Excited-State Identification of a Nickel-Bipyridine Photocatalyst by Time-Resolved X-ray Absorption Spectroscopy.

Photoassisted catalysis using Ni complexes is an emerging field for cross-coupling reactions in organic synthesis. However, the mechanism by which light enables and enhances the reactivity of these complexes often remains elusive. Although optical techniques have been widely used to study the ground and excited states of photocatalysts, they lack the specificity to interrogate the electronic and structural changes at specific atoms. Herein, we report metal-specific studies using transient Ni L- and K-edge X-ray absorption spectroscopy of a prototypical Ni photocatalyst, (dtbbpy)Ni(o-tol)Cl (dtb = 4,4'-di-tert-butyl, bpy = bipyridine, o-tol = ortho-tolyl), in solution. We unambiguously confirm via direct experimental evidence that the long-lived (∼5 ns) excited state is a tetrahedral metal-centered triplet state. These results demonstrate the power of ultrafast X-ray spectroscopies to unambiguously elucidate the nature of excited states in important transition-metal-based photocatalytic systems.

Absorption Correction for 3D Elemental Distributions of Dental Composite Materials Using Laboratory Confocal Micro-X-ray Fluorescence Spectroscopy.

Confocal micro-X-ray fluorescence (micro-XRF) spectroscopy facilitates three-dimensional (3D) elemental imaging of heterogeneous samples in the micrometer range. Laboratory setups using X-ray tube excitation render the method accessible for diverse research fields but interpretation of results and quantification remain challenging. The attenuation of X-rays in composites depends on the photon energy as well as on the composition and density of the material. For confocal micro-XRF, attenuation severely impacts elemental distribution information, as the signal from deeper layers is distorted by superficial layers. Absorption correction and quantification of fluorescence measurements in heterogeneous composite samples have so far not been reported. Here, an absorption correction approach for confocal micro-XRF combining density information from microcomputed tomography (micro-CT) data with laboratory X-ray absorption spectroscopy (XAS) and synchrotron transmission measurements is presented. The energy dependency of the probing volume is considered during the correction. The methodology is demonstrated on a model composite sample consisting of a bovine tooth with a clinically used restoration material.

In situ/operando plug-flow fixed-bed cell for synchrotron PXRD and XAFS investigations at high temperature, pressure, controlled gas atmosphere and ultra-fast heating.

A plug-flow fixed-bed cell for synchrotron powder X-ray diffraction (PXRD) and X-ray absorption fine structure (XAFS) idoneous for the study of heterogeneous catalysts at high temperature, pressure and under gas flow is designed, constructed and demonstrated. The operating conditions up to 1000°C and 50 bar are ensured by a set of mass flow controllers, pressure regulators and two infra-red lamps that constitute a robust and ultra-fast heating and cooling method. The performance of the system and cell for carbon dioxide hydrogenation reactions under specified temperatures, gas flows and pressures is demonstrated both for PXRD and XAFS at the P02.1 (PXRD) and the P64 (XAFS) beamlines of the Deutsches Elektronen-Synchrotron (DESY).

Scan-Free GEXRF in the Soft X-ray Range for the Investigation of Structured Nanosamples.

Scan-free grazing-emission X-ray fluorescence spectroscopy (GEXRF) is an established technique for the investigation of the elemental depth-profiles of various samples. Recently it has been applied to investigating structured nanosamples in the tender X-ray range. However, lighter elements such as oxygen, nitrogen or carbon cannot be efficiently investigated in this energy range, because of the ineffective excitation. Moreover, common CCD detectors are not able to discriminate between fluorescence lines below 1 keV. Oxygen and nitrogen are important components of insulation and passivation layers, for example, in silicon oxide or silicon nitride. In this work, scan-free GEXRF is applied in proof-of-concept measurements for the investigation of lateral ordered 2D nanostructures in the soft X-ray range. The sample investigated is a Si3N4 lamellar grating, which represents 2D periodic nanostructures as used in the semiconductor industry. The emerging two-dimensional fluorescence patterns are recorded with a CMOS detector. To this end, energy-dispersive spectra are obtained via single-photon event evaluation. In this way, spatial and therefore angular information is obtained, while discrimination between different photon energies is enabled. The results are compared to calculations of the sample model performed by a Maxwell solver based on the finite-elements method. A first measurement is carried out at the UE56-2 PGM-2 beamline at the BESSY II synchrotron radiation facility to demonstrate the feasibility of the method in the soft X-ray range. Furthermore, a laser-produced plasma source (LPP) is utilized to investigate the feasibility of this technique in the laboratory. The results from the BESSY II measurements are in good agreement with the simulations and prove the applicability of scan-free GEXRF in the soft X-ray range for quality control and process engineering of 2D nanostructures. The LPP results illustrate the chances and challenges concerning a transfer of the methodology to the laboratory.

