Kbeta X-ray Research Articles

Experimental and theoretical correlations between vanadium K-edge X-ray absorption and Kβ emission spectra.

A series of vanadium compounds was studied by K-edge X-ray absorption (XAS) and Kbeta X-ray emission spectroscopies (XES). Qualitative trends within the datasets, as well as comparisons between the XAS and XES data, illustrate the information content of both methods. The complementary nature of the chemical insight highlights the success of this dual-technique approach in characterizing both the structural and electronic properties of vanadium sites. In particular, and in contrast to XAS or extended X-ray absorption fine structure (EXAFS), we demonstrate that valence-to-core XES is capable of differentiating between ligating atoms with the same identity but different bonding character. Finally, density functional theory (DFT) and time-dependent DFT calculations enable a more detailed, quantitative interpretation of the data. We also establish correction factors for the computational protocols through calibration to experiment. These hard X-ray methods can probe vanadium ions in any oxidation or spin state, and can readily be applied to sample environments ranging from solid-phase catalysts to biological samples in frozen solution. Thus, the combined XAS and XES approach, coupled with DFT calculations, provides a robust tool for the study of vanadium atoms in bioinorganic chemistry.Electronic supplementary materialThe online version of this article (doi:10.1007/s00775-016-1358-7) contains supplementary material, which is available to authorized users.

Persistent Fe moments in the normal-state collapsed-tetragonal phase of the pressure-induced superconductorCa0.67Sr0.33Fe2As2

Using non-resonant Fe K-beta x-ray emission spectroscopy, we reveal that Sr-doping of CaFe2As2 decouples the Fe moment from the volume collapse transition, yielding a collapsed-tetragonal, paramagnetic normal state out of which superconductivity develops. X-ray diffraction measurements implicate the c-axis lattice parameter as the controlling criterion for the Fe moment, promoting a generic description for the appearance of pressure-induced superconductivity in the alkaline-earth-based 122 ferropnictides (AFe2As2). The evolution of the superconducting critical temperature with pressure lends support to theories for superconductivity involving unconventional pairing mediated by magnetic fluctuations.

A statistical approach to fit Gaussian part of full-energy peaks from Si(PIN) and SDD X-ray spectrometers

A new statistical fitting approach, named Statistical Distribution-Based Analytic (SDA) method, is proposed to fit single Gaussian-shaped K-alpha and K-beta X-ray peaks recorded by Si(PIN) and silicon drift detector (SDD). In this method, we use the discrete distribution theory to calculate standard deviation of energy resolution sigma. The calibration of sigma and energy (E) for two detectors between the energy ranges of 4.5-26 keV are also completed by measuring characteristic X-ray spectra of nineteen types of pure elements. With the spectrum fraction (SF) parameter proposed in this paper, the SDA method can be used to resolve overlapping peaks. In measured spectra, the Gaussian part of X-ray peaks can be fitted by a Gaussian function with two parameters, sigma and SF. This new fitting approach is simpler than traditional methods and it achieves relatively good results when fitting the complex X-ray spectra of national standard alloy samples detected by Si(PIN) and SDD detectors. The chi (r) (2) values are obtained for each spectrum to assess fitting results, and the SDA fitting method gives a preferable fit for the SDD detector.

Probing Valence Orbital Composition with Iron Kβ X-ray Emission Spectroscopy

A systematic study of 12 ferric and ferrous Kbeta X-ray emission spectra (XES) is presented. The factors contributing to the Kbeta main line and the valence to core region of the spectra are experimentally assessed and quantitatively evaluated. While the Kbeta main line spectra are dominated by spin state contributions, the valence to core region is shown to have greater sensitivity to changes in the chemical environment. A density functional theory (DFT) based approach is used to calculate the experimental valence spectra and to evaluate the contributions to experimental intensities and energies. The spectra are found to be dominated by iron np to 1s electric dipole allowed transitions, with pronounced sensitivity to spin state, ligand identity, ligand ionization state, hybridization state, and metal-ligand bond lengths. These findings serve as an important calibration for future applications to iron active sites in biological and chemical catalysis. Potential applications to Compound II heme derivatives are highlighted.

