Br K-edge Research Articles

Analyzing the Intensities of K-Edge Transitions in X2 Molecules (X = F, Cl, Br) for Use in Ligand K-Edge X-ray Absorption Spectroscopy.

Ligand K-edge X-ray absorption spectroscopy (XAS) is regularly used to determine the ligand contribution to metal-ligand bonds. For quantitative studies, the pre-edge transition intensities must be referenced to an intensity standard, and pre-edge intensities obtained from different ligand atoms cannot be compared without standardization due to different cross sections at each absorption edge. In this work, the intensities of the 1s → σ* transitions in F2, Cl2, and Br2 are analyzed for their use as references for ligand K-edge XAS. We show that the intensities of these transitions are equal to the intensities of the 1s → np transitions in the unbound halogens. This finding is supported by a comparison between the normalized experimental intensities for the molecules and the calculated oscillator strengths for the atoms. These results highlight the potential for these molecules to be used as intensity standards in F, Cl, and Br K-edge XAS experiments.

Coupled X-ray Absorption/UV-vis Monitoring of a Prototypical Oscillating Reaction.

Oscillating reactions are among the most intriguing phenomena in chemistry, but many questions on their mechanisms still remain unanswered, due to their intrinsic complexity and to the low sensitivity of the most common spectroscopic techniques toward the reaction brominated species. In this work, we investigate the cerium ion-catalyzed Belousov-Zhabotinsky (BZ) oscillating reaction by means of time-resolved X-ray absorption spectroscopy (XAS), in combination with UV-vis spectroscopy and unsupervised machine learning, multivariate curve resolution, and kinetic analyses. Altogether, we provide new insights into the collective oscillatory behavior of the key brominated species involved in the classical BZ reaction and measure previously unreported oscillations in their concentrations through Br K-edge XAS, while simultaneously tracking the oscillatory Ce4+-to-Ce3+ transformation by coupling XAS with UV-vis spectroscopy. Our work evidences the potential of the XAS technique to investigate the mechanisms of oscillatory chemical systems whose species are often not detectable with conventional experimental methods.

X-ray induced electron and ion fragmentation dynamics in IBr.

Characterization of the inner-shell decay processes in molecules containing heavy elements is key to understanding x-ray damage of molecules and materials and for medical applications with Auger-electron-emitting radionuclides. The 1s hole states of heavy atoms can be produced by absorption of tunable x rays and the resulting vacancy decays characterized by recording emitted photons, electrons, and ions. The 1s hole states in heavy elements have large x-ray fluorescence yields that transfer the hole to intermediate electron shells that then decay by sequential Auger-electron transitions that increase the ion's charge state until the final state is reached. In molecules, the charge is spread across the atomic sites, resulting in dissociation to energetic atomic ions. We have used x-ray/ion coincidence spectroscopy to measure charge states and energies of Iq+ and Brq'+ atomic ions following 1s ionization at the I and Br K-edges of IBr. We present the charge states and kinetic energies of the two correlated fragment ions associated with core-excited states produced during the various steps of the cascades. To understand the dynamics leading to the ion data, we develop a computational model that combines Monte-Carlo/Molecular-Dynamics (MC/MD) simulations with a classical over-the-barrier model to track inner-shell cascades and redistribution of electrons in valence orbitals and nuclear motion of fragments.

Operando Br K-Edge Dispersive X-ray Absorption Fine Structure Analysis for Br–/Br3– Redox Mediator for Li–Air Batteries

The behavior of Br–/Br3– redox mediator (RM), which has been shown to suppress charging overpotential in lithium–air batteries, has been detected for the first time using operando Br K-edge dispersive X-ray absorption fine structure. Br3– ions in the electrolyte increase monotonically after ∼30% charging, which indicates that a part of Br3– formed via the oxidation of Br– during charging does not oxidize Li2O2 as RM but gets accumulated in the cell. The accumulated Br3– is estimated at ∼20% of the total amount of charging. Consistent with this result, the O2 evolution efficiency gradually decreases from the initial 100% as the charging progresses, and CO2 is emitted at the end of the charging. The integrated amount of O2 evolution during charging accounts for ∼83% of the theoretical two-electron reaction. All these results clearly show that the Br–/Br3– RM functions effectively at the beginning of charging but gradually loses its function as charging proceeds.

