Nature of the Oxygen Species at Ni(110) and Ni(100) Surfaces Revealed by Exposure to Oxygen and Oxygen-Ammonia Mixtures: Evidence for the Surface Reactivity of O- Type Species

Adsorption of dioxygen at clean Ni(110) and Ni(100) surfaces gives rise to two prominent features in the O(1s) spectra at 530 and 531 eV due to O2- and O- type species, respectively. Interaction of ammonia with a Ni(100)-O surface where theta(oxygen) < 0.1 ML favors the dissociation of NH3 giving NHn, (n = 1, 2) and N(a) species. This is accompanied by a decrease in the intensity of the 531 eV feature. On the other hand. a Ni(100)-O surface where the oxygen species are mainly of the O2- type is unreactive, Coadsorption studies of NH3-O-2 mixtures show that at Ni(110) surfaces the uptake of both oxygen and ammonia increase with the proportion of oxygen in the NH3-O-2 mixture. The surface concentrations of the O- species and the NHn species also increase with the increase in the O-2/NH3 ratio while the slope of the plot of sigma(N) versus sigma(O-) is around unity. The results demonstrate the high surface reactivity of the O- species and its role in the dissociation of ammonia. Based on these observations, the possibility of the formation of a surface complex between ammonia and oxygen (specifically O-) is suggested. Results from vibrational spectroscopic studies of the coadsorption of NH3-O-2 mixtures are consistent with those from core-level spectroscopic studies.

Ultrasonic wave characteristics of a monolayer graphene on silicon substrate

The present work deals with an ultrasonic type of wave propagation characteristics of monolayer graphene on silicon (Si) substrate. An atomistic model of a hybrid lattice involving a hexagonal lattice of graphene and surface atoms of diamond lattice of Si is developed to identify the carbon-silicon bond stiffness. Properties of this hybrid lattice model is then mapped into a nonlocal continuum framework. Equivalent force constant due to Si substrate is obtained by minimizing the total potential energy of the system. For this equilibrium configuration, the nonlocal governing equations are derived to analyze the ultrasonic wave dispersion based on spectral analysis. From the present analysis we show that the silicon substrate affects only the flexural wave mode. The frequency band gap of flexural mode is also significantly affected by this substrate. The results also show that, the silicon substrate adds cushioning effect to the graphene and it makes the graphene more stable. The analysis also show that the frequency bang gap relations of in-plane (longitudinal and lateral) and out-of-plane (flexural) wave modes depends not only on the y-direction wavenumber but also on nonlocal scaling parameter. In the nonlocal analysis, at higher values of the y-directional wavenumber, a decrease in the frequency band gap is observed for all the three fundamental wave modes in the graphene-silicon system. The atoms movement in the graphene due to the wave propagation are also captured for all the tree fundamental wave modes. The results presented in this work are qualitatively different from those obtained based on the local analysis and thus, are important for the development of graphene based nanodevices such as strain sensor, mass and pressure sensors, atomic dust detectors and enhancer of surface image resolution that make use of the ultrasonic wave dispersion properties of graphene. (C) 2011 Elsevier Ltd. All rights reserved.

Optical studies of SN 2009jf: a Type Ib supernova with an extremely slow decline and aspherical signature

Optical UBVRI photometry and medium-resolution spectroscopy of the Type Ib supernova SN 2009jf, during the period from similar to -15 to +250 d, with respect to the B maximum are reported. The light curves are broad, with an extremely slow decline. The early post-maximum decline rate in the V band is similar to SN 2008D; however, the late-phase decline rate is slower than other Type Ib supernovae studied. With an absolute magnitude of M-V = -17.96 +/- 0.19 at peak, SN 2009jf is a normally bright supernova. The peak bolometric luminosity and the energy deposition rate via the 56Ni -> 56Co chain indicate that similar to 0.17+0.03(-0.03) M-circle dot of 56Ni was ejected during the explosion. The He i 5876 A line is clearly identified in the first spectrum of day similar to -15, at a velocity of similar to 16 000 km s-1. The O i] 6300-6364 A line seen in the nebular spectrum has a multipeaked and asymmetric emission profile, with the blue peak being stronger. The estimated flux in this line implies that greater than or similar to 1.34 M-circle dot oxygen was ejected. The slow evolution of the light curves of SN 2009jf indicates the presence of a massive ejecta. The high expansion velocity in the early phase and broader emission lines during the nebular phase suggest it to be an explosion with a large kinetic energy. A simple qualitative estimate leads to the ejecta mass of M-ej = 4-9 M-circle dot and kinetic energy E-K = 3-8 x 1051 erg. The ejected mass estimate is indicative of an initial main-sequence mass of greater than or similar to 20-25 M-circle dot.

