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Figure: Photo of the spectrometer at CLAESS beamline (Top). Energy resolution of the spectrometer as a function of the energy studied (Left). Energy dispersion of CLEAR as a function of the energy studied (Right).
Cerdanyola del Vallès, 7th of December 2022. Spectroscopic techniques are nowadays widely exploited in the study of matter at the atomic scale. Particularly, X-ray absorption spectroscopy (XAS) is the most extended one, though other techniques such as X-ray emission spectroscopy (XES) are becoming increasingly popular in the study of several materials. These two techniques are, in fact, two sides of the same phenomena but require different methods to be used. In XAS, samples are illuminated with photons of specific energies selected to trigger some specific electronic transitions from occupied to empty levels, while in XES the photons used to illuminate are well above a particular absorption edge and what is probed are the occupied states. The first method is used to unveil the local electronic and structural properties of the material via the study of the evolution of its absorption coefficient as a function of energy, while the second one gives access to complementary information studying the decay which follow the absorption.
Conceived by ALBA researchers and installed at CLÆSS beamline, the CLEAR spectrometer allows to combine both techniques to achieve more reliable results. In this way the entangled parameters of the material (oxidation state, local structure, spin state…) are resolved to yield a unique and undoubted interpretation. Moreover, this experimental method allows real time study of dynamic processes with high reliability due to the direct correlation of the measures performed by both techniques.
The CLEAR X-ray spectrometer is an energy dispersive spectrometer based on a 1m radius Rowland circle geometry assembled in vacuum, which is highly convenient to avoid misleading signals coming from the sample environment. It has a wide Bragg angle range (40º-80º) and the disposition is a fully back scattering one relative to the sample. A dynamically bent Si(1,1,1) diced analyzer crystal is located inside the spectrometer, which allows to cover an energy range from 6 to 22 keV by means of exploiting the different reflections allowed in Si(1,1,1). The X-ray beam used to study the sample passes through the two halves of the analyzer.
The energy resolution of CLEAR depends on the size of the beam used, the Bragg angle picked for the experiment and the particular reflection being exploited to make the measurement, but it is typically in the range of 0.5-2 eV. Since that range is almost continuous, and since the post processing of the collected data is not to be performed by the user of the spectrometer who can rely on simple ascii files, it proves to be user friendly.
CLEAR is optimized for short time measurements of both emission lines and high-resolution absorption spectra of highly concentrated systems (around half an hour) but for highly diluted systems or weak emission lines, as the ones corresponding to the valence-to-core transitions, it is still not competitive and needs longer measurement times.
In light of the features and results of this pioneer spectrometer designed and built at ALBA, it is clear that it constitutes a promising alternative to other spectrometers. An exciting new tool implemented at CLÆSS that, due to its characteristics, is highly suitable for scientists willing to study highly correlated systems or researching in spintronics and battery design. An exciting step towards revealing more usually hidden details and characteristics of the fundamental structure of materials.
Link to the complete article published in Journal of Synchrotron Radiation: http://doi.org/10.1107/S1600577522009821
