Resonant X-ray Emission Spectroscopy Research Articles

The insufficiency of the charge transfer model for resonant XES in UF4

The insufficiency of the charge transfer model for resonant XES in UF4

The role of carboxylate ligand orbitals in the breathing dynamics of a metal-organic framework by resonant X-ray emission spectroscopy.

Metal-organic frameworks (MOFs) exhibit structural flexibility induced by temperature and guest adsorption, as demonstrated in the structural breathing transition in certain MOFs between narrow-pore and large-pore phases. Soft modes were suggested to entropically drive such pore breathing through enhanced vibrational dynamics at high temperatures. In this work, oxygen K-edge resonant X-ray emission spectroscopy of the MIL-53(Al) MOF was performed to selectively probe the electronic perturbation accompanying pore breathing dynamics at the ligand carboxylate site for metal-ligand interaction. It was observed that the temperature-induced vibrational dynamics involves switching occupancy between antisymmetric and symmetric configurations of the carboxylate oxygen lone pair orbitals, through which electron density around carboxylate oxygen sites is redistributed and metal-ligand interactions are tuned. In turn, water adsorption involves an additional perturbation of π orbitals not observed in the structural change solely induced by temperature.

Resonant X-ray emission spectroscopy using self-seeded hard X-ray pulses at PAL-XFEL.

Self-seeded hard X-ray pulses at PAL-XFEL were used to commission a resonant X-ray emission spectroscopy experiment with a von Hamos spectrometer. The self-seeded beam, generated through forward Bragg diffraction of the [202] peak in a 100 µm-thick diamond crystal, exhibited an average bandwidth of 0.54 eV at 11.223 keV. A coordinated scanning scheme of electron bunch energy, diamond crystal angle and silicon monochromator allowed us to map the Ir Lβ2 X-ray emission lines of IrO2 powder across the Ir L3-absorption edge, from 11.212 to 11.242 keV with an energy step of 0.3 eV. This work provides a reference for hard X-ray emission spectroscopy experiments utilizing self-seeded pulses with a narrow bandwidth, eventually applicable for pump-probe studies in solid-state and diluted systems.

Resonant Excitation Unlocks Chemical Selectivity of Platinum Lβ Valence-to-Core X-ray Emission Spectra.

Valence-to-core X-ray emission spectroscopy (VtC XES) is an emerging technique that uses hard X-rays to probe the valence electronic structure of an absorbing atom. Despite finding varied applications for light elements and first row transition metals, little work has been done on heavier elements such as second and third row transition metals. This lack of application is at least partially due to the relatively low resolution of the data at the high energies required to measure these elements, which obscures the useful chemical information that can be extracted from the lower energy, higher resolution spectra of lighter elements. Herein, we collect data on a set of platinum-containing compounds and demonstrate that the VtC XES resolution can be dramatically enhanced by exciting the platinum atom in resonance with its L3-edge white line absorption. Whereas spectra excited using standard nonresonant absorption well above the Pt L3-edge display broad, unfeatured VtC regions, resonant XES (RXES) spectra have more than twofold improved resolution and are revealed to be rich in chemical information with the ability to distinguish between even closely related species. We further demonstrate that these RXES spectra may be used to selectively probe individual components of a mixture of Pt-containing compounds, establishing this technique as a viable probe for chemically complex samples. Lastly, it is shown that the spectra are interpretable using a molecular orbital framework and may be calculated using density functional theory, thus suggesting resonant excitation as a general strategy for extracting chemically useful information from heavy element VtC spectra.

