Band 28, Heft 6

The theory of light scattering by fluctuations has been extended to optically active fluids. The new feature is the "gyration parameter", a second rank asymmetric tensor, the fluctuations of which must be counted along with the familiar isotropic (scalar) and anisotropic (traceless and symmetric) fluctuations of the electric permeability tensor. Scattering equations are derived and solved in order to obtain the spectrum of scattered light. It is found that the angular dependence of scattering caused by fluctuations of permeability alone does not involve the propagation vectors of the incident or scattered fields whereas that which involves fluctuations of the gyration parameter depends explicitly upon these two vectors. This gyration tensor also has distinguishable effects upon the depolarization, ellipticity, and rotation or tilt of the scattered quasi-monochromatic light. The total intensity is resolved into a sum of five distinct contributions, each associated with different irreducible components of the gyration and permeability tensors and each with a different dependence upon the scattering angle and polarizations of the incident and scattered light. Experiments are suggested which emphasize effects originating from the optical activity of the scattering medium.

The magnetic Senftleben-Beenakker effect of the heat conductivity is considered for a gas of p-H 2 molecules. The magnitude of the saturation value is expressed in terms of collision integrals of the linearized Waldmann-Snider collision term. The collision integrals are, in turn, connected with molecular cross sections describing the production of a tensor polarization of the molecular rotational angular momenta. These orientation cross sections depend essentially on the nonspherical part of the intermolecular potential. For small nonsphericity of the interaction, valid in the case of H 2 , it is sufficient to take into account only energetically elastic collisions For a simple potential model (nearly spherical rigid ellipsoids of revolution) the scattering amplitude is analytically evaluated in first order DWBA. The relevant collision integral which describes the coupling between the rotational heat flux and the Kagan polarization is calculated for room temperature. The comparison with the experimental value shows a good agreement.

Expressions which are exact in the distorted wave Born approximation are derived for a number of kinetic theory collision cross sections, viz. This is done for a number of nonspherical potentials, namely for single-P 1 , single-P 2 , dipole-dipole, dipolequadrupole and quadrupole-quadrupole interactions. A number of additional approximations is used to obtain approximate relations between some of these cross sections and to derive a approximate law of corresponding states. A simple model is described which makes the approximate evaluation of collision cross sections for multipole-multipole interactions very straightforward.

A study is presented of the effect of a magnetic field on the viscosity of HD, N 2 , CO and CH 4 . The temperature dependence of this viscomagnetic effect has been measured in the range between 300 K and the boiling point of these gases. The results are presented in terms of effective cross sections which depend directly on the angle dependent part of the intermolecular potential. The cross sections for reorientation of angular momentum have been compared with those obtained from N.M.R. measurements.

A study is presented of the viscomagnetic effect in binar mixtures consisting of diatomics and noble gas atoms. The following systems have been investigated: HD -He, HD -Ne, HD-Ar and N 2 -He, N 2 -Ne, N 2 -Ar. The purpose of this research is to obtain effective cross sections for the relatively simple interaction between monatomic and diatomic molecules. Various effective cross sections are presented.

Heat-flow birefringence is described by a constitutive law which links the anisotropic part of the dielectric tensor with the gradient of the heat flux or the 2nd spatial derivative of the temperature field. For a gas of linear molecules, this relation is derived from transport-relaxation equations which, in turn, were obtained from the Waldmann-Snider equation. The magnitude of the heat-flow birefringence can be inferred from the Senftleben-Beenakker effect of the viscosity and of the heat conductivity. It is found to be of measurable size.

The behaviour of rarefied polyatomic gases is described by transport relaxation equations obtained from the Waldmann-Snider equation and by an appropriate set of boundary conditions which are derived from the interfacial entropy production. The influence of the magnetic field on the transport properties of the gas is due to the coupling of the tensor polarization and the tensor polarization flux with the frictional pressure tensor and the heat fluxes respectively. As an application the flow and the heat conduction of a gas between parallel plates in the presence of a magnetic field are treated, and from this two major results are obtained: (a) Knudsen corrections for the Senftleben-Beenakker effect of viscosity, (b) a theory of the thermomagnetic pressure difference. A comparison with experimental data of Hulsman e. a. and Hermans e. a. yields boundary coefficients which describe both phenomena rather well for the gases HD, CH 4 , CO and N 2 .

Self-diffusion measurements on molten LiCl in the temperature range 610 °C ≦ ϑ ≦ 760 °C, obtained with a capillary-reservoir technique, result in the expressions D + = [10.6 + 0.0307 ϑ - 610) ±0.81 × 10 -5 cm 2 /s, D_ = [ 5.8 + 0.0117 ϑ - 610) ±0.3] × 10 -5 cm 2 /s. These results fit well into the pattern of the other alkali halides. As with these, it is observed that (D + /R T) + (D_/R T) - (Λ/F 2 ) >0, the difference increasing with rising temperature.

