Empirical valence bond models for reactive potential energy surfaces: a parallel multilevel genetic program approach

We describe a new method for constructing empirical valence bond potential energy surfaces using a parallel multilevel genetic program (PMLGP). Genetic programs can be used to perform an efficient search through function space and parameter space to find the best functions and sets of parameters that fit energies obtained by ab initio electronic structure calculations. Building on the traditional genetic program approach, the PMLGP utilizes a hierarchy of genetic programming on two different levels. The lower level genetic programs are used to optimize coevolving populations in parallel while the higher level genetic program (HLGP) is used to optimize the genetic operator probabilities of the lower level genetic programs. The HLGP allows the algorithm to dynamically learn the mutation or combination of mutations that most effectively increase the fitness of the populations, causing a significant increase in the algorithm's accuracy and efficiency. The algorithm's accuracy and efficiency is tested against a standard parallel genetic program with a variety of one-dimensional test cases. Subsequently, the PMLGP is utilized to obtain an accurate empirical valence bond model for proton transfer in 3-hydroxy-gamma-pyrone in gas phase and protic solvent.     

Absence of proton weak coupling states in93Nb

The reactions⁹³Nb(p, p)⁹³Nb,⁹³Nb(p, n)⁹³Mo, and⁹³Nb(p, p′) through isobaric analog resonances in⁹⁴Mo have been used to study the structure of low-lying⁹³Nb states, specifically the degree to which these states can be described as single protons coupled to the core states of⁹²Zr. Excitation curves were obtained from 5.8 to 7.6 MeV incident proton energy, for eight states in⁹³Nb. Results of this work are compared to results from Coulomb excitation studies,⁹³Nb(n, n′γ),⁹³Nb(d,p),⁹²Zr(³He,d),⁹⁴Mo(d,³He) and other related work.     

Four-wave mixing in microstructure fiber

We report what we believe to be the first experimental demonstration of nondegenerate four-wave mixing in a microstructure fiber. The effect of the chi((3)) nonlinearity is enhanced in such a fiber because of the small core area, and we achieve phase matching by operating near the zero-dispersion wavelength (?750 nm) . We have observed parametric gains of more than 13 dB in 6.1-m-long fiber with a pump peak power of only 6 W. We compare our experimental gain results with those predicted by theory and explore the effects of Raman shift and (or) amplification and cascaded nonlinear mixing.     

Mechanisms of antioxidant action: Sulphur compounds as inhibitors of mechanochemical degradation of rubber

It is shown that the stability of sulphur vulcanised natural rubber to mechanodegradation (fatigue) is associated with the formation of labile oxidation products. The addition of sulphides and their oxidation products to a non-sulphur vulcanisate has a similar effect. Stress relaxation shows that the initial rate of chain scission under stress is directly related to the fatigue resistance of the vulcanisate. However, the oxidation process also produces peroxidolytic antioxidants (PD-C). It is concluded that the fatigue resistance of sulphur vulcanisates is due to a combination of facile restructurisation and an oxidation chain-breaking mechanism.     

DWBA approach to inelastic scattering at medium energies

Medium energy proton elastic and inelastic scattering to states of ⁵⁸Ni and ²⁰⁸Pb, and ⁴He elastic and inelastic scattering to states of ⁴⁰Ca, are analyzed using the partial wave approach, by solving the Schrödinger equation with relativistic kinematics and using the distorted wave Born approximation. Our results can be compared with results of several previous analyses of the nucleon inelastic data using the Glauber approximation. Our calculations are absolute, using nuclear collective parameters obtained from a survey of a large number of low-energy analyses of inelastic scattering of electrons, nucleons and nuclei from ⁴⁰Ca, ⁵⁸Ni and ²⁰⁸Pb.     

Frictional sliding modes along an interface between identical elastic plates subject to shear impact loading

Frictional sliding along an interface between two identical isotropic elastic plates under impact shear loading is investigated experimentally and numerically. The plates are held together by a compressive stress and one plate is subject to edge impact near the interface. The experiments exhibit both a crack-like and a pulse-like mode of sliding. Plane stress finite element calculations modeling the experimental configuration are carried out, with the interface characterized by a rate and state dependent frictional law. A variety of sliding modes are obtained in the calculations depending on the impact velocity, the initial compressive stress and the values of interface variables. For low values of the initial compressive stress and impact velocity, sliding occurs in a crack-like mode. For higher values of the initial compressive stress and/or impact velocity, sliding takes place in a pulse-like mode. One pulse-like mode involves well-separated pulses with the pulse amplitude increasing with propagation distance. Another pulse-like mode involves a pulse train of essentially constant amplitude. The propagation speed of the leading pulse (or of the tip of the crack-like sliding region) is near the longitudinal wave speed and never less than times the shear wave speed. Supersonic trailing pulses are seen both experimentally and computationally. The trends in the calculations are compared with those seen in the experiments.     

Laboratory astrophysics and non-ideal equations of state: the next challenges for astrophysical MHD simulations

Laboratory astrophysics holds great promise not only as a highly effective validation tool for astrophysical magneto-hydrodynamics (MHD) codes but it also presents a unique challenge for these codes. The high-density plasmas found in these experiments are not well modeled by the ideal equations of state (EOS) found in most astrophysical simulation codes. To solve this problem, we replaced the ideal EOS scheme in an existing MHD code, AstroBEAR, with a non-ideal EOS method and validated our implementation with van der Waals shock tube tests. The improved code is also able to model flows that contain more than one material, as required in laboratory experiments. Simulations of jet experiments performed at the OMEGA Laser reproduce the morphology of the jet much better than when the code used a single material and an ideal EOS.     

An HPTLC method for the quantitative determination of zearalenone in maize

Summary The high performance liquid chromatographic method adopted by the AOAC [1] for the quantitative analysis of zearalenone has been modified for application to maize, with quantification by high performance thin layer chromatography (HPTLC). The method has been validated by spiking uncontaminated extracts of maize with zearalenone over the range 10 to 320 μg kg⁻¹. A linear relationship was found between 10 and 80 μg kg⁻¹, but at higher levels, the observed values were below the fitted line.     

The88Sr(d,n∼p)88Sr reaction

The⁸⁸Sr(d, n～p)⁸⁸Sr cross section has been measured at 170 ° for deuteron energies from 7.5 to 10.0 MeV, and the total⁸⁸Sr(d, n～p) cross section obtained. The total cross section is compared with results of coupled-channel Born approximation calculations using both empirical and shell-model-reaction-theory form factors to describe the decaying isobaric analog state. It is shown also that the new data and calculations are consistent with earlier⁸⁸Sr(d, p)⁸⁹Sr data.