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Abstract: Illuminating complexity: Time-resolved, microreactor-based, EDE/mass spectrometry (EDE=energy dispersive extended X-ray absorption fine structure) was used to probe the behaviour of both Rh and Pd components of alloyed nanoparticles during NO reduction by H2. The picture shows the NO conversion as a function of temperature. This surface enrichment in Pd is shown to curtail rapid oxidation of the Rh component by NO and leads to a promotion of NO reduction.
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Jul 2004
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Abstract: The high-pressure behavior of synthetic end-member IIb clinochlore, Mg5Al(Si3Al)O10(OH)8, has been studied by synchrotron X-ray powder diffraction to 27 GPa at 300 K. A non-quenchable, reversible transformation occurs between 9 and 10 GPa that is dominated by compression normal to the structural layering and has an associated small but significant shear of the β angle from 97.2 to 96.3°. The high-pressure chlorite is more compressible than the low-pressure phase. Diffraction patterns of the high-pressure chlorite are very similar from 10 to 27 GPa, indicating that it persists stably with no significant change in β to very high pressures: β is effectively locked at the transformation to the high-pressure structure. It is proposed that the transformation is not polytypic and that the distortion reflects reorganization of the interlayer hydrogen bonding, possibly involving novel proton behavior as adjacent sheets of cations of the brucite-like and tetrahedral layers close down on the sheet of interlayer protons. The transformation is likely driven by O atom close-packing requirements imposed by pressure.
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Aug 2004
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Detectors
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Abstract: Photon counting detectors have to work at high count rates at synchrotron radiation sources. It is well known that at high count rates, counts may be lost because the time difference between the arrival of two X‐rays in the detector can be too small to resolve each count. This is coventionally modelled in the paralyzable deadtime model. In a previous paper, we introduced a new model — the pulse overlap model. This model predicts that there is a limit to the statistical accuracy that can be achieved by a measurement made in fixed counting time by a given photon counting detector. At counting rates above the point where maximum statistical accuracy is achieved, the statistical accuracy decreases. We present data obtained from computer Monte Carlo simulation and measurements with a real detector and compare with the predictions of the pulse overlap model.
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Aug 2004
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Abstract: A new X-ray Michelson interferometer based on the BBB interferometer of Bonse and Hart and designed for X-rays of wavelength similar to1 Angstrom was described in a previous paper. Here, a further test carried out at the SPring-8 1 km beamline BL29XUL is reported. One of the BBB's mirrors was displaced by a piezo to introduce the required path-length difference. The resulting variation of intensity with piezo voltage as measured by an avalanche photodiode could be ascribed to the phase variation resulting from the path-length change, with a small additional contribution from the change of the position of the lattice planes of the front mirror relative to the rest of the crystal. This 'Michelson fringe' interpretation is supported by the observed steady movement across the output beam of the interference fringes produced by a refractive wedge when the piezo voltage was ramped. The front-mirror displacement required for one complete fringe at the given wavelength is only 0.675 Angstrom; therefore, a quiet environment is vital for operating this device, as previous experiments have shown.
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Sep 2004
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Abstract: In their Comment, G. Faigel, M. Tegze, and G. Bortel claim that low-pass filtering can be used to remove the long-range order contributions to the reconstructed images of objects with crystalline internal structure. In this Reply, we present further results, clarifying the problem and supporting the conclusions drawn in the commented paper. We show that the Comment does not provide any evidence to contradict our results.
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Sep 2004
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Abstract: We present a detailed study of the interaction of chlorine with the Si(I 13) surface by means of scanning tunneling microscopy. Upon chlorine adsorption at a substrate temperature of 600 degreesC, the most prominent feature is the presence of (2 x n) reconstructions with n = 2, 3, 5, 7 or 9 in the order of decreasing chlorine coverage. A concurrent process is the rearrangement of step edges from an initially rough appearance to an atomically smooth configuration perpendicular to the [33 (2) over bar] direction. (C) 2004 Elsevier B.V. All rights reserved.
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Sep 2004
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Abstract: The recrystallisation of organic compounds from solution under high-pressure conditions is shown to be a versatile method for the formation of new polymorphs and solvates. The technique is illustrated by the crystallisation of a new polymorph of phenanthrene from dichloromethane at a pressure of 0.7 GPa, and the crystallisation of a novel dihydrate of paracetamol from water at a pressure of 1.1 GPa. These phases have been characterised by single crystal X-ray diffraction. We also demonstrate that the technique can be used to prepare a polymorph that is metastable under ambient conditions. Thus the orthorhombic form of paracetamol was crystallised from ethanol at a pressure of 1.1 GPa.
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Sep 2004
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Abstract: Synchronous, time resolved, infra-red, XAFS, and mass spectroscopies are simultaneously applied in situ to the investigation of the dynamic behaviour of Rh/Al2O3 catalysts during NO reduction by CO; NO conversion, and its kinetic character are closely correlated to the conversion of Rh(I) (predominantly Rh-I(CO)(2)) to Rh(0).
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Sep 2004
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Accelerator Physics
Survey
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Abstract: The paper will cover the survey and alignment techniques selected for the installation and alignment of the diamond accelerators together with a current status report.
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Oct 2004
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Vacuum
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Abstract: The ion to neutral ratio of the sputtered material have been studied for high power pulsed magnetron sputtering and compared with a continuous direct, current (dc) discharge using the same experimental setup except for the power source. Optical emission spectroscopy (OES) was used to study the optical emission from the plasma through a side window. The emission was shown to be dominated by emission from metal ions. The distribution of metal ionized states clearly differed from the distribution of excited states, and we suggest the presence of a hot dense plasma surrounded by a cooler plasma. Sputtered material was ionized close to the target and transported into a cooler plasma region where the emission was also recorded. Assuming a Maxwell-Boltzmann distribution of excited states the emission from the plasma was quantified. This showed that the ionic contribution to the recorded spectrum was over 90% for high pulse. powers. Even at relatively low applied pulse powers, the recorded spectra were dominated by emission from ions. OES analysis of the discharge in a continuous dc magnetron discharge was also made, which demonstrated much lower ionization. (C) 2005 American Vacuum Society.
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Nov 2004
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