Enabling Coarse X-ray Fluorescence Imaging Scans with Enlarged Synchrotron Beam by Means of Mosaic Crystal Defocusing Optics.

Trace elements, functionalized nanoparticles and labeled entities can be localized with sub-mm spatial resolution by X-ray fluorescence imaging (XFI). Here, small animals are raster scanned with a pencil-like synchrotron beam of high energy and low divergence and the X-ray fluorescence is recorded with an energy-dispersive detector. The ability to first perform coarse scans to identify regions of interest, followed by a close-up with a sub-mm X-ray beam is desirable, because overall measurement time and X-ray dose absorbed by the (biological) specimen can thus be minimized. However, the size of X-ray beams at synchrotron beamlines is usually strongly dependent on the actual beamline setup and can only be adapted within specific pre-defined limits. Especially, large synchrotron beams are non-trivial to generate. Here, we present the concept of graphite-based, convex reflection optics for the one-dimensional enlargement of a 1 mm wide synchrotron beam by a factor of 5 to 10 within a 1 m distance. Four different optics are tested and characterized and their reflection properties compared to ray tracing simulations. The general shape and size of the measured reflection profiles agree with expectations. Enhancements with respect to homogeneity and efficiency can be expected with improved optics manufacturing. A mouse phantom is used for a proof-of-principle XFI experiment demonstrating the applicability of coarse and fine scans with the suggested optics design.

Towards Poisson noise limited optical pump soft X-ray probe NEXAFS spectroscopy using a laser-produced plasma source: erratum.

We provide corrections for our previous publication [Opt. Express27, 36524 (2019)10.1364/OE.27.036524].

Laboratory quick near edge x-ray absorption fine structure spectroscopy in the soft x-ray range with 100 Hz frame rate using CMOS technology.

In laboratory based x-ray absorption fine structure (XAFS) spectroscopy, the slow readout speed of conventional CCD cameras can prolong the measuring times by multiple orders of magnitude. Using pulsed sources, e.g., laser-based x-ray sources, the pulse repetition rate often exceeds the frame rate of the CCD camera. We report the use of a scientific CMOS (sCMOS) camera for XAFS spectroscopy with a laser-produced plasma source facilitating measurements at 100 Hz. With this technological improvement, a new class of experiments becomes possible, starting from the time consuming analysis of samples with small absorption to pump-probe investigations. Furthermore, laboratory quick soft x-ray absorption fine structure (QXAFS) measurements with 10 ms time resolution are rendered feasible. We present the characterization of the sCMOS camera concerning noise characteristics and a comparison to conventional CCD camera performance. The feasibility of time resolved QXAFS measurements is shown by analyzing the statistical uncertainty of single shot spectra. Finally, XAFS spectroscopy on a complex sandwich structure with minute amounts of NiO exemplifies the additional merits of fast detectors.

Transient Sub-nanosecond Soft X-ray NEXAFS Spectroscopy on Organic Thin Films.

We demonstrate visible pump soft X-ray probe near-edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements at the carbon K edge on thin molecular films in the laboratory. This opens new opportunities through the use of laboratory equipment for chemical speciation. We investigate the metal-free porphyrin derivative tetra(tert-butyl)porphyrazine as an ideal model system to elucidate electronic properties of tetrapyrroles like chlorophyll or heme. In contrast to measurements in gas or liquid state, the investigation of thin films is of high interest in the field of optoelectronic and photovoltaic devices though challenging due to the low damage thresholds of the samples upon excitation. With a careful pre-characterization using optical techniques, successful measurements were performed using a NEXAFS spectrometer based on a laser-produced plasma source and reflection zone plates with a resolving power of 1000 and a time resolution of 0.5 ns. In combination with density functional theory calculations, first insights into a long-lived excitonic state are gained and discussed.