Soft X-ray characterization of halide perovskite film by scanning transmission X-ray microscopy

Organic–inorganic metal halide perovskites (MHPs) have recently been receiving a lot of attention due to their newfound application in optoelectronic devices, including perovskite solar cells (PSCs) which have reached power conversion efficiencies as high as 25.5%. However, the fundamental mechanisms in PSCs, including the correlation of degradation with the excellent optoelectrical properties of the perovskite absorbers, are poorly understood. In this paper, we have explored synchrotron-based soft X-ray characterization as an effective technique for the compositional analysis of MHP thin films. Most synchrotron-based studies used for investigating MHPs so far are based on hard X-rays (5–10 keV) which include various absorption edges (Pb L-edge, I L-edge, Br K-edge, etc.) but are not suited for the analysis of the organic component in these materials. In order to be sensitive to a maximum number of elements, we have employed soft X-ray-based scanning transmission X-ray microscopy (STXM) as a spectro-microscopy technique for the characterization of MHPs. We examined its sensitivity to iodine and organic components, aging, or oxidation by-products in MHPs to make sure that our suggested method is suitable for studying MHPs. Furthermore, methylammonium triiodide with different deposition ratios of PbI2 and CH3NH3I (MAI), and different thicknesses, were characterized for chemical inhomogeneity at the nanoscale by STXM. Through these measurements, we demonstrate that STXM is very sensitive to chemical composition and homogeneity in MHPs. Thus, we highlight the utility of STXM for an in-depth analysis of physical and chemical phenomena in PSCs.

Relativistic Corrections to Phase Shift Calculation in the GNXAS Package

Modern XAFS (X-ray Absorption Fine Structure) data-analysis is based on accurate multiple-scattering (MS) calculations of the X-ray absorption cross-section. In this paper, we present the inclusion and test of relativistic corrections for the multiple-scattering calculations within the GnXAS suite of programs, which is relevant to the treatment of the XAFS signals when atoms with high atomic number are contained into the system. We present a suitable strategy for introducing relativistic corrections without altering the basic structure of the programs. In particular, this is realized by modifying only the Phagen program calculating the atomic absorption cross sections and scattering t-matrices for the selected cluster. The modification incorporates a pseudo-Schrödinger Equation (SE) replacing the Dirac relativistic form. The phase-shift calculations have been put to a test in two known molecular and crystalline cases: molecular bromine Br2 and crystalline Pb. Calculations in an extended energy range have been shown to be very close to the non-relativistic case for Br2 (Br K-edge) while corrections have been found to exceed 25% for amplitude and phases of the XAFS multiple-scattering signals (Pb L3-edge). Benefits in the structural refinement using relativistic corrections are discussed for crystalline Pb at room temperature.

Resonant x-ray absorption of strong-field-ionized CF3Br

We report on an experimental and theoretical study of strong-field laser ionization of CF3Br followed by resonant x-ray absorption at the Br K-edge. Distinct 1s → 4p, 5p Rydberg transitions of Br q+ (q = 1–4) atomic ions are observed and identified with Hartree–Fock–Slater and relativistic configuration interaction calculations. Time-dependent density functional theory and ab initio molecular dynamics calculations were performed to simulate the dissociative ionization process and the molecular orbitals for the q = 1–4 charge states. Measurements were made with both parallel and perpendicular linear polarizations of the laser and x-rays, but dichroism was not observed, indicating negligible alignment by the laser ionization process. This result is explained by calculations on atomic Br and the molecular simulation.

