EMIL
PINK: X-Ray Emission Spectroscopy
X-ray emission spectroscopy (XES) has been a focus of study for transition metals in both the Kb main line and the valence-to-core (VtC) spectral regions. These regions offer valuable, element-selective insights into spin states and coordination environments, respectively. Recent experiments have demonstrated that analyzing weak VtC spectra can aid in identifying ligands surrounding the probed atom (Lancaster 2011) (Pollock 2013)({Pollock, 2015 #38} {Cutsail, 2019 #39}, {Levin, 2020 #40}.
In contrast to other X-ray absorption spectroscopy (XAS) and extended X-ray absorption fine structure (EXAFS) techniques, which investigate unoccupied orbitals and atomic scatters surrounding the metal, VtC XES predominantly explores the filled, ligand-based orbitals of a metal complex. The integration of VtC XES with density functional theory (DFT) calculations enables the differentiation of light ligand elements such as H, C, N, and O.
The high-flux PINK branch has been specifically designed for non-resonant XES in a tender to hard X-ray energy range spanning 2.1 keV to 9.5 keV (Peredkov). This energy range facilitates comprehensive XES and X-ray absorption spectroscopy (XAS) studies of transition metals from Ti to Cu (Ka, Kb lines) and Zr to Ag (La, Lb), as well as light elements like P, S, Cl, K, and Ca (Ka, Kb).
In future XAS measurements will also be possible combining both multilayer and DCM monochromators.
PINK station
Experiments conducted in the tender to hard x-ray region present specific challenges, including higher photon absorption in media, increased radiation damage to samples, and a more pronounced scattering background. These challenges introduce additional requirements for the design of the endstation, vacuum system, and sample environment.
The PINK station is split into three isolated vacuum areas. The ultra-high vacuum (UHV) segment comprises the beamline, a flexible bellow, a beam position monitor, and terminates with a 13µm beryllium window. The second area, the diagnostic chamber, is separated from the rest of the vacuum system, including a sample environment chamber, by a 20µm chemical vapor deposition (CVD) diamond window. The vacuum security
Figure 1: Scheme of the PINK endstation with installed cryogenic sample environment chamber CryoSEC.
system and gas supply system are controlled by a Programmable Logic Controller (PLC), which reads analogue signals from vacuum sensors and manages vacuum valves securely.
The setup's alignment is achieved using three beam position monitors: one 3.5 meters downstream of the multilayer mirror (BPM1), another 2 meters before the sample position (BPM2), and a third 0.7 meter after the sample position (BPM3). The BPMs are equipped with motorized 100 µm YAG and 20 µm CVD thick diamond screens monitored by high resolution video cameras. The diagnostic chamber houses an X-Y slit unit, a set of filters for flux reduction (diamond/Al/Ta), an intensity monitor (I0), and a fast shutter for controlling sample exposure. Additionally, for absolute flux measurements, two PIN photodiodes are employed – one in the diagnostic chamber and another at the end of the beamline.
The PINK setup features two in-house designed dispersive von Hamos X-ray spectrometers. A key advantage of dispersive spectrometers is their ability to capture the entire spectrum simultaneously. This feature translates into faster data acquisition times, which is particularly crucial when studying radiation-sensitive samples.
In designing these spectrometers, we aimed to strike a balance between resolution, efficiency, the available space for the sample environment, and cost considerations. Initially, our primary focus was on achieving the highest efficiency while maintaining a moderate energy resolution, given that the natural width of Kb, Lb, and VtC lines typically exceeds 1-1.5 eV. By employing a short-radius analyzer crystal with a bending radius of R=250 mm, we achieved an energy resolution ranging from 0.3 to 0.8 eV, covering a solid angle of about 0.05 sr or 9e-5 to 1.5e-4 sr/eV, depending on the span of Bragg angles and the crystal used.
One of the spectrometers is optimized for measurements in the lower energy range of 2.1-6 keV. It is integrated into a vacuum chamber and equipped with a CCD detector. The other spectrometer, used in ambient air, is designed for measurements between 6 and 9.5 keV and employs an Eger2 R 500K detector. Both analysers can operate simultaneously. This dual design enables two-color XES data collection capabilities, providing an opportunity for more detailed investigations of catalysts and metalloenzymes with more than one metal at the active sites.