Origin of carrier-type reversal in Pb-Ge-Se glasses: A detailed thermal, electrical, and structural study

A long-standing and important problem in glass science has been carrier-type reversal (CTR) in semiconducting glasses. This phenomenon is exhibited by Pb-Ge-Se glasses also. It has been addressed here by carrying out detailed electrical, thermal, and spectroscopic investigations. PbxGe42-xSe58 (x = 0-20) glasses were prepared by a two stage melt-quenching process and characterized using x-ray diffraction, high-resolution electron microscropy, and energy dispersive analysis of x-rays. Thermoelectric power and high-pressure electrical resistivity have been measured. IR, Raman, and X-ray adsorption near edge structure spectroscopies have been used for examining the glass structures as well as differential scanning calorimetry (DSC) for studying the thermal properties. A structural model based on the chemical nature of the constituents has been proposed to account for the observed properties of these glasses. Effect of Pb incorporation on local structures and qualitative consequences on the energy band structures of Ge-Se glasses has been considered. The p -->n transition has been attributed to the energetic disposition of the sp(3)d(2) band of Pb atoms, which is located closely above the lone pair band of selenium. This feature makes Pb unique in the context of p -->n transition of chalcogenide glasses. The model can be extended successfully to account for the CTR behavior observed in Bi containing chalcogenide glasses also.

Photoemission study of pyrite-type transition-metal chalcogenides MS2-xSex (M=Fe, Co, Ni)

The electronic structures of pyrite-type transition-metal chalcogenides MS2-xSex (M = Fe, Co, Ni) has been investigated by photoemission and inverse-photoemission spectroscopy. The valence-band spectrum of ferromagnetic CoS2 does not show exchange splitting of the Co 3d peak, in disagreement with band-structure calculations. High-resolution photoemission spectra of NiS1.55Se0.45 shows spectral weight transfer from low (similar or equal to 50 meV) to high (0.2-0.5 eV) binding energies, in going from the metallic to the insulating phase.

Reverse flotation of silica from Kudremukh iron ore .2. Optimization of flotation process

An experimental programme based on statistical analysis was used for optimizing the reverse Rotation of silica from non-magnetic spiral preconcentrate of Kudremukh iron ore. Flotation of silica with amine and starch as the Rotation reagents was studied to estimate the optimum reagent levels at various mesh of grind. The experiments were first carried out using a two level three factor design. Analysis of the results showed that two parameters namely, the concentration level of the amine collector and the mesh of grind, were significant. Experiments based on an orthogonal design of the hexagonal type were then carried out to determine the effects of these two variables, on recovery and grade of the concentrate. Regression equations have been developed as models. Response contours have been plotted using the 'path of steepest ascent', maximum response has been optimized at 0.27 kg/ton of amine collector, 0.5 kg/ton of starch and mesh of grind of 48.7% passing 300 mesh to give a recovery of 83.43% of Fe in the concentrate containing 66.6% Fe and 2.17% SiO2.

Similarity solution of unsteady boundary layer equations with a magnetic field

The unsteady laminar incompressible boundary layer flow of an electrically conducting fluid in the stagnation region of two-dimensional and axisymmetric bodies with an applied magnetic field has been studied. The boundary layer equations which are parabolic partial differential equations with three independent variables have been reduced to a system of ordinary differential equations by using suitable transformations and then solved numerically using a shooting method. Here, we have obtained new solutions which are solutions of both the boundary layer and Navier-Stokes equations.

Synthesis based on cyclohexadienes .24. A new total synthesis of pupukean-2-one and a facile entry to copa and ylanga type sesquiterpene skeletons

A novel tandem 5-exo-trig allyl and 3-exo-trig radical cyclisation and rearrangement to copa and ylanga type sesquiterpene skeletons from easily prepared cyclohexadienes is reported, A new total synthesis of pupukean-2-one 8, which belongs to a novel class of sesquiterpenes, involving a 5-exo-trig allyl radical cyclisation as the key step is also reported.

The effect of temperature on the surface tension and adsorption functions of the Fe-S-O melts - an analysis based on interaction parameters

The present investigation analyses the thermodynamic behaviour of the surfaces and adsorption as a function of temperature and composition in the Fe-S-O melts based on the Butler's equations. The calculated-values of the surface tensions exhibit an elevation or depression depending on the type of the added solute at a concentration which coincides with that already present in the system. Generally, the desorption of the solutes as a function of temperature results in an initial increase followed by a decrease in the values of the surface tension. The observations are analyzed based on the surface interaction parameters which are derived in the present research.

