Core-hole Lifetime Research Articles

High-energy-resolution off-resonant spectroscopy with self-seeded x-ray free-electron laser pulses.

This paper presents the implementation of high-energy-resolution off-resonant spectroscopy (HEROS) measurements using self-seeded x-ray free-electron laser (XFEL) pulses. This study systematically investigated XFEL conditions, including photon energy and accumulated shot numbers, to optimize the measurement efficiency for copper foil samples near the K-edge. The x-ray absorption spectra reconstructed using HEROS were compared with those derived from fluorescence-yield measurements. The HEROS-based spectra exhibited consistent line shapes independent of the sample thickness. The potential application of HEROS to high-temperature copper was also explored. HEROS offers distinct advantages including scan-free measurement of x-ray absorption spectra with reduced core-hole lifetime broadening and self-absorption effects. Using self-seeded XFEL pulses, HEROS facilitates single-shot-based pump-probe measurements to investigate the ultrafast dynamics in various materials and diverse conditions.

Time-Dependent Resonant Inelastic X-ray Scattering of Pyrazine at the Nitrogen K-Edge: A Quantum Dynamics Approach.

We calculate resonant inelastic X-ray scattering spectra of pyrazine at the nitrogen K-edge in the time domain including wavepacket dynamics in both the valence and core-excited state manifolds. Upon resonant excitation, we observe ultrafast non-adiabatic population transfer between core-excited states within the core-hole lifetime, leading to molecular symmetry distortions. Importantly, our time-domain approach inherently contains the ability to manipulate the dynamics of this process by detuning the excitation energy, which effectively shortens the scattering duration. We also explore the impact of pulsed incident X-ray radiation, which provides a foundation for state-of-the-art time-resolved experiments with coherent pulsed light sources.

The AXEAP2 program for Kβ X-ray emission spectra analysis using artificial intelligence.

The processing and analysis of synchrotron data can be a complex task, requiring specialized expertise and knowledge. Our previous work addressed the challenge of X-ray emission spectrum (XES) data processing by developing a standalone application using unsupervised machine learning. However, the task of analyzing the processed spectra remains another challenge. Although the non-resonant Kβ XES of 3d transition metals are known to provide electronic structure information such as oxidation and spin state, finding appropriate parameters to match experimental data is a time-consuming and labor-intensive process. Here, a new XES data analysis method based on the genetic algorithm is demonstrated, applying it to Mn, Co and Ni oxides. This approach is also implemented as a standalone application, Argonne X-ray Emission Analysis 2 (AXEAP2), which finds a set of parameters that result in a high-quality fit of the experimental spectrum with minimal intervention. AXEAP2 is able to find a set of parameters that reproduce the experimental spectrum, and provide insights into the 3d electron spin state, 3d-3p electron exchange force and Kβ emission core-hole lifetime.

Operando High Energy Resolution Fluorescence Detected (HERFD) X-Ray Absorption Spectroscopic Study of Toyota Mirai Gen.1 Catalysts

