Analogy of the slow dynamics between the supercooled liquid and the supercooled plastic crystal states of difluorotetrachloroethane

Slow dynamics of difluorotetrachloroethane in both supercooled plastic crystal and supercooled liquid states have been investigated from Molecular Dynamics simulations. The temperature and wave-vector dependence of collective dynamics in both states are probed using coherent dynamical scattering functions $S(Q,t)$. Our results confirm the strong analogy between molecular liquids and plastic crystals for which $\alpha$-relaxation times and non-ergodicity parameters are controlled by the non trivial static correlations $S(Q)$ as predicted by the Mode Coupling Theory. The use of infinitely thin needles distributed on a lattice as model of plastic crystals is discussed

Onset of slow dynamics in difluorotetrachloroethane glassy crystal

Complementary Neutron Spin Echo and X-ray experiments and Molecular Dynamics simulations have been performed on difluorotetrachloroethane (CFCl2-CFCl2) glassy crystal. Static, single-molecule reorientational dynamics and collective dynamics properties are investigated. The orientational disorder is characterized at different temperatures and a change in nature of rotational dynamics is observed. We show that dynamics can be described by some scaling predictions of the Mode Coupling Theory (MCT) and a critical temperature $T_{c}$ is determined. Our results also confirm the strong analogy between molecular liquids and plastic crystals for which $\alpha$-relaxation times and non-ergodicity parameters are controlled by the non trivial static correlations as predicted by MCT

Test beam Characterizations of 3D Silicon Pixel detectors

3D silicon detectors are characterized by cylindrical electrodes perpendicular to the surface and penetrating into the bulk material in contrast to standard Si detectors with planar electrodes on its top and bottom. This geometry renders them particularly interesting to be used in environments where standard silicon detectors have limitations, such as for example the radiation environment expected in an LHC upgrade. For the first time, several 3D sensors were assembled as hybrid pixel detectors using the ATLAS-pixel front-end chip and readout electronics. Devices with different electrode configurations have been characterized in a 100 GeV pion beam at the CERN SPS. Here we report results on unirradiated devices with three 3D electrodes per 50 x 400 um2 pixel area. Full charge collection is obtained already with comparatively low bias voltages around 10 V. Spatial resolution with binary readout is obtained as expected from the cell dimensions. Efficiencies of 95.9% +- 0.1 % for tracks parallel to the electrodes and of 99.9% +- 0.1 % at 15 degrees are measured. The homogeneity of the efficiency over the pixel area and charge sharing are characterized.Comment: 5 pages, 7 figure

Sodium p-nitrophenolate tetrahydrate

The structure of sodium p-nitrophenolate tetrahydrate, Na+�C6H4NO3 -�4H2O, is presented. The nature of the hydrogen and coordination bonds in this structure is discussed and compared with that of sodium p-nitrophenolate dihydrate

Longitudinal patterns in an Arkansas River Valley stream: an Application of the River Continuum Concept

The River Continuum Concept (RCC) provides the framework for studying how lotic ecosystems vary from headwater streams to large rivers. The RCC was developed in streams in eastern deciduous forests of North America, but watershed characteristics and land uses differ across ecoregions, presenting unique opportunities to study how predictions of the RCC may differ across regions. Additionally, RCC predictions may vary due to the influence of fishes, but few studies have used fish taxa as a metric for evaluating predictions of the RCC. Our goal was to determine if RCC predictions for stream orders 1 through 5 were supported by primary producer, macroinvertebrate, and fish communities in Cadron Creek of the Arkansas River Valley. We sampled chlorophyll a, macroinvertebrates, and fishes at five stream reaches across a gradient of watershed size. Contrary to RCC predictions, chlorophyll a did not increase in concentration with catchment size. As the RCC predicts, fish and macroinvertebrate diversity increased with catchment size. Shredding and collecting macroinvertebrate taxa supported RCC predictions, respectively decreasing and increasing in composition as catchment area increased. Herbivorous and predaceous fish did not follow RCC predictions; however, surface-water column feeding fish were abundant at all sites as predicted. We hypothesize some predictions of the RCC were not supported in headwater reaches of this system due to regional differences in watershed characteristics and altered resource availability due to land use surrounding sampling sites

Source gases: Concentrations, emissions, and trends

Source gases are defined as those gases that influence levels of stratospheric ozone (O3) by transporting species containing halogen, hydrogen, and nitrogen to the stratosphere. Examples are the CFC's, methane (CH4), and nitrous oxide (N2O). Other source gases that also come under consideration in an atmospheric O3 context are those that are involved in the O3 or hydroxyl (OH) radical chemistry of the troposphere. Examples are CH4, carbon monoxide (CO), and nonmethane hydrocarbons (NMHC's). Most of the source gases, along with carbon dioxide (CO2) and water vapor (H2O), are climatically significant and thus affect stratospheric O3 levels by their influence on stratospheric temperatures. Carbonyl sulphide (COS) could affect stratospheric O3 through maintenance of the stratospheric sulphate aerosol layer, which may be involved in heterogeneous chlorine-catalyzed O3 destruction. The previous reviews of trends and emissions of source gases, either from the context of their influence on atmospheric O3 or global climate change, are updated. The current global abundances and concentration trends of the trace gases are given in tabular format

Dehydration mechanism of a small molecular solid: 5-nitrouracil hydrate

Previous studies of the dehydration of 5-nitrouracil (5NU) have resulted in it being classified as a ‘‘channel hydrate’’ in which dehydration proceeds principally by the exit of the water molecules along channels in the structure. We have re-examined this proposal and found that in fact there are no continuous channels in the 5NU structure that would contribute to such a mechanism. Product water molecules would be immediately trapped in unlinked voids in the crystal structure and would require some additional mechanism to break loose from the crystal. Through a detailed structural analysis of the macro and micro structure of the 5NU as it dehydrates, we have developed a model for the dehydration process based on the observed development of structural defects in the 5NU crystal and the basic crystallography of the material. The model was tested against standard kinetic measurements and found to present a satisfactory account of kinetic observations, thus defining the mechanism. Overall, the study shows the necessity of complementing standard kinetic studies with a parallel macro and micro examination of the dehydrating material when evaluating the mechanisms of dehydration and decomposition processes

Random Sequential Adsorption: From Continuum to Lattice and Pre-Patterned Substrates

The random sequential adsorption (RSA) model has served as a paradigm for diverse phenomena in physical chemistry, as well as in other areas such as biology, ecology, and sociology. In the present work, we survey aspects of the RSA model with emphasis on the approach to and properties of jammed states obtained for large times in continuum deposition versus that on lattice substrates, and on pre-patterned surfaces. The latter model has been of recent interest in the context of efforts to use pre-patterning as a tool to improve selfassembly in micro- and nanoscale surface structure engineering

Critical behavior of thermopower and conductivity at the metal-insulator transition in high-mobility Si-MOSFET's

This letter reports thermopower and conductivity measurements through the metal-insulator transition for 2-dimensional electron gases in high mobility Si-MOSFET's. At low temperatures both thermopower and conductivity show critical behavior as a function of electron density which is very similar to that expected for an Anderson transition. In particular, when approaching the critical density from the metallic side the diffusion thermopower appears to diverge and the conductivity vanishes. On the insulating side the thermopower shows an upturn with decreasing temperature.Comment: 4 pages with 3 figure