K-edge Absorption Spectra Research Articles

Reduced Density Matrix Formulation of Quantum Linear Response.

The prediction of spectral properties via linear response (LR) theory is an important tool in quantum chemistry for understanding photoinduced processes in molecular systems. With the advances of quantum computing, we recently adapted this method for near-term quantum hardware using a truncated active space approximation with orbital rotation, named quantum linear response (qLR). In an effort to reduce the classic cost of this hybrid approach, we here derive and implement a reduced density matrix (RDM) driven approach of qLR. This allows for the calculation of spectral properties of moderately sized molecules with much larger basis sets than so far possible. We report qLR results for benzene and R-methyloxirane with a cc-pVTZ basis set and study the effect of shot noise on the valence and oxygen K-edge absorption spectra of H2O in the cc-pVTZ basis.

A new parameterization of the DFT/CIS method with applications to core-level spectroscopy.

Time-dependent density functional theory (TD-DFT) within a restricted excitation space is an efficient means to compute core-level excitation energies using only a small subset of the occupied orbitals. However, core-to-valence excitation energies are significantly underestimated when standard exchange-correlation functionals are used, which is partly traceable to systemic issues with TD-DFT's description of Rydberg and charge-transfer excited states. To mitigate this, we have implemented an empirically modified combination of configuration interaction with single substitutions (CIS) based on Kohn-Sham orbitals, which is known as "DFT/CIS." This semi-empirical approach is well-suited for simulating x-ray near-edge spectra, as it contains sufficient exact exchange to model charge-transfer excitations yet retains DFT's low-cost description of dynamical electron correlation. Empirical corrections to the matrix elements enable semi-quantitative simulation of near-edge x-ray spectra without the need for significant a posteriori shifts; this should be useful in complex molecules and materials with multiple overlapping x-ray edges. Parameter optimization for use with a specific range-separated hybrid functional makes this a black-box method intended for both core and valence spectroscopy. Results herein demonstrate that realistic K-edge absorption and emission spectra can be obtained for second- and third-row elements and 3d transition metals, with promising results for L-edge spectra as well. DFT/CIS calculations require absolute shifts that are considerably smaller than what is typical in TD-DFT.

Structural and chemical analysis of hard carbon negative electrode for Na-ion battery with X-ray Raman scattering and solid-state NMR spectroscopy

This study explores the structural changes of hard carbon (HC) negative electrodes in sodium-ion batteries induced by insertion of Na ions during sodiation. X-ray Raman spectroscopy (XRS) was used to record both C and Na K-edge absorption spectra from bulk HC anodes carbonized at different temperatures and at several points during sodiation and desodiation. Comparing the π∗/σ∗ regions in the C K-edge spectra sp2/sp3 hybridization ratio of material was determined. Higher carbonization temperatures led to increased order in graphitic structure and shorter ▪ bond lengths. Sodiation caused a decrease in graphitic layer order due to inserted Na ions. Complementary operando solid state 23Na nuclear magnetic resonance (ssNMR) studies confirmed the structural changes, while showing pore filling mechanism, which is not observed in ex situ measurements, primarily at higher carbonization temperatures. XRS analysis of Na K-edge spectra revealed systematic variations in the solid electrolyte interface (SEI) composition during cycling. Changes in XRS spectra were attributed to both SEI composition alterations, accompanied by the insertion/adsorption of Na ions at defect sites within the carbon structure.

Ultrafast Energy Transfer from Photoexcited Tryptophan to the Haem in Cytochrome c.

We report femtosecond Fe K-edge absorption (XAS) and nonresonant X-ray emission (XES) spectra of ferric cytochrome C (Cyt c) upon excitation of the haem (>300 nm) or mixed excitation of the haem and tryptophan (<300 nm). The XAS and XES transients obtained in both excitation energy ranges show no evidence for electron transfer processes between photoexcited tryptophan (Trp) and the haem, but rather an ultrafast energy transfer, in agreement with previous ultrafast optical fluorescence and transient absorption studies. The reported (J. Phys. Chem. B 2011, 115 (46), 13723-13730) decay times of Trp fluorescence in ferrous (∼350 fs) and ferric (∼700 fs) Cyt c are among the shortest ever reported for Trp in a protein. The observed time scales cannot be rationalized in terms of Förster or Dexter energy transfer mechanisms and call for a more thorough theoretical investigation.