Zn incorporation and band gap shrinkage in p-type GaAs

Dimethylzine (DMZn) was used as a p-type dopant in GaAs grown by low pressure metalorganic vapor phase epitaxy using trimethylgallium and arsine (AsH3) as source materials, The hole carrier concentrations and zinc (Zn) incorporation efficiency are studied by using the Hall effect, electrochemical capacitance voltage profiler and photoluminescence (PL) spectroscopy, The influence of growth parameters such as DMZn mole fraction, growth temperature, and AsH, mole fraction on the Zn incorporation have been studied. The hole concentration increases with increasing DMZn and AsH3 mole fraction and decreases with increasing growth temperature. This can be explained by vacancy control model. The PL experiments were carried out as a function of hole concentration (10(17)-1.5 x 10(20) cm(-3)). The main peak shifted to lower energy and the full width at half maximum (FWHM) increases with increasing hole concentrations. We have obtained an empirical relation for FWHM of PL, Delta E(p)(eV) = 1.15 x 10(-8)p(1/3). We also obtained an empirical relation for the band gap shrinkage, Delta E-g in Zn doped GaAs as a function of hole concentration. The value of Delta E-g(eV) = -2.75 x 10(-8)p(1/3), indicates a significant band gap shrinkage at high doping levels, These relations are considered to provide a useful tool to determine the hole concentration in Zn doped GaAs by low temperature PL measurement. The hole concentration increases with increasing AsH3 mole fraction and the main peak is shifted to a lower energy side. This can be explained also by the vacancy control model. As the hole concentration is increased above 3.8 x 10(18) cm(-3), a shoulder peak separated from the main peak was observed in the PL spectra and disappears at higher concentrations. (C) 1997 American Institute of Physics.

Si incorporation and Burstein-Moss shift in n-type GaAs

Silane (SiH4) was used as an n-type dopant in GaAs grown by low pressure metalorganic vapor phase epitaxy using trimethylgallium (TMGa) and arsine (AsH3) as source materials. The electron carrier concentrations and silicon (Si) incorporation efficiency are studied by using Hall effect, electrochemical capacitance voltage profiler and low temperature photoluminescence (LTPL) spectroscopy. The influence of growth parameters, such as SiH4 mole fraction, growth temperature, TMGa and AsH3 mole fractions on the Si incorporation efficiency have been studied. The electron concentration increases with increasing SIH4 mole fraction, growth temperature, and decreases with increasing TMGa and AsH3 mole fractions. The decrease in electron concentration with increasing TMGa can be explained by vacancy control model. The PL experiments were carried out as a function of electron concentration (10(17) - 1.5 x 10(18) cm(-3)). The PL main peak shifts to higher energy and the full width at half maximum (FWHM) increases with increasing electron concentrations. We have obtained an empirical relation for FWHM of PL, Delta E(n) (eV) = 1.4 x 10(-8) n(1/3). We also obtained an empirical relation for the band gap shrinkage, Delta E-g in Si-doped GaAs as a function of electron concentration. The value of Delta E-g (eV) = -2.75 x 10(-8) n(1/3), indicates a significant band gap shrinkage at high doping levels. These relations are considered to provide a useful tool to determine the electron concentration in Si-doped GaAs by low temperature PL measurement. The electron concentration decreases with increasing TMGa and AsH3 mole fractions and the main peak shifts to the lower energy side. The peak shifts towards the lower energy side with increasing TMGa variation can also be explained by vacancy control model. (C) 1999 Elsevier Science S.A. All rights reserved.

Solutions of cubic equations in quadratic fields

Let K be any quadratic field with O-K its ring of integers. We study the solutions of cubic equations, which represent elliptic curves defined over Q, in quadratic fields and prove some interesting results regarding the solutions by using elementary tools. As an application we consider the Diophantine equation r + s + t = rst = 1 in O-K. This Diophantine equation gives an elliptic curve defined over Q with finite Mordell-Weil group. Using our study of the solutions of cubic equations in quadratic fields we present a simple proof of the fact that except for the ring of integers of Q(i) and Q(root 2), this Diophantine equation is not solvable in the ring of integers of any other quadratic fields, which is already proved in [4].

Tidally compressed gas in centers of early-type and ultraluminous galaxies

In this paper we propose that the compressive tidal held in the centers of flat-core early-type galaxies and ultraluminous galaxies compresses molecular clouds producing dense gas observed in the centers of these galaxies. The effect of galactic tidal fields is usually considered disruptive in the literature. However, for some galaxies, the mass profile flattens toward the center and the resulting galactic tidal field is not disruptive, but instead it is compressive within the flat-core region. We have used the virial theorem to determine the minimum density of a molecular cloud to be stable and gravitationally bound within the tidally compressive region of a galaxy. We have applied the mechanism to determine the mean molecular cloud densities in the centers of a sample of flat-core, early-type galaxies and ultraluminous galaxies. For early-type galaxies with a core-type luminosity profile, the tidal held of the galaxy is compressive within half the core radius. We have calculated the mean gas densities for molecular gas in a sample of early-type galaxies which have already been detected in CO emission, and we obtain mean densities of [n] similar to 10(3)-10(6) cm(-3) within the central 100 pc radius. We also use our model to calculate the molecular cloud densities in the inner few hundred parsecs of a sample of ultraluminous galaxies. From the observed rotation curves of these galaxies we show that they have a compressive core within their nuclear region. Our model predicts minimum molecular gas densities in the range 10(2)-10(4) cm(-3) in the nuclear gas disks; the smaller values are applicable typically for galaxies with larger core radii. The resulting density values agree well with the observed range. Also, for large core radii, even fairly low-density gas (similar to 10(2) cm(-3)) can remain bound and stable close to the galactic center.