The well-known relation of Fresnel and Lorentz for the effect of material motion on the propagation of light is extended to guided waves propagating in a system of dielectric layers differing in refractive index as well as in the magnitude of their velocities. The analysis is based only on the existence of a transverse dispersion (determinantal equation) which is invariant with respect to sliding motion at least to first order in β. A one-to-one correspondence ("transmodification") of wave vectors in the moving and in the quiescent state is used to demonstrate that nonuniform motion can lead to (convectively) unstable modes. The theory is applied to a moving parallel-plate waveguide and to the model of a glow discharge enclosed in a circular waveguide.

It had been shown 1, 2 recently that it may be possible to reach densities up to 10 3 g/cm 3 by imploding a solid hydrogen pellet with the aid of a symmetric arrangement of multiple laser beams irradiating the pellet simultaneously from all sides. The purpose of this proposed scheme was to reduce the energy input requirements for the controlled release of thermonuclear energy delivered by laser ignited T - D micro-explosion systems. It is shown in this paper that by applying the foregoing concept of high density compression to a pellet consisting of fissionable material such as U235, Pu239, or U223, the critical mass for the initiation of a fission chain reaction in these materials can be reduced by many orders of magnitude raising the possibility of micro-fission-explosions with laser energy inputs for compression within the expected reach of laser technology. Alternatively, the pellet compression may be also achieved by the employment of intense relativistic electron beams. In addition, by surrounding the fis­sionable pellet with a layer of dense T - D material to be compressed together with the pellet, the minimum pellet size can be furthermore reduced by neutron reflection from the high neutron albedo T - D shell. For fissionable pellets without a reflector the critical mass can be as small as ∼ 0.3 g requiring a laser energy input for compression of several megajoule. For a pellet surrounded by a T - D reflector the critical mass can be reducel down to ∼10 -3 g with a laser energy input for compression of several 10 5 joule. In addition to the neutron albedo effect of the T - D shell there is a bootstrap mechanism where­by the T - D mantle achieves thermonuclear temperatures resulting in the emission of thermo­nuclear reaction neutrons into the fissionable pellet causing more fission reactions and thereby raising the temperature of the pellet. This then in turn will lead to an additional heating of the T - D blanket resulting in an increased flux of thermonuclear neutrons raising the fission rate even further. Because of the greatly increased material density, a fission chain reaction initiated in it will grow in proportion much faster than in uncompressed fissionable material as it is being used in conventional fission bombs. The system described may have important usefulness as a small fission power plant especially for space propulsion applications but also for a more effective burn in a thermonuclear micro-bomb reactor as a fission supported fusion reaction.

The electromagnetic field of a point charge moving uniformly in a uniaxial dispersive medium is studied in the rest frame of the charge. It is shown that the Fourier integral for the scalar potential breaks up into three integrals, two of which are formally identical to the isotropic integral and yield the ordinary and extraordinary cones. Using the convolution theorem of the Fourier transform, the third integral is reduced to an integral over the isotropic field. Dispersion is explicitly introduced into the problem and the isotropic field is evaluated on the basis of a simplified dispersion formula. The effect of dispersion on the field cone is studied as a function of the cut-off frequency.

In this paper a group theoretical method for the determination of the optimal factorization of the Hamiltonian matrix is presented.

In this paper a group theoretical method for the determination of the optimal factorization of the Hamiltonian matrix is presented.

The intermolecular coupling of molecular vibrations in molecular crystals is usually treated by the exciton concept. This is identical with the assumption that a potential, which describes the interaction of a vibrating molecule with its neighbours, perturbs the isolated molecule energies and thereby couples the motions of molecules. In most cases this potential will be the transition-dipoletransition- dipole interaction. If one assumes validity of the pair interaction approximation this concept should be able to describe the effects in the liquid state, too. The aim of these considerations is to show, how collective excitation can influence the shape of vibrational bands. Because dipoledipole interaction is strongly orientation dependent, molecular reorientations perturb the coupling of the vibrations and cause motional narrowing. The behaviour of the shapes of the vibration bands of CH 3 J is discussed on the basis of these considerations. One finds that the band shape of ν 2 is mainly broadened by vibration coupling. As expected a distinct motional narrowing is observed, if one increases the temperature from the melting point to the boiling point.

Various consequences of the supposition that the value of the resonance integral β AB depends on the nature of the bond are evaluated within the framework of the simple Hückel method. The resulting equation for β AB in function of the polarity 1 of the AB bond is: β AB =(1-I 2 )- ½ [C+(½) ∫ (α A -α B )dI] where αX is the Coulomb integral. A solution is proposed corresponding with β AB = β AA · β BB ) ½. Further elaboration then suggests that the simple Hückel method in this form is in favour of the ionic approximation to chemical bonding. The Coulomb integral turns out to be the electron affinity, which must now be considered as the electronegativity of elements when forming chemical bonds.

