Band 53, Heft 12

A linear pinch discharge above the Pease-Braginskii current and stabilized by axial shear flow can radiatively collapse to high densities. A thermonuclear detonation wave can then be launched from one end of the discharge channel by ignition with a powerful laser pulse. Axial shear flow stabilization may be realized by injecting a fast moving jet along the pinch discharge channel, possibly in combination with a frozen DT fiber positioned on the pinch axis.

The Rayleigh-Taylor (R-T) instability of two superposed plasmas, consisting of interacting ions and neutrals, in a horizontal magnetic field is investigated. The usual magnetohydrodynamic equations, including the permeability of the medium, are modified for finite Larmor radius (FLR) corrections. From the relevant linearized perturbation equations, using normal mode analysis, the dispersion relation for the two superposed fluids of different densities is derived. This relation shows that the growth rate unstability is reduced due to FLR corrections, rotation and the presence of neutrals. The horizontal magnetic field plays no role in the R-T instability. The R-T instability is discussed for various simplified configurations. It remains unaffected by the permeability of the porous medium, presence of neutral particles and rotation. The effect of different factors on the growth rate of R-T instability is investigated using numerical analysis. Corresponding graphs are plotted for showing the effect of these factors on the growth of the R-T instability.

Electron Paramagnetic Resonance spectra of Cu 2+ doped into cadmium barium formate dihydrate single crystal were studied at 300 K. The powder spectrum of this material was studied at 300 to 133 K. The values of the g and A hyperfine tensors were found to be temperature dependent. The temperature dependence of the Cu 2+ EPR spectrum is discussed in terms of the dynamic Jahn-Teller effect. The angular variation of the spectra indicates the substitution of the host Cd 2+ with Cu 2+ . The spectra were fitted with a tetragonal spin-Hamiltonian, the parameters of which were determined at 300 K. The ground-state wave function of the copper complex has been constructed at 113 K and 300 K.

Activated and non-activated states of the zeolites NH 4 NaY and NH 4 CaA, and their NH 4 forms were prepared with an (NH 4 ) 2 SO 4 solution and were investigated by EPR after 7-irradiation. The formed radicals were attributed to O 2 - supposed to be trapped near a trigonally coordinated framework of 27 Al (I = 5/2). Anisotropic O 2 - spectra were observed, consisting of hyperfine structure due to the 27 Al framework, even at room temperature. This was attributed to the scarceness of NH 4 sites or trap sites because of the preparation of the NH 4 forms of the zeolites with an (NH 4 ) 2 SO 4 solution. The crystal field splittings were found and interpreted.

In a hollow cathode He-Zn discharge it was observed that addition of Ne stops laser oscillation at the 492.4 nm Zn II line and increases significantly the spontaneous intensity at the 210 nm Zn II line, which originates from the 4d 2 D 5/2 lower level of the laser transition. Based on a rate equation model, the cross-section of the Ne ion-Zn atom charge transfer collisions populating the 4d 2 D 5/2 level was determined. The cross-section 3.5x 10 -15 cm 2 found by us is in acceptable agreement with the value 2.3x 10 -15 cm 2 reported in the literature. This high cross-section together with existing lower level depopulation points to the possibility of obtaining laser oscillation at 210 nm in a Ne-Zn discharge.

Former experiments on molten FeCl 3 have shown that, as for AlCl 3 , melting is accompanied by a transition from sixfold to essentially fourfold coordination. However, in contrast to AICI 3 , the FeCl 3 melt near freezing has an appreciable ionic conductivity. We propose a model for the structure of FeCl 3 melt as consisting of closely packed Fe 2 Cl 6 bitetrahedral molecules in equilibrium with (Fe 2 Cl 5 ) + and (Fe 2 Cl 7 ) - ionised species.

Microscopic expressions for the elastic constants of binary liquid crystalline mixtures composed of short rigid uniaxial molecules are derived in the thermodynamic limit at small distorsions and a small density. Uniaxial and biaxial nematic phases are considered. The expressions involve the one-particle distribution functions and the potential energy of two-body short-range interactions. The theory is used to calculate the phase diagram of a mixture of rigid prolate and oblate molecules. The concentration dependence of the order parameters and the elastic constants are obtained. The possibility of phase separation is not investigated.

