X-ray Absorption Profiles Research Articles

Cu site differentiation in tetracopper(i) sulfide clusters enables biomimetic N2O reduction.

Copper clusters feature prominently in both metalloenzymes and synthetic nanoclusters that mediate catalytic redox transformations of gaseous small molecules. Such reactions are critical to biological energy conversion and are expected to be crucial parts of renewable energy economies. However, the precise roles of individual metal atoms within clusters are difficult to elucidate, particularly for cluster systems that are dynamic under operating conditions. Here, we present a metal site-specific analysis of synthetic Cu4(μ4-S) clusters that mimic the Cu Z active site of the nitrous oxide reductase enzyme. Leveraging the ability to obtain structural snapshots of both inactive and active forms of the synthetic model system, we analyzed both states using resonant X-ray diffraction anomalous fine structure (DAFS), a technique that enables X-ray absorption profiles of individual metal sites within a cluster to be extracted independently. Using DAFS, we found that a change in cluster geometry between the inactive and active states is correlated to Cu site differentiation that is presumably required for efficient activation of N2O gas. More precisely, we hypothesize that the Cu δ+⋯Cu δ- pairs produced upon site differentiation are poised for N2O activation, as supported by computational modeling. These results provide an unprecedented level of detail on the roles of individual metal sites within the synthetic cluster system and how those roles interplay with cluster geometry to impact the reactivity function. We expect this fundamental knowledge to inform understanding of metal clusters in settings ranging from (bio)molecular to nanocluster to extended solid systems involved in energy conversion.

Metal Site-Specific Electrostatic Field Effects on a Tricopper(I) Cluster Probed by Resonant Diffraction Anomalous Fine Structure (DAFS).

Studies of multinuclear metal complexes are greatly enhanced by resonant diffraction measurements, which probe X-ray absorption profiles of crystallographically independent metal sites within a cluster. In particular, X-ray diffraction anomalous fine structure (DAFS) analysis provides data that can be interpreted akin to site-specific XANES, allowing for differences in metal K-edge resonances to be deconvoluted even for different metal sites within a homometallic system. Despite the prevalence of Cu-containing clusters in biology and energy science, DAFS has yet to be used to analyze multicopper complexes of any type until now. Here, we report an evaluation of trends using a series of strategically chosen Cu(I) and Cu(II) complexes to determine how energy dependencies of anomalous scattering factors are impacted by coordination geometry, ligand shell, cluster nuclearity, and oxidation state. This calibration data is used to analyze a formally tricopper(I) complex that was found by DAFS to be site-differentiated due to the unsymmetrical influence on different Cu sites of the electrostatic field from a proximal K+ cation.

Bonding wire characterization using non-destructive X-ray imaging

Counterfeiting in the semiconductor industry poses a significant threat to the economy and can lead to lower performance and reliability of electronic devices. X-ray technology in this respect is commonly used for non-destructive testing to identify physical defects and anomalies in integrated circuits. However, for accurate material identification, destructive methods such as spectroscopic techniques including X-ray diffraction and X-ray fluorescence are typically required. In this work, a fast and non-destructive method is presented for material characterization of bonding wires in ICs and other SMD components by measuring X-ray absorption profiles. The results demonstrated that the X-ray imaging technique can effectively distinguish between bonding wires made of gold, silver, copper, and aluminum. Moreover, the thickness of these materials can also be detected. This technique exhibits high potential for utilization as a simple and quick method for non-destructive material characterization in the industrial environment.

Sinogram Inpainting with Generative Adversarial Networks and Shape Priors

X-ray computed tomography is a widely used, non-destructive imaging technique that computes cross-sectional images of an object from a set of X-ray absorption profiles (the so-called sinogram). The computation of the image from the sinogram is an ill-posed inverse problem, which becomes underdetermined when we are only able to collect insufficiently many X-ray measurements. We are here interested in solving X-ray tomography image reconstruction problems where we are unable to scan the object from all directions, but where we have prior information about the object’s shape. We thus propose a method that reduces image artefacts due to limited tomographic measurements by inferring missing measurements using shape priors. Our method uses a Generative Adversarial Network that combines limited acquisition data and shape information. While most existing methods focus on evenly spaced missing scanning angles, we propose an approach that infers a substantial number of consecutive missing acquisitions. We show that our method consistently improves image quality compared to images reconstructed using the previous state-of-the-art sinogram-inpainting techniques. In particular, we demonstrate a 7 dB Peak Signal-to-Noise Ratio improvement compared to other methods.