Synthesis, Structure, and Properties of Copper(II) Bromide Complexes with N-Disubstituted Thiocarbamoyl-N'-cyclohexylsulfenamides

The [CuLBr2] complexes, where L is RRN–C(=S)–S–NHC6H11 and RR = (C2H5)2, (CH2)5, or (CH2)2O(CH2)2, have been synthesized by reacting equimolar amounts of CuBr2 in methanol and L in diethyl ether. The compounds have been studied by elemental analysis, IR spectroscopy, EPR, X-ray absorption spectroscopy, conductometry, magnetochemistry, and thermal analysis. According to IR spectroscopy data, the ligands in the complexes are bidentately coordinated to Cu(II) through the thione sulfur sulfenamide and nitrogen atoms. Exact structural parameters of the nearest environment of Cu(II) have been determined from analysis of Cu and Br K-edge EXAFS spectra. The Cu–N, Cu–S, and Cu–Br bond lengths are within 2.06–2.08, 2.24–2.49, and 2.33–2.38 A, respectively. The EPR spectra of the complexes in a DMF solution at 293 K are described by the isotropic spin Hamiltonian with spin S = 1/2, including hyperfine coupling to the nuclear spin of the central copper atom and additional hyperfine coupling to the nuclear spins of two equivalent bromine atoms and one nitrogen atom.

EXAFS, ab Initio Molecular Dynamics, and NICIS Spectroscopy Studies on an Organic Dye Model at the Dye-Sensitized Solar Cell Photoelectrode Interface.

The organization of dye molecules in the dye layer adsorbed on the semiconductor substrate in dye-sensitized solar cells has been studied using a combination of theoretical methods and experimental techniques. The model system is based on the simple D-π-A dye L0, which has been chemically modified by substituting the acceptor group CN with Br (L0Br) to offer better X-ray contrast. Experimental EXAFS data based on the Br K-edge backscattering show no obvious difference between dye-sensitized titania powder and titania film samples, thus allowing model systems to be based on powder slurries. Ab initio molecular dynamic (aiMD) calculations have been performed to extract less biased information from the experimental EXASF data. Using the aiMD calculation as input, the EXAFS structural models can be generated a priori that match the experimental data. Our study shows that the L0Br dye adsorbs in the trans-L0Br configuration and that adsorption involves both a proximity to other L0Br dye molecules and the titanium atoms in the TiO2 substrate. These results indicate direct coordination of the dye molecules to the TiO2 surface in contrast to previous results on metal-organic dyes. The molecular coverage of L0Br on mesoporous TiO2 was also estimated using NICIS spectroscopy. The NICISS results emphasized that the L0Br dye on nanoporous titania mainly forms monolayers with a small contribution of multilayer coverage.

In Situ XAFS Observation of Chemical Species Near Solid/Liquid Interface in a Model Reaction of Pitting Process

The corrosion of metal and alloy under a wet environment includes a series of process: ionization of constituent elements by electron transfer reaction (oxidation) at the solid/liquid interface between a solution and a metal surface, and dissolution of metal into the solution, diffusion of the dissolved ions, and formation of hydroxides. These reactions progress depending on the environments near the interface, which change with the progress of the corrosion reaction itself. One of the most important environments is the concentration distribution of chemical species near the solid/liquid interface that is different from that of uniform corrosion. This is the case of pitting corrosion, and it is difficult to understand its mechanism by only using ex situ observation technique. Therefore, an in situ measurement technique with time and spatial resolution is required. In this study, an unique cell for an in situ electrochemical-XAFS measurement was developed to observe diffusion behavior of dissolved ions and reactions among them near the solid/liquid interface. Figure shows the schematic diagram of an in situ electrochemical cell [1], which can detect the state of dissolved metal ions in a minute space simulating pitting corrosion. The X-ray absorption spectrum (XAS) measurement was performed using the cell. The change in concentrations and structures of dissolved ions were measured at different distances from the solid/liquid interface as a function of reaction time. A sheet of SUS304 (Cr 18 %, Ni 8 %) with a thickness of 200 μm was used as a sample, and worked as an electrode. LiBr aqueous solution (1.0 M) was used as an electrolyte. A typical measurement of corrosion was carried out by applying potential of 0.8 V vs. Ag/AgCl on the specimen. The size of an X-ray beam was set to 500 μm (horizontal) × 90 μm (vertical) (FWHM). As the corrosion progressed, the interface shifted to –Z direction (Fig.), but the shape of the interface, the metal surface shape, along the horizontal direction remained sufficiently smooth compared with the beam size. XAS for Fe, Cr, and Br K-edges were measured by the transmission geometry using QXAFS technique at BL-9A of Photon Factory (PF), High Energy Accelerator Research Organization (KEK). The X-ray absorption spectrum changed according to the progression of the corrosion reaction. From the spectrum, the change of absorption (Δμt) at the energy of white line at different distances from the solid/liquid interface as a function of the reaction time: t was obtained.These results showed that the absorption of Fe rapidly increases at the early stage of reaction and as the reaction advances, the change in the absorption becomes small and reaches an equilibrium state. It was also shown that the absorption change also heavily depends on the distance from the interface. The same measurements were also performed for Cr and Br. These results will be discussed by paying a special attention to the relationship of the concentration change of Fe, Cr, and Br and their coordination structures.