Recent advances in the theory of electromagnetic retardation have made it possible to derive the basic equations of the special relativity theory and to duplicate the most important practical results of this theory without using the concepts of relativistic length contraction and time dilation. Thus the reality of these concepts appears to be questionable. It is imperative therefore to reexamine the experimental evidence supporting these concepts. The calculations presented in this paper show that some of the experiments allegedly proving the reality of length contraction and time dilation can be unambiguously interpreted as manifestations of velocity-dependent dynamical interactions taking place within the systems involved in the experiments rather than as manifestations of length contraction or time dilation.

Ionic motion and local structure of phenethylammonium ion (C 6 H 5 CH 2 CH 2 NH 3+ ) in a series of phenethylammonium lead(II) halides, [C 6 H 6 CH 2 CH 2 NH 3 ] 2 PbX 4 (X = Cl, Br, I) were studied by means of 13 C cross polarization/magic angle sample spinning (CP/MAS) NMR technique. Among the three salts, remarkable differences in the spectra were observed in the signals corresponding to the phenyl carbons. The peaks from C2, C3, C5, and C6 in the phenyl group were split into three for the chloride and into two for the bromide, although in the iodide those were observed as a single peak. Coalescence of those peaks, as well as line broadening were observed on heating. This indicated that peak split brings about disorder of the orientation in the phenyl group around the C1-C4 axis, suggesting to have three and two orientations for the chloride and the bromide, respectively. Above room temperature the phenyl group undergoes chemical exchange among these orientations, and at higher temperature, reorientation with a large amplitude takes place around its axis. The apparent activation energies of the reorientation of the phenyl group for the chloride and bromide were estimated from the temperature dependence of the linewidth of the resonance peaks to be about 24 kJ mol -1 , 25 kJ mol -1 , respectively, which is similar to in the iodide (25.0 kJ mol -1 ). Ab initio molecular orbital energy calculation was carried out to evaluate the potential barrier of the internal rotation of the phenyl group in a free phenethylammonium ion. The intramolecular interaction was evaluated to be 13.9 kJ mol -1 from the calculation, and the intermolecular interaction results to be 10 kJ mol -1 .

Dielectric spectra have been measured for the title systems up to 72 GHz at 20 °C with salt concentrations ≤1 mol/1 and DMSO mole fractions of the mixed solvent in the 0.3 to 0.6 range. Results can be described by a superposition of two Debye terms (at lower frequencies) plus one Cole-Davidson term (at higher frequencies). The discussion is particularly focused on the lowest frequency term, which is related to ionic species. These are likely to consist in nonsymmetrically solvated cations rather than ion pair complexes. While Cu, Ni and Mg nitrates behave similarly, the Zn salt shows some differences which possibly are due to a change of the coordination number with increasing salt content.

The absolute configuration of the naphthylisoquinolines isoancistrocladine and isohamatine at the biaryl axis was determined by comparing the measured and the calculated CD spectra. In order to calculate these spectra we used the CNDO/S method in combination with a Boltzmann weighting of conformers with different dihedral angles at the biaryl axis. The Boltzmann factors were obtained with the AM 1 method.

A thermoanalytical (TGA/DSC) and diffractometric (XRD) study has been performed on the solid state reaction system Li 2 CO 3 -Fe 2 O 3 in the x Li range 0.10±0.50. A detailed analysis of the results shows that the data are in agreement with a reaction model where the carbonate decomposition is regulated by the formation of both LiFeO 2 and LiFe 5 O 8 , and the relative amount of the two phases depends on the initial composition. The DSC evidence offers the possibility to directly quantify the LiFe 5 O x phase. Furthermore it allows one to obtain the enthalpies of formation of both LiFeO 2 and LiFe 5 O 8 .

