Volume 45, Issue 1

The relaxation of velocity-space-anisotropies due to Coulomb collisions is studied within the class of bi-Maxwellian distributions in the presence of an external homogeneous magnetic field, both analytically and numerically. It is found that the anisotropy dependence of the energy exchange collision frequency well separates from its magnetic field dependence. In a certain region of magnetic field strength, the latter can be approximated by a substitution of the thermal electron's Larmorradius for the Debye-length in the Coulomb-logarithm, as has been conjectured earlier.

The viscosity tensor of biaxial-nematic liquid crystals contains 16 independent elements. A complete set of these viscosity coefficients is introduced and related to experimentally accessible quantities. Furthermore, the flow alignment and its stability against an arbitrary infinitesimal disturbance are discussed. By the affine transformation model, formerly established for the uniaxial symmetric case, one can express the viscous anisotropy of perfectly ordered biaxial ellipsoids in terms of the two viscosities of an isotropic reference system and the axes ratios of the nonspherical particles.

The Dirac equation with scalar potential and fourth component of vector potential of the Gaussian form is solved numerically for potential parameters obtained by a least squares fitting of the ground state binding energies of the A in a number of hypernuclei. The binding energies in the ground and excited states for various hypernuclei are determined. The spacings between the various levels are also given

A decomposition theory for positive sesquilinear forms densely defined in Hilbert spaces is developed. On decomposing such a form into its closable and singular part and using Bochner's theorem it is possible to derive the central decomposition of the associated gauge-invariant quasifree state on the boson C*-Weyl algebra. The appearance of a classical field part of the boson system is studied in detail in the GNS-representation and shown to correspond to the so-called singular subspace of a natural enlargement of the one-boson testfunction space. In the example of Bose-Einstein condensation a non-trivial central decomposition (or equivalently a non-trivial classical field part) is directly related to the occurrence of the condensation phenomenon.

By EPR, two nematic liquid crystals (MBCA and EPAPU) were investigated using a steroidal nitroxide spin probe. In both liquid crystals the isotropic-nematic phase transition is of first order. The observed variation of the order parameter with temperature is compared with predictions from the Maier-Saupe and Humphries-James-Luckhurst models and with results obtained by several other experimental techniques.

Temperature dependences of 1 H and 19 F second moment and spin-lattice relaxation times for polycrystalline pyridinium tetrafluoroborate were measured. A phase transition was discovered at 202 K. A model of cation reorientation between inequivalent (low-temperature phase) and equivalent (high-temperature phase) equilibrium positions is proposed. Whether the anion reorients isotropically or about a symmetry axis cannot be decided. An analysis of cross-relaxation effects yielded activation parameters for cation and anion reorientation. The rotational correlation times for both ions converge just at the phase transition reaching the value of 10 -10 s.

A survey of various methods to measure the elastic constants K i of nematic liquid crystals is given. To determine K 1 , K 3 and the anisotropy of the diamagnetic susceptibility Δ x of two members of the 5n-hexyl-2-[4n-alkyloxy-phenyl]-pyrimidines and a mixture of both, we used a combined electromagneto- optical method, consisting in independent measurements of the optical phase difference in electric and magnetic fields acting on the same cell. The temperature dependence of the K 1- and K 3 -values for these phenylpyrimidines can be explained by common theories. The Δ x data show the same temperature dependence as the values of the orientational order parameter S obtained by 1 H-NMR.

The effects of skin depth on gamma detected single pulse Nuclear Magnetic Resonance on Oriented Nuclei (NMRON) signals are theoretically explored for narrow, intermediate and broad line metallic samples, using the density matrix approach describing a pure Zeeman system. It is shown that the skin effect distortion of the signal can dominate over intermediate to broadline distortions for that range of experimental conditions generally applicable to ferromagnetic hosts. In particular, the skin effect distortions of the first maximum, obtained when the excitation pulse width is lengthened, are significant and can determine the accuracy of calibration of the radiofrequency "(rf)" field amplitude at the resonating nuclei when assigning an average turn angle to this maximum.

The environmental radial distribution functions (RDFs) around Cu and Fe as well as the ordinary RDF of an as-quenched Al 65 Cu 20 Fe 15 alloy have been determined by anomalous x-ray scattering (AXS) and conventional x-ray diffraction techniques. The experimental information indicates that a strong directional dependence along the symmetrical axes exists for the Cu atoms but not for the Fe atoms. Almost identical RDFs were also obtained in both the as-quenched and fully-annealed states of the Al 65 Cu 20 Fe 15 alloy. Thus, the small differences observed between diffraction profiles of the as-quenched and annealed samples of this alloy may be attributed to defects or strains introduced during the rapid quenching process.

Deuterium nmr has been used to study the rotational dynamics of the ammonium ion in ammonium tetraphenylborate and of the cation in a series of methylammonium tetraphenylborates. The results of variable temperature 2 H relaxation measurements (T 1Z and T 1Q ) on the parent compound, ND 4 B(C 6 H 5 ) 4 , lead to a low activation energy for reorientation of the ND 4+ ion, 5.0 ± 0.5 kJ mol -1 , indicative of weak cation-anion interactions. Related studies of the tetramethyl-d 12 ammonium salt indicate rapid jumps of the whole cation (τ ≪ 10 -5 S) over the complete temperature range investigated, 137 to 385 K, with an activation energy of 14.1 + 0.5 kJ mol -1 . Deuterium nmr lineshape studies of the methylammonium salt indicate rapid reorientation of the CD 3 and ND 3 groups about their C 3 axes; however, the motion of the C - N axis is restricted to large amplitude librations. Similar studies of the dimethylammonium salt indicate methyl group jumps at 245 K; at temperatures above 320 K the cation also undergoes two-site jumps about the C 2 axis. In the case of the trimethylammonium salt, the nmr lineshapes indicate methyl group jumps superimposed on C 3 rotations of the complete cation about the N - H bond axis over the temperature interval 185-348 K. In all the tetraphenylborate salts studied, the 2 H nmr lineshapes indicate substantial librational motion of the cation at room temperature.

By coprecipitation of ZnS and MnS in excess of Na 2 S, luminescent centres are built differing considerably from those in zinc salt excess. ESR data give evidence of centres of T d symmetry.

The numbers of benzenoid chemical isomers, represented by the formulas C n H s , are reviewed. The data which have been deduced from the available literature are supplemented.