In XES and XAS studies of metalloproteins, the impact of radiation damage is a significant consideration (Van Schooneveld 2015). The PINK beamline delivers a high photon flux, particularly in the tender X-ray energy range. This enhances the spectral signal but concurrently escalates the rate of radiation-induced damage. This damage is particularly notable at lower photon energies, specifically within the range of 2-5 keV, primarily due to the reduction in the attenuation length of the material. Three approaches are commonly used for measurements of radiation sensitive specimens: subjecting samples to cryogenic temperatures (Meents 2010) employing scanning techniques on the sample and utilizing high flow rate liquid cells.
There are two sample environment chambers that can be incorporated in to the PINK vacuum system. The Cryogenic Sample Environment Chamber (CryoSEC), developed in collaboration with HZB's sample environment group, facilitates sample cooling through helium exchange gas at a pressure of 10 mbar and a temperature of 30 Kelvin. An interchangeable sample carrier can accommodate up to 7 samples, allowing convenient loading in either liquid nitrogen or a glovebox. The loaded sample carrier can be housed in a vessel with liquid nitrogen. The sample exchange process, after venting the chamber with helium, typically takes 15-20 seconds and can be performed by one person. This procedure can be carried out while the cryo-chamber remains at 30 Kelvin.
For room temperature experiments, the Liquid Sample Environment Chamber (LiquidSEC) is available, operating under vacuum conditions of 2-10 mbar or in a helium atmosphere. This chamber can hold 4 samples and a liquid flow or electrochemical cell. It features two feedthroughs for liquid supplies and a DSUB-9 socket for connecting a potentiostat, along with an LED light source for studying photoinduced reactions. A miniature camera inside the chamber aids in alignment and controls the operation of the flow/electrochemical cells.
Both chambers are equipped with X-Y linear stages, enabling rapid sample scanning at speeds up to 1000 µm/s. By varying the scanning speed, the instantaneous irradiation time of the specimen can be reduced to 100-50 ms during a single scan over the sample area. This approach allows for signal collection while keeping the absorbed dose minimal.
The PINK branch entered the commissioning phase and initiated in-house experiments in the spring of 2019. The XES studies of P, S, Ru, Ti, Fe and Cu were conducted and subsequently published (Levin 2020) (Mathe 2021) (Geoghegan 2022) (Gerz 2021) (Hou 2023) (Liu 2023) (Peredkov). As of 2022, the facility is available for external users interested in conducting non-resonant XES experiments. The second – monochromatic mode – that giving access to resonant-XES and enhanced XAS – presently is under commissioning.
Where to learn more:
[1] U. Bergmann and P. Glatzel, "X-ray emission spectroscopy", Photosynth. Res., 255 (2009)
[2] M. A. Beckwith, M. Roemelt, M.-N. Collomb, C. DuBoc, T.-C. Weng U. Bergmann, P. Glatzel, F. Nesse and S. DeBeer, "Manganese Kb x-ray emission spectroscopy as a probe of metal-ligand interactions", Inorg. Chem, 50, 8397, (2011)
[3] C. J. Pollock, K. Grubel, P. H. Holland, S. DeBeer, "Experimentally quantifying small-molecule bond activation using valence-to-core x-ray emission spectroscopy", J. of the Am. Chem. Soc., 135, 32, 11803 (2013)
[4] J. Szlachetko, M. Nachtegaal, E. de Boni, M. Willimann, O. Safonova, J. Sa, G. Smolentsev, M. Szlachetko, J. A. van Bokhoven, J.-Cl. Dousse, J. Hoszowska, Y. Kayser, P. Jagodzinski, A. Bergamaschi, B. Schmitt, C. David, and A. Lücke, "A von Hamos x-ray spectrometer based on a segmented-type diffraction crystal for single-shot x-ray emission spectroscopy and time-resolved resonant inelastic x-ray scattering studies", Rev. Sci. Inst., 83, 103105 (2012)