An investigation of nonequilibrium kinetic isotope effects in chemically activated vinyl radicals

New experimental data have been obtained for H + C₂H₂, D + C₂H₂, H + C₂D₂, and D + C₂D₂ at room temperature. Two previously described apparatus were used in order to measure the pressure dependence of the reactions. The absolute rate constants are compared to results from other laboratories. The present results and those of Payne and Stief are used to obtain the high-pressure limiting rate constant at room temperature. When the activation energy from the work of Payne and Stief is considered, it is shown that the A factor for H + C₂H₂ is too low by a factor of ～20. If a transmission coefficient is introduced which is constant for all isotopic variations, the pressure dependence can be explained in terms of the randomly energized radicals. RRKM theory is then invoked to explain the observed statistical nonequilibrium kinetic isotope effects.     

A second-order radiation boundary condition for the shallow water wave equations on two-dimensional unstructured finite element grids

A second-order radiation boundary condition (RBC) is derived for 2D shallow water problems posed in ‘wave equation’ form and is implemented within the Galerkin finite element framework. The RBC is derived by matching the dispersion relation for the interior wave equation with an approximate solution to the exterior problem for outgoing waves. The matching is correct to second order, accounting for curvature of the wave front and the geometry. Implementation is achieved by using the RBC as an evolution equation for the normal gradient on the boundary, coupled through the natural boundary integral of the Galerkin interior problem. The formulation is easily implemented on non-straight, unstructured meshes of simple elements. Test cases show fidelity to solutions obtained on extended meshes and improvement relative to simpler first-order RBCs.     

The Properties of a cellulolytic microbial community growing on pure cellulose and lignocellulose

A microbial community capable of degrading pure cellulose has been isolated from soil using a continuous flow chemostat. The component organisms have been identified as Penicillium nigricans, Paecilomyces liliacinus, Fusarium oxysporum (3 strains), Aspergillus fumigatus, Gliocladium roseum, and Penicillium simplicissimum. Extracellular enzyme production (exoglucanase, endoglucanase, and (Β-glucosidase) and cellulose breakdown by the whole community and the component organisms in pure culture was measured during growth on pure cellulose in batch culture. After 30 d, 50–60% degradation was achieved by Penicillium simplicissimum, Aspergillus fumigatus, and the mixed culture. These cultures also produced the highest levels of extracellular endo- and exoglucanase and were able to produce the full enzyme complex necessary for the degradation of crystalline cellulose. The other isolates were capable of 0–20% degradation and produced lower levels of enzyme activity. Most were unable to produce the full enzyme complex. The results indicated that the whole community was no better at degrading pure cellulose than two members in pure culture. Since the full potential of the community may not be expressed on pure cellulose, breakdown and enzyme production was investigated during growth on lignocellulose. A large proportion of lignocellulosic material consists of xylan and enzymes capable of degrading this component were investigated. The highest percentage degradation and enzyme production (with the exception of Β-glucosidase) after 15 d growth on straw was achieved by the same three cultures. Studies of their growth on hay and straw were extended over a 60-d period. The rate and extent of degradation of hay by P. simplicissimum and A. fumigatus was similar to that of the mixed culture, more than 40% degradation occurring in 40 d. However, differences in levels of enzyme activity were observed. The mixed culture produced lower levels of enzyme activity (with the exception of Β-glucosidase) particularly during the initial states of degradation. Differences in enzyme production by A. fumigatus and P. simplicissimum were also observed. A. fumigatus produced high levels of xylanase and endoglucanase while P. simplicissimum produced high levels of exoglucanase. Similar results were observed on straw. The composition of the lignocellulosic material was measured by sequential chemical extraction of the solubles, hemicellulose, cellulose, and lignin. Changes in the composition during degradation were investigated. During the growth of all three cultures on hay, the total percentage of hemicellulose and cellulose decreased from 84% to less than 45% in 40 d. Despite significant differences in the enzyme activity, P. simplicissimum, A. fumigatus and the mixed culture were capable of degrading lignocellulosic material at the same rate. The production of a full cellulase complex appears to be more important than the production of high levels of individual enzymes.     

Wave equation hydrodynamics on deforming elements

The shallow water wave equation is derived in a general deforming co-ordinate system. A weak form is developed which displays the natural boundary condition prominently and which may be implemented on C⁰ elements. A time-stepping algorithm is implemented with clastic mapping of interior node motion. Lossless test cases show agreement with analytic solutions. A simple hypothetical test case shows intuitively good behaviour at length scales approaching those required of estuarine simulations.     

Thermoreflectance of single crystal Gd

Thermoreflectance spectra for E∥c and E⊥c were measured on a Gd single crystal. The thermoconductance spectra, Δσ_∥ and Δσ_t], are very anisotropic. The strong interband edge at about 0.5 eV is shown to arise from a transition to or from the Fermi level for E⊥c. For E∥c additional effects occur.     

Lyman-α absorption photometry at high pressure and atom density kinetic results for H recombination

Atomic absorption and fluorescence spectrophotometry have been routinely used in kinetic investigations as probes of relative, rather than absolute, atom concentration. The calibration of a Lyman-α photometer for measurement of absolute hydrogen atom concentrations at levels [H] ι ≤ 1.8 × 10¹⁴ atoms/cm² and total pressure of 1.5 torr He is described. The photometer is characterized in terms of a two-level emission source and an absorption region in which only Doppler broadening of the transition is considered. The modifications due to pressure broadening by high pressures (500 ≤ P ≤ 1500 torr) in the absorption region are discussed in detail. Application of the technique is reported for the recombination of hydrogen atoms in the presence of six nonreactive heat bath gases. Experiments were performed in a static reaction cell at pressures of 500–1500 torr of heat bath gas, and hydrogen atoms were produced by Hg (³P₁) photosensitization of H₂. The technique is critically evaluated and the mechanistic implications of the hydrogen atom recombination results are examined. The measured room temperature recombination rate constants in H₂, He, Ne, Ar, Kr, and N₂ are 8.5 ± 1.2, 6.9 ± 1.5, 5.9 ± 1.5, 8.0 ± 0.8, 10.2 ± 0.9, and 9.6 ± 1.4, respectively, where the units are 10³³ cm⁶/molec² · sec.