Electron energy-loss spectra of functionalized titanium and zirconium MXene monolayers, including excitonic effects

Using the full-potential linearized augmented plane wave (FP-LAPW)-based energy-loss near-edge structure (ELNES), the present study deals with the electronic properties of titanium and zirconium MXene monolayers. Specifically, the carbon and nitrogen K-edges of M2CT2 and M2NT2 (M = Ti, Zr; T = F, O, OH) MXene monolayers have been calculated and compared with those of the existing experimental spectra. On going from the bulk systems to MXene monolayers, the main spectral features occur at higher energies as a result of the shorter M-X bond lengths in the monolayers. As fingerprints of the terminal groups, in the C K- and N K-edges of Ti- and Zr-based terminated MXenes, the oscillator strengths and overall spectral shapes are changed, and in comparison to bare MXenes, the main spectral features are red-shifted. Moreover, on going from O termination to OH and then to F, one can observe a slight red-shift in the K-edge spectra of Ti2C(N) and Zr2C(N). While the peaks originating from the px + py states are modified through the appending of terminal elements, showing an obvious red-shift, the terminal substituent has no significant effect on features engendered by pz character. In addition, including excitonic effects on the K-edge absorption spectra considerably changes the response of MXene semiconductors, resulting in red-shifts of the ELNES spectral features.

Transition-potential coupled cluster II: optimisation of the core orbital occupation number

The issue of orbital relaxation in computational core-hole spectroscopy, specifically x-ray absorption, has been a major problem for methods such as equation-of-motion coupled cluster with singles and doubles (EOM-CCSD). The transition-potential coupled cluster (TP-CC) method is utilised to address this problem by including an explicit treatment of orbital relaxation via the use of reference orbitals with a fractional core occupation number. The value of the fractional occupation parameter λ was optimised for both TP-CCSD and XTP-CCSD methods in an element-specific manner due to the differences in atomic charge and energy scale. Additionally, TP-CCSD calculations using the optimised parameters were performed for the K-edge absorption spectra of gas-phase adenine and thymine. TP-CCSD reproduces the valence region well and requires smaller overall energy shifts in comparison to EOM-CCSD, while also improving on the relative position and intensities of several absorption peaks.

Characterization of Electrochemical Processes in Metal-Organic Batteries by X-ray Raman Spectroscopy.

X-ray Raman spectroscopy (XRS) is an emerging spectroscopic technique that utilizes inelastic scattering of hard X-rays to study X-ray absorption edges of low Z elements in bulk material. It was used to identify and quantify the amount of carbonyl bonds in a cathode sample, in order to track the redox reaction inside metal–organic batteries during the charge/discharge cycle. XRS was used to record the oxygen K-edge absorption spectra of organic polymer cathodes from different multivalent metal–organic batteries. The amount of carbonyl bond in each sample was determined by modeling the oxygen K-edge XRS spectra with the linear combination of two reference compounds that mimicked the fully charged and the fully discharged phases of the battery. To interpret experimental XRS spectra, theoretical calculations of oxygen K-edge absorption spectra based on density functional theory were performed. Overall, a good agreement between the amount of carbonyl bond present during different stages of battery cycle, calculated from linear combination of standards, and the amount obtained from electrochemical characterization based on measured capacity was achieved. The electrochemical mechanism in all studied batteries was confirmed to be a reduction of double carbonyl bond and the intermediate anion was identified with the help of theoretical calculations. X-ray Raman spectroscopy of the oxygen K-edge was shown to be a viable characterization technique for accurate tracking of the redox reaction inside metal–organic batteries.