Observation of Raman components in resonant X-ray emission spectra of Fe cyanide

There were two theoretical calculations to reproduce the Fe 2p absorption spectrum of hexacyanoferrate [R. K. Hocking et al., J. Am. Chem. Soc. 128 (2006) 10442, and Y. Nanba and K. Okada: J. Phys. Soc. Jpn. 79 (2010) 114722], but there was a big difference in the parameters used in both. To confirm which of the theoretical calculations is more suitable, we have carried out the experiments of the resonant X-ray emission spectroscopy (RXES) around the Fe Kβ emission lines for K4Fe(CN)6 and K3Fe(CN)6, in order to acquire the electronic structure of Fe atom as a model compound of hexacyanoferrate. In the RXES results of K4Fe(CN)6, three Raman scattering peaks are observed at 6.5, 9.5, and 15.4 eV near the elastic peak, respectively, called Raman Peak (RP) 1, RP2, and RP3. By comparing theoretical calculations of Raman scattering peaks of hexacyanoferrate [Y. Nanba: Doctoral Thesis, Okayama University, 2012 (in Japanese)], the Raman shift energies of RP1 were close to those of the metal-to-ligand charge transfer (MLCT). On the other hand, the Raman shift energies of RP3 were close to those of the ligand-to-metal charge transfer (LMCT). A similar result was also obtained in those of K3Fe(CN)6. It was found that the features of the MLCT excitation and the LMCT one were observed using the RXES experiments.

Sulfur-Ligated [2Fe-2C] Clusters as Synthetic Model Systems for Nitrogenase.

Metal clusters featuring carbon and sulfur donors have coordination environments comparable to the active site of nitrogenase enzymes. Here, we report a series of di-iron clusters supported by the dianionic yldiide ligands, in which the Fe sites are bridged by two μ2-C atoms and four pendant S donors.The [L2Fe2] (L = {[Ph2P(S)]2C}2-) cluster is isolable in two oxidation levels, all-ferrous Fe2II and mixed-valence FeIIFeIII. The mixed-valence cluster displays two peaks in the Mössbauer spectra, indicating slow electron transfer between the two sites. The addition of the Lewis base 4-dimethylaminopyridine to the Fe2II cluster results in coordination with only one of the two Fe sites, even in the presence of an excess base. Conversely, the cluster reacts with 8 equiv of isocyanide tBuNC to give a monometallic complex featuring a new C-C bond between the ligand backbone and the isocyanide. The electronic structure descriptions of these complexes are further supported by X-ray absorption and resonant X-ray emission spectroscopies.

A compact von Hámos spectrometer for parallel X-ray Raman scattering and X-ray emission spectroscopy at ID20 of the European Synchrotron Radiation Facility.

A compact spectrometer for medium-resolution resonant and non-resonant X-ray emission spectroscopy in von Hámos geometry is described. The main motivation for the design and construction of the spectrometer is to allow for acquisition of non-resonant X-ray emission spectra while measuring non-resonant X-ray Raman scattering spectra at beamline ID20 of the European Synchrotron Radiation Facility. Technical details are provided and the performance and possible use of the spectrometer are demonstrated by presenting results of several X-ray spectroscopic methods on various compounds.

High-efficiency X-ray emission spectroscopy of cold-compressed Fe2O3and laser-heated pressurized FeCO3using a von Hámos spectrometer

Fast and efficient (resonant) X-ray emission spectroscopy setup for the electronic structure at lower mantle conditions: high spin in laser-heated FeCO3and a two-step spin transition in pressurized Fe2O3with increasing valence to core intensity.

Resonant Inelastic Soft X-ray Scattering and X-ray Emission Spectroscopy of Solid Proline and Proline Solutions.

The amino group of proline is part of a pyrrolidine ring, which makes it unique among the proteinogenic amino acids. To unravel its full electronic structure, proline in solid state and aqueous solution is investigated using X-ray emission spectroscopy and resonant inelastic soft X-ray scattering. By controlling the pH value of the solution, proline is studied in its cationic, zwitterionic, and anionic configurations. The spectra are analyzed within a "building-block principle" by comparing with suitable reference molecules, i.e., acetic acid, cysteine, and pyrrolidine, as well as with spectral calculations based on density functional theory. We find that the electronic structure of the carboxyl group of proline is very similar to that of other amino acids as well as acetic acid. In contrast, the electronic structure of the amino group is significantly different and strongly influenced by the ring structure of proline.

The MagneDyn beamline at the FERMI free electron laser.

