Volume 34, Issue 7

The possible IR- and Raman-active phonons of α-boron. β-boron, and boron carbide have been obtained by group theoretical methods. Reflectivity and absorption measurements in the spectral range from 10 to 1450 cm -1 and a classical dispersion analysis yield the entire spectra of the anisotropic optical constants in the lattice vibration range of β-boron and beyond. From the reflectivity spectrum of boron carbide the resonance frequencies of the phonons and estimated values of their other characteristic values have been derived. The absorption spectrum of amor­phous boron has been included in the discussion. - Certain frequency ranges can be attributed to vibrations of the icosahedra split in the crystal field. Single vibrations with E∥c displaced to higher frequencies are attributted to the central atoms in the unit cells of β-boron and boron carbide. The phonon shift depending on temperature can be described by the Grüneisen shift. Three- and four-phonon processes are necessary to describe the temperature dependent damping constants of the lattice vibrations.

The temperature dependence of absorption, emission and fluorescence-decay in EBBA has been investigated. The total absorption power μ=μ ∥ +2·μ ⊥ was found to be constant over the whole investigated temperature range. For perylene a decrease of the quantum-efficiency with growing temperature was observed, but not for 4-dimethyl-amino-4′-nitrostilbene (DMANS). In the binary system an energy transfer via dipol-dipol interaction was observed. In the ordered phase energy-transfer distances about 30 Å and less were computed.

A comparative survey is made of the neutron and light scattering techniques as applied to structural investigations in the vicinity of critical points in fluids. For neutron scattering as applied to fluctuations in binary mixtures, some criteria for a good contrast in order to optimize the scattering parameters such as the isotopic composition or the concentration are considered. This leads us to show how to write the partial structure factors in binary mixtures in order to remove the coupling between concentration and density fluctuations artificially introduced bv Bhatia and Thornton when the components are of different size. Finally, the amplitudes of the concentration fluctuations near a critical point are discussed.

The activities of aluminium in Al-Ga, Al-Ge and Al-Ga-Ge systems were measured between 933 K and 1273 K using an e.m.f. method with a molten salt electrolyte. The results concerning Al-Ga and Al-Ge systems are in good agreement with those previously obtained. The E(T) curves of the 36 ternary alloys studied give also some liquid-solid equilibrium tem­peratures and allow isothermal curves of the ternary liquidus surface to be drawn.

The simple shear flow of a homeotropic nematic layer in an electric field normal to the layer is studied theoretically and experimentally. Several material constants of MBBA are determined as a function of temperature.

The electrical resistivities of liquid selenium and seleniumrich Na-Se alloys have been measured. The results of dilute alloys are interpreted in terms of percolation theory. A previously determined phasediagram was found to be incomplete.

Solid-liquid and liquid-liquid equilibria in the reciprocal ternary system K, Li/Br, F were fully measured. The projection of the miscibility gap occupies 20.8% of the composition square; the upper critical point of the gap is along the stable diagonal at 953°C and x LIF = 0.70. Attempts to predict the liquid-liquid equilibria by means of the conformal ionic solution theory using temperature independent interaction parameters lead to an incorrect symmetry of the gap. The symmetry can be improved if the temperature dependence of the interaction parameters of the binary mixtures is taken into account.

A molecular dynamics simulation of an aqueous ammonium chloride solution has been carried out at a mass density of 1.027 g/cm 3 , corresponding to a 2.2 m solution, and at a temperature of T = 301 K. The effective pair potentials are based on the ST2 model of water, a rigid tetra­hedral four point charge model of NH 4 + and a single point charge model of Cl - . The calculated structural properties of this solution are discussed and compared with those obtained for alkali halide solutions. The first hydration shell of the ammonium ion contains on the average eight water molecules, it has a more complex structure than the hydration shells of cations with spherical symmetry. The static orientational order of water dipole moments indicate a structure breaking ability of NH 4 + .

We report on the measurement of T 2 for several rotational transitions of CH 3 C 15 N. The transient emission signal is investigated at different temperatures for the pure gas and mixtures with He and H 2 , and effective collision diameters are derived from the pressure dependence of T 2 .

A quantitative assessment of the barrier of rotation in singlet trimethylene is made by an energy partitioning analysis within the MINDO/2 approximation, including 3X3 configuration interaction between HOMO and LUMO. The dominating factors originate from (a) the non­bonding repulsion between 1,3-positioned atoms and (b) the weak attraction between the 1,5- positioned cis-hydrogens in the EE conformation. The energy difference between the EE and EF conformation is lowered by radical stabilizing substituents, such as a CH 3 group (attached to the carbon atom bearing one unpaired electron), because one of the unpaired electrons is stabilized in the EF conformation.