Detailed study of reactively sputtered ScN thin films at room temperature

To contemplate an alternative approach for the minimization of diffusion at high temperature, present findings impart viability of room-temperature deposited reactively sputtered ScN thin films. The adopted route endows precise control over the RN2 flow for a methodical structural phase evolution from Sc→ScN and probe the correlated physical aspects of the highly textured ScN samples. In the nitrided regime i.e. at RN2 = 2.5–100% flow, incorporation of unintentional oxygen defects was evidenced from surface sensitive soft x-ray absorption spectroscopy study, though less compared to their metal (RN2 = 0%) and interstitial (RN2 = 1.6%) counterparts, due to higher Gibbs free energy for Sc-O-N formation with no trace of ligand field splitting around the O K-edge spectra. To eradicate the skepticism of appearance of N K-edge (401.6 eV) and Sc L-edge (402.2 eV) absorption spectra adjacent to each other, the first-ever Sc K-edge study has been adopted to validate complementary insight on the metrical parameters of the Sc-N system. Optical bandgaps of the polycrystalline ScN thin film samples were found to vary between 2.25 and 2.62 eV as obtained from the UV–vis spectroscopy, whereas, the nano-indentation hardness and modulus of the as-deposited samples lie between 15–34 GPa and 152–476 GPa, respectively following a linearly increasing trend of resistance to plastic deformations with an exception at 34 GPa in case of RN2 = 5% sample. Besides, contrary to other early 3d transition metal nitrides (TiN, VN, CrN), a comprehensive comparison of noticeably large homogeneity range in Sc-N has been outlined to apprehend the minuscule lattice expansion over the large RN2 realm.

Probing chirality across the electromagnetic spectrum with the full semi-classical light-matter interaction.

We present a formulation and implementation of anisotropic and isotropic electronic circular dichroism (ECD) using the full semi-classical light-matter interaction operator within a four-component relativistic framework. Our treatment uniquely accounts for both beyond-first-order light-matter interactions and relativistic effects, enabling us to investigate the ECD response across the electromagnetic spectrum from optical to x-ray wavelengths where relativistic selection rules and spatial field variations gain increasing importance. We consider the isotropic and oriented ECD across the valence transition and sulfur L- and K-edge transitions in the simplest disulfides, H2S2 and (CH3S)2, and evaluate the influence of the full interaction by comparing to a traditional truncated formulation in the Coulomb gauge (velocity representation). Additionally, we demonstrate that in the relativistic formalism, it is possible to work in the velocity representation, hence keeping order-by-order gauge-origin invariance, contrary to the multipolar gauge, yet being able to distinguish electric and magnetic multipole contributions. Going beyond a first-order treatment in the wave vector is mandatory in the higher-energy end of the soft x-ray region and beyond where the consequent intensity redistribution becomes significant. While the sulfur K-edge absorption spectrum is essentially unaffected by this redistribution, the signed differential counterpart is not: At least third-order contributions are required to describe the differential absorption profile that is otherwise overestimated by a factor of about two. The first-order description deteriorates at higher transition energies (beyond ∼1000eV) where it may even fail to predict the signof individual differential oscillator strengths.

On-Surface Synthesis of Boroxine-Based Molecules

The on-surface synthesis of boroxine-containing molecules can be a convenient method of introducing specific functionalities. Here, we show the validity of a previously described synthesis protocol on the Au (111) surface by applying it to a different molecular precursor. We study in detail the assembly of the precursor, highlighting possible intermediate stages of the condensation process. We combine scanning tunneling microscopy and X-ray spectroscopies to fully characterize both the morphology and the electronic properties of the system. DFT calculations are presented to assign the main electronic transitions originating the B K-edge absorption spectrum. The study paves the way to a facile strategy for functionalizing a surface with molecules of tailored sizes and compositions.

