Time-resolved gas-phase kinetic and quantum chemical studies of the reaction of silylene with oxygen

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with O-2. The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF6 bath gas, at five temperatures in the range 297-600 K. The second order rate constants at 10 Torr were fitted to the Arrhenius equation: log(k/cm(3) molecule(-1) s(-1)) = (-11.08 +/- 0.04) + (1.57 +/- 0.32 kJ mol(-1))/RT ln10 The decrease in rate constant values with increasing temperature, although systematic is very small. The rate constants showed slight increases in value with pressure at each temperature, but this was scarcely beyond experimental uncertainty. From estimates of Lennard-Jones collision rates, this reaction is occurring at ca. 1 in 20 collisions, almost independent of pressure and temperature. Ab initio calculations at the G3 level backed further by multi-configurational (MC) SCF calculations, augmented by second order perturbation theory (MRMP2), support a mechanism in which the initial adduct, H2SiOO, formed in the triplet state (T), undergoes intersystem crossing to the more stable singlet state (S) prior to further low energy isomerisation processes leading, via a sequence of steps, ultimately to dissociation products of which the lowest energy pair are H2O + SiO. The decomposition of the intermediate cyclo-siladioxirane, via O-O bond fission, plays an important role in the overall process. The bottleneck for the overall process appears to be the T -> S process in H2SiOO. This process has a small spin orbit coupling matrix element, consistent with an estimate of its rate constant of 1 x 10(9) s(-1) obtained with the aid of RRKM theory. This interpretation preserves the idea that, as in its reactions in general, SiH2 initially reacts at the encounter rate with O-2. The low values for the secondary reaction barriers on the potential energy surface account for the lack of an observed pressure dependence. Some comparisons are drawn with the reactions of CH2 + O-2 and SiCl2 + O-2

On Love-type waves in a finitely deformed magnetoelastic layered half-space

In this paper, the propagation of Love-type waves in a homogeneously and finitely deformed layered half-space of an incompressible non-conducting magnetoelastic material in the presence of an initial uniform magnetic field is analyzed. The equations and boundary conditions governing linearized incremental motions superimposed on an underlying deformation and magnetic field for a magnetoelastic material are summarized and then specialized to a form appropriate for the study of Love-type waves in a layered half-space. The wave propagation problem is then analyzed for different directions of the initial magnetic field for two different magnetoelastic energy functions, which are generalizations of the standard neo-Hookean and Mooney–Rivlin elasticity models. The resulting wave speed characteristics in general depend significantly on the initial magnetic field as well as on the initial finite deformation, and the results are illustrated graphically for different combinations of these parameters. In the absence of a layer, shear horizontal surface waves do not exist in a purely elastic material, but the presence of a magnetic field normal to the sagittal plane makes such waves possible, these being analogous to Bleustein–Gulyaev waves in piezoelectric materials. Such waves are discussed briefly at the end of the paper

The First VERITAS Telescope

The first atmospheric Cherenkov telescope of VERITAS (the Very Energetic Radiation Imaging Telescope Array System) has been in operation since February 2005. We present here a technical description of the instrument and a summary of its performance. The calibration methods are described, along with the results of Monte Carlo simulations of the telescope and comparisons between real and simulated data. The analysis of TeV $\gamma$-ray observations of the Crab Nebula, including the reconstructed energy spectrum, is shown to give results consistent with earlier measurements. The telescope is operating as expected and has met or exceeded all design specifications.Comment: Accepted by Astroparticle Physic

Infrared spectra of the products of interaction of tungsten and molybdenum with fluorine isolated in solid argon

The IR spectra of the products formed when molecular fluorine passed over molybdenum and tungsten (heated to 100-1100 degreesC) and isolated in inert matrices at 12 K were recorded. In the W + F-2 system, the major product was tungsten hexafluoride. For the Mo + F-2 system, bands of MoF6, MoOF4, (MoF5)(3), MoF5, MoF4, and, possibly, MoF3 were identified. The spectra were interpreted with the use of thermodynamic calculations of the equilibrium composition of the gas phase at various temperatures. The structure of metal fluorides of different compositions is discussed

The infrared spectra of products of tantalum and ruthenium reactions with fluorine isolated in solid argon

The IR spectra of the products formed when molecular fluorine was passed over tantalum and ruthenium heated to 100-1100 degreesC and isolated in inert matrices at 12 K were studied. In the Ta + F-2 system, bands of (TaF5)(3), TaF5, and TaF4 were observed. In the Ru + F-2 system, the major reaction product was monomeric ruthenium pentafluoride. The structures of metal fluorides of various compositions are discussed

EXAFS and optical spectroscopy characterisation of reduction products of binary silver-copper ion mixture in mordenite

Ag+, Cu2+ and Ag+-Cu2+ binary mixtures with different Ag/Cu ratios were supported on mordenite with Si/Al ratio equal to 10 and reduced in the temperature range 323-673 K. Diffuse Reflectance UV-Visible spectra reveal appearance of Ag-8 clusters and small copper metal particles. Cu K-edge EXAFS show for Cu-rich non-reduced samples the presence of only oxygen shell; but for the Ag-rich sample, the presence of heavier backscatterer is also indicated producing a different EXAFS envelope. Silver EXAFS analysis suggests that, for the reduced samples, there are both Ag - 0 and Ag - Ag distances (Ag - Ag and Ag - O coordination numbers of approx 2 and of 1.3 respectively for the Ag/Cu = 3:1. and 1:3 samples). Importantly. for the reduced sample, the EXAFS indicates that Ag appears to promote/stabilise Cu cluster formation whereas Cu appears to impede/destabilise Ag cluster formation