Phase transitions in (NH 4 , M)Cl with M=Tl, Rb or Cs and (ND 4 , Tl)Cl as well as (NH 4 , M)Br and (ND 4 , M)Br are measured using birefringence or differential thermal analysis. The phase diagrams of the mixed systems with M=Tl having decreasing lattice constant with increasing amount of Tl are similar to the well known T(P) diagrams. Therefore the diagrams of the sys­tems with M=Rb or Cs having increasing lattice constants correspond to T(P) diagrams in the range of fictitious negative pressure P. Substitution of D for H produces an effect similar to reducing the lattice constant. The extraordinary large negative isotope effect of II-III transitions and the appearance of the birefringent phase III in the chlorides are now understood.

A complete vibrational analysis is performed for a heptamolybdate, Mo 7 O 24 , niodel of C 2V symmetry. The symmetry coordinates are classified into those of (a) framework vibrations, (b) ligand vibrations, (c) framework-ligand couplings, and (d) interligand vibrations. A simple valence force field is estimated, and a relative scale for the force constants of the angles is pre­sented. Vibrational frequencies are calculated and the influence of inclusion of redundancies is investigated. The final result is compared with experimental data from infrared and Raman. In the calculation of species A 1 a correlation is found to all except one of the polarized Raman frequencies.

A new force field is developed for the in-plane vibrations of condensed aromatics using pyrene as a test molecule. Only five parameters are employed in this force field, which improves a four- parameter approximation previously investigated. Still the simple five-parameter approximation gives calculated vibrational frequencies in good agreement with those from the more elaborate Califano-Neto force field. In the course of the normal coordinate analysis the Hüekel molecular orbitals have been de­duced. The CC stretching parameters were modified by means of the calculated bond orders. A complete set of independent symmetry coordinates (both in-plane and out-of-plane) for the pyrene molecular model was developed. New experimental data from infrared and Raman spectra for pyrene are reported. Also the mean amplitudes of vibration are studied in detail. It is concluded that the simple five-parameter approximation gives reliable mean amplitudes, which undoubtedly are accurate enough for the interpretation of gas electron diffraction data. Finally the calculated frequencies and mean amplitudes of benzene are given. The mean amplitudes are again found to be very accurate in spite of some substantial inaccuracies in the frequencies.

Mean amplitudes of vibration from spectroscopic computations for coronene are presented for the first time. Good agreement is found for the results from (a) the simple five-parameter in-plane force field approximation and (b) the more elaborate Califano-Xeto force field. Mean amplitudes from the former method (a) for naphthalene and anthracene are also given.

This paper reports our results on the magnetic suscep­tibility (Χ m ) of light rare-earth (viz. Ce, Pr, Nd, Sm, Eu and Gd) tungstates in the temperature range of 300 to 900 K. As expected the variation of Χ m with temperature obeys the Curie-Weiss law Χ m = C/T - θ). The values of the Curie-constant (C), paramagnetic Curie temperature (θ) and magneton number (p) for the magnetic ions of these tungstates have been evaluated. The experimental p values are in good agreement with the theoretical values of p for these ions. Thus it is concluded that the magnetic (4f) electrons remain intact with the respective ions at high temperature and their participation in electrical conduction is improbable. Negative values of θ normally indicate anti- ferromagnetic ordering in these compounds at low tem­peratures; however in these tungstates the crystal field seems to be the prime reason for Curie-Weiss law behaviour and negative values of θ. The magnetic behaviour of light rare-earth tungstates in briefly discussed.

The Lorentz invariance of the fine-grained one-particle distribution function of a gas, as well as that of multiple­time many-particle distribution functions, is shown to be a consequence of the very definition of a physical event, so that it requires no formal proof. Since it is independent of any special property of Lorentz transformations, this invariance holds quite generally for arbitrary transforma­tions of space-time coordinates.

Vibro-rotational excitation processes, in particle-diatom scattering, are considered from a field-theoretical point of view. It is shown that a relaxation of the relationship directly connecting boundary conditions and reference state, of the Green's functions of the theory’, can lead to a formulation of ab initio “scaling laws”, for both elastic and inelastic scattering.