Core electron spectroscopic studies on new structure type iron based superconductors CaKFe4As4 and KCa2Fe4As4F2: DFT predictions

Theoretical predictions on X-ray absorption near edge structure (XANES) for two different newly discovered hybrid iron-based superconducting compounds, CaKFe4As4 and KCa2Fe4As4F2, are presented. The Fe and As K-edge absorption spectra of these hybrid compounds as well as their parent building block materials KFe2As2, CaFe2As2, and CaFeAsF are theoretically calculated and contrasted with each other. In particular, for the hybrid compounds, As K-edge absorption spectra are qualitatively different for two distinct types of As atoms belonging to dissimilar coordination environments. No such differences are found in the XANES of the building block compounds, in which all of the As atoms belong to an equivalent coordination environment. Our density functional theory-based first principles studies in the presence and absence of the core-hole effect are presented separately, to understand the role of the core-hole effect in determining the XANES for these compounds. In general, the core-hole effect is most significant in CaKFe4As4 compared to the other compounds, for which its effect is moderate. The distinct peak features of the absorption spectra signify specific electronic transitions, which are thoroughly explained in terms of the unoccupied atom projected partial density of states. Very prominent pre-edge features found in KFe2As2, CaFeAsF, and CaKFe4As4 imply perfect tetrahedral coordination of the absorbing Fe atom in these compounds, while this is slightly distorted in CaFe2As2 and KCa2Fe4As4F2. The interrelationship between the inter-atomic charge transfer and the shift in the Fe K-edge absorption for various compounds is established through Bader charge analysis. We find that the Fe atom transfers relatively less charge in the case of KFe2As2 compared to KCa2Fe4As4F2, which shifts the corresponding absorption edge of KCa2Fe4As4F2 to higher photon energy.

Core electron spectroscopic studies for 11 iron-based superconductors

Using density functional theory (DFT)-based first principal calculations we have investigated the electron energy loss near edge spectra (ELNES) of two different iron-based superconducting material from 11 family (FeSe and FeTe). The origin of spectral features in Fe K-edge absorption spectra are thoroughly described in light of Fe/Se/Te atom projected partial density of states (PDOS). When include the “core–hole effect”, we found drastic change in absorption spectra. Our theoretically calculated results including core–hole effect agrees well with experimentally observed X-ray Absorption Near Edge Structure (XANES) data available in literature.

Theoretical description of optical and x-ray absorption spectra of MgO including many-body effects

Here we report the optical and x-ray absorption (XAS) spectra of the wide-band-gap oxide MgO using density functional theory (DFT) and many-body perturbation theory (MBPT). Our comprehensive study of the electronic structure shows that while the band gap is underestimated with the exchange-correlation functional PBEsol (4.58 eV) and the hybrid functional HSE06 (6.58 eV) compared to the experimental value (7.7 eV), it is significantly improved (7.52 eV) and even overcompensated (8.53 eV) when quasiparticle corrections are considered. Inclusion of excitonic effects by solving the Bethe-Salpeter equation (BSE) yields the optical spectrum in excellent agreement with experiment. Excellent agreement is observed also for the O and Mg K-edge absorption spectra, demonstrating the importance of the electron-hole interaction within MBPT. Projection of the electron-hole coupling coefficients from the BSE eigenvectors on the band structure allows us to determine the origin of prominent peaks and identify the orbital character of the relevant contributions. The real space projection of the lowest energy exciton wavefunction of the optical spectrum indicates a Wannier-Mott type, whereas the first exciton in the O K-edge is more localized.

Highly efficient soft x-ray spectrometer for transient absorption spectroscopy with broadband table-top high harmonic sources.

We present a novel soft x-ray spectrometer for ultrafast absorption spectroscopy utilizing table-top femtosecond high-order harmonic sources. Where most commercially available spectrometers rely on spherical variable line space gratings with a typical efficiency on the order of 3% in the first diffractive order, this spectrometer, based on a Hettrick–Underwood design, includes a reflective zone plate as a dispersive element. An improved efficiency of 12% at the N K-edge is achieved, accompanied by a resolving power of 890. The high performance of the soft x-ray spectrometer is further demonstrated by comparing nitrogen K-edge absorption spectra from calcium nitrate in aqueous solution obtained with our high-order harmonic source to previous measurements performed at the electron storage ring facility BESSY II.