The development of proton exchange membrane fuel cells (PEMFCs) have received increasing attention for its potential to get rid of dependence on fossil fuels and curb carbon emissions. However, the application of PEMFC has been limited by the consumption of platinum due to the sluggish kinetics of the cathode reaction and limiting platinum resources. It is well-known that the oxygen species generated on the Pt surface hiders ORR reaction at high potential region. Therefore, it is very important to evaluate the interaction between the oxygen species and the catalyst based on the electronic structure. However, the differences in XAFS spectra due to the presence of oxygen species are relatively small, and conventional detection methods, such as transmittance and total fluorescence yield, have difficulty distinguishing differences in electronic structure because the fine structure of the spectra is filled in by the lifetime width of the Pt 2p 3/2 core hole. In contrast, the new high-energy-resolution fluorescence-detection X-ray absorption spectroscopy (HERFD-XAS) monochromatized fluorescence lines (e.g., Pt Lα1), which are broadened only by Pt 3d 5/2 (~2.4 eV) rather than by Pt 2p 3/2 (~5.2 eV) core hole lifetime, are used to monitor core High-resolution X-ray absorption spectra with extended hole lifetimes can now be extracted. The present study covers conventional platinum catalysts supported on Vulcan carbon and TOYOTA MIRAI Gen.1 catalysts. The HERFD-XAS method was applied to elucidate the cause of the superior electrochemical properties of the TOYOTA MIRAI Gen.1 catalyst. o perando Pt LIII edge HERFD-XAS measurements were performed on Pt/C and TOYOTA MIRAI Gen.1 catalysts under oxygen reduction conditions, and the high energy resolution revealed the interaction of oxygen species with the Pt surface.Pt/C (29.1 wt %; TEC10V30E, average particle size of 3 nm, observed by TEM) catalyst was purchased from TKK Co. Ltd., Japan. The Pt/C catalyst ink contained water and 2-propanol in a 3:2 ratio (99.99%, FUJIFILM Wako Pure Chemical Corporation, Japan), and the amount of Pt/C was adjusted to form a 20 μg carbon cm–2 catalyst layer after dropping 10 μL on a glassy carbon RDE (GC RDE, HOKUTO DENKO, Japan; 0.196 cm2 area). TOYOTA MIRAI Gen.1 catalyst was also prepared by the above-mentioned preparation method. operando XAS measurements for Pt LΙΙΙ-edge of the catalysts were carried out at BL39XU of SPring-8, Hyogo, Japan. Prior to the experiments, the samples were electrochemically cleaned by CV in the range 0.0–1.1 V (vs. RHE). The electrode potential was swept cathodically from 0.5 to 1.1 V (vs. RHE).Figure 1 shows the results for in situ HERFD-XAS spectra of Pt and MIRAI Gen.1 catalyst. For both samples, the white line intensity changes depending on the adsorption conditions. The obtained HERFD-XAS spectra were decomposed into oxidized (Pt-OH) and reduced (Pt metal) components. The ratio of the peak area intensity of the two components is proportional to the surface coverage assuming similar charge transfer among the catalysts. It is noted that enhancement of oxidation is significantly suppressed on MIRAI Gen.1 catalysts. Figure 1. operando HERFD-XAS spectra for (a) TOYOTA MIRAI Gen.1 catalyst. (b) Pt/C (TEC10V30E) Acknowledgement This work was supported by a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Unraveling higher-order contributions to spin excitations probed using resonant inelastic x-ray scattering

Resonant inelastic x-ray scattering (RIXS) is an evolving tool for investigating the spin dynamics of strongly correlated materials, which complements inelastic neutron scattering. In isotropic spin-$\frac{1}{2}$ Heisenberg antiferromagnetic (HAFM) spin chains, both techniques have observed non-spin-conserving (NSC) excitations confined to the two-spinon phase space. However, a recent O $K$-edge RIXS study of the one-dimensional HAFM ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$ observed spin-conserving (SC) four-spinon excitations outside the two-spinon phase space. Here, we demonstrate that analogous four-spinon excitations can also be accessed at the Cu ${L}_{3}$ edge in the related material ${\mathrm{SrCuO}}_{2}$. Through detailed modeling, we establish that these excitations appear in both the SC and NSC channels of the Cu ${L}_{3}$ edge, and are only captured by higher-order terms in the ultrashort core-hole lifetime expansion. Since these terms encode information about spin-spin correlations extending beyond nearest neighbors, our results offer different possibilities for studying nonlocal spin correlations in quantum magnets.

Near K-edge Photoionization and Photoabsorption of Singly, Doubly, and Triply Charged Silicon Ions

Experimental and theoretical results are presented for double, triple, and quadruple photoionization of Si+ and Si2+ ions and for double photoionization of Si3+ ions by a single photon. The experiments employed the photon–ion merged-beams technique at a synchrotron light source. The experimental photon-energy range 1835–1900 eV comprises resonances associated with the excitation of a 1s electron to higher subshells and subsequent autoionization. Energies, widths, and strengths of these resonances are extracted from high-resolution photoionization measurements, and the core-hole lifetime of K-shell ionized neutral silicon is inferred. In addition, theoretical cross sections for photoabsorption and multiple photoionization were obtained from large-scale multiconfiguration Dirac–Hartree–Fock calculations. The present calculations agree with the experiment much better than previously published theoretical results. The importance of an accurate energy calibration of laboratory data is pointed out. The present benchmark results are particularly useful for discriminating between silicon absorption in the gaseous and in the solid component (dust grains) of the interstellar medium.

High-energy resolution X-ray spectroscopy at actinide M4,5 and ligand K edges: what we know, what we want to know, and what we can know.

