Descripción: The potential energy for the diffusion of positive and negative defects in ice is calculated by a SCF-MOLCAO procedure. The resulting height of the potential energy barrier for positive defects is much lower than for negative ones, thus explaining qualitatively the greater mobility of the H 3O+ ions. It is also found that the potential-energy heights are very sensitive to the distance between the oxygens, the diffusion being greater when the oxygens are nearer. The results obtained suggest therefore that the diffusion of positive defects is correlated to the vibrations of the lattice.

Descripción: The adiabatic potential energy surface (PES) of β-4-linked mannobiose was obtained using the MM3 force field at ε=3 and ε=80, and plotted as contour maps and as 2D graphs representing the energy vs. the ψ angle. The surfaces of the corresponding trisaccharide were also obtained and represented by a single 3D contour map for which the energy is plotted against the two ψ glycosidic angles. The PES of the disaccharide contains a low-energy well comprising two different minima, and three more minima in different locations. No major change was observed by changing the dielectric constant. For the trisaccharide, four main minima were observed, located within one minimum-energy region. The minima have a geometry close to that experimentally obtained for mannobiose, mannotriose and mannan I in solid state, but differ from that expected in aqueous solutions. The flexibility of the glycosidic linkage increases at higher dielectric constant, whereas it decreases for the linkage closer to the reducing end when passing from the di- to the trisaccharide. ©ARKAT.

Descripción: An SCF-MO-LCAO calculation for the four electrons involved in the straight hydrogen bond in ice is performed using a limited set of Slater-type orbitals. The energy of the four electrons plus the interaction between the "core potentials" is calculated as a function of the position of the proton along the line joining the oxygens. The energy of the hydrogen bond is obtained-8.2 kcal/mole-in good agreement with experiment. Electrostatic and delocalization contributions to the energy of the hydrogen bond are in excellent agreement with previous theoretical estimates. The potential energy for the motion of the proton results in a very asymmetric curve, with only one minimum. The dipole moment increases from 1.68 D for a free water molecule to 2.40 D thus confirming previous estimates.

Descripción: Our previously developed analytical infinite order sudden quantum theory of triatomic photodissociation is generalized to compute fragment internal energy distributions when the initial triatomic rotational state has K ≠ 0. The dependence of product rotational energy distributions on initial rotational and vibrational state is illustrated through model computations for the direct NOCl photodissociation from the ground to the T1(1 3A″) potential energy surface. The calculations consider all J,K ≤ 9 and employ a repulsive potential that is fit to ab initio computations. Comparisons of fragment rotational distributions with previous semiclassical approximations further elucidate the role of the mapping of the initial state bending wave function onto the fragment rotational distributions and the influence of parent rotations on this mapping. The infinite order sudden quantum-mechanical distributions exhibit a more complex structure, but upon thermal averaging they are already transformed at T = 3 K into fairly broad rotational distributions. The present theory readily permits the calculations of energy distributions for initial states of high J and K. © 1994 American Institute of Physics.

Descripción: We discuss the global gravity wave (GW) activity expressed by the specific potential energy in the altitude range from 5 km below to 10 km above the tropopause, derived from GPS radio occultation data from CHAMP (2001-2008). The GW analysis is based on vertical detrending of the individual measured temperature profiles by applying a Gaussian filter in two different ways: (i) filtering of the complete temperature profiles and (ii) separate filtering of the profiles for the tropospheric and lower stratospheric parts. The separate filtering method significantly reduces the usually observed wave activity enhancement in the tropopause region which highly depends on the performance of the complete filtering method to reproduce the change in the temperature gradient at the tropopause. We only consider vertical wavelengths less than 10 km. The global mean potential energy in the tropopause region deduced with these different background temperatures will be analyzed, differences will be emphasized and possible error sources of the new method will be considered. Copyright 2008 by the American Geophysical Union.