The scope of this paper is to outline the main marks and performances of the MagneDyn beamline, which was designed and built to perform ultrafast magnetodynamic studies in solids. Open to users since 2019, MagneDyn operates with variable circular and linear polarized femtosecond pulses delivered by the externally laser-seeded FERMI free-electron laser (FEL). The very high degree of polarization, the high pulse-to-pulse stability, and the photon energy tunability in the 50-300eV range allow performing advanced time-resolved magnetic dichroic experiments at the K-edge of light elements, e.g., carbon and at the M- and N-edge of the 3d-transition-metals and rare earth elements, respectively. To this end, two experimental end-stations are available. The first is equipped with an in situ dedicated electromagnet, a cryostat, and an extreme ultraviolet Wollaston-like polarimeter. The second, designed for carry-in user instruments, hosts also a spectrometer for pump-probe resonant x-ray emission and inelastic spectroscopy experiments with a sub-eV energy resolution. A Kirkpatrick-Baez active optics system provides a minimum focus of ∼20×20μm2 FWHM at the sample. A pump laser setup, synchronized with the FEL-laser seeding system, delivers sub-picosecond pulses with photon energies ranging from the mid-IR to near-UV for optical pump-FEL probe experiments with a minimal pump-probe jitter of few femtoseconds. The overall combination of these features renders MagneDyn a unique state-of-the-art tool for studying ultrafast magnetic and resonant emission phenomena in solids.

Metal-insulator transition in CuIr2S4 observed by Cu Kα resonant x-ray emission spectroscopy

We have investigated the Cu-derived electronic structure of iridium spinel compound ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ with a metal-insulator transition (MIT) at ${T}_{\mathrm{MIT}}\ensuremath{\sim}230$ K by means of high-energy resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS) and Cu $K\ensuremath{\alpha}$ resonant x-ray emission spectroscopy (RXES) around the Cu $K$ absorption edge. The Cu $K$ HERFD-XAS spectrum in the metallic phase exhibits a peak in the pre-edge region at an incident photon energy of $h{\ensuremath{\nu}}_{\mathrm{in}}=8981$ eV. The peak is ascribed to the Cu $1s--3d$ quadrupole transition and is abruptly shifted to higher energy by about 0.5 eV on going from metallic to insulating phases. The energy shift of the unoccupied Cu $3d$ states is related to the insulator gap formation of ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ via the hybridization with the Ir $5d$--S $3p$ bands. A hysteresis behavior is observed in the Cu $1s--3d$ peak intensity between cooling and heating processes around ${T}_{\mathrm{MIT}}$, consistent with those observed in the transport properties. The abrupt change in the unoccupied Cu $3d$ states across MIT is also clearly detected in the Cu $K\ensuremath{\alpha}$ RXES spectra.

AXEAP: a software package for X-ray emission data analysis using unsupervised machine learning.

The Argonne X-ray Emission Analysis Package (AXEAP) has been developed to calibrate and process X-ray emission spectroscopy (XES) data collected with a two-dimensional (2D) position-sensitive detector. AXEAP is designed to convert a 2D XES image into an XES spectrum in real time using both calculations and unsupervised machine learning. AXEAP is capable of making this transformation at a rate similar to data collection, allowing real-time comparisons during data collection, reducing the amount of data stored from gigabyte-sized image files to kilobyte-sized text files. With a user-friendly interface, AXEAP includes data processing for non-resonant and resonant XES images from multiple edges and elements. AXEAP is written in MATLAB and can run on common operating systems, including Linux, Windows, and MacOS.

Intermediate valence state in YbB4 revealed by resonant x-ray emission spectroscopy

We report the temperature dependence of the Yb valence in the geometrically frustrated compound from 12 to 300 K using resonant x-ray emission spectroscopy at the Yb transition. We find that the Yb valence, v, is hybridized between the v = 2 and v = 3 valence states, increasing from at 12 K to at 300 K, confirming that is a Kondo system in the intermediate valence regime. This result indicates that the Kondo interaction in is substantial, and is likely to be the reason why does not order magnetically at low temperature, rather than this being an effect of geometric frustration. Furthermore, the zero-point valence of the system is extracted from our data and compared with other Kondo lattice systems. The zero-point valence seems to be weakly dependent on the Kondo temperature scale, but not on the valence change temperature scale T v .