Laboratory Soft X-Ray Microscopy with an Integrated Visible-Light Microscope-Correlative Workflow for Faster 3D Cell Imaging.

Laboratory transmission soft X-ray microscopy (L-TXM) has emerged as a complementary tool to synchrotron-based TXM and high-resolution biomedical 3D imaging in general in recent years. However, two major operational challenges in L-TXM still need to be addressed: a small field of view and a potentially misaligned rotation stage. As it is not possible to alter the magnification during operation, the field of view in L-TXM is usually limited to a few tens of micrometers. This complicates locating areas and objects of interest in the sample. Additionally, if the rotation axis of the sample stage cannot be adjusted prior to the experiments, an efficient workflow for tomographic imaging cannot be established, as refocusing and sample repositioning will become necessary after each recorded projection. Both these limitations have been overcome with the integration of a visible-light microscope (VLM) into the L-TXM system. Here, we describe the calibration procedure of the goniometer sample stage and the integrated VLM and present the resulting 3D imaging of a test sample. In addition, utilizing this newly integrated VLM, the extracellular matrix of cryofixed THP-1 cells (human acute monocytic leukemia cells) was visualized by L-TXM for the first time in the context of an ongoing biomedical research project.

Toroidal multilayer mirrors for laboratory soft X-ray grazing emission X-ray fluorescence.

Efficient soft X-ray spectroscopy in the laboratory is still a challenging task. Here, we report on new toroidal multilayer optics designed and applied with the laser-produced plasma (LPP) source of the Berlin Laboratory for innovative X-ray technologies. The optics are described and characterized, and the application of the updated source to scanning-free grazing emission X-ray fluorescence is demonstrated on thermoelectric gold-doped copper oxide nanofilms. The comparison with synchrotron measurements allows estimating a flux on the sample of approximately 7.5 × 109 photons/s in the 1 keV range on a 100 µm × 100 µm spot, emphasizing the suitability of the updated LPP source for the application in photon hungry experiments.

The effect of post-deposition annealing conditions on structural and thermoelectric properties of sputtered copper oxide films.

The development of thin-film thermoelectric applications in sensing and energy harvesting can benefit largely from suitable deposition methods for earth-abundant materials. In this study, p-type copper oxide thin films have been prepared on soda lime silicate glass by direct current (DC) magnetron sputtering at room temperature from a pure copper metallic target in an argon atmosphere, followed by subsequent annealing steps at 300 °C under various atmospheres, namely air (CuO:air), nitrogen (CuO:N) and oxygen (CuO:O). The resultant films have been studied to understand the influence of various annealing atmospheres on the structural, spectroscopic and thermoelectric properties. X-ray diffraction (XRD) patterns of the films showed reflexes that could be assigned to those of crystalline CuO with a thin mixed Cu(I)Cu(II) oxide, which was also observed by near edge X-ray absorption fine structure spectroscopy (NEXAFS). The positive Seebeck coefficient (S) reached values of up to 204 µV K-1, confirming the p-type behavior of the films. Annealing under oxygen provided a significant improvement in the electrical conductivity up to 50 S m-1, resulting in a power factor of 2 µW m-1 K-2. The results reveal the interplay between the intrinsic composition and the thermoelectric performance of mixed copper oxide thin films, which can be finely adjusted by simply varying the annealing atmosphere.

Selective mineral transport barriers at Cuscuta-host infection sites.

The uptake of inorganic nutrients by rootless parasitic plants, which depend on host connections for all nutrient supplies, is largely uncharted. Using X-ray fluorescence spectroscopy (XRF), we analyzed the element composition of macro- and micronutrients at infection sites of the parasitic angiosperm Cuscuta reflexa growing on hosts of the genus Pelargonium. Imaging methods combining XRF with 2-D or 3-D (confocal) microscopy show that most of the measured elements are present at similar concentrations in the parasite compared to the host. However, calcium and strontium levels drop pronouncedly at the host/parasite interface, and manganese appears to accumulate in the host tissue surrounding the interface. Chlorine is present in the haustorium at similar levels as in the host tissue but is decreased in the stem of the parasite. Thus, our observations indicate a restricted uptake of calcium, strontium, manganese and chlorine by the parasite. Xylem-mobile dyes, which can probe for xylem connectivity between host and parasite, provided evidence for an interspecies xylem flow, which in theory would be expected to carry all of the elements indiscriminately. We thus conclude that inorganic nutrient uptake by the parasite Cuscuta is regulated by specific selective barriers whose existence has evaded detection until now.