A short working distance multiple crystal x-ray spectrometer

For x-ray spot sizes of a few tens of microns or smaller, a millimeter-sized flat analyzer crystal placed approximately 1 cm from the sample will exhibit high energy resolution while subtending a collection solid angle comparable to that of a typical spherically bent crystal analyzer (SBCA) at much larger working distances. Based on this observation and a nonfocusing geometry for the analyzer optic, we have constructed and tested a short working distance (SWD) multicrystal x-ray spectrometer. This prototype instrument has a maximum effective collection solid angle of 0.14 sr, comparable to that of 17 SBCA at 1 m working distance. We find good agreement with prior work for measurements of the Mn Kbeta x-ray emission and resonant inelastic x-ray scattering for MnO, and also for measurements of the x-ray absorption near-edge structure for Dy metal using Lalpha(2) partial-fluorescence yield detection. We discuss future applications at third- and fourth-generation light sources. For concentrated samples, the extremely large collection angle of SWD spectrometers will permit collection of high-resolution x-ray emission spectra with a single pulse of the Linac Coherent Light Source. The range of applications of SWD spectrometers and traditional multi-SBCA instruments has some overlap, but also is significantly complementary.

Absence of Mn-centered oxidation in the S(2) --> S(3) transition: implications for the mechanism of photosynthetic water oxidation.

A key question for the understanding of photosynthetic water oxidation is whether the four oxidizing equivalents necessary to oxidize water to dioxygen are accumulated on the four Mn ions of the oxygen-evolving complex (OEC), or whether some ligand-centered oxidations take place before the formation and release of dioxygen during the S(3) --> [S(4)] --> S(0) transition. Progress in instrumentation and flash sample preparation allowed us to apply Mn Kbeta X-ray emission spectroscopy (Kbeta XES) to this problem for the first time. The Kbeta XES results, in combination with Mn X-ray absorption near-edge structure (XANES) and electron paramagnetic resonance (EPR) data obtained from the same set of samples, show that the S(2) --> S(3) transition, in contrast to the S(0) --> S(1) and S(1) --> S(2) transitions, does not involve a Mn-centered oxidation. On the basis of new structural data from the S(3)-state, manganese mu-oxo bridge radical formation is proposed for the S(2) --> S(3) transition, and three possible mechanisms for the O-O bond formation are presented.

Mn K-edge XANES and Kbeta XES studies of two Mn-oxo binuclear complexes: investigation of three different oxidation states relevant to the oxygen-evolving complex of photosystem II.