In recent years, scientists have progressively recognized the role of electronic structures in the characterization of chemical properties for actinide containing materials. High-energy resolution X-ray spectroscopy at the actinide M4,5 edges emerged as a promising direction because this method can probe actinide properties at the atomic level through the possibility of reducing the experimental spectral width below the natural core-hole lifetime broadening. Parallel to the technical developments of the X-ray method and experimental discoveries, theoretical models, describing the observed electronic structure phenomena, have also advanced. In this feature article, we describe the latest progress in the field of high-energy resolution X-ray spectroscopy at the actinide M4,5 and ligand K edges and we show that the methods are able to (a) provide fingerprint information on the actinide oxidation state and ground state characters (b) probe 5f occupancy, non-stoichiometry, defects, and ligand/metal ratio and (c) investigate the local symmetry and effects of the crystal field. We discuss the chemical aspects of the electronic structure in terms familiar to chemists and materials scientists and conclude with a brief description of new opportunities and approaches to improve the experimental methodology and theoretical analysis for f-electron systems.

Femtosecond x-ray absorption spectroscopy of pyrazine at the nitrogen K-edge: on the validity of the Lorentzian limit

We calculate the femtosecond x-ray absorption spectrum of pyrazine at the nitrogen K-edge including the wavepacket dynamics in both the valence and core-excited state manifolds. We do not invoke the widely used short-time (or Lorentzian) approximation which neglects the nuclear dynamics after the x-ray probe excitation. Instead, we calculate the x-ray-induced polarization in the time-domain where the optical pump as well as x-ray probe pulses are explicitly described. While the non-adiabatic population transfer following the optical excitation is well reproduced in the Lorentzian limit the transient x-ray absorption spectra obtained under this approximation lack some vibronic features, even when considering the short core-hole lifetime of nitrogen. We further demonstrate the effect of an increasingly longer pulse on the observed photo-triggered wavepacket dynamics which are blurred to the point that the x-ray probe response becomes effectively time-independent.

X-ray spectroscopic and first-principles investigation of lead tungstate under pressure

High-energy-resolution fluorescence-detected (HERFD) near-edge x-ray-absorption fine-structure measurements performed at the Pb and W ${L}_{3}$ absorption edges have been used to study the pressure dependence of the local atomic structure of $\mathrm{PbW}{\mathrm{O}}_{4}$ from 1 bar up to 22 GPa. Comparison of measured spectra with density-functional theory structural calculations and simulations based on solution of the Bethe-Salpeter equation establish an unambiguous $\mathrm{PbW}{\mathrm{O}}_{4}\text{\ensuremath{-}}\mathrm{I}$ to $\mathrm{PbW}{\mathrm{O}}_{4}\text{\ensuremath{-}}\mathrm{III}$ phase transition around 7 GPa. No evidence of either the Raspite or Fergusonite structures is found. The smaller core-hole lifetime broadening afforded by HERFD allows unique experimental determination of crystal-field splitting and its sensitivity to the oxygen cage geometry surrounding the Pb and W atoms thus giving definitive identification of the high-pressure phase of $\mathrm{PbW}{\mathrm{O}}_{4}$.

Description of Resonant Inelastic X-Ray Scattering in Correlated Metals

To fully capitalize on the potential and versatility of resonant inelastic x-ray scattering (RIXS), it is essential to develop the capability to interpret different RIXS contributions through calculations, including the dependence on momentum transfer, from first-principles for correlated materials. Toward that objective, we present new methodology for calculating the full RIXS response of a correlated metal in an unbiased fashion. Through comparison of measurements and calculations that tune the incident photon energy over a wide portion of the Fe L$_3$ absorption resonance of the example material BaFe$_2$As$_2$, we show that the RIXS response in BaFe$_2$As$_2$ is dominated by the direct channel contribution, including the Raman-like response below threshold, which we explain as a consequence of the finite core-hole lifetime broadening. Calculations are initially performed within the first-principles Bethe-Salpeter framework, which we then significantly improve by convolution with an effective spectral function for the intermediate-state excitation. We construct this spectral function, also from first-principles, by employing the cumulant expansion of the Green's function and performing a real-time time dependent density functional theory calculation of the response of the electronic system to the perturbation of the intermediate-state excitation. Importantly, this allows us to evaluate the indirect RIXS response from first-principles, accounting for the full periodicity of the crystal structure and with dependence on the momentum transfer.

Controlling core-hole lifetime through an x-ray planar cavity

It has long been believed that core-hole lifetime (CHL) of an atom is an intrinsic physical property, and controlling it is significant yet is very hard. Here, CHL of the 2p state of W atom is manipulated experimentally through adjusting the emission rate of a resonant fluorescence channel with the assistance of an x-ray thin-film planar cavity. The emission rate is accelerated by a factor linearly proportional to the cavity field amplitude, that can be directly controlled by choosing different cavity modes or changing the angle offset in experiment. This experimental observation is in good agreement with theoretical predictions. It is found that the manipulated resonant fluorescence channel even can dominate the CHL. The controllable CHL realized here will facilitate the nonlinear investigations and modern x-ray scattering techniques in hard x-ray region.