The effects of random internal strains on the paraelectric resonance spectra for off-center Li in KCl are investigated quantitatively. Diagrams giving the theoretical line positions and relative intensities are presented for the principal orientations 〈100〉, 〈110〉, and 〈111〉. Where the model for an ideal host crystal predicts only one line, up to six lines are obtained in the present calcula­tion. A direct comparison with the published experimental spectra, taken at 9, 24, 29, 35, 63 and 74 GHz is performed. Very good agreement is found and the magnitude of internal strain has been inferred, yielding values between 18 and 42 bar for different samples. Random internal electric fields are found to be of the order 3kV/cm. For the halfwidth of the main signal, the orientation and frequency dependence is examined and explained by the calculations. The possible use of paraelectric Li as a microprobe for internal strains and internal electric fields in KCl is discussed. The electric dipole moment of Li 7 is found to be 5.9±0.3 Debye in agreement with previous deter­minations. The “cube edge” tunneling model with equidistant zero field levels, separated by 22.2 GHz for Li 7 , is fully confirmed.

The inhomogeneous excitation resulting from strong absorption of the incident light as well as the low carrier mobility resulting in nearly negligible diffusion is taken into account in converting the directly measured conductance and number of spins into a time- and intensity of illuminationdependence of the concentrations of localized states and carriers. Then a model as simple as possible is searched out which will be able to explain simultaneously the behaviour of the conductivity and density of carriers and spin carrying centres. The resulting four coupled nonlinear differential equations have been solved analytically and numerically. Within the accuracy of the measurements, the observations can be reproduced.

A gradient coil has been constructed for exact relative measurements of the longitudinal (Dʟ) and transverse (Dᴛ) diffusion coefficients of nematic liquid crystals and of small molecules dissolved therein. Some small molecules dissolved in the nematic phase of Licristal IV showed a Dʟ/Dᴛ ratio varying from about 1.1 to 1.25, depending on the concentration. In contrast, tetramethylsilane dis­solved in p-methoxy-benzylidene-p'-n-butyl-aniline showed no anisotropy in the nematic phase.

The relative frequency stabilities and the modulation spectra of differently stabilized lasers are compared. The measurements were made by a Michelson interferometer with an internal optical de­lay line. For 6328 Å He - Ne-lasers stabilized by means of a Zeeman-absorption-cell and by the Lamb-dip-method frequency-standard-deviations of 1.3 × 10 5 and 9.0 × 10 5 Hz for observation times between 1 s and 20 s were measured. The frequency modulation spectra are composed of a low frequency noise part and many discrete frequencies, which can be related to the stabilization mechanism.

Pyrene single crystals have been grown from zone refined material by sublimation and from the melt. The absorption edge has been measured for thick and thin crystals at room temperature. Thin crystals have been successfully cooled below the phase transition temperature (123 °K) without cracking. The sharp absorption peak at 376 nm vanishes at this temperature while the fluorescence spectrum remains unchanged. Using polarized light below 50 °K, a band with a maximum at 410 nm appears whose intensity increases with decreasing temperature. It is concluded that this band is due to ground-to-excimer state absorption in the ground state equilibrium distance of the pyrene molecules. The consequences of this finding for excimer-exciton transport is discussed.

The static correlation function governing elastic scattering of X-rays or neutrons by a defective crystal is discussed for three degrees of imperfections, that is for slight, severe, and medium distortions of the scattering crystal. In the exponent of this correlation function the main term, which is linear in the defect concentration, is shown to be fairly independent of the particular type of statistics describing the random distribution of the defects. One condition for a successful analysis of defect structures using diffuse scattering data is that the scattering function can be split into the individual contributions of all single defects. For two regions, that is for the immediate vicinity of Bragg reflections (Huang scattering) and for the asymptotic regions of distortion scattering, this "single-defect approximation" is shown to be useful even for higher concentrations. In this case the correlations due to a single defect have to be corrected by a factor which takes into account the average correlation reduction by all other defects (static Debye-Waller factor in the Huang regions and a similar but variable factor in the asymptotic regions). The formulas given in this paper are applied to the sacttering by isotropic crystals containing point defects of spherical symmetry.

A glass sphere filled with a nematic liquid crystal has been suspended in a homogeneous magnetic field by a thin thread with known restoring moment. Sudden changes of the field direction with respect to the director of the nematic liquid crystal cause rotary oscillations of the sphere. This motion is studied for various initial conditions as a function of field strength. In general, we observe damped oscillations superimposed by an exponential decay function, a theoretical description of which is given in terms of coupled differential equations. Their solution provides two parameters: The field dependence of the oscillation frequency yields the anisotropy of the magnetic susceptibility Χa . The twist viscosity γ 1 can be calculated from the damping of the oscillations as well as from the exponential decay function obtained with high magnetic fields. For MBBA we find Χa = 1.32 × 10 -7 at 22 °C. γ 1 differs for both methods which is being discussed.