Two structurally homologous Mn compounds in different oxidation states were studied to investigate the relative influence of oxidation state and ligand environment on Mn K-edge X-ray absorption near-edge structure (XANES) and Mn Kbeta X-ray emission spectroscopy (Kbeta XES). The two manganese compounds are the di-mu-oxo compound [L'2Mn(III)O2Mn(IV)L'2](ClO4)3, where L' is 1,10-phenanthroline (Cooper, S. R.; Calvin, M. J. Am. Chem. Soc. 1977, 99, 6623-6630) and the linear mono-mu-oxo compound [LMn(III)OMn(III)L](ClO4)2, where L- is the monoanionic N,N-bis(2-pyridylmethyl)-N'-salicylidene-1,2-diaminoethane ligand (Horner, O.; Anxolabéhère-Mallart, E.; Charlot, M. F.; Tchertanov, L.; Guilhem, J.; Mattioli, T. A.; Boussac, A.; Girerd, J.-J. Inorg. Chem. 1999, 38, 1222-1232). Preparative bulk electrolysis in acetonitrile was used to obtain higher oxidation states of the compounds: the Mn(IV)Mn(IV) species for the di-mu-oxo compound and the Mn(III)Mn(IV) and Mn(IV)Mn(IV) species for the mono-mu-oxo compound. IR, UV/vis, EPR, and EXAFS spectra were used to determine the purity and integrity of the various sample solutions. The Mn K-edge XANES spectra shift to higher energy upon oxidation when the ligand environment remains similar. However, shifts in energy are also observed when only the ligand environment is altered. This is achieved by comparing the di-mu-oxo and linear mono-mu-oxo Mn-Mn moieties in equivalent oxidation states, which represent major structural changes. The magnitude of an energy shift due to major changes in ligand environment can be as large as that of an oxidation-state change. Therefore, care must be exercised when correlating the Mn K-edge energies to manganese oxidation states without taking into account the nature of the ligand environment and the overall structure of the compound. In contrast to Mn K-edge XANES, Kbeta XES spectra show less dependence on ligand environment. The Kbeta1,3 peak energies are comparable for the di-mu-oxo and mono-mu-oxo compounds in equivalent oxidation states. The energy shifts observed due to oxidation are also similar for the two different compounds. The study of the different behavior of the XANES pre-edge and main-edge features in conjunction with Kbeta XES provides significant information about the oxidation state and character of the ligand environment of manganese atoms.

Trace chemical characterization using monochromatic X-ray undulator radiation

An efficient Johansson-type X-ray fluorescence spectrometer has been developed for advanced X-ray spectroscopic analysis with third-generation synchrotron radiation. Kalpha and Kbeta X-ray fluorescence spectra for trace metals have been collected by a Ge(220) analyzing crystal with a Rowland radius of 150 mm, under monochromatic X-ray excitation at the undulator beamline at the SPring-8. The energy resolution is approximately 10 eV for most of the K lines for 3d transition metals. In light of the greatly improved efficiency, as well as the excellent signal-to-background ratio, the relative and absolute detection limits achieved are 1 ppm and 1.2 ng of copper in a carbon matrix, respectively. The energy resolution of the present spectrometer permits the observation of some chemical effects in Kbeta spectra. It has been demonstrated that the changes in Kbeta5 and Kbeta'' intensity for iron and cobalt compounds can be used for the analysis of chemical states. Resonant X-ray fluorescent spectra are another important application of monochromatic excitation. In view of trace chemical characterization, the present spectrometer can be a good alternative to a conventional Si(Li) detector system when combined with highly brilliant X-rays.

Layered structure analysis of GMR multilayers by X-ray reflectometry using the anomalous dispersion effect.

As a basic layered structure for giant magnetoresistive (GMR) heads, NiFe/Cu/NiFe/Ta/Si substrate was measured by X-ray reflectometry at Cu Kalpha, Cu Kbeta and Cu K-absorption-edge energies. The accuracy of both the Cu thickness and the interface width between the upper NiFe and the Cu layers was found to improve in the order Cu Kalpha < Cu Kbeta < Cu K-edge. The final thickness and interface width values obtained from Cu Kbeta reflectivity are in good agreement with those from the Cu K-edge. The anomalous-dispersion effect is useful in the more accurate analysis of the layered structure of transition metal multilayers because it causes a large difference in the refractive indices of specific elements near the absorption edge. The Kbeta X-rays, which can be produced from conventional X-ray sources, are also available for the accurate analysis of reflectivity measurements.

The KβX-ray emission spectrum of argon: a theoretical account

Detailed calculations on the Kbeta X-ray emission spectrum of atomic argon are reported, using CI wavefunctions based on MCDF-EAL calculations for a number of configurations of varying degrees of excitation and ionisation. Crude approximations to the total widths of states, initial-state populations from shake theory and radiative transition probabilities calculated in a transition orbitals approach were used to obtain line intensities which were synthesised into a predicted spectrum which could be compared with experiment. It is shown that the Rydberg spectator transition (1s)((3p)4p 1S) 2S-(3p2 1D)34p 2P0 contributes substantially to the Kbeta v satellite, altering its shape from that predicted in a simple model which includes only primary hole states, and bringing it into agreement with the experimentally determined shape. It was not possible to identify the major contributions the the Kbeta satellite, but the (1s3s) transitions could be excluded by energy considerations, and the most likely contributions are considered to be the (1s3p2)-(3p3) transitions with or without Rydberg spectator satellites.