Localized holes and delocalized electrons in photoexcited inorganic perovskites: Watching each atomic actor by picosecond X-ray absorption spectroscopy.

We report on an element-selective study of the fate of charge carriers in photoexcited inorganic CsPbBr3 and CsPb(ClBr)3 perovskite nanocrystals in toluene solutions using time-resolved X-ray absorption spectroscopy with 80 ps time resolution. Probing the Br K-edge, the Pb L3-edge, and the Cs L2-edge, we find that holes in the valence band are localized at Br atoms, forming small polarons, while electrons appear as delocalized in the conduction band. No signature of either electronic or structural changes is observed at the Cs L2-edge. The results at the Br and Pb edges suggest the existence of a weakly localized exciton, while the absence of signatures at the Cs edge indicates that the Cs+ cation plays no role in the charge transport, at least beyond 80 ps. This first, time-resolved element-specific study of perovskites helps understand the rather modest charge carrier mobilities in these materials.

Anomalous small-angle X-ray scattering study on the spatial distribution of hydrophobic molecules in polymer micelles

The spatial distribution of bromine-labeled hydrophobic molecules—bromobenzene (BrBz) and 4-bromobenzylalcohol (BrBzOH)—incorporated in polymer micelles of poly(N,N-dimethylaminoethyl methacrylate)-block-poly(methyl methacrylate) (PDMAEMA-b-PMMA) was investigated using anomalous small-angle X-ray scattering (ASAXS) near the Br K-edge. ASAXS analyses revealed that BrBzOH was homogeneously dispersed in the hydrophobic PMMA core of the PDMAEMA-b-PMMA micelle, whereas BrBz was excluded from the vicinity of the core–shell interface of the PMMA core. The results indicate that the spatial distribution of hydrophobic molecules in polymer micelles strongly depend on the polarity of the encapsulated species.

Spatial distribution of hydrophobic compounds in polymer micelles as explored by anomalous small-angle X-ray scattering near Br K-edge

Spatial distributions of bromine-labeled hydrophobic molecules, 9-bromofluorene (BrFL) or ethyl 2-bromoisobutylate (BrEB), incorporated in micelles of amphiphilic block copolymers composed of hydrophilic poly(N,N-dimethylaminoethyl methacrylate) and hydrophobic poly(methyl methacrylate) (PDMAEMA-b-PMMA) were explored with anomalous small-angle X-ray scattering (ASAXS) near Br K-edge. ASAXS analyses clarified that BrFL was concentrated into small domain within hydrophobic PMMA core, whereas BrEB, which had similar molecular structure to the repeating unit of PMMA, was homogeneously dispersed in PMMA core. The concentration of BrFL into small domain within PMMA core was interpreted due to segregation of BrFL molecules from water penetrating into the PMMA core. On the other hand, the BrEB molecules were homogeneously dispersed in the PMMA cores because the molecular structure of BrEB was similar to the repeating unit of PMMA. The results in this work indicated that the differences of spatial distributions of hydrophobic compounds in polymer micelles were derived by the differences of intermolecular interactions between the hydrophobic molecules and the other components, such as a hydrophobic polymer and solvent, in hydrophobic cores of polymer micelles.