Descripción: The excited states of the double hydrogen bond in the adenine-thymine nucleotide base pair has been investigated in the semiempirical CNDO/S-CI approximation. Double-minimum potential curves are obtained for several nuclear configurations characterizing simultaneous tautomeric rearrangements of the NH⋯N and O⋯HN bonds. The energy profiles for the coupled movement of the hydrogen bonding show that the Watson-Crick configuration of the adenine-thymine base pair is the most stable for all of the excited states studied. Estimates are made within the WKB approximation of the tunneling rate and tunneling probability. The results indicate that increasing the energy of the excited states would increase the probability of double protonic transfer by tunnel effect and thus for irreversible mutation. A comparison of the composition of the potentials for the single movement of the protons with the double-minimum potential of the concerted movement shows that the potential is nonseparable. The shortcomings that follow from the WKB approximation as applied to the present problem are discussed. © 1981 American Chemical Society.

Descripción: We study a k-essence field evolving linearly with the cosmic time and the atypical k-essence model on a homogeneous and isotropic flat 3-brane. We show that the k-field is driven by an inverse quadratic polynomial potential. The solutions represent expanding, contracting or bouncing universes with a finite time span and some of them end in a big crunch or a big rip. Besides, by selecting the extended tachyonic kinetic functions we analyze the high and low energy limits of our model, obtaining the nearly power law solution. We introduce a tachyon field with negative energy density and show that the universe evolves between two singularities. © 2010 IOP Publishing Ltd.

Descripción: The potential energy of the lowest 1 ∑u + state of the hydrogen molecule is given as a function of the internuclear distance. Calculations were carried out by the valence bond method using Slater 1s and 2p atomic Orbitals. Both ionic and covalent structures were considered. The 1s orbitals of the ionic and covalent functions included a variational parameter but the orbital exponent of the 2p orbital was kept constant. It is found that the introduction of the 2p orbital into the wave function results in a considerable improvement of the dissociation energy and that further the state is essentially covalent in character rather than ionic, as previously reported.

Descripción: We investigate an interacting two-fluid cosmological model and introduce a scalar field representation by means of a linear combination of the individual energy densities. Applying the integrability condition to the scalar field equation we show that this "exotic quintessence" is driven by an exponential potential and the two-fluid mixture can be considered as a model of three components. These components are associated with baryonic matter, dark matter and dark energy respectively. We use the Simon, Verde and Jimenez [J. Simon, L. Verde, R. Jimenez, Phys. Rev. D 71 (2005) 123001] determination of the redshift dependence of the Hubble parameter to constrain the current density parameters of this model. With the best fit density parameters we obtain the transition redshift between non-accelerated and accelerated regimes zacc = 0.66 and the time elapsed since the initial singularity t0 = 19.8 Gyr. We study the perturbation evolution of this model and find that the energy density perturbation decreases with the cosmological time. © 2008 Elsevier B.V. All rights reserved.

Descripción: Adsorption on single planar walls and filling of slits with identical planar walls are investigated in the frame of the density functional theory. In this sort of slits the external potential is symmetric with respect to its central plane. Calculations were carried out by applying both the canonical and grand canonical ensembles (CE and GCE, respectively). The behavior is analyzed by varying the strength of the adsorbate-substrate attraction, the temperature T, and the coverage Results obtained for physisorption of Xe on alkaline surfaces are reported in the present work. Prewetting (PW) lines and wetting temperatures, T w, are determined from the analysis of adsorption on single walls. The filling of slits is analyzed for temperatures T T w. It is found that whenever for a given Xe-substrate combination the adsorption on a single wall exhibits a first-order wetting transition then asymmetric profiles that break the left-right symmetry of the external potential appear in the filling of an equivalent slit. These spontaneously symmetry breaking (SSB) solutions occur in a restricted range of with a T-dependent width. In the case of closed slits analyzed in the CE scheme, the obtained asymmetric profiles exhibit lower Helmholtz free energies than the symmetric species and, therefore, could be stabilized in this geometry. For open slits, the GCE scheme yields all the symmetric and SSB states in the corresponding convex regimes of the free energy. It is shown that both the CE and the GCE frames yield three coexistent states, two symmetric and one asymmetric twofold degenerate. Both a PW line and the related SSB effect terminate at the same temperature. For rather strongly attractive surfaces reentrant SSB states are found at a fixed value of T. © Copyright 2011 Author(s).