High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy: An Analytical Method for Selenium Speciation.

Selenium is in many ways an enigmatic element. It is essential for health but toxic in excess, with the difference between the two doses being narrower than for any other element. Environmentally, selenium is of concern due to its toxicity. As the rarest of the essential elements, its low levels often provide challenges to the analytical chemist. X-ray absorption spectroscopy (XAS) provides a powerful tool for in situ chemical speciation but is severely limited by poor spectroscopic resolution arising from core-hole lifetime broadening. Here we explore selenium Kα1 high energy resolution fluorescence detected XAS (HERFD-XAS) as a novel approach for chemical speciation of selenium, in comparison with conventional Se K-edge XAS. We present spectra of a range of selenium species relevant to environmental and life science studies, including spectra of seleno-amino acids, which show strong similarities with S K-edge XAS of their sulfur congeners. We discuss strengths and limitations of HERFD-XAS, showing improvements in both speciation performance and low concentration detection. We also develop a simple method to correct fluorescence self-absorption artifacts, which is generally applicable to any HERFD-XAS experiment.

Photon-in/photon-out spectroscopy at the I20-scanning beamline at diamond light source

A scanning multi-crystal x-ray emission spectrometer to perform photon-in/photon-out spectroscopy at the I20-Scanning beamline at Diamond Light Source is described. The instrument, equipped with three analyzer crystals, is based on a 1 m Rowland circle spectrometer operating in the vertical plane. The energy resolution of the spectrometer is of the order of 1 eV, having sufficient resolving power to overcome the core-hole lifetime broadening of most of the transition metals K-edges. Examples showing the capability of the beamline for performing high energy resolution fluorescence detection x-ray absorption spectroscopy (HERFD-XAS), non-resonant x-ray emission spectroscopy (XES) and resonant x-ray emission spectroscopy are presented. The comparison of the Zn and Mn K-edge HERFD–XANES of ZnO and MnO with ab initio calculations shows that the technique provides enhanced validation of the models by making subtle spectral features more visible.

Determining electronic properties from L -edge x-ray absorption spectra of transition metal compounds with artificial neural networks

X-ray absorption spectroscopy at the L-edge probes transitions of 2p-electrons into unoccupied d-states. Applied to transition metal atoms, this experimental technique can provide valuable information about the electronic structure of d-states. However, multiplet effects, spin-orbit coupling, a large number of possible transitions can cause a rather involved nature of 2p XAS spectra, which can often complicate extracting of information directly from them. Here, artificial neural networks trained on simulated spectra of a 2p XAS model Hamiltonian are presented that can directly determine information about atomic properties and the electronic configuration of d-states from L-edge X-ray absorption spectra. Moreover, the adaptable nature of artificial neural networks (ANNs) allows extending their capability to obtain information about the electronic ground state and core hole lifetimes from 2p XAS spectra as well as to incorporate external factors, such as temperature and experimental convolution that can affect details in spectral features. The effects of noise and background contributions in spectra on the accuracy of ANNs are discussed and the method is validated on experimental spectra of transition metal compounds, including metal-organic molecules and metal oxides.

Argon KLL Auger spectrum: Initial states, core-hole lifetimes, shake, and knock-down processes

State-of-the-art argon $KLL$ Auger spectra measured using photon energies of $h\ensuremath{\nu}=3216$ and 3400 eV are presented along with an Ar $[1s]$ photoelectron spectrum (square brackets indicate holes in the respective orbital). The two different photon energies used for measuring the Auger spectra allow distinguishing between the shake transitions during the Auger decay and the Auger transitions of the photoelectron satellites. A complete assignment of satellite transitions is provided, partially based on configuration-interaction calculations. In addition, Ar $[1s3(s,p)]{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}\ensuremath{\rightarrow}[2{p}^{2}{(}^{1}{D}_{2})]$ transitions are observed, which can be explained by knock-down transitions leading to a direct exchange of angular momentum between the excited electron and the Auger electron. The lifetime broadenings of the Ar $[2s]$ single-core-hole state and the $[2{s}^{2}]$ and $[2s2p]$ double-core-hole states are also determined, confirming previously observed trends for double-core-hole states.

