Yasuhiro Niwa

High-speed x-ray reflectometory in multiwavelength-dispersive mode

The potential of a high speed x-ray reflectometer for time-resolved studies on the subsecond to millisecond timescales is demonstrated by recording x-ray reflection curves from a small area (1mm×10mm) of a 14.3nm thick gold film on a silicon substrate with data collection times of 0.05–1s. A horizontally convergent x-ray beam having a one-to-one correlation between ray direction and energy is produced by a curved crystal polychromator, and the beam is incident on and vertically reflected by a specimen placed at the focus. The x-ray reflectivity is measured as a function of the x-ray energy downstream of the focus using a one dimensional detector with no need for angle scan of the specimen and detector.

Dynamics of Photoelectrons and Structural Changes of Tungsten Trioxide Observed by Femtosecond Transient XAFS

The dynamics of the local electronic and geometric structures of WO3 following photoexcitation were studied by femtosecond time-resolved X-ray absorption fine structure (XAFS) spectroscopy using an X-ray free electron laser (XFEL). We found that the electronic state was the first to change followed by the local structure, which was affected within 200 ps of photoexcitation.

Time-Resolved Dispersive XAFS Instrument at NW2A Beamline of PF-AR

The configuration and performance of the time‐resolved dispersive XAFS (DXAFS) instrument, which has been constructed at the NW2A beamline of PF‐AR (KEK), are presented. The DXAFS instrument is mainly composed of a polychromator part, a position control part for sample, and a linear detector part. The Bragg‐ and Laue‐type polychromator crystal (Si(111) or Si(311)) is bent using the holder with fixed bending radius, in which the thermostated water is circulated to prevent the temperature change of crystal due to the heat load. The photodiode array (PDA) with and without phosphor screen is used as the linear X‐ray detector, and the minimum exposure time is 2 ms for the 1024‐element PDA. The phosphor screen on the PDA detector prevents the damage of the chip especially for high energy X‐rays but the existence reduces the energy resolution because of the scattering of the visible light converted on the phosphor. The DXAFS instrument was applied to the mechanistic study of the reduction processes of Cu support...

Ternary intermetallic LaCoSi as a catalyst for N 2 activation

AbstractActivating high-energy multiple bonds using earth-abundant metals is one of the most significant challenges in catalysis. Here, we show that LaCoSi—a ternary intermetallic compound—is an efficient and stable catalyst for N2 activation to produce NH3. The ammonia synthesis is significantly promoted by shifting the reaction bottleneck from the sluggish N2 dissociation to NHx formation, which few catalysts have achieved. Theoretical calculations reveal that the negatively charged cobalt mediates electron transfer from lanthanum to the adsorbed N2, which further reduces the activation barrier of N2 dissociation. Most importantly, the specific LaCoSi geometric configuration stabilizes the N2 adsorption with a strong exothermic effect, which dramatically decreases the apparent energy barrier of N2 activation. Consequently, LaCoSi shows a superior activity (1,250 μmol g−1 h−1), with a 60-fold increase over the activity of supported cobalt catalysts under mild reaction conditions (400 °C, 0.1 MPa).Ammonia synthesis is an energy-intensive process due to the high activation barrier for N2 dissociation. Here, Hosono and co-workers show that the intermetallic compound LaCoSi can lower the energy requirement for N2 activation and shift the rate-determining step of the process to NHx formation under mild conditions.

X-ray-induced reduction of Au ions in an aqueous solution in the presence of support materials and in situ time-resolved XANES measurements.

In situ time-resolved XANES measurements of Au ions in an aqueous solution in the presence of support materials were performed under synchrotron X-ray irradiation. The synchrotron X-ray-induced reduction of Au ions leads to the formation of Au nanoparticles on the carbon particles, acrylic cell or polyimide window. The deposited Au metallic spots were affected by the wettability of carbon particles.

Magnetic field-induced spin-crossover transition in [MnIII(taa)] studied by x-ray absorption spectroscopy

X-ray absorption near-edge structure (XANES) spectra of Mn in a spin-crossover compound, [MnIII(taa)], was studied in pulsed high magnetic fields up to 37 T. The significant changes in the XANES spectra, related to the spin-crossover from the low-spin (LS) state to high-spin (HS) state, were found to be induced by magnetic fields at temperatures down to 17 K. The fraction of the field-induced HS states was, at most, 30%, and the magnetic field dependence of the HS fraction exhibited hysteresis. The observed field-induced spin-crossover at T = 17 K (T/Tc ∼ 0.4, where Tc (= 46 K) is the spin-crossover transition temperature) cannot be understood as the field-induced macroscopic phase transition, because the transition magnetic field is known to be higher than 55 T at low temperatures. The observed field-induced LS-HS transition is likely the local microscopic transition at the single-molecule level.

In situ observation of RedOx reactions of Pd/Sr-Fe-O catalysts for automotive emission

We investigated the change in structures during reduction and oxidation of a new type of Pd on Sr-Fe-O catalyst for exhaust emission. In situ DXAFS techniques have shown changes in palladium states and oxygen concentration simultaneously during reduction and oxidation reactions. Quantitative analysis has shown that palladium atoms change their states between Pd(II*) and Pd(0) reversibly according to the gas atmosphere accompanying changes in the oxygen concentration. This reaction explains the high performance of the catalyst.

Newly designed double surface bimorph mirror for BL-15A of the photon factory

BL-15A is a new x-ray undulator beamline at the Photon Factory. It will be dedicated to two independent research activities, simultaneous XAFS/XRF/XRD experiments, and SAXS/WAXS/GI-SAXS studies. In order to supply a choice of micro-focus, low-divergence and collimated beams, a double surface bimorph mirror was recently developed. To achieve further mirror surface optimization, the pencil beam scanning method was applied for “in-situ” beam inspection and the Inverse Matrix method was used for determination of optimal voltages on the piezoelectric actuators. The corrected beam profiles at every focal spot gave good agreement with the theoretical values and the resultant beam performance is promising for both techniques. Quick and stable switching between highly focused and intense collimated beams was established using this new mirror with the simple motorized stages.

Time-resolved observation of structural change of copper induced by laser shock using synchrotron radiation with dispersive XAFS

ABSTRACT We report a single-shot nanosecond time-resolved dispersive XAFS measurement of copper foil under laser-induced shock compression. Obtained EXAFS results indicated clearly the structural evolution, though XANES spectra showed that the laser irradiation does not affect the copper electronic state. The nearest (ca. 0.22 nm) and longer (ca. 0.44 nm) Cu–Cu distances are changed at 4 and 200 ns, respectively, in the radial structure functions (RSFs). It was revealed that an average compression of 2% occurs at 4 ns and is released within 200 ns. It was found that the fragmentation of the copper begins at around 200 ns according to the increase in distance of the RSF.

Curved crystal X-ray optics for a new type of high speed, multiwavelength dispersive X-ray reflectometer

As the key element of a new type of high speed, multiwavelength dispersive X-ray reflectometer, the performance of a curved crystal optical system having an extremely small radius of curvature was studied by realizing and characterizing a convergent X-ray beam that has a one-to-one correlation between its energy and direction. Such an X-ray beam covered an energy range of 8?45 keV, which is sufficiently wide to cover a reasonably wide range of perpendicular momentum transfer in X-ray reflectometory. The energy resolution is estimated to be 0.1?0.7% in the range of 8?45 keV.