9,216 research outputs found
The method of Gaussian weighted trajectories. V. On the 1GB procedure for polyatomic processes
In recent years, many chemical reactions have been studied by means of the
quasi-classical trajectory (QCT) method within the Gaussian binning (GB)
procedure. The latter consists in "quantizing" the final vibrational actions in
Bohr spirit by putting strong emphasis on the trajectories reaching the
products with vibrational actions close to integer values. A major drawback of
this procedure is that if N is the number of product vibrational modes, the
amount of trajectories necessary to converge the calculations is ~ 10^N larger
than with the standard QCT method. Applying it to polyatomic processes is thus
problematic. In a recent paper, however, Czako and Bowman propose to quantize
the total vibrational energy instead of the vibrational actions [G. Czako and
J. M. Bowman, J. Chem. Phys., 131, 244302 (2009)], a procedure called 1GB here.
The calculations are then only ~ 10 times more time-consuming than with the
standard QCT method, allowing thereby for considerable numerical saving. In
this paper, we propose some theoretical arguments supporting the 1GB procedure
and check its validity on model test cases as well as the prototype four-atom
reaction OH+D_2 -> HOD+D
Physical processes leading to surface inhomogeneities: the case of rotation
In this lecture I discuss the bulk surface heterogeneity of rotating stars,
namely gravity darkening. I especially detail the derivation of the omega-model
of Espinosa Lara & Rieutord (2011), which gives the gravity darkening in
early-type stars. I also discuss the problem of deriving gravity darkening in
stars owning a convective envelope and in those that are members of a binary
system.Comment: 23 pages, 11 figure, Lecture given to the school on the cartography
of the Sun and the stars (May 2014 in Besan\c{c}on), to appear in LNP, Neiner
and Rozelot edts V2: typos correcte
Defect chemistry and transport properties of BaxCe0.85M0.15O3-d
The site-incorporation mechanism of M3+ dopants into A2+B4+O3 perovskites controls the overall defect chemistry and thus their transport properties. For charge-balance reasons, incorporation onto the A2+-site would require the creation of negatively charged point defects (such as cation vacancies), whereas incorporation onto the B4+-site is accompanied by the generation of positively charged defects, typically oxygen vacancies. Oxygen-vacancy content, in turn, is relevant to proton-conducting oxides in which protons are introduced via the dissolution of hydroxyl ions at vacant oxygen sites. We propose here, on the basis of x-ray powder diffraction studies, electron microscopy, chemical analysis, thermal gravimetric analysis, and alternating current impedance spectroscopy, that nominally B-site doped barium cerate can exhibit dopant partitioning as a consequence of barium evaporation at elevated temperatures. Such partitioning and the presence of significant dopant concentrations on the A-site negatively impact proton conductivity. Specific materials examined are BaxCe0.85M0.15O3-d (x = 0.85 - 1.20; M = Nd, Gd, Yb). The compositional limits for the maximum A-site incorporation are experimentally determined to be: (Ba0.919Nd0.081)(Ce0.919Nd0.081)O3, (Ba0.974Gd0.026)(Ce0.872Gd0.128)O2.875, and Ba(Ce0.85Yb0.15)O2.925. As a consequence of the greater ability of larger cations to exist on the Ba site, the H2O adsorption and proton conductivities of large-cation doped barium cerates are lower than those of small-cation doped analogs
Moving embedded lattice solitons
It was recently proved that isolated unstable "embedded lattice solitons"
(ELS) may exist in discrete systems. The discovery of these ELS gives rise to
relevant questions such as the following: are there continuous families of
ELS?, can ELS be stable?, is it possible for ELS to move along the lattice?,
how do ELS interact?. The present work addresses these questions by showing
that a novel differential-difference equation (a discrete version of a complex
mKdV equation) has a two-parameter continuous family of exact ELS. The
numerical tests reveal that these solitons are stable and robust enough to
withstand collisions. The model may apply to the description of a Bose-Einstein
condensate with dipole-dipole interactions between the atoms, trapped in a deep
optical-lattice potential.Comment: 13 pages, 11 figure
Symmetry Nonrestoration at High Temperature in Little Higgs Models
A detailed study of the high temperature dynamics of the scalar sector of
Little Higgs scenarios, proposed to stabilize the electroweak scale, shows that
the electroweak gauge symmetry remains broken even at temperatures much larger
than the electroweak scale. Although we give explicit results for a particular
modification of the Littlest Higgs model, we expect that the main features are
generic. As a spin-off, we introduce a novel way of dealing with scalar
fluctuations in nonlinear sigma models, which might be of interest for
phenomenological applications.Comment: 23 pages, LaTeX, 4 figure
Dose Calibration and Track Diameter Distribution for 241Am-Be Neutron Source, Using CR-39 Nuclear Track Methodology
In neutron detection, the more common method is using electronic instrumentation associate with Bonner spheres, however, currently the Nuclear Tracks Methodology (NTM) is coming popular because of the simplicity, flexibility in size of the detector, no requirement for sophisticated instrumentation and installation, and low cost. In this work, a preliminary result of the dose calibration and track diameter distribution of Americium-Beryllium (241Am-Be) source using Nuclear Track Methodology is presented. As material detector, CR-39 polycarbonate, cut in 1.8 × 0.9 cm2 chips was chosen, and two step chemical etchings after neutron exposure was used to develop the tracks. The irradiations were made in environmental normal conditions, in the ORNL neutron calibration facilities. The CR-39 chips were placed in a phantom, with 3mm plastic (Lexan) sheet in between the source and detectorsto increase the proton generation. The total track density and track diameter distribution was performing in a Counting and Analysis Digital Image System (CADIS), developed at the Institute of Physics of the University of Mexico UNAM. The results are compared with a standard survey instrument and energy reference spectra of the International Atomic Energy Agency (IAEA)
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