Volume 47, Issue 9

An expansion of the velocity distribution function in a series of spherical harmonics is used to transform the nonlinear Boltzmann equation into a system of moment equations. The close connection between the moment equations of zeroth and first order with the transport equations for mass, momentum and energy is pointed out. By comparing the order of magnitude of the various moments it is shown that the P 2 approximation is adequate for systems with small mean free path. Simplifications of the collision terms of the moment equations are discussed, where attention is payed to the conservation laws and the H theorem

A Klein-Gordon-type equation is derived for the wave propagation in an ideal, uniform waveguide, and its quantum-mechanical interpretation is given. The "cross-section" concept is introduced for a waveguide and the power transmission factor is obtained by using standard methods of quantum mechanics. The spinorial formalism is also employed for deriving the equivalent Dirac-type equation, and the perturbation theory is applied for computing the frequency shifts. The general applicability of the quantum-mechanical concepts to the waveguides theory is discussed

We analyse the effect of an Ergodic Magnetic Limiter on the magnetic field line dynamics in the edge of a large aspect-ratio Tokamak. We model the limiter action as an impulsive perturbation and use a peaked-current model for the Tokamak equilibrium field. The theoretical analysis is made through the use of invariant flux functions describing magnetic surfaces. Results are compared with a numerical mapping of the field lines

The polarisation effect on the isotropisation of a magnetized bi-Maxwellian plasma is investigated in the weak coupling approximation. Based on the Rostoker collision operator, which reduces in the magnetic field free limit to the Balescu-Lenard operator, the equipartition frequency v is evaluated numerically for B→0 and analytically for B→∞, respectively. This frequency, depending nonlinearly on ε and B, describes the approach to thermodynamic equilibrium of the anisotropy parameter ε = T ⊥ /T ‖ via d ε /d t - (1 - ε) ν(ε, B). It is found that in both limits dynamical screening makes a negligible contribution to the relaxation process. For finite B a simplified, yet unexploited expression for ν(ε, B) is presented, which invokes three new functions

Single crystals of sulphamethoxazole were X-irradiated at 273 K. ESR and ENDOR spectra were obtained at 100 K. The free radicals stable at room temperature are formally formed by abstraction of a hydrogen atom from the methyl group of the molecule. The unpaired electron is delocalized in the isoxazole ring. The assignment is supported by comparisons of spin densities obtained experimentally and by semiempirical molecular orbital calculations

A recently devised new method for numerical Abel inversion is compared with four other commonly used methods. One of them, the convolution method, is employed in computer tomography for reconstructing asymmetrical objects. It is investigated whether this method can be adapted for the case of radial symmetry. As a first approach the comparison is performed by computer simulation. Special attention is given to the propagation of errors according to their origin. The result is a recipe for minimizing errors and for choosing the optimal method for reconstruction. The second step is a comparison of experimentally obtained radial profiles with functions resulting from Abel inversion of measured side-on data. Thus it is shown that the concept developed by computer simulation can be applied in practice

The influence of the moment of inertia on the rotational fluorescence depolarization is discussed. Based on experimental results obtained for five luminescent compounds: 2,5-diphenyloxazole (PPO), 2,2'-p-phenylene-bis(5-phenyloxazole) (POPOP), p-bis[2-(5-α-naphthyloxazolyl)]-benzene (α-NOPON), 4-dimethylamino-ω-methylsulphonyl-trans-styrene (3a) in n-parafines at low viscosity (from 0.22 x 10 -3 Pa • s to 0.993 x 10 -3 Pa • s) and diphenylenestilbene (DPS) in different solvents, a semi-empirical equation is proposed, yielding moments of inertia that are only two to five times higher than those estimated from the molecular geometry

The redox potential (E) of the couple Ce 4+ /Ce 3+ has been determined up to 368 K by means of cyclic voltammetric measurement in aqueous HClO 4 solutions with c HClO4 decreasing from 7.45 to 0.023 mol kg -1 . A constant potential of (1.741 V) 298 K , resp. (1.836 V) 368K , indicating the existence of pure unhydrolysed Ce 4+ was obtained at c HClO4 ≥ 6.05 m. At lower HClO 4 concentration, the potential as a function of the HClO 4 molality, as well as of the pH shows 4 further distinct steps. At 298 K, for instance, the potential became nearly constant at pH values of 0.103, 0.735,1.115, after which it drastically decreased, respectively at 1.679, just before the precipitation of Ce(OH) 4 occurred. The curves indicate obviously the stepwise formation of the Ce(IV) mono-, di-, tri- and tetrahydroxo complexes. The slope of the curves E vs. pH increased gradually with increasing temperature. Δ S and Δ H of the redox reaction were determined as functions of T at the different HClO 4 concentrations. ΔS is positive at c HClO4 > 1.85 m and turns to be negative at lower concentrations. ΔH is negative at all HClO 4 concentrations studied. The cumulative formation constants ß i , of the Ce(IV) hydroxo complexes and the corresponding hydrolysis constants (K h ) i were calculated. An unusual decrease of ß i with increasing temperature has been discussed

First, the following rate constants (in dm 3 mol -1 s -1 ) were reported: k(e aq - + C 2 H 5 OH) = (1.2 ± 0.3) x 10 3 , k(H + C 2 H 5 OH) = (2.8 ± 0.2) x 10 7 and k(OH + C 2 H 5 OH) = (1.85 + 0.1) x 10 9 The major radiolytic products resulting from aqueous 10 -2 , 1 and 10 mol dm -3 ethanol were studied in the absence and presence of CO as a function of dose at pH 2 and 7. Probable reaction mechanisms are presented

The size dependence and convergence rates of thermodynamic averages during Monte Carlo simulations of supercritical water in a wide range of densities have been analysed and compared with simulations under normal conditions. It is shown that the fluctuations of internal energy (enthalpy) in NPT-ensemble Monte Carlo simulations of high-temperature water are strongly correlated with fluctuations of volume and the convergence of the thermodynamic averaging under such conditions depends on the frequency of volume-changing moves and may be considerably improved by increasing this frequency by a factor of 3 - 10 relative to generally accepted values

The temperature dependence of the rotational relaxation of propionitrile, CH 3 CH 2 C 15 N, has been investigated by means of microwave transient emission signals. From analysis of the pressure dependence of the coherence decay rates for the rotational lines (J, K - , K + ) = (2, 0, 2)-(3, 0, 3), (2,1, 2)-(3, 1, 3), and (2, 2, l)-(3, 2, 2) pressure broadening parameters have been derived for mixtures with nitrogen in the 210-300 K range and for the pure gas at ambient temperatures

The intramolecular and liquid structures of tetramethylurea (TMU) were investigated by the liquid x-ray diffraction method. It is confirmed that in the liquid state the four methyl groups are out of the plane which is constructed by the O, C, and the two N atoms, the molecular structure being similar to what had been found in the gas phase. All structural parameters are in good agreement with those of TMU determined in the gas phase and with those of amide derivatives in the liquid phase. The liquid is characterized by a random distribution of molecules as predominant, but a weak molecular ordering characterized by approximately two intermolecular nonbonding Me ··· O distances at 340 pm is proposed