DISCOVERY OF BROAD SOFT X-RAY ABSORPTION LINES FROM THE QUASAR WIND IN PDS 456

ABSTRACT High-resolution soft X-ray spectroscopy of the prototype accretion disk wind quasar, PDS 456, is presented. Here, the XMM-Newton reflection grating spectrometer spectra are analyzed from the large 2013–2014 XMM-Newton campaign, consisting of five observations of approximately 100 ks in length. During the last observation (OBS. E), the quasar is at a minimum flux level, and broad absorption line (BAL) profiles are revealed in the soft X-ray band, with typical velocity widths of km s−1. During a period of higher flux in the third and fourth observations (OBS. C and D, respectively), a very broad absorption trough is also present above 1 keV. From fitting the absorption lines with models of photoionized absorption spectra, the inferred outflow velocities lie in the range . The absorption lines likely originate from He and H-like neon and L-shell iron at these energies. A comparison with earlier archival data of PDS 456 also reveals a similar absorption structure near 1 keV in a 40 ks observation in 2001, and generally the absorption lines appear most apparent when the spectrum is more absorbed overall. The presence of the soft X-ray BALs is also independently confirmed by an analysis of the XMM-Newton EPIC spectra below 2 keV. We suggest that the soft X-ray absorption profiles could be associated with a lower ionization and possibly clumpy phase of the accretion disk wind, where the latter is known to be present in this quasar from its well-studied iron K absorption profile and where the wind velocity reaches a typical value of 0.3c.

X-Ray Absorption Measured in the Resonant Auger Scattering Mode

We report both experimental and theoretical studies on x-ray absorption measured in the resonant Auger scattering mode of gas phase carbon monoxide near the O1s-->2pi region. Both experiment and theory display a crucial difference between the x-ray absorption profiles obtained in the conventional and resonant scattering modes. Lifetime vibrational interference is the main source of the difference. It is demonstrated that such interference, which arises from a coherent excitation to overlapping intermediate levels, ruins the idea for obtaining x-ray absorption spectra in a lifetime broadening free regime.

PQCT study on diffusion and equilibrium distribution of iodinated anionic contrast agent in human articular cartilage – associations to matrix composition and integrity

X-ray imaging of articular cartilage using anionic contrast agents has been introduced for quantification of tissue glycosaminoglycan (GAG) concentration. In this in vitro study we investigated diffusion and equilibrium distribution of an anionic contrast agent in human articular cartilage and related the results to tissue composition and integrity. Osteochondral cylinders (d=4.0mm, n=24) were prepared from femoral medial condyles (FMCs, cartilage thickness 2.13+/-0.54 mm, mean+/-standard deviation [SD]), and tibial medial plateaus ([TMPs]1.99+/-0.38 mm) of human cadaver knees. Samples were immersed for 24h at room temperature in 21 mM concentration of anionic contrast agent Hexabrix. The X-ray absorption maps and profiles were measured before immersion, and after every 2h of immersion using clinical peripheral quantitative computed tomography (pQCT). An increase in X-ray attenuation along cartilage depth, indicating a characteristic density profile increasing from superficial to deep tissue, could be seen in pQCT images acquired without contrast agent. The complete diffusion of the contrast agent into cartilage took more than 12h. However, the uronic acid concentration correlated with the contrast agent concentration in femoral cartilage (r=-0.76, n=12, P=0.004) as early as after 2h of immersion, and the linear correlation was virtually unchanged during the remaining 22 h. Similarly, the histological tissue integrity (Mankin score) correlated positively with the contrast agent concentration in tibial cartilage (r=+0.75, P=0.005) after 2h of immersion. The X-ray absorption profiles before immersion, i.e., without the contrast agent, and after 24h of immersion were significantly correlated (r=-0.76+/-0.34, mean+/-SD). Although the complete contrast agent diffusion into human articular cartilage in vitro took more than 12h, significant apparent correlations were revealed between the spatial proteoglycan (PG) and contrast agent distributions already after 2h of immersion. At the stage of incomplete penetration, however, the spatial contrast agent concentration distribution cannot directly reflect the true PG distribution as the Donnan equilibrium has not been reached. However, in degenerated cartilage the diffusion rate increases. Obviously, this can lead to the reported correlation between the bulk PG content and the bulk contrast agent concentration already at the early stages of diffusion.

The Kinematics and Physical Conditions of the Ionized Gas in Markarian 509. I.ChandraHigh Energy Grating Spectroscopy