Descripción: We analyze a universe filled with interacting dark matter, a scalar field accommodated as dark radiation along with dark energy plus a decoupled radiation term within the framework of the spatially flat Friedmann-Robertson-Walker (FRW) spacetime. We work in a three-dimensional internal space spanned by the interaction vector and use a transversal interaction Qt for solving the source equation in order to find all the interacting component energy densities. We asymptotically reconstruct the scalar field and potential from an early radiation era to the late dominate dark energy one, passing through an intermediate epoch dominated by dark matter. We apply the χ2 method to the updated observational Hubble data for constraining the cosmic parameters, contrast with the Union 2 sample of supernovae, and analyze the amount of dark energy in the radiation era. It turns out that our model fulfills the severe bound of Ωφ{symbol}(z≃1100)<0.018 at 2σ level, is consistent with the recent analysis that includes cosmic microwave background anisotropy measurements from the Atacama Cosmology Telescope and the South Pole Telescope along with the future constraints achievable by Planck and CMBPol experiments, and satisfies the stringent bound Ωφ{symbol}(z≃1010)<0.04 at 2σ level in the big-bang nucleosynthesis epoch. © 2013 Springer-Verlag Berlin Heidelberg and Società Italiana di Fisica.

Descripción: The effective action for quantum fields on a d-dimensional spacetime can be computed using a non-local expansion in powers of the curvature. We show explicitly that, for conformal fields and up to quadratic order in the curvature, the non-local effective action is equivalent to the d + 1 action for classical gravity in AdSd+1 restricted to a (d - 1)-brane. This generalizes previous results about quantum corrections to the Newtonian potential and provides an alternative method for making local a non-local effective action. The equivalence can be easily understood by comparing the Kallen-Lehmann decomposition of the classical propagator with the spectral representation of the non-local form factors in the quantum effective action. © 2001 Published by Elsevier Science B.V.

Descripción: Dielectronic recombination data for nitrogen-like ions forming oxygen-like ions has been calculated as part of the assembly of a level-resolved dielectronic recombination database necessary for the modelling of dynamic finite-density plasmas (Badnell et al. 2003). Dielectronic recombination rate coefficients for a selection of ions from O+ to Xe47+ are presented and the results discussed.

Descripción: A numerical solution for the metallic-plasma-neutral-gas structure generated in a low-pressure arc is presented. The equations correspond to a spherically symmetric fluid-like steady model, valid for the outer region of the arc, and describe the ion slowing down by elastic scattering with the neutral particles. Technically, the obtention of the profiles of different magnitudes is complicated due to the existence of a critical point in the steady-state system of equations. The proposed approach to overcome this difficulty is to solve instead a pseudotransient system of equations which rapidly and efficiently relax to the stationary state. By employing this numerical method of second-order accuracy in space, the plasma and neutral gas density, the electron and ion drift velocities, the electron and neutral temperatures, and the electrostatic potential profiles are obtained from the border of the arc channel up to the discharge chamber wall. It is found that the value of the neutral gas filling pressure strongly influences the plasma density and plasma potential distributions. An important result is that metallic ions emitted from the arc channel deliver their kinetic energy to the filling gas in a gradual manner, up to a pressure-dependent point beyond which they move to the walls sustained against collisions with the gas by a self-consistent electric field. Near the mentioned point, the metallic ion density presents a peculiar behavior, showing an increase that is more pronounced at high pressures; a pattern also evident in the electrostatic potential. © 2000 American Institute of Physics.

