Double meson production in ultraperipheral heavy-ion collisions

The double-meson production in ultraperipheral heavy-ions collisions is addressed, focusing on the particular case of from two-photon reactions. The cross section at photon level is obtained using distinct parameterizations for the gluon distribution on the light meson. The resulting estimates for the nuclear case are presented and discussed. As a by-product, we estimate the double production cross section using the pomeron-exchange factorization relations. Received: 20 March 2003 / Published online: 2 June 2003     

An integrated vision of the Green Chemistry evolution along 25 years

The objective of the present review on the evolution of Green Chemistry (GC), since its emergence until 2016 (25th anniversary), aimed an integrated vision of its progress along the three phases of its development: emergence, divulgation and consolidation. The methodology involved the analysis of a selection of bibliography on the evolution of GC collected from issues of the ACS symposia series; editorials in specialized GC journals; and commemorative birthday papers/editorials of these journals and of the GC itself. The analysis allowed to identify and discuss the characteristics and conceptions of GC agents about fundamental concepts and procedures, in order to visualize and better understand the state of knowledge and the subject areas that make up the field. The main alterations identified in the evolution of GC were: it acquired a better defined identity, with more participation of academy and increased scope; was progressively framed in sustainability; has been aspiring to a holistic frame (but it is still dominated by its original reductionism) and to a change on the nature of the innovation for its implementation, from incremental to transformative. The analysis suggests that for supporting expansion and consolidation of GC the following aspects deserve attention: full validation of chemical greenness by metrics; better characterization of the frontiers of GC research areas/themes; increased integration between academy and industry; and penetration in chemistry curricula. Its basic philosophy being the practice of a chemistry more focused on the environment preservation, GC means a new stage in nature and in the history of Chemistry.

     

A Fuzzy Valuation-Based Local Search Framework for Combinatorial Problems

A novel local search method is presented. One of the new elements of this Fuzzy Adaptive Neighborhood Search (FANS) algorithm is a fuzzy valuation, which is used to measure the degree to which the solutions that are considered at the decision stages accomplish a certain qualitative property. FANS is analyzed from two perspectives: first, it is shown how FANS may be adapted to behave like other traditional local search techniques by means of suitable definitions for the fuzzy valuation component. Second, comparisons are made to show the potential of the method as a general purpose optimization tool, when none or minimal knowledge of the problem being solved is available.Both aspects make FANS a valuable tool regarding further developments within the context of decision support systems involving heuristic algorithms.     

Absence of relaxor-like ferroelectric phase transition in (Pb,Sr)TiO3 thin films

We have performed dielectric and micro-Raman spectroscopy measurements in the 298–673 K temperature range in polycrystalline Pb_0.50Sr_0.50TiO₃ thin films prepared by a soft chemical method. The phase transition have been investigated by dielectric measurements at various frequencies during the heating cycle. It was found that the temperature corresponding to the peak value of the dielectric constant is frequency-independent, indicating a non-relaxor ferroelectric behavior. However, the dielectric constant versus temperature curves associated with the ferroelectric to paraelectric phase transition showed a broad maximum peak at around 433 K. The observed behavior is explained in terms of a diffuse phase transition. The obtained Raman spectra indicate the presence of a local symmetry disorder, due to a higher strontium concentration in the host lattice. The monitoring of some modes, conducted in the Pb_0.50Sr_0.50TiO₃ thin films, showed that the ferroelectric tetragonal phase undergoes a transition to the paraelectric cubic phase at around 423 K. However, the Raman activity did not disappear, as would be expected from a transition to the cubic paraelectric phase. The strong Raman spectrum observed for this cubic phase is indicative that a diffuse-type phase transition is taking place. This behavior is attributed to distortions of the perovskite structure, allowing the persistence of low-symmetry phase features in cubic phase high above the transition temperature. This result is in contrast to the forbidden first-order Raman spectrum, which would be expected from a cubic paraelectric phase, such as the one observed at high temperature in pure PbTiO₃ perovskite. PACS 78.30.-j; 77.80.Bh; 64.70.Kb; 68.55.-a; 77.22.-a; 77.55.+f     

Experimental and theoretical study of the homogeneous,unimolecular gas‐phase elimination kinetics of 2‐furoic acid

The kinetics of the gas‐phase elimination kinetics of CO₂ from furoic acid was determined in a static system over the temperature range 415–455°C and pressure range 20–50 Torr. The products are furan and carbon dioxide. The reaction, which is carried out in vessels seasoned with allyl bromide and in the presence of the free‐radical suppressor toluene and/or propene, is homogeneous, unimolecular, and follows a first‐order rate law. The observed rate coefficient is expressed by the following Arrhenius equation: log k₁(s^?1) = (13.28 ± 0.16) ? (220.5 ± 2.1) kJ mol^?1 (2.303 RT)^?1. Theoretical studies carried out at the B3LYP/6‐31++G** computational level suggest two possible mechanisms according to the kinetics and thermodynamic parameters calculated compared with experimental values. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 298–306, 2007     

Analysis of hypersonic flows using finite elements with Taylor–Galerkin scheme

The aim of the present work is to introduce a formulation for the numerical analysis of three‐dimensional thermochemical non‐equilibrium hypersonic flows, using the finite element method and the Taylor–Galerkin scheme and adopting Park's 2‐temperature, 5‐species (N2, O2, NO, N and O) and 17‐reaction model. Examples using Euler and Navier–Stokes equations are included and compared with experimental and numerical works presented by other authors. The results are close to those analysed by other researches and a good computational performance was obtained. Copyright © 2004 John Wiley & Sons, Ltd.     

Deeply virtual Compton scattering and saturation approach

We investigate the deeply virtual Compton scattering (DVCS) in the color dipole approach, implementing the dipole cross section through the saturation model, which interpolates successfully between soft and hard regimes. The imaginary and real part of the DVCS amplitude are obtained and the results are compared to the available data. Received: 12 February 2003, Revised: 2 May 2003, Published online: 11 June 2003     

Photoionization of CF4 in the VUV region

We present calculated results of photoionization cross sections and photoelectron angular distributions for ionization out of the five outermost valence orbitals of CF₄ for photon energies ranging from near threshold to 55 eV. The Schwinger variational iterative method, using an exact static-exchange plus a model correlation–polarization potential, is applied to obtain the continuum photoelectron orbitals. The quantitative agreement between our calculated results and the experimental data is fair. Moreover, our study is capable of identifying most structures seen in experimental results for both cross sections and asymmetry parameters.     

Nuclear exclusive vector meson photoproduction

We calculate the nuclear cross section for vector meson exclusive photoproduction within the QCD color dipole picture and in the Regge approach. For the former approach, we have considered the phenomenological saturation model, whereas for the latter we use a model based on the dipole pomeron framework. Theoretical estimates for scattering on both light and heavy nuclei are given over a large range of energy.Received: 17 April 2004, Revised: 21 September 2004, Published online: 18 November 2004