Bromine speciation in hydrous silicate melts at high pressure

Br speciation in hydrous silicate melts at high pressure has been investigated up to 7.6GPa using X-ray absorption spectroscopy (XAS) at the Br K-edge in a Paris–Edinburgh press. Br in silicate melts is surrounded by an average of 6 Na cations, a number slightly increasing with pressure (5.8 to 6.6), with a Br–Na distance increasing from 3.49 to 3.72Å. Two oxygens, either from a water, an –OH molecule or from the tetrahedral silicate network, with an average Br–O distance of 1.80Å, form the closest coordination shell around Br ions. The persistence of an alkali shell around Br, in a structure similar to crystalline NaBr, throughout the pressure range investigated shows that Br can be retained in the melt structure at relatively high pressure and supports the idea of its deep recycling. Finally, our results confirm that Br could be efficiently degassed with water at low pressures and that Br may also have been efficiently degassed along with water during the early stages of an oxidized magma ocean.

X-ray Birefringence Imaging of Materials with Anisotropic Molecular Dynamics.

The X-ray birefringence imaging (XBI) technique, reported very recently, is a sensitive tool for spatially resolved mapping of the local orientational properties of anisotropic materials. In this paper, we report the first XBI measurements on materials that undergo anisotropic molecular dynamics. Using incident linearly polarized X-rays with energy close to the Br K-edge, the X-ray birefringence is dictated by the orientational properties of the C-Br bonds in the material. We focus on two materials (urea inclusion compounds containing 1,8-dibromooctane and 1,10-dibromodecane guest molecules) for which the reorientational dynamics of the brominated guest molecules (and hence the reorientational dynamics of the C-Br bonds) are already well characterized by other experimental techniques. The XBI results demonstrate clearly that, for the anisotropic molecular dynamics in these materials, the effective X-ray optic axis for the X-ray birefringence phenomenon is the time-averaged resultant of the orientational distribution of the C-Br bonds.

Evidence for halogen bond covalency in acyclic and interlocked halogen-bonding receptor anion recognition.

The synthesis and anion binding properties of novel halogen-bonding (XB) bis-iodotriazole-pyridinium-containing acyclic and [2]catenane anion host systems are described. The XB acyclic receptor displays selectivity for acetate over halides with enhanced anion recognition properties compared to the analogous hydrogen-bonding (HB) acyclic receptor. A reversal in halide selectivity is observed in the XB [2]catenane, in comparison to the acyclic XB receptor, due to the interlocked host’s unique three-dimensional binding cavity, and no binding is observed for oxoanions. Notable halide anion association constant values determined for the [2]catenane in competitive organic–aqueous solvent mixtures demonstrate considerable enhancement of anion recognition as compared to the HB catenane analogue. X-ray crystallographic analysis of a series of halide catenane complexes reveal strong XB interactions in the solid state. These interactions were studied using Cl and Br K-edge X-ray Absorption Spectroscopy (XAS) indicating intense pre-edge features characteristic of charge transfer from the halide to its bonding partner (σAX←X–* ← X1s), and providing a direct measure of the degree of covalency in the halogen bond(s). The data reveal that the degree of covalency is similar to that which is observed in transition metal coordinate covalent bonds. These results are supported by DFT results, which correlate well with the experimental data.

Communication: Coordination structure of bromide ions associated with hexyltrimethylammonium cations at liquid/liquid interfaces under potentiostatic control as studied by total-reflection X-ray absorption fine structure

Total-reflection X-ray absorption fine structure (TR-XAFS) technique was applied for the first time to an interface between two immiscible electrolyte solutions under potentiostatic control. The hydration structure of bromide ions was investigated at polarized 2-octanone/water interfaces. TR-XAFS spectra at Br K-edge measured in the presence of hexyltrimethylammonium bromide (C6TAB) were slightly modified depending on the Galvani potential difference (Δ(o)(w)φ). The extended X-ray absorption fine structure analysis exposed hydration structure changes of bromide ions at the polarized interface. The coordination structure of bromide ions at the interface could be analyzed as compared with bromide ions dissolved in aqueous solution and Br(-)-exchanged resin having quaternary ammonium groups. The results indicated that bromide ions were associated with C6TA(+) at the polarized interface. The relative contribution of ion association form of bromide ions with quaternary ammonium groups was enhanced at a potential close to the ion transfer of C6TA(+), where the interfacial concentration of C6TA(+) is increased as a function of Δ(o)(w)φ.