Descripción: We obtain a black hole solution in the Einstein-Gauss-Bonnet theory for the string cloud model in a five-dimensional spacetime. We analyze the event horizons and naked singularities. Later, we compute the Hawking temperature TH, the specific heat C, the entropy S, and the Helmholtz free energy F of the black hole. The entropy was computed using the Wald formulation. In addition, the quantum correction to the Wald's entropy is considered for the string cloud source. We mainly explore the thermodynamical global and local stability of the system with vanishing or non-vanishing cosmological constant. The global thermodynamic phase structure indicates that the Hawking-Page transition is achieved for this model. Further, we observe that there exist stable black holes with small radii and that these regions are enlarged when choosing small values of the string cloud density and of the Gauss-Bonnet parameter. Besides, the rate of evaporation for these black holes are studied, determining whether the evaporation time is finite or not. Then, we concentrate on the dynamical stability of the system, studying the effective potential for s-waves propagating on the string cloud background. © 2010 Elsevier B.V. All rights reserved.

Descripción: Grand-canonical like descriptions of many electron atomic and molecular open systems which are characterized by a non-integer number of electrons are presented. Their associated reduced density matrices (RDMs) are obtained by introducing the contracting mapping for this type of distributions. It is shown that there is loss of information when connecting RDMs of different order by partial contractions. The energy convexity property of these systems simplifies the description. Consequently, this formulation opens the possibility to a new look for chemical descriptors such as chemical potential and reactivity among others. Examples are presented to discuss the theoretical aspects of this work. © 2013 AIP Publishing LLC.

Descripción: A two-component theory for shielding calculations starting from a four-component Rayleigh-Schrodinger perturbation theory (RSPT) formalism is presented. Thus, a set of operators entering the RSPT expressions in terms of the Schrodinger molecular spectrum are derived by expanding such four-component expression as a power series in c-1. All formal expressions are retained, without neglecting any terms in the intermediate steps of the derivation.

Descripción: Using high energy, pulsed X-rays, generated by a 4.7 kJ Plasma Focus operated with deuterium - argon admixtures, images of fast rotating metallic pieces were obtained. This illustrates the potential application of Plasma Focus devices for non invasive scanning of moving metallic objects. The samples for radiographic imaging were located outside the plasma focus chamber. High-sensitivity, fast-response commercial radiographic film was used as X-ray detector. The use of small doses of argon helps to enhance both the neutron production and the hard X-ray emission. By analyzing the mean attenuation of the studied radiation in different metals, an X-ray effective energy of about 100 keV was obtained.

Descripción: In the moth, Agrotis ipsilon, newly mated males cease to be attracted to the female-produced sex pheromone, preventing them from re-mating until the next night, by which time they would have refilled their reproductive glands for a potential new ejaculate. The behavioural plasticity is accompanied by a decrease in neuron sensitivity within the primary olfactory centre, the antennal lobe (AL). However, it was not clear whether the lack of the sexually guided behaviour results from the absence of sex pheromone detection in the ALs, or if they ignore it in spite of detection, or if the sex pheromone itself inhibits attraction behaviour after mating. To test these hypotheses, we performed behavioural tests and intracellular recordings of AL neurons to non-pheromonal odours (flower volatiles), different doses of sex pheromone and their mixtures in virgin and newly mated males. Our results show that, although the behavioural and AL neuron responses to flower volatiles alone were similar between virgin and mated males, the behavioural response of mated males to flower odours was inhibited by adding pheromone doses above the detection threshold of central neurons. Moreover, we show that the sex pheromone becomes inhibitory by differential central processing: below a specific threshold, it is not detected within the AL; above this threshold, it becomes inhibitory, preventing newly mated males from responding even to plant odours. Mated male moths have thus evolved a strategy based on transient odour-selective central processing, which allows them to avoid the risk-taking, energy-consuming search for females and delay re-mating until the next night for a potential new ejaculate. © 2010. Published by The Company of Biologists Ltd.