Interfacial electronic coupling and band alignment of P3HT and exfoliated black phosphorous van der Waals heterojunctions

The band alignment and the interfacial electronic coupling of van der Waals heterojunctions formed by the Poly[3-hexylthiophene-2,5-diyl] (P3HT) semiconductor polymer and the mechanically exfoliated black phosphorous (BP) multilayers were investigated using different spectroscopies techniques. The energy band alignment was assessed using Ultraviolet (UPS) and X-ray (XPS) photoelectron spectroscopies. The result of this analysis shows that the P3HT-BP interface presents a type I band alignment with conduction band minimum and the valence band maximum located in the BP layer. A valence band offset of 0.45 eV and a conduction band offset of 1.25 eV parameters were found. Near-Edge X-ray Absorption Fine Structure (NEXAFS) and resonant Auger (RAS) synchrotron-based spectroscopies were used to investigate the interfacial electronic coupling between BP and P3HT. Interfacial charge transfer times obtained from core hole clock approach and using the S1s core–hole lifetime as a reference time were used as quantitative parameters to measure the degree of electronic coupling. We found that the π*(SC) electronic state is the faster electron delocalization pathway from the P3HT to the BP conduction band and therefore there is a strong electronic coupling between these states. This result was supported from density functional theoretical calculations.

Observing photo-induced chiral edge states of graphene nanoribbons in pump-probe spectroscopies

Photo-induced edge states in low-dimensional materials have attracted considerable attention due to the tunability of topological properties and dispersion. Specifically, graphene nanoribbons have been predicted to host chiral edge modes upon irradiation with circularly polarized light. Here, we present numerical calculations of time-resolved angle resolved photoemission spectroscopy and trRIXS of a graphene nanoribbon. We characterize pump-probe spectroscopic signatures of photo-induced edge states, illustrate the origin of distinct spectral features that arise from Floquet topological edge modes, and investigate the roles of incoming photon energies and finite core–hole lifetime in RIXS. With momentum, energy, and time resolution, pump-probe spectroscopies can play an important role in understanding the behavior of photo-induced topological states of matter.

3d-4f Resonant Inelastic X-ray Scattering of Actinide Dioxides: Crystal-Field Multiplet Description.

A theoretical overview of the core-to-core (3d-4f) resonant inelastic X-ray scattering (RIXS) spectra of actinide dioxides AnO2 (An = Th, U, Np, Pu, Am, Cu, Bk, Cf) is provided. The 3d-4f RIXS maps were calculated using crystal-field multiplet theory and turned out to be significantly different at the An M5 vs M4 edges, because of selection rules and final state effects. The results of the calculations allowed for a general analysis of so-called high-energy-resolution fluorescence-detected X-ray absorption (HERFD-XAS) spectra. The cuts of the calculated RIXS maps along the incident energy axis at the constant emitted energy, corresponding to the maximum of the RIXS intensity, represented the HERFD spectra and provided their comparison with calculated conventional X-ray absorption (XAS) spectra with a reduced core-hole lifetime broadening at the An M5 and M4 edges. Although the An M5 HERFD profiles were found to depart from the X-ray absorption cross-section, in terms of appearing additional transitions, the results of calculations for the An M4 edges demonstrate overall better agreement between the HERFD and XAS spectra for most dioxides, keeping in mind a restricted HERFD resolution, because of the core–hole lifetime broadening in the final state. The results confirm the utility of HERFD for the An chemical state determination and indicate the importance of calculating the entire RIXS process in order to interpret the HERFD data correctly.

Hv2-concept breaks the photon-count limit of RIXS instrumentation.

Upon progressive refinement of energy resolution, the conventional resonant inelastic X-ray scattering (RIXS) instrumentation reaches the limit where the bandwidth of incident photons becomes insufficient to deliver an acceptable photon-count rate. Here it is shown that RIXS spectra as a function of energy loss are essentially invariant to their integration over incident energies within the core-hole lifetime. This fact permits RIXS instrumentation based on the hv2-concept to utilize incident synchrotron radiation over the whole core-hole lifetime window without any compromise on the much finer energy-loss resolution, thereby breaking the photon-count limit.

Deep-core photoionization of krypton atoms below and above the 1s ionization threshold

Electronic relaxation of atomic Kr below and above the $1s$ ionization threshold is investigated experimentally using hard x-ray photoelectron spectroscopy. The experimental results are interpreted with the aid of relativistic Dirac-Fock calculations. The $1s$ orbital core-hole lifetime is extracted and the satellite's structures accompanying the photoelectron main line are assigned. Auger spectra recorded below and above the $K$ edge are also investigated. In particular, the Auger cascade originating from vacancies of different origin in the $L$ shell is analyzed in great detail. Competition between radiative $KL$ versus nonradiative $KLL$ Auger emission is emphasized.