Local Coordination and Electronic Structure Ramifications of Guest-Dependent Spin Crossover in a Metal-Organic Framework: A Combined X-ray Absorption and Emission Spectroscopy Study.

The porous Hoffman-type 3D lattice Fe(pz)[NiII(CN)4] exhibits thermally induced spin-crossover (SCO) behavior that is dependent on the solvent guest species occupying the pores. Here, in situ Fe K-edge X-ray absorption spectroscopy (XAS) and both non-resonant and resonant Kβ X-ray emission spectroscopy (XES) methods are used to probe this framework under two solvent environments that yield different extremes of spin crossover temperature: acetonitrile and toluene. While the acetonitrile pore environment engenders an SCO response around room temperature, toluene guests stabilize the high spin state and effectively suppress SCO behavior throughout the ambient temperature range. The multipronged X-ray spectroscopy approach simultaneously confirmed this spin crossover behavior and provided new local coordination and electronic structural insights of the framework under these two solvent environments. Extended X-ray absorption fine structure analysis revealed spin state and solvent guest-dependent differences in coordination bond lengths and structural disorder. Resonant XES measurements produced high-resolution XAS spectra with distinct pre-edge and edge features, whose assignment was established using both simple ligand field theory and time-dependent density-functional theory calculations and further supported by their observed resonance behavior in the 2D RXES plane. Edge feature variation with the Fe spin state was interpreted to reveal changes in specific metal-linker bond covalency.

Challenges and Opportunities for Applications of Advanced X-ray Spectroscopy in Catalysis Research

X-ray spectroscopy has had a significant and continually growing impact on catalysis research for nearly 50 years. In particular, the ability to obtain element selective electronic and geometric structural information via the X-ray absorption (XAS) edge and extended X-ray absorption fine structure regions, respectively, has been a major asset for catalysis research. In the last two decades, the development of dedicated synchrotron-based X-ray emission spectrometers has greatly expanded the range of possible experiments, enabling both nonresonant and resonant X-ray emission spectroscopy experiments that can provide greater selectivity and more detailed electronic and structural information. Herein, we briefly review the range of hard X-ray photon-in, photon-out experiments that are presently possible, and highlight their recent applications in catalysis research. We also discuss the ongoing need for conventional XAS applications, either in standalone applications or in combination with more advanced approaches. The open opportunities and ongoing challenges for applying these methods, and ultimately for analyzing and interpreting the data, are also discussed.

Valence transition of the intermetallic compound Ce2Rh2Ga probed by resonant x-ray emission spectroscopy

We have investigated the Ce valence of an intermetallic compound ${\mathrm{Ce}}_{2}{\mathrm{Rh}}_{2}\mathrm{Ga}$ with a phase transition at ${T}_{t}=128.5$ K by means of high-energy resolution fluorescence-detected x-ray absorption spectroscopy (HERFD-XAS) and resonant x-ray emission spectroscopy (RXES) around the Ce ${L}_{3}$ edge. The HERFD-XAS spectra in the whole temperature range from 300 to 3 K exhibit weak ${f}^{0}({\mathrm{Ce}}^{4+})$ and ${f}^{2}({\mathrm{Ce}}^{2+})$ peaks as well as the ${f}^{1}({\mathrm{Ce}}^{3+})$ main peak, indicating that the Ce $4f$ electrons hybridize with the conduction electrons. With decreasing temperature from 125 to 100 K across ${T}_{t}$, the ${f}^{1}$ peak is reduced, while the ${f}^{0}$ peak is enhanced. Concomitantly, the Ce $L{\ensuremath{\alpha}}_{1}$ RXES intensity taken at the ${f}^{2}$ peak of the HERFD-XAS spectrum decreases. These spectroscopic results indicate that the phase transition at ${T}_{t}$ of ${\mathrm{Ce}}_{2}{\mathrm{Rh}}_{2}\mathrm{Ga}$ is driven by a valence transition of the Ce ions.