A Comparison of Detection Systems for Trace Phase Analysis

Since the introduction of semiconductor detectors with sufficient energy resolution to resolve K-alpha X-rays from the K-beta X-rays for first row transition metals, there have been several attempts (1,2) to replace traditional detector systems (scintillation detector/graphite monochromator or proportional counter/nickel filter) in X-ray diffraction with semiconductor detectors. The goal of these experiments was to increase the total system detection efficiency by combining the K-beta discrimination and X-ray detection into a single operation. These early attempts showed that the semiconductor detector based system was more efficient, however, the problem of large dead-time losses hampered development in this area.

Foil thickness dependence of Ar K X-rays from 56 MeV Ar12+bombarding carbon foils

The average X-ray production cross sections and energies for Ar projectile Kalpha , Kbeta and K REC (radiative electron capture) rays were measured for C target foils of effective thicknesses 8-450 mu g cm-2. A model is constructed which describes these yields, the several parameters employed being fitted to the data by a chi-square fit program. The parameters involved represent the main Ar K vacancy production (a single cross section parameter, sigma v'), the Auger and X-ray decay modes for projectiles within and after the foil, and the in-foil radiative and non-radiative electron capture (REC and NREC, respectively) decay modes. Variations in these parameters with projectile energy are estimated, and in-foil collisionally induced configuration alteration effects are considered. From the fit parameters it is possible to estimate several projectile characteristics, e.g. Ar K vacancy and Kalpha and Kbeta X-ray production cross section, in-foil and post-foil average fluorescence yields and the NREC cross section. From the energy measurements the average Ar projectile configuration of an ion with a K vacancy in the thin target foil limit is estimated.

K shell X-ray production of eight elements from Ti to Ge by14N-ion bombardment

The energy shifts of Kbeta X-rays and the Kbeta /Kalpha intensity ratios by 2.8 MeV N+-ion bombardments have been measured for eight elements from Ti to Ge. The Kbeta /Kalpha ratios are expressed as relative values to the ratios for 2 MeV proton impact. The experimental results indicate single K- plus multiple L- and M-shell ionisation, and strongly reflect the relative importance of M-shell vacancies in the Kbeta /Kalpha ratios at the time of K X-ray emission. These data are compared with the Hartree-Fock-Slater calculations and used to deduce information about the vacancy configurations of ionised atoms. The present results can be qualitatively explained within the framework of the direct Coulomb ionisation theory.

Use of Cr K-Beta X-Rays and Position Sensitive Detector for Residual Stress Measurement in Stainless Steel Pipe

Residual stresses on the inner surface of stainless steel pipe used in nuclear reactors are of exceptional importance. Apparatus for measuring these in situ, in welded lengths of 10-inch diameter austenitic (304) stainless pipe has been developed at the University of Denver Research Institute under the sponsorship of the Electric Power Research Institute.

Production of monoenergetic X-rays from 8 to 87 keV (for use in chemical analysis)

A secondary target system was employed to study the production of monoenergetic X-rays using an X-ray generator and a series of secondary targets of Cu, Ge, Mo, Ag, Cd, Te, Sm, Gd, W and Pb. The bremsstrahlung output of the X-ray tube was used to picture stimulated X-ray fluorescence in the secondary targets. The fluorescence spectra of the above targets were recorded with a high resolution Ge X-ray detector, and were found to be composed of 85-90%Kalpha and Kbeta X-rays of the secondary target at optimum X-ray tube potentials.