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

Summary Discovering novel materials and attaining higher performance are the eternal pursuit of thermoelectric materials research. Here, we report a material series, (Cu1−xAgx)2(Te1−ySy) (0.16 ≤ x ≤ 0.24, 0.16 ≤ y ≤ 0.24), which adopts a complex orthorhombic structure differing from any known crystal structure of (Cu/Ag)2(S/Te). This material series is featured by the crystal-amorphicity duality induced by the large anionic size mismatch: a crystalline sublattice of highly size-mismatched anions Te/S coexists with an amorphous-like sublattice of cations Cu/Ag. In the context of structure-property correlation, the crystal-amorphicity duality gave rise to not only interesting electrical properties but also exceptionally low lattice thermal conductivities from 300 to 1,000 K. A state-of-the-art figure of merit zT of 2.0 is obtained in the x = y = 0.22 sample at 1,000 K. These results give insights into crystal-amorphicity duality as a paradigm-shifting materials design approach to develop high-performance thermoelectric materials.

High pressure core electron spectroscopy in 111 Fe-based superconducting materials: A first principles study

Based on particle swarm optimization algorithm, symmetry constraints on structural generation, crystal structures of 111 Fe-based superconducting materials at very high pressures were predicted by Zhang et al. (2012). Using these crystal structures we calculate Fe K-edge absorption spectra for NaFeAs and LiFeAs materials at very high pressures ranging from 22 GPa to 240 GPa using density functional theory. Theoretically computed Fe K-edge absorption spectra at different high pressures contain non-identical spectral features, which corresponds to different local structural environment around the absorbing atom due to applied pressure. At higher energies and higher pressures LiFeAs exhibits larger number of prominent features than that in NaFeAs. Origin of all the core electron absorption features is thoroughly analyzed in terms of the unoccupied partial density of states of the absorbing and ligand atoms. The role of core–hole effect on absorption spectra is also studied in detail. Bifurcation of spectral features in presence of core–hole effect and the corresponding transfer of spectral weights are also discussed in detail. At a moderate pressure 40 GPa, NaFeAs has higher Fe-K edge absorption energy (by ~3 eV) in comparison to that of LiFeAs; in contrast, LiFeAs shows larger inter layer charge transfer than NaFeAs. In general, the interrelationship between the interlayer charge transfer and the shift in the Fe K-edge absorption at various pressures is established through Bader charge analysis, where smaller the inter-layer charge transfer larger the shifts of the absorption edge to higher photon energies is found. In absence of any high pressure experimental X-ray absorption near edge structure (XANES) data, we show that our calculation agrees with available experimental data at atmospheric pressure reasonably well.

In Situ X-ray Absorption Spectroscopy to Monitor the Degradation of Fe/N/C Cathode Catalyst in Proton Exchange Membrane Fuel Cells

Understanding the degradation mechanism of Fe/N/C cathode catalysts in proton exchange membrane fuel cells (PEMFCs) is important. We studied the degradation of an Fe/N/C catalyst prepared from polyimide nanoparticles in an in situ cell by X-ray absorption spectroscopy (XAS). This technique enables real-time monitoring of the Fe species during a fuel cell operation. The Fe K-edge absorption spectra were recorded during the continuous operation of the fuel cell. Initially during the fuel cell operation, the Fe species were atomically isolated and their valence state was found to be 3+. The spectra gradually changed during the first few hours of operation, suggesting the dissolution of the Fe species from the active sites, whereas the fuel cell performance continued to decrease during the eight hours of operation. The demetallation from the FeN x centers during the first few hours has been successfully monitored in real time, while the remaining FeN x centers seem to be stable in the following fuel cell operating condition.