We observed the Seyfert 1 galaxy Mrk 509 for ~59 ks with the Chandra high-energy transmission gratings, simultaneously with HST/STIS and RXTE. Here we present a detailed analysis of the soft X-ray spectrum observed with Chandra. We measure strong absorption lines from He-like Ne and Mg and from H-like N, O, and Ne. Weaker absorption lines may also be present. The lines are unresolved except for Ne X Lyα (λ12.134) and Ne IX 1s2-1s2p (λ13.447), which appear to be marginally resolved. The profiles are blueshifted with respect to the systemic velocity of Mrk 509, indicating an outflow of ~-200 km s-1. There is also a hint that the profiles may have a velocity component near systemic. The soft X-ray spectrum can be described in remarkable detail with a simple, single-zone photoionized absorber having an equivalent neutral hydrogen column density of 2.06 × 1021cm-2 and an ionization parameter of log ξ = 1.76 (or log U = 0.27). Although the photoionized gas almost certainly is comprised of matter in more than one ionization state and may consist of several kinematic components, data with better spectral resolution and signal-to-noise ratio would be required to justify a more complex model. The UV data, on the other hand, have a velocity resolution of ~10 km s-1 and can easily detect eight kinematic components, covering roughly the same velocities as the X-ray absorption profiles. Even though the X-ray and UV absorbers share the same velocity space, the UV absorbers have a much smaller column density and ionization state. We show that models of the X-ray data do not predict significant UV absorption and are therefore consistent with the UV data. Finally, we do not detect any soft X-ray emission lines.

Ce Electronic Structure Studied by Resonant Electron Spectroscopies

In Ce compounds, strong resonant effects are observed in high energy photoemission spectra, both at the Ce M4, 5 and Ce L2, 3 absorption edges for the Ce 4d and Ce 3d photoemission peaks, respectively. In the mixed valent compounds, for the photoemission peaks corresponding to mainly 4f0, 4f1 and 4f2 character of the final state, the resonance occurs at different photon energies making it possible to unravel the X-ray absorption profiles. Moreover resonant L2, 3M4, 5N4, 5 Raman Auger lines are observed up to 10 eV above the L2 and L3 absorption edge jumps, showing the strong localization of the core excited state. These results help to understand the validity and the limits of atomic models for the description of the electronic and magnetic properties of these systems.

High energy resonant photoemission and resonant Auger spectroscopy in mixed valent Ce compounds

Abstract In CeRh3 and Ce7Rh3 compounds, strong resonant effects are observed in the Ce 3d and 4d photoemission spectra at the Ce L3 and M4,5 absorption edges respectively. These resonances show unambiguously that interference effects exist in core level spectroscopies. In the CeRh3 mixed valent compound, the resonances of photoemission peaks corresponding to 4f0, 4f1 and 4f2 final state character occur at different photon energies, making it possible to unravel the X-ray absorption profiles. Moreover, resonant L3M4,5N4,5 Raman Auger lines are observed up to 10 eV above the L3 absorption edge, showing the strong localized character of the core excited state.

Multiatom Resonant Photoemission: Theory and Systematics

A first-principles calculation of the recently discovered interatomic multiatom resonant photoemission (MARPE) effect is presented. In this phenomenon, core photoelectron intensities are enhanced when the photon energy is tuned to a core-level absorption edge of nonidentical neighboring atoms, thus enabling direct determination of near-neighbor atomic identities. Both the multiatom character of MARPE and retardation effects in the photon and electron interactions in the resonant channel are shown to be crucial. Measured peak-intensity enhancements of $40%$ in MnO and spectral shapes similar to the corresponding x-ray absorption profiles are well reproduced by this theory.

L3 and M4,5 absorption edges of intermediate valent cerium unravelled by resonant photoemission and resonant Auger spectroscopy.

In CeRh3 and CeTRh3 compotmds, strong resonant effects are observed in the Ce 3d and 4d photoemission spectra at the Ce L3 and M4,5 absorption edges respectively. These resonm~ces show unambiguously that interference effects exist in core level spectroscopies. In the CeRh~ mixed valent compound, the resonances of photoenfission peaks corresponding to 4f °, 4t a and 4f 2 final state character occur at different photon energies, making it possible to mu'avel the X-ray absorption profiles. Moreover, resonant L31Vh,sN4.5 Rmnan Auger lines are observed up to 10 eV above the L3 absorption edge, showing the strong localized character of the core excited state.

Soft x-ray lithographic studies of interphase chromosomes.

Soft x-ray absorption lithograph patterns of purified interphase human nuclei and chromosome arrays, imaged on PMMA resists, were examined by scanning EM. The patterns obtained were compared to those utilizing more conventional sources, including transmission EM, scanning EM, high voltage EM, and various light microscopic techniques. The x-ray resist images revealed orderly arrays of absorption profiles in the 3-dimensional specimen with both mild and more extensive developments of the resist. Dense chromatin at the edge of interphase nuclei revealed aligned periodic peaks on the order of 2200 A diameter, with substructure. The periodicity and alignment of interphase chromosomes were entirely consistent with birefringent data on nuclei indicating a high degree of 3-dimensional order. This degree of 3-dimensional order was observed in nuclei containing essentially DNA and histones with only very few other minor (probably structural) proteins. Sonication and nuclease treatment to disperse interphase chromosomes revealed similar absorption periodicities in individual chromosome fibers. Analysis of x-ray absorption profiles thus appears to offer significant new insights into the ordered structure of these defined biological specimens.