Towards Poisson noise limited optical pump soft X-ray probe NEXAFS spectroscopy using a laser-produced plasma source.

We present a laboratory setup for near edge X-ray absorption spectroscopy (NEXAFS) in the soft X-ray regime between 284 eV to 1303 eV with a resolving power of up to 1370. Based on a laser-produced plasma source, a pair of identical reflection zone plates and an X-ray CCD camera, the setup is intended for optical pump X-ray probe NEXAFS measurements with a detectable change in absorption of the excited sample down to 10-4 and 500 ps time resolution. Because of the high stability of the source the statistical error only depends on the detector response and the number of photons detected and can reach the detector noise limit after a couple of thousands single shots. Thus, structure-function relationship investigations of bio-molecules are rendered feasible in the laboratory.

Scanning transmission X-ray microscopy with efficient X-ray fluorescence detection (STXM-XRF) for biomedical applications in the soft and tender energy range.

Scanning transmission X-ray microscopy, especially in combination with X-ray fluorescence detection (STXM-XRF) in the soft X-ray energy range, is becoming an increasingly important tool for life sciences. Using X-ray fluorescence detection, the study of biochemical mechanisms becomes accessible. As biological matrices generally have a low fluorescence yield and thus a low fluorescence signal, high detector efficiency (e.g. large solid angle) is indispensable for avoiding long measurement times and radiation damage. Here, the new AnImaX STXM-XRF microscope equipped with a large solid angle of detection enabling fast scans and the first proof-of-principle measurements on biomedical samples are described. In addition, characterization measurements for future quantitative elemental imaging are presented.

A laboratory spectrometer for high throughput X-ray emission spectroscopy in catalysis research.

We have built a laboratory spectrometer for X-ray emission spectroscopy. The instrument is employed in catalysis research. The key component is a von Hamos full cylinder optic with Highly Annealed Pyrolytic Graphite (HAPG) as a dispersive element. With this very efficient optic, the spectrometer subtends an effective solid angle of detection of around 1 msr, allowing for the analysis of dilute samples. The resolving power of the spectrometer is approximately E/ΔE = 4000, with an energy range of ∼2.3 keV-10 keV. The instrument and its characteristics are described herein. Further, a comparison with a prototype spectrometer, based on the same principle, shows the substantial improvement in the spectral resolution and energy range for the present setup. The paper concludes with a discussion of sample handling. A compilation of HAPG fundamentals and related publications are given in a brief Appendix.

On the Electronic Structure of Cu Chlorophyllin and Its Breakdown Products: A Carbon K-Edge X-ray Absorption Spectroscopy Study.

Using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, the carbon backbone of sodium copper chlorophyllin (SCC), a widely used chlorophyll derivative, and its breakdown products are analyzed to elucidate their electronic structure and physicochemical properties. Using various sample preparation methods and complementary spectroscopic methods (including UV/Vis, X-ray photoelectron spectroscopy), a comprehensive insight into the SCC breakdown process is presented. The experimental results are supported by density functional theory calculations, allowing a detailed assignment of characteristic NEXAFS features to specific C bonds. SCC can be seen as a model system for the large group of porphyrins; thus, this work provides a novel and detailed description of the electronic structure of the carbon backbone of those molecules and their breakdown products. The achieved results also promise prospective optical pump/X-ray probe investigations of dynamic processes in chlorophyll-containing photosynthetic complexes to be analyzed more precisely.

Advanced Absorption Correction for 3D Elemental Images Applied to the Analysis of Pearl Millet Seeds Obtained with a Laboratory Confocal Micro X-ray Fluorescence Spectrometer.

