Band 30, Heft 2

The influence of a high magnetic field on the viscosity of diamagnetic gases of linear, symmetric and asymmetric top molecules is studied within the frame-work of the kinetic theory based on the Waldmann-Snider equation. The quadratic Zeeman effect associated with the anisotropic magnetic susceptibility is taken into account in addition to the usual linear Zeeman splitting.

The binary scattering amplitude matrix is derived from the general interaction potential between linear molecules. The first order distorted wave Born approximation (DWBA) is used which is applicable for small nonsphericity of the interaction. The molecular cross sections determining the most important Waldmann-Snider collision integrals are calculated. In particular, the scattering cross section, the orientation cross sections for vector-and tensor polarization and the reorientation cross sections for the tensor polarization of the rotational angular momenta are treated. For a simple interaction model for HD (HT) molecules the DWBA-scattering amplitude is evaluated analytically. The relaxation cross section of the tensor polarization, σT , and the coupling cross section of friction pressure tensor and tensor polarization, ση,T, are calculated for room temperature and compared with experimental values.

The behaviour of a dilute vapour, diffusing in a non-condensable gas, has been investigated. The vapour condenses on, and evaporates from, a spherical droplet, and the appropriate Boltzmann equation has been formulated thus enabling the condensation rate to be calculated in a precise manner. In contrast to previous work on this problem, the saturation temperature of the droplet differs from that of the non-condensable gas and this leads to certain differences and difficulties in the calculations. Despite these difficulties, an approximate method has been developed which uses the integral form of the transport equation to express the condensation rate in a form which includes any type of vapour-gas interaction. This is an advance on previous work which was confined to simple model problems and restricted to isotropic scattering. An expression for the heat flux to the droplet is obtained and a method for calculating the vapour density and temperature is outlined. Finally, we stress the limitations of the method and suggest ways in which these may be overcome.

We extend the New Tamm-Dancoff method by introducing intermediate states. In this way we are able to treat with the Green's function method the effect of nearby levels in many-body systems. We formulate the η-and the ζ-function method with intermediate states. Aldready in first order, the ζ-function method yields a whole series of new approximations in addition to known theories such as the Hartree-Fock theory and the Hartree-Bogoliubov theory. As an example we study intensively "the Hartree-Fock theory with intermediate states". The ζ-function method which is based on the η-function method yields in first order in addition to RPA, quasi-particle RPA and other approximations "RPA with intermediate states". We apply the Hartree-Fock theory with intermediate states and RPA with intermediate states to the exactly solvable Lipkin model.

It is pointed out that a considerable amount of evidence exists to support the concept of a ground state rotational band based on tetrahedral deformation for 16 O, 40 Ca and 208 Pb. The possible meaning of this interpretation as well as other well-known alternatives are discussed.

The thermoluminescence glow curve of γ-irradiated silica gel investigated between 77 °K and 600 °K is a broad band extending from 80 °K to ~ 400 °к. The emission occurs at ~ 450 nm. Shallow traps (0.1 - 0.2 eV) are responsible for most of the light emitted. The distribution of trap depth and the increase of trap density toward the band edge is tentatively attributed to the dis-ordered structure of silica gel in accordance with previous suggestions concerning glasses. In contrast to the growth with radiation dose of color-and paramagnetic center concentration, luminescence is already saturated at about 1 Mrad. The luminescence decay follows the hyperbolic relation derived by Adirowich for a bimolecular process in the case of retrapping for which evidence is presented. At very low radiation doses the initial intensity of luminescence was proportional to the square of the dose rate as expected for a bimolecular reaction. Removal of surface OH groups as accomplished by raising the degassing temperature of the gel results in an increase of luminescence intensity by several orders of magnitude. Hydrogen and oxygen when adsorbed prior to or subsequent to irradiation reduce the luminescence intensity to about one per cent of that observed in vacuum or in the presence of helium. The results are dis-cussed in terms of surface reactions involving free and trapped charge carriers generated during irradiation.

