Speaker
Description
At the MPI CEC, high energy resolution X-ray spectrometers are used for investigation molecular systems designed for storing and releasing energy in chemical bonds. X-ray absorption spectroscopy is, among the different spectroscopic techniques available at the institute, one option for interrogating the electronic and geometric structure of systems, permitting to investigate on an element-sensitive basis changes in oxidation state, coordination or interatomic distances upon catalytic cycling. In contrast to synchrotron radiation-based experiments, investigations are currently performed in transmission mode only. However, the flexible access to the instrumentation allows for short-notice experiments, while avoiding long-distance transfer of samples.
Depending on the required information, the near-edge or the extended range in the X-ray absorption fine structure can be explored, each with dedicated instruments. In the hard X-ray regime, XANES experiments can be realized using either a Johann- [1] or a von Hamos-type spectrometer, while for EXAFS a von Hamos-type spectrometer is available [2]. In the XANES instruments pure crystals with a rather large bending radius are used to make sure the required resolving power can be achieved, while for the EXAFS spectrometer the detection efficiency was prioritized in the design of the instrument. To this end, a highly annealed pyrolytic graphite crystal with a rather small radius of curvature is used. This design will prove beneficial in the realization of an in situ set up for thermal catalysis, with first experiments being designed around investigations of the reformation of ammonia.
Complementarily to the hard X-ray range, experiments in the soft X-ray range are possible as well on the laboratory level. These experiments are enabled by using a laser produced plasma source [3] instead of conventional X-ray tubes. These sources deliver high flux in the energy range from 200 eV to 1000 eV. Coupling them with reflective zoneplates in a dispersive scheme, allows for investigating the NEXAFS range around the K-ionization threshold of low-Z materials or the L-ionization threshold of 3d transition metals [4], or in other words studying the ligands or the metallic centers of molecular systems. Thus, complementary insights to the K-edge experiments on the 3d transition metals is offered. Further, due to the pulsed time pattern of the source even transient experiments are enabled, an option that is currently being commissioned for solid and liquid sample environments
The present status of the different instruments mentioned will be detailed in this contribution, along with selected examples and future development plans. The data that is collected using in-house X-ray spectrometers presents a valuable resource for delivering training data, at the premise that the instrumentation parameters are well controlled and accounted for. For this aspect curated data is essential. Reversely, a growing community of users of laboratory-based X-ray spectrometers will profit from advanced modelling tools.
References
[1] E. Jahrmann, et al. (2019). An improved laboratory-based x-ray absorption fine structure and x-ray emission spectrometer for analytical applications in materials chemistry research, Rev. Sci. Instrum., 90, 024106.
[2] C. Schlesiger, et al. (2020). Recent progress in the performance of HAPG based laboratory EXAFS and XANES spectrometers, J. Anal. At. Spectrom., 35, 2298-2304.
[3] I. Mantouvalou et al. (2015). High average power, highly brilliant laser-produced plasma source for soft X-ray spectroscopy, Rev. Sci. Instrum., 86, 035116.
[4] A. Jonaset al. (2019), Towards Poisson noise limited optical pump soft X-ray probe NEXAFS spectroscopy using a laser-produced plasma source, Opt. Express, 27, 36524-36537.