We present a novel absorption correction approach for elemental distribution images obtained with a laboratory confocal micro X-ray fluorescence spectrometer. The procedure is suited especially for biological samples, as a constant dark matrix with a varying minor or trace element distribution is assumed. The constant absorption in the sample is extracted from depth dependent measurements. By using the concept of an effective excitation energy, depth-dependent, and element-specific excitation energy values are calculated. For each voxel of a full 3D measurement, a correction is performed taking into account the actual number of voxels in the excitation and detection path. As proof of concept, the embryonic region of pearl millet seeds is investigated. Data are measured from the top and bottom side, resulting in a good agreement after the application of the absorption correction procedure. The distribution of elemental micronutrients is compared in seeds of two pearl millet genotypes. The corrected images illustrate different localization patterns of the micronutrient elements in pearl millet seed tissues.

Magnesium K-Edge NEXAFS Spectroscopy of Chlorophyll a in Solution.

The interaction of the central magnesium atom of chlorophyll a (Chl a) with the carbon and nitrogen backbone was investigated by magnesium K near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in fluorescence detection mode. A crude extract of Chl a was measured as a 1 × 10-2 mol/L ethanol solution (which represents an upper limit of concentration without aggregation) and as dried droplets. For the first time, the investigation of Mg bound to Chl a in a liquid environment by means of X-ray absorption spectroscopy is demonstrated. A pre-edge feature in the dissolved as well as in dried Chl a NEXFAS spectra has been identified as a characteristic transition originating from Mg in the Chl a molecule. This result is confirmed by theoretical DFT calculations leading to molecular orbitals (MO) which are mainly situated on the magnesium atom and nitrogen and carbon atoms from the pyrrole rings. The description is the first referring to the MO distribution with respect to the central Mg ion of Chl a and the surrounding atoms. On this basis, new approaches for the investigations of dynamic processes of molecules in solution and structure-function relationships of photosynthetic pigments and pigment-protein complexes in their native environment can be developed.

Full-Field Calcium K-Edge X-ray Absorption Near-Edge Structure Spectroscopy on Cortical Bone at the Micron-Scale: Polarization Effects Reveal Mineral Orientation.

Here, we show results on X-ray absorption near edge structure spectroscopy in both transmission and X-ray fluorescence full-field mode (FF-XANES) at the calcium K-edge on human bone tissue in healthy and diseased conditions and for different tissue maturation stages. We observe that the dominating spectral differences originating from different tissue regions, which are well pronounced in the white line and postedge structures are associated with polarization effects. These polarization effects dominate the spectral variance and must be well understood and modeled before analyzing the very subtle spectral variations related to the bone tissue variations itself. However, these modulations in the fine structure of the spectra can potentially be of high interest to quantify orientations of the apatite crystals in highly structured tissue matrices such as bone. Due to the extremely short wavelengths of X-rays, FF-XANES overcomes the limited spatial resolution of other optical and spectroscopic techniques exploiting visible light. Since the field of view in FF-XANES is rather large the acquisition times for analyzing the same region are short compared to, for example, X-ray diffraction techniques. Our results on the angular absorption dependence were verified by both site-matched polarized Raman spectroscopy, which has been shown to be sensitive to the orientation of bone building blocks and by mathematical simulations of the angular absorbance dependence. As an outlook we further demonstrate the polarization based assessment of calcium-containing crystal orientation and specification of calcium in a beta-tricalcium phosphate (ß-Ca3(PO4)2 scaffold implanted into ovine bone. Regarding the use of XANES to assess chemical properties of Ca in human bone tissue our data suggest that neither the anatomical site (tibia vs jaw) nor pathology (healthy vs necrotic jaw bone tissue) affected the averaged spectral shape of the XANES spectra.

High average power, highly brilliant laser-produced plasma source for soft X-ray spectroscopy.

In this work, a novel laser-produced plasma source is presented which delivers pulsed broadband soft X-radiation in the range between 100 and 1200 eV. The source was designed in view of long operating hours, high stability, and cost effectiveness. It relies on a rotating and translating metal target and achieves high stability through an on-line monitoring device using a four quadrant extreme ultraviolet diode in a pinhole camera arrangement. The source can be operated with three different laser pulse durations and various target materials and is equipped with two beamlines for simultaneous experiments. Characterization measurements are presented with special emphasis on the source position and emission stability of the source. As a first application, a near edge X-ray absorption fine structure measurement on a thin polyimide foil shows the potential of the source for soft X-ray spectroscopy.