63 Cu nuclear magnetic resonance and viscosity studies of 0.064 M copper (I) Perchlorate solutions have been made at 298 K in binary mixtures of acetonitrile (AN) with dimethylsulphoxide (DMSO), hexamethylphosphotriamide (HMPA), N,N-dimethylacetamide (DMA), nitromethane (NM), propylene carbonate (PC) and 3-hydroxypropionitrile (3 HPN) at several compositions of the mixtures using a 500 MHz NMR Spectrometer and Ubbelohde viscometer, respectively. The chemical shift (δ), linewidth (Δ) and line intensity (I) of the 63 Cu NMR signal in these mixed solvents have been measured relative to the 63 Cu signal in 0.064 M copper (I) Perchlorate (CuClO 4 ) solution in pure AN. The quadrupolar re-laxation rates (1/T 2 )Q, reorientational correlation times (τ R ) and quadrupolar coupling constants (QCC) of the copper (I) solvates have also been estimated from the data. The QCC values show a big variation in all solvent systems with the change of solvent composition, indicating the formation of mixed com-plexes. The variation of all NMR parameters with solvent composition shows strong effects of DMSO, HMPA, and DMA on the solvation behaviour of Cu + in the first three mixtures, and relatively much weaker effects of PC, NM and 3HPN in the other three mixtures.

To answer the question about the way our visual system processes images it has to work with every day, it is necessary to investigate the statistical structure of these pictures. For this purpose we investigated several ensembles of artificial and real-world greyscale images to find different invariance properties: translation invariance by determining an average pair-correlation function, scale invariance by investigating the power spectrum and the coarse graining of the images, and a new hierarchical invariance recently proposed [D. L. Ruderman, Network 5, 517 (1994)]. The results of our work indicated that the assumption of translational invariance can be taken for granted. Our results concerning the scale invariance are qualitatively the same as those found by Ruderman and others. The deviations of the distributions of the logarithmically transformed images from a Gaussian distribution cannot be seen as clearly as stated by Ruderman. This results from the fact that for a correct determination of the deviations the non-linear transformation must be considered. Depending on the preprocessing of the images the results concerning the hierarchical invariance differed widely. It seems that this new invariance can be confirmed only for logarithmically transformed images.

Synchronisation of chaotic oscillators offers a way of practical application of the theory of Chaos in obtaining secure communication. In this work we introduce a nonautonomous chaotic system with sinusoidal external force for communication of binary signals. The information is applied to the phase position of the sinusoidal forcing signal of the chaotic oscillator using a quadrature difference phase shift keying (QDPSK) modulation. An inverse synchronisation system approach with direct modulation is applied. We describe the system in detail and discuss the requirements of a secure communication system. Issues related to bit error rate, transfer rate, signal to noise ratio, channel bandwidth, bandwidth efficiency and channel capacity are discussed, and the properties of the realized communication system are placed in relation to the requirements of a secure communication system.

The control method of Ott, Grebogi, and Yorke (OGY) is used to stabilize the unstable periodic orbits of a chaotic relay system. Small variations in the height of the relay output are used as control input. The influence of the control activation bound is studied in detail via the one-dimensional Poincare map of the controlled system. The reduced sensitivity of the multi-step OGY method for higher period orbits can thus directly be verified.

Much experimental and theoretical effort has gone into revealing the internal complexities of molten salts for the past two decades. In this paper we shall show how neutron diffraction and computer simulation techniques have helped in gaining a better understanding of these systems at the microscopic level. Firstly, a short review on the structure of molten halide systems as revealed by these techniques will be presented. Complementarity of using X-rays with neutrons and, some recent results on the structure of molten DyCl 3 obtained by combining neutron and X-ray diffraction with molecular dynamic simulations will be discussed. Neutron diffraction isotopic substitution techniques have played an important role in elucidating the interatomic structure of a diversity of molten salts. Pair distribution functions (PDFs), determined for a number of 1:1 and 2 :1 halide melts, provided theorists with a critical test of their model potentials. It is now clear that for 1:1 molten systems theoretical models based on Fumi-Tosi potentials can adequately describe many of the structural features. Nevertheless, the chal-lenge is two fold: (i) to determine real interatomic potentials for 2:1 and 3 :1 molten systems capable of reproducing not only the microscopic structural details obtained at the partial PDF, g αβ (r) level, but also their macroscopic behaviour and, (ii) to characterise the structure in binary molten salt mixtures.