Extended quasiparticle approach to non-resonant and resonant X-ray emission spectroscopy.

The initial states of the secondary processes of X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) are highly excited eigenstates with a deep core hole after a X-ray photoelectron spectroscopy (XPS) process and a X-ray photoabsorption spectroscopy (XAS) process, respectively, so that the XES and RIXS calculation offers a good example of extended quasiparticle theory (EQPT) (K. Ohno, S. Ono and T. Isobe, J. Chem. Phys., 2017, 146, 084108) which is applicable to any initial exited eigenstate. We apply the standard one-shot GW + Bethe-Salpeter equation (BSE) approach in MBPT to this problem on the basis of EQPT and analyze XES and RIXS spectra for CH4, NH3, H2O, and CH3OH molecules. We also suggest a simpler approach only using the GW calculation without solving the BSE to compute the XES and RIXS energies, although it cannot give the spectral intensity. Moreover, according to extended Kohn-Sham theory (T. Nakashima, H. Raebiger and K. Ohno, Phys. Rev. B, 2021, 104, L201116), we give a justification and comment of applying the method relying on time-dependent density functional theory as well as the one-shot GW + BSE approach to this problem.

Operando resonant soft X-ray emission spectroscopy of the LiMn2O4 cathode using an aqueous electrolyte solution.

The Mn 3d electronic-structure change of the LiMn2O4 cathode during Li-ion extraction/insertion in an aqueous electrolyte solution was studied by operando resonant soft X-ray emission spectroscopy (RXES). The Mn L3 RXES spectra for the charged state revealed the Mn4+ state with strong charge-transfer from the O 2p to Mn 3d orbitals dominates, while for the open-circuit-voltage and discharged states it is ascribed to the mixture of sites with Mn3+ and Mn4+ states. The degree of charge transfer is significantly different between the Mn3+ and Mn4+ states, indicating that the redox reaction takes place on the strongly-hybridized Mn 3d-O 2p orbital rather than the localized Mn 3d orbital.

Resonant X-ray Emission Spectroscopy with a SASE Beam

Aqueous iron (III) oxide nanoparticles were irradiated with pure self-amplified spontaneous emission (SASE) X-ray free-electron laser (XFEL) pulses tuned to the energy around the Fe K-edge ionization threshold. For each XFEL shot, the incident X-ray pulse spectrum and Fe Kβ emission spectrum were measured synchronously with dedicated spectrometers and processed through a reconstruction algorithm allowing for the determination of Fe Kβ resonant X-ray emission spectroscopy (RXES) plane with high energy resolution. The influence of the number of X-ray shots employed in the experiment on the reconstructed data quality was evaluated, enabling the determination of thresholds for good data acquisition and experimental times essential for practical usage of scarce XFEL beam times.

Resonant X-ray emission spectroscopy from broadband stochastic pulses at an X-ray free electron laser

Hard X-ray spectroscopy is an element specific probe of electronic state, but signals are weak and require intense light to study low concentration samples. Free electron laser facilities offer the highest intensity X-rays of any available light source. The light produced at such facilities is stochastic, with spikey, broadband spectra that change drastically from shot to shot. Here, using aqueous ferrocyanide, we show that the resonant X-ray emission (RXES) spectrum can be inferred by correlating for each shot the fluorescence intensity from the sample with spectra of the fluctuating, self-amplified spontaneous emission (SASE) source. We obtain resolved narrow and chemically rich information in core-to-valence transitions of the pre-edge region at the Fe K-edge. Our approach avoids monochromatization, provides higher photon flux to the sample, and allows non-resonant signals like elastic scattering to be simultaneously recorded. The spectra obtained match well with spectra measured using a monochromator. We also show that inaccurate measurements of the stochastic light spectra reduce the measurement efficiency of our approach.