allow for the detection of the weak valence-to-core lines even in dilute samples.

The laboratory XES spectrometer is based on the von Hamos principle (Link zu Unterkategorie 3). It is a scanning free approach; hence, the entire spectrum is acquired simultaneously. In order to achieve the high efficiency required for XES with laboratory sources, a 360° collection geometry is utilized. Thus, the X-ray lines are mapped as rings onto the ccd, which is used as a position sensitive detector. To further maximize the sensitivity, a mosaic crystal with high integral reflectivity, Highly Annealed Pyrolytic Graphite (HAPG) (Link zu Unterkategorie 4), was used as a dispersive element. The effective solid angle of such a ring optic has a magnitude of around 1 msr.

Based on a prototype, which is still located at the BLiX/TU Berlin [1] an optimized spectrometer by means of maximized spectral resolving power [2] has been developed together with and then transferred to the Max-Planck-Institute for chemical energy conversion, research group of Prof. Serena DeBeer (Link). The main components are an excillum liquid metal jet that excites the sample to emit fluorescence radiation, a full cylinder HAPG optic with a diameter of 60 cm and a Princeton Instruments ccd camera with a size of 1 inch times 1 inch. The goal was to be able to investigate the emission lines in the range of sulfur to zinc radiation (in first order of reflection), as well as air and moisture sensitive samples. Therefore, a glove box including a sample transfer and cooling system into the vacuum vessel that holds the optic and detector was added to the setup. In comparison with the prototype instrument, the current yields much higher quality of the XES spectra. The spectral resolving power is now E/ΔE = 4000 instead of E/ΔE = 2000. This resolving power is sufficient to resolve energetic position, width and intensity ratio of satellite lines and therefore different chemical environments can be distinguished, see figure 2 for exemplary spectra of various Ca compounds [3]. Additionally, on the right side, calculated spectra are shown. This helps to model the electronic structure of the molecule.

Point of contact: Wolfgang Malzer, Christopher Schlesiger

Relevant publications:

[1] L. Anklamm, C. Schlesiger, W. Malzer, D. Grötzsch, M. Neitzel and B. Kanngießer, Rev. Sci. Instrum. 85, 053110 (2014). Link

[2] W. Malzer, D. Grötzsch, R. Gnewkow, C. Schlesiger, F. Kowalewski, B. Van Kuiken, S. DeBeer, B. Kanngießer, Rev. Sci. Instrum. 89, 113111 (2018). Link

[3] Z. Mathe, D. Patazis, H. B. Lee, R. Gnewkow, B. Van Kuiken, T. Agapie, S. DeBeer, Inorg. Chem 2019, 58, 23 16292-16301. Link

 

 

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