Influence of temperature dependence and Li substitution on the electronic structure of delafossite CuFeO2 and CuFeS2 semiconductors

The electronic structure properties of the delafossite CuFeO2 and Chalcopyrite CuFeS2 semiconductors were studied by XAFS technique theoretically. Calculations were performed with the XAFS code FEFF 8.20 under increasing temperature conditions for 300 K, 350 K, 450 K, 500 K, and for 20% Li substitution at room temperature. High stability in the electronic structure of both materials addressed was observed as a result of the hybridized states built with the contribution of the overlapped outer shell electrons and levels of the neighboring atoms, i.e., Cu, Fe, O or S. The hybridized levels were determined as supporting the stability of the molecular bands via rich quantum symmetries. For the second step of the studies, 20% Li substitution in copper coordination was processed to probe the response of the materials to substitution mechanisms. In the CuFeO2 semiconductors, 0.5 eV shift was observed with the Li substitution at the Fe K-edge absorption spectra which caused a loose in the Li-O bonds as a result of the weaker electronegativity which increased the strength of the Fe-O bonds.

Correlation of Local Crystal Structural and Physical Properties of the Delafossite CuCr1-xFexO2 (0 ≤ x ≤ 1) Series.

Cu-based delafossites offer multifunctional properties that include electrical, magnetic, optical, and thermal transport, and they also act as a photocathode for energy-harvesting applications. Such properties can be modified by bringing about subtle changes in the chemical environment, like introducing holes or excess electrons in the system. With the aim to understand the evolution of physical properties that take place upon systematically replacing 3d3 (Cr3+) with 3d5 (Fe3+), we performed a comprehensive study of structural, electrical and thermal transport, and magnetic properties of the CuCr1-xFexO2 series. In agreement with the different ionic radii of trivalent cations, high-intensity X-ray diffraction confirms a systematic increase of the unit cell parameters. Extended X-ray absorption fine structure spectroscopy confirms the uniform solution of mixed trivalent Cr3+/Fe3+ cations and demonstrates the changes in the hybridization between Cu 3d and O 2p orbitals. Cu K-edge near-edge spectra reflect a sharp 1s → 4p transition associated with the ligand-metal charge transfer "shakedown" process in the collinear O-Cu-O bond along c-axis, whereas the Cr and Fe K-edge absorption spectra show small reciprocal shifts in the edge energies. The charge carrier concentration increases with Fe substitution as confirmed from Hall effect measurement; both p-type conduction and a decrease in the electrical resistivity are observed. The electrical conduction follows a 3D variable range hopping at low-temperature and thermally activated transport at high temperature. The two end members of the series are well-studied, geometrically frustrated antiferromagnetic systems; the present study of magnetic properties of intermediate compositions show a systematic increase of FM interaction with rising Fe concentration and hence a competing magnetic state. A sharp transition in high-temperature (600 K) region identifies a truly paramagnetic state for Fe rich compounds. Thermal conductivity values are drastically affected by the spin fluctuation and spin-phonon scattering taking place in these compositions.

A portable data-collection system for soft x-ray absorption spectroscopy in the Shanghai Synchrotron Radiation Facility.

Based on the Experimental Physics and Industrial Control System, a portable data-collection system for soft x-ray absorption spectroscopy has been developed at the BL02B and BL08U beamlines of the Shanghai Synchrotron Radiation Facility. The data-collection system can be used to carry out total electron yield (TEY) and total fluorescence yield (TFY) experiments simultaneously. The hardware consists of current preamplifiers, voltage-to-frequency converters, and a multi-channel counter, which are aimed at improving the signal-to-noise ratio. The control logic is developed using Python and Java. The novelty of this control system is its designed portability while being extensible and readable and having low noise and high real-time capabilities. The oxygen K-edge absorption spectra of SrTiO3 were obtained using the TEY and TFY technology at the BL02B beamline. Furthermore, the TEY and TFY spectra of the relaxor ferroelectric single-crystal of lead magnesium niobate-lead titanate measured by the present data-collection system have lower peak-to-peak noise amplitude than the ones measured by using a picoammeter. The experimental results show that the spectral signal-to-noise ratio recorded by the present system is 5.7-12.4 dB higher than that with the picoammeter detector.