Luminescence observed on adsorption of gases and vapours (RSL) on silica gel previously ex-posed to y-rays has been investigated. The dependence of RSL intensity on absorbed radiation dose and degassing temperature of the gel prior to irradiation parallels the dependence of radio-thermoluminescence (RTL) intensity: A limiting value is reached around 1 Mrad and the intensity increases steeply on raising the degassing temperature. The RSL intensity is largest on adsorption of H 2 and decreases in the order H 2 ; O 2 ; N 2 O; N 2 ; He. The stronger the RSL effect, the more is the subsequent measured RTL reduced. The RSL emission spectrum has a maximum at 450 nm and is identical with the spectrum of RTL. In some cases bands at longer wavelengths have been observed. The RSL decays with hyperbolic kinetics which may be related to the logarithmic rate of adsorption as given by the Elovich equation. The results are discussed in terms of detrapping of charges, in this case stimulated by adsorption, and the recombination of carriers leading to excitation of luminescent centers as in RTL. The emission bands at longer wavelengths are thought to involve electron transfer from interacting molecules neutralizing holes trapped in the solid.

The set-up of digital-out-put for HR-CW-NMR-spectrometers and computer-programs suitable for Lorentzian-line-shape-analysis is reported. The values for 5 J HF AND 6 J HF IN [C 6 F 5 Si(CH 3 ) 2 ] 2 are determined.

Experiments are reported which extend the useful Radiofrequency Microwave Double Resonance Method to higher Radiofrequencies. A special absorption cell was constructed for this purpose.

The microwave spectra of following isotopic species 12. 12 C 32 S 19 F 35 Cl, 12 C 32 S 19 F 37 Cl, 12 C 34 S 19 F 35 Cl, 13 C 32 S 19 F 35 Cl have been measured. The rotational constants and the centrifugal distortion constants have been determined and a r 0 structure calculated. The structure parameters are: r CS 1.595° Å, r CF 1.326° Å, r CCl 1.715° Å, ∢ SCF 123.82°, SCCl 127.12°.

In connection with the problem of radio frequency heating of plasma, the interaction of a high power wave (0.1 GWatts) with a cylindrical plasma is investigated experimentally for frequencies near the ion cyclotron frequency and in the region between the ion cyclotron frequency and the lower hybrid frequency. It is found that a geometric resonance can be excited in which more than 90% of the wave energy is absorbed by the plasma. At frequencies around the ion cyclotron frequency, the macroscopic properties of the plasma, including distortion of the wave by large amplitude effects, can be fairly well explained by a MHD-model assuming an anomalous resistivity. The enlarged collision frequency is attributed to the presence of turbulence which is excited by drift currents connected with the wave.

Absolute Measurement of Power Transfer to a Deuterium Plasma Near the Lower Hybrid Resonance The power transfer to a deuterium plasma generated by a high frequency discharge near the lower hybrid resonance is investigated in the range of several hundred watts. The results are compared with linear theories. Reasonable agreement is found between measurements and the linear theory of a bounded plasma as far as the wings of the resonance are concerned. But the dominant radial modes predicted by theory are not observed. Introduction of an empirical effective collision frequency ν* = 0.2 ω pe , however, results in good agreement between measurements and theory in the domain of these radial modes.

The half-widths of five A III and two A IV lines have been measured in a pulsed arc operated in an argon-nitrogen mixture. Electron densities of 3.8 to 8.0 × 10 16 cm -3 were determined by laser interferometry at a single wavelength while electron temperatures from 20750° to 23100 °K were measured from the relative intensities of All lines. The experimental A III and A IV Stark profile halfwidths were compared with calculated values using various theoretical approximations.

The electron concentration, the excitation temperatures (from Ar-I-, Ar-II-and Cu-lines) and the rotation-and vibration temperatures (from N 2 -and N 2 + -bands) have been measured in a glow discharge lamp. Spectral lines of neutral molecules indicate temperatures in the range of 3000 K. Using ion lines as indicator it could be shown that T exc > T vib > T rot (30 000; 9300; 1190 K). The former are hardly affected by discharge parameters, the latter ones are strongly affected. At the edge of the glow the temperature is distinctly lower. The electron concentration is approx. 10 14 cm -3 . The results are discussed.

An attempt is made to explain the E-C formalism for ionic interactions in terms of the ionic approximation to chemical bonding. Dravo's E-C equation is seen to be a first approximation to the bond energy equation as it is given by the ionic bonding approach. The meaning of the ratio C/E is discussed and its relation with the hardness and softness of interacting species, as these occur in Pearson's HSAB rule, shows that the electron affinity or electronegativity of elements completely determines the chemical behaviour of ionic species. This analysis illustrates the consistency of the ionic approximation to chemical bonding.

