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New von Hámos spectrometer at ID20


A new, compact spectrometer for medium-resolution resonant and non-resonant X-ray emission spectroscopy in von Hámos geometry is available at ID20 and can be used in parallel with the X-ray Raman scattering spectrometer. Combining both techniques in a single experiment extends the spectroscopic capabilities, in particular for in-situ / operando measurements.

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X-ray energies typically used in X-ray Raman scattering (XRS) spectroscopy are usually well above the K- and L- absorption edges of heavier elements. It means that the very same incident X-ray beam can be used for simultaneous measurement of XRS and non-resonant core-to-core and valence-to-core emission lines via X-ray emission spectroscopy (XES), yielding complementary information from the low-Z and, for example, 3d transition metal elements in the investigated sample. A new, compact, dispersive spectrometer in the von Hámos geometry has been installed in the limited space around the XRS spectrometer on beamline ID20 (Figure 1). It enables the recording of an entire emission spectrum in a single image, and thus can be used as an extra diagnostic tool to assess the evolution of the chemical state of the sample during a series of XRS measurements, e.g., under in-situ conditions.

The new von Hámos spectrometer is equipped with three cylindrically bent analyser crystals with a bending radius of R = 250 mm. The analyser crystals are housed in a compact vacuum chamber for high signal-to-noise ratio and maximum ease of use and alignment. Each analyser crystal is 25 mm high and 110 mm wide. At present, Si(n,n,n), Si(n,n,0) and Si(n,0,0) crystals are available and can easily be exchanged depending on the required energy range of the emission line, with Bragg angles comprised between 45 and 85 degrees. Both the analyser crystals and the detector are equipped with motorised translations to align them into diffraction and focusing positions. The X-rays are detected by a 5 x 1 Maxipix hybrid pixel detector (500 mm Si sensor thickness) with 55 µm x 55 µm pixel size and 14 mm x 70 mm active surface area. The solid angle of detection is of the order of 1% of 4π.



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Fig. 1: a-b) Three-dimensional renderings of technical drawings of the von Hámos spectrometer. a) An orthographic view of the entire chamber including the Maxipix pixel detector. b) Same as (a) but with a transparent vacuum chamber. c) A schematic drawing of the von Hámos geometry. For clarity, only the central analyser crystal is shown (S: source/sample; R: crystal-analyser bending radius; D: detector; dz: detector position; A: analyser; az: vertical analyser position; ay: horizontal analyser position; θB: Bragg angle; H: analyser height; w: analyser width; δaz: vertical deviation from the ideal sample position; δay: horizontal deviation from the ideal sample position). d-f) Sections of typical detector images showing the footprints of non-resonant XES from a germanium single-crystal sample. Possible regions of interest are indicated with thin dashed black lines. Images are shown on a logarithmic intensity scale.

The great potential of the new spectrometer is the combination of two spectroscopic techniques to provide complementary information on the electronic structure of the studied sample. Both methods make use of hard X-ray photons, which are compatible with complex sample environments such as in-situ and operando cells for the study of chemical processes and reactions, and/or high-pressure diamond anvil cells for the investigation of samples under extreme pressure conditions.

The compact design and portability, including a stand-alone control rack, makes the instrument fully mobile, facilitating its integration into other ESRF beamlines for combination with other synchrotron-based experimental techniques using hard X-rays.

Principal publication and authors
A compact von Hámos spectrometer for parallel X-ray Raman scattering and X-ray emission spectroscopy at ID20 of the European Synchrotron Radiation Facility, C.J. Sahle (a), F. Gerbon (a), C. Henriquet (a), R. Verbeni (a), B. Detlefs (a), A. Longo (a), A. Mirone (a), M.-C. Lagier (a), F. Otte (b,c), G. Spiekermann (d), S. Petitgirard (d), J. Synchrotron Rad. 30, 251-257 (2023);
(a) ESRF
(b) Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Dresden (Germany)
(c) The Rossendorf Beamline at ESRF
(d) Department of Earth Sciences, ETH Zürich, Zürich (Switzerland)