The hypersonic velocity of molten sodium and potassium chlorides has been measured over a temperature range of about 100°C above the melting points. The measurements were carried out at scattering angles 90° and 140°, cor­responding to a 6-10 GHz frequency range. No deviation from previous ultrasonic values of velocity was observed at these frequencies, indicating that the measurements were performed at frequencies lower than any relaxation fre­quency.

Binding energies are calculated using the orbital energies within the Kohn-Sham-Gaspar statistical exchange approx­imation for the Xe and Ar isoelectronic series, respectively. The results are generally in good agreement with the available Hartree-Fock orbital energies.

Recently we published a construction of a micro­wave-microwave double resonance spectrometer [1], which allows a variety of experiments. An essential part of it is a “stripline” absorption cell. Although Stark-Spectroscopy [2] gave a very important impetus to the investigation of rotation spectra and is in use for three decades, measure­ments below 6.5 GHz are few in number compared to the frequency region above 6.5 GHz. The reason is that generally the line intensity decreases at lower frequencies and the cross sections of the standard waveguides are larger. If one does not use special waveguides * a higher Stark voltage has to be applied to produce the necessary Stark field for modulation. This requires a rather powerful Stark square wave generator** to load and unload the capacity of the absorption cell. Measurements in the low frequency region may be useful since at these lower frequencies the density of rotational spectra is sometimes considerably lower than at higher frequencies, which faciliates the assignement. We noticed that the stripline cell, which transmits from DC to 18 GHz in a coaxial type mode, may be used for Stark-Spektroscopy by feeding the Stark voltage and the microwave simultaneously to the septum. After having finished our experiments, we noticed that S. Weisbaum [4] et al. measured the transitions 3 2 - 3 a and 5 2 - 5 3 of HDO at 486.50 and 824.64 MHz, respectively, by a similar tech­nique. The radiofrequency was connected together with the Stark voltage to the septum of a conven­tional Stark cell. In Fig. 1 we give the details of the set up. To combine the microwave and the square wave we use “monitor tees” *. The square wave is fed into the monitor port which transmits a 100 kHz square wave. For example, a residual square wave of 140 V is observed at the microwave port when applying a 400 V square wave. To prevent any influence of the square wave on the microwave sourcje we use a piece of waveguide as a high pass filter. At the output of the absorption cell another piece af waveguide acting as a high pass filter protects the detector cristal. The cell we used for the measurements presented in Table 1 and Fig. 2 is made of an X-band waveguide R100 (cut off for the TE 10 -mode 6.56GHz). The construction is similar to that given in Fig. 1 of the previous paper [1], but TNC connectors** are used. The cell has been tested for a voltage of 2500 V between septum and the waveguide walls. The attenuation of the microwave on the stripline between 0.4 and 2.4 GHz is about 1.5 dB.

The commercially available cement Picein 80 has been tested and successfully used for the determination of g- factors and spin concentrations at X- and Q-band frequencies. The main advantages compared to other standards are a very small space requirement and the fact that the concentra­tion of the standard can be easily adjusted to the signal intensities of the sample. This standard exhibits a single line at a g-factor of 2.0041 with a halfwidth of .47 mT and a spin concentration of 2.10 15 spins/mg Picein. An additional weaker resonance occurs at g = 2.0177, and at Q-band a partly overlapping multiplet with 5.8 mT separation is observable, but the intensities of both signals are low com­pared to that of the main signal. This standard is applicable over the whole range of useful spin concentrations, and the linewidth makes it particularly useful for measurements on transition metal ions in diluted single crystals.

Compounds like III(4,9,10,11,12,13,13-Heptachloro-8b,9,12,12a-tetrahydro-9,12-methano-benzo[e]pyrene) are ex­amples for systems with heavy atom substituents in two different bonding situations. The halogen directly connected with the 7-electronic system causes an internal heavy atom effect whilst the halogens isolated from the .7-system by aliphatic carbons produce a “quasi external” heavy atom effect. The observation that the internal heavy atom effect in III is weaker than in halo phenanthrenes is explained with the lower electron density in the phenanthrene unit of III compared to phenanthrene. this lower electron density being a result of field and inductive effects of the peripherically arranged chlorine atoms.

CXDO/2 MO calculations show the elektron withdra­wing ability of the CF 3 group versus the electron releasing of the CH 3 group to be the primary source of the consi­derable S-C bond lengthening in CF 3 SO 2 Cl as compared with CH 3 SO 2 Cl.