Structural motifs of pre-nucleation clusters

Structural motifs of pre-nucleation clusters prepared in single, optically levitated supersaturated aqueous aerosol microparticles containing CaBr2 as a model system are reported. Cluster formation is identified by means of X-ray absorption in the Br K-edge regime. The salt concentration beyond the saturation point is varied by controlling the humidity in the ambient atmosphere surrounding the 15-30 μm microdroplets. This leads to the formation of metastable supersaturated liquid particles. Distinct spectral shifts in near-edge spectra as a function of salt concentration are observed, in which the energy position of the Br K-edge is red-shifted by up to 7.1 ± 0.4 eV if the dilute solution is compared to the solid. The K-edge positions of supersaturated solutions are found between these limits. The changes in electronic structure are rationalized in terms of the formation of pre-nucleation clusters. This assumption is verified by spectral simulations using first-principle density functional theory and molecular dynamics calculations, in which structural motifs are considered, explaining the experimental results. These consist of solvated CaBr2 moieties, rather than building blocks forming calcium bromide hexahydrates, the crystal system that is formed by drying aqueous CaBr2 solutions.

Re and Br X-ray Absorption Near-Edge Structure Study of the Ground and Excited States of [ReBr(CO)3(bpy)] Interpreted by DFT and TD-DFT Calculations

X-ray absorption spectra of fac-[ReBr(CO)3(bpy)] near the Re L3- and Br K-edges were measured in a steady-state mode as well as time-resolved at 630 ps after 355 nm laser pulse excitation. Relativistic spin-orbit time-dependent density functional theory (TD-DFT) calculations account well for the shape of the near-edge absorption (the ″white line″) of the ground-state Re spectrum, assigning the lowest-lying transitions as core-to-ligand metal-to-ligand charge transfer from Re 2p(3/2) into predominantly π*(bpy) molecular orbitals (MOs) containing small 5d contributions, followed in energy by transitions into π* Re(CO)3 and delocalized σ*/π* MOs. Transitions gain their intensities from Re 5d and 6s participation in the target orbitals. The 5d character is distributed over many unoccupied MOs; the 5d contribution to any single empty MO does not exceed 29%. The Br K-edge spectrum is dominated by the ionization edge and multiple scattering features, the pre-edge electronic transitions being very weak. Time-resolved spectra measured upon formation of the lowest electronic excited state show changes characteristic of simultaneous Re and Br electronic depopulation: shifts of the Re and Br edges and the Re white line to higher energies and emergence of new intense pre-edge features that are attributed by TD-DFT to transitions from Re 2p(3/2) and Br 1s orbitals into a vacancy in the HOMO-1 created by electronic excitation. Experimental spectra together with quantum chemical calculations provide a direct evidence for a ReBr(CO)3 → bpy delocalized charge transfer character of the lowest excited state. Steady-state as well as time-resolved Re L3 spectra of [ReCl(CO)3(bpy)] and [Re(Etpy)(CO)3(bpy)](+) are very similar to those of the Br complex, in agreement with similar (TD) DFT calculated transition energies as well as delocalized excited-state spin densities and charge changes upon excitation.

X-ray Absorption Spectroscopy of Ground and Excited Rhenium–Carbonyl–Diimine Complexes: Evidence for a Two-Center Electron Transfer

Steady-state and picosecond time-resolved X-ray absorption spectroscopy is used to study the ground and lowest triplet states of [ReX(CO)(3)(bpy)](n+), X = Etpy (n = 1), Cl, or Br (n = 0). We demonstrate that the transient spectra at both the Re L(3)- and Br K-edges show the emergence of a pre-edge feature, absent in the ground-state spectrum, which is associated with the electron hole created in the highest occupied molecular orbital following photoexcitation. Importantly, these features have the same dynamics, confirming previous predictions that the low-lying excited states of these complexes involve a two-center charge transfer from both the Re and the ligand, X. We also demonstrate that the DFT optimized ground and excited structures allow us to reproduce the experimental XANES and EXAFS spectra. The ground-state structural refinement shows that the Br atom contributes very little to the latter, whereas the Re-C-O scattering paths are dominant due to the so-called focusing effect. For the excited-state spectrum, the Re-X bond undergoes one of the largest changes but still remains a weak contribution to the photoinduced changes of the EXAFS spectrum.