In this work we adopt the RMMV potential function proposed by Davis and calculate third virial coefficients for Ar, Kr, and Xe. Nonadditive effects due to dipole-dipole-dipole and dipole-dipole-quadrupole terms are included. We found good agreement between theoretical and experi-mental values for Ar and Kr, but not for Xe.

The elastic constants of a nematic liquid and of a solid crystal were derived by comparing the Gibbs free energy with the elastic energy. The expressions for the elastic constants of the nematic and the solid crystal are isomorphic: k ∝ ∣ D ∣ /1. In the nematic phase ∣ D ∣ and I are the mean field energy and the molecular length. In the solid crystal, ∣ D ∣ and l correspond to the curvature of the potential and to the lattice constant, respectively. The measured nematic elastic constants show the predicted l dependence.

X-ray Diffraction Study of Molten Tellurium An X-ray diffraction apparatus with focusing geometry used for the investigation of free surfaces of melts and a computer programme for the analysis of scattering intensities are discribed. The results of our measurements on molten tellurium at 480, 550 and 620 °C are presented and compared with a model of the temperature change of the short range order in molten tellurium as described by Cabane, Tourand and Breuil 14-17 . The important result is that the number of nearest neighbours is approximately 3 at all the 3 temperatures. This suggests that the short range order is changing at the melting point from a chain structure to an antimony-like structure.

Mo-Kα X-rays have been used as a radiation probe to evaluate the interference functions l(K) (also called structure factors) of liquid Ag, Cu and Ag-Cu alloys with 16.5, 28, 37, 50, 57, 71 and 85 at.% Cu at temperatures about 50 °C above the liquidus. Employing the transmission technique, I(K) has been determined in the range of K = 4 π sin θ/λ between 0.8 and 12.5 Å -1 . The partial interference functions I ij (K) have been calculated, and it was found that the assumption of concentration independence of I ij (K) yielded reduced partial distribution functions G ij (r), the weighted sum of which were in excellent agreement with G(r), the Fourier transform of F(K) =K[l(K)-1]. The position of the first peak in G(r) and the coordination number η show a positive deviation from a straight line when plotted as a function of concentration.

Isotope Effect and Pressure Coefficient of the Superconducting Transition Temperature within the Eliashberg-Theory Starting from the observation that both the mass of the ions and their volume control the phonon frequencies by stretching the phonon spectrum, the change of the Eliashberg-equations of the theory of superconductivity regarding these stretchings is considered. General expressions for the isotopic exponent and the pressure coefficient of the transition temperature are derived in which only derivatives of the transition temperature with respect to the fundamental parameters of the theory are involved. A comparison with experimental values shows that lead is not the metal with the highest transition temperature possible under the simple metals.

The straight forward application of the Ritz variational technique has been shown to be a very convenient method for obtaining numerically the first few discrete eigenvalues of the Schroedinger operator with certain special types of potentials. This method solves essentially the (finite) matrix eigenvalue problem obtained by truncating the infinite matrix representing the Schroedinger operator with respect to the Coulomb wave functions. The Ritz theorem justifies the validity of this truncation procedure.

In 1. c. 1 war gezeigt worden, daß Circobiphenyl (I) die aufgrund seines relativ kleinen S 1 -T 1 -Inter-valls von ca. 4kK zu erwartende E-Typ-verzögerte Fluoreszenz zeigt.

Efficient superradiant wavelength conversion and focus-sing in the vacuum UV region below 300 Å is described for Carbon and Nitrogen plasmas. In this scheme a plasma containing predominantly Helium-like ions is pumped with optically thick resonance radiation from Hydrogen-like ions in a neighbouring plasma of higher temperature. Such a two-plasma system could exist in the earth's atmosphere, or in the laboratory by irradiating a suitable target with a high-powered laser. The power requirements in the latter case are within present capabilities.

Backscattering of 20 MeV electrons from lead is simulated by a Monte Carlo code for varied angles of incidence. For glancing incidence, the most probable exit angle approaches the reflection angle, and the most probable energy approaches the initial energy. The underlying mechanism is derived from the calculations. Beam collimators for electron therapy may be improved by consideration of this effect.

NaNO 3 , Sr(NO 3 ) 2 , BaS, and MgC 2 O 4· 2 H 2 O powders have been gamma-irradiated at room temperature and their electron spin resonance spectra examined for paramagnetic species. The species identified were NO 3 and NO 2 radicals in the nitrates, SO 2 - and S in BaS, and H 2 C 2 O 4 - in magnesium oxalate. Theoretical predictions agree with these assignments.