Three partial structure factors S ij (Q) have been evaluated from the scattered X-ray intensities of liquid Na -K and Al - Mg alloys assuming that the S ij (Q) are independent of the relative abundance of the respective elements in the alloys. The functions S ii (Q) and S jj (Q) and the reduced radial distribution functions G ii (r) and G jj (r) obtained in this work are very similar to those observed in the respective pure liquid metals. In both cases, S ij (Q) and G ij (r) have maxima which lie in between those of the pure elements. From these results, liquid Na -K and Al -Mg alloys are interpreted as random mixing fluids. A comparison between the partial structure factors obtained in this work and those calculated from the hard sphere model was made. Adequate agreement was obtained on the low angle side of the first peak, but agreement on the whole pattern is not necessarily found. The electrical resistivity was calculated using Faber-Ziman’s theory and compared with experimental data.

Electrocatalytic activities of metal-phthalocyanines (Me-Pc) and -dibenzotetraazaannulenes (Me-TAA) for oxygen reduction have been measured by standardized dispersion electrode in 4.5-n. H 2 SO 4 . The most active chelates are Fe-Pc and Co-TAA. Normally substitution of the ringsystem leads to a strong decrease in activity. Exceptions are - COOH, - NO 2 with Me-Pc, -Cl with Me-TAA. In alcaline media, the Co-chelates are especially active. Electrocatalytic activity and catalase effect are not correlated. A mechanism is given, which is called “redox-catalysis”. In a chemical step, the electrocatalyst is oxydized by the oxygen to the cation, which is reduced in an electrochemical follow up step. This gives a satisfactory explanation for the complementarity of activity and stability of these electrocatalysts.

The photoionization curves and the threshold energies for the molecules vinylchloride, vinylfluoride, 1,1-difluoroethylene and their abundant fragment ions have been measured with synchrotron radiation in the photon energy range from 10 - 23 eV. Appearance potentials were used to calculate heats of formation, bond energies and ionization potentials. The structure of ion efficiency curves is discussed in terms of different ionization processes.

An attempt is made, to investigate the reactivity of aromatic compounds by a stepwise approach of the NO 2 -cation to the aromatic system, by means of the MINDO-II method.

The approximation of the stable core has been investigated, using small basis sets of the SCF -MO -LCGO-method. The results of the calculated three Gauss-functions and the total energies for the configuration (1s) 2 (2s) 2 of B + , C 2+ , N 3+ , O 4+ and F 5+ are given and compared with the basis sets of five Gauss-functions. A discussion on the quality of these basis sets for molecular calculations is given.

Explicit formulas are derived for the eigenvectors of polaritons (quasi-normal coordinates and electric field). With these eigenvectors Raman scattering cross sections can be calculated directly.

It was shown in previous papers that trapped charge carriers can be used at low temperatures for detection and free path determination of single phonons. Using this method it is shown now that because of the high phonon density of a phonon pulse generated by one alpha-particle the action of such phonons on trapped charge carriers is mostly a multiphonon action so that the carriers can be released from trap depths which are much greater than the energy of a single phonon. Because of the great free paths of phonons at low temperatures this action can take place in distances up to 1 mm from the path of the alpha-particle itself.

Far infrared transmission and reflexion measurements were performed on polycrystalline samples of α-RbAg 4 I 5 solid electrolyte. As in α-AgI, an anomalous broad low frequency ab­sorption band centred at 5 · 10 11 Hz is found in addition to the lattice absorption at ca. 3 · 10 12 Hz. Since it is rather tempera­ture insensitive, the low frequency band indicates the exis­tence of very slow high amplitude lattice vibrations in this material.

The magnetic field induced deformations of a homeotropic nematic liquid crystal layer are studied by means of the electric conductance. Its time dependence observed after rotating the magnetic field direction by 180° is explained by the formation of alignment inversion walls.

In a binary system of cholesteric liquid crystals with righthanded sense of helix a twofold helical inversion at two different molar fractions ϰ n,1 and ϰ n, 2 has been observed by measurements of circular dichroism and optical rotatory power. In the region of composition between ϰ n,1 and ϰ n, 2 all mixtures exhibit a left-handed cholesteric structure.

Delayed β - γ -coincidence measurements of the 667.0 keV- level in 131 Xe populated by the decay of 131 I were carried out with a Plastic-Ge (Li)-detector-combination and resulted in the following value: T ½ of 667.0 keV-level in 131 Xe < 0.5 ns.