Theoretical Investigation of Phase Transitions of Graphite and Cubic 3C Diamond Into Hexagonal 2H Diamond Under High Pressures

Possible techniques for experimentally obtaining hexagonal diamond are studied in the scope of the density functional theory method. It has been found that hexagonal diamond may be created as a result of structural transition at 61 68 GPa uniaxial compression from orthorhombic AB graphite and at 57 66GPa from hexagonal AA graphite. Also the formation of hexagonal diamond is shown to take place in case of very strong 300 380 GPa compression of cubic diamond. X ray and electron microscopic data on nanodiamonds from meteorite craters are analyzed for the presence of hexagonal diamond. The analysis shows that impact origin carbon materials do not contain pure cubic and hexagonal diamonds, and the layers of nascent crystals of diamond polytypes are randomly packe

Hard x ray spectroscopy and imaging by a reflection zone plate in the presence of astigmatism

The feasibility of an off axis x ray reflection zone plate to perform wavelength dispersive spectroscopy, on axis point focusing, and two dimensional imaging is demonstrated by means of one and the same diffractive optical element DOE at a synchrotron radiation facility. The resolving power varies between 30 and 400 in the range of 7.6 keV to 9.0 keV, with its maximum at the design energy of 8.3 keV. This result is verified using an adjustable entrance slit, by which horizontal H and vertical V focusing to 0.85 amp; 956;m H and 1.29 amp; 956;m V is obtained near the sagittal focal plane of the astigmatic configuration. An angular and axial scan proves an accessible field of view of at least 0.6 arcmin 0.8 arcmin and a focal depth of plus minus 0.86 mm. Supported by the grating efficiency of around 17.5 and a very short pulse elongation, future precision x ray fluorescence and absorption studies of transition metals at their K edge on an ultrashort timescale could benefit from our finding

The New HZB X Ray Microscopy Beamline U41 PGM1 XM at BESSY II

At the BESSY II electron storage ring running by the Helmholtz Zentrum Berlin HZB a transmission soft X ray microscope TXM is operated very successfully for both tomographic imaging of cells [1,2] and for NEXAFS studies in materials science [3 5]. Here, we present the setup of a newly designed beamline Fig. 1 that will significantly enhance the performance of the HZB TXM. Faster data acquisition over the whole accessible photon energy range together with a possible extension into the tender X ray range is given. Furthermore, within the photon energy tuning range, two new important absorption edges, namely sulfur and phosphorus, will become accessible for element specific 3D imaging, in addition to those already available, e.g. at carbon, calcium, titanium and oxygen. Another advantage of this new beamline is that phase contrast X ray microscopy for thicker specimens will become possible in the tender X ray photon energy rang

Monochromatization of femtosecond XUV light pulses with the use of reflection zone plates

We report on a newly built laser based tabletop setup which enables generation of femtosecond light pulses in the XUV range via employing the process of high order harmonic generation HHG in a gas medium. The spatial, spectral, and temporal characteristics of the XUV beam are presented. Monochromatization of XUV light with minimum temporal pulse distortion is the central issue of this work. Off center reflection zone plates are shown to be superior to gratings when selection of a desired harmonic is carried out with the use of a single optical element. A cross correlation technique was applied to characterize the performance of zone plates in the time domain. By using laser pulses of 25 fs length to pump the HHG process, a pulse duration of 45 fs for monochromatized harmonics was achieved in the present setu