Volume 47, Issue 7-8

The fluorescence decay from thin coronene films deposited on Si (100) and Si/SiO 2 substrates are measured as a function of coronene coverage at helium temperature. As the film thickness is decreased (1000 Å-10 Å) the decays are shortened and increasingly deviate from monoexponential. Excellent fits to the experimental curves are achieved by superimposing the monoexponential decay by an empirical function of the Kohlrausch type. Possible quenching processes are modeled in terms of energy transport through the layer followed by trapping in surface states and direct energy transfer from the excited molecules to the substrate

The fluorine spin-lattice relaxation time as well as NMR second moment for perdeuterated guanidinium tetrafluoroborate were studied over a wide range of temperature. An analytical solution of a set of coupled differential equations describing the time variation of nuclear magnetisations for four unlike spin systems was applied to analyse all cross-relaxation effects in the compound. Activation parameters E F a = 19.3 kJ/mole and τ F U = 9 • 10"14 s for the isotropic anion reorientation were derived. A coupling of rotational modes of cation and anion was found. Significant lowering of the melting point explained by a weakening of the hydrogen bonds involves diminishing of the ion activation energies due to the large positive isotope effect

A method of determining the angles between the emission and absorption dipole moments and the molecular symmetry axis is described. The order parameters <P 2 > and <P 4 > of nematic-dye mixtures are also calculated. The method is applied in investigations of tricarboxylic acid derivatives dissolved in the nematic mixture E 18 (Merck).

The optical absorption spectrum of Ni(II) ions doped in magnesium thallium sulphate hexahydrate has been studied at room- and liquid nitrogen-temperature. The crystal shows characteristic absorption of Ni(II) ion in the visible and near infrared region. The observed bands are assigned as transitions from the ground state 3 A 2g (F) to various excited triplet and singlet levels of the Ni(II) ion in octahedral symmetry. The splitting in one of the bands at liquid nitrogen temperature has been explained to be due to spin-orbit splitting. All the observed band positions have been fitted with the parameters B, C, Dq, and ξ

The occurrence of phase transitions in dynamical systems is intimately related to the analyticity properties of the thermodynamical functions. Such effects are believed to be of generic nature (for example, homoclinic tangency points). For one-dimensional maps, their existence has been proven for both the probabilistic and the dynamical scaling functions (that is, the / (a) and <P (A) spectra, respectively). With the help of simple model systems, we elucidate the different combinations in which the two types of phase transitions (namely, in the f(x) and <ϕ(λ) spectra) can appear. Moreover, we demonstrate the numerical evidence of phase-transition-like behavior in both spectra derived from experimental time series of laser and semiconductor systems. Finally, we discuss new aspects for the evaluation of the thermodynamic functions from time series

Amorphous Ni 50 Ta 50 -samples with their high crystallization temperature of 985 K were investigated. To evaluate the three partial structure factors of amorphous Ni 50 Ta 50 one X-ray diffraction experiment was performed with Ni 50 Ta 50 and two neutron diffraction experiments with Co 10 Ni 40 Ta 50 and with Ni 50 Ta 50 , respectively. The Bhatia-Thornton partial structure factor S CC (Q) indicates rather strong chemical short range order which also explains the premaximum observed in the Faber-Ziman partial S NiNi (Q)-function. The nearest neighbor distance is 2.82 Å for Ni-Ni, 2.91 Å for Ta-Ta, and is shortest for Ni-Ta, 2.44 Å. The coordination numbers are N NiNi = 4.9, N TaTa = 8.2, and N NiTa = 6.0. We report on the chemical short range order and the possible binding behavior in a-Ni 50 Ta 50 and compare the present results with those reported in the literature on a-Ni 40 Ti 60 as well as on a-Ni 55 Ta 45

We present the results of neutron and X-ray scattering experiments on liquid tellurium. No dependence on temperature of the radius and the coordination number of the first neighbour shell has been found in the temperature range between 460 °C and 550 °C. We investigate the influence of the truncation of the Fourier transformation on the determined coordination number of nearest neighbours and on the shape of the second coordination maximum at both, the lower and upper integration limit. The structure of liquid tellurium can be described in terms of a chain-like atomic arrangement with two bonds per atom

The structure of aqueous zinc(II) bromide solutions with [H 2 O/[ZnBr 2 ] molar ratios 5 and 10 in the supercooled state and at ambient temperature has been investigated by Raman spectroscopy and X-ray scattering. Raman spectra of glassy solutions have also been measured for comparison. The Raman and X-ray data suggest that tetrabromo and hexaaqua zinc(II) complexes are favoured in the supercooled and glassy state, while dibromo- and tribromozinc(II) complexes are formed as main species in the solutions at ambient temperature. Probably the reactions and occur in the supercooled and glassy solutions. These equilibrium shifts are discussed.

The cubic (high temperature) phase of some langbeinites, A + 2 B 2 2+ (SO 4 ) 3 with 11 different combinations of A and B type cations, doped with Mn 2+ , is investigated by EPR. Powder or single crystal spectra are measured at X-band. They indicate centers of axial symmetry in all cases. In 6 of the langbeinites two centers are found which differ considerably in intensity. Both centers are substitutional defects (Mn in the crystallographically nonequivalent divalent B ion sites). For the more intense one the zero field splitting parameter is proved to be negative in all cases. That center is assigned to the more spacious site which, depending on the cation size, allows for local relaxation. It is shown by comparison with predictions of the superposition model that relaxation appears to be effective

Proton and oxygen-17 nmr relaxation rates of CD 3 17 OH as well as deuteron nmr relaxation rates of CH 3 OD were measured in mixtures with carbon tetrachloride at different compositions and temperatures. By varying the 170-content different contributions to the proton relaxation rate could be separated and from the 17 O-H dipolar interaction contribution the correlation time τ 2 OH of the OH bond was determined. Using these correlation times the composition dependence of the deuterium and oxygen-17 quadrupole coupling constants of methanol was derived. A strong variation of the coupling constants over the measured composition range is evident

The microwave spectra of cyano-pentafluorobenzene, C 6 F 5 CN, and isocyano-pentafluorobenzene, C 6 F 5 NC , have been assigned by molecular beam microwave Fourier transform spectroscopy. The 14 N nuclear quadrupole coupling constants have been determined for both isomers

X-ray structure determination of the enamine (Z)-4-(6'-t-butoxycarbonyl-2',2'-dimethyl-5'-phenyl- 3'-hexen-3'-yl)morpholine (1) reveals that certain bulky groups may enforce a relative orientation of the molecular subunits without conjugative interaction between the nitrogen lone pair and the olefinic double bond. According to the results of quantum chemical ab initio calculations the experimentally found arrangement would be the least favourable one in the absence of such substituents. A careful analysis of the molecular structure of 1 shows that this unusual arrangement is due to the presence of both, the a- and the ß-substituent. In (E)-4-(3'-t-butoxycarbonylmethyl-1'-phenyl-1'-penten-1'-yl)morpholine (2) rather the nitrogen lone pair than the phenyl n system is in conjugation with the olefinic double bound. The results of ab initio calculations on model compounds show that conjugation of the double bond with the nitrogen lone pair is by 2 - 6 kcal/mol more favourable than conjugative interaction between the phenyl group and the C = C bond. Closer examination of the molecular structure of 2, however, led to the conclusion that it is predominantly the ß-substituent which forces the phenyl ring in a position where conjugation with the enamine double bond is not possible

Photophysical properties of 9,10-di[(l-naphthyl)-methyl]-9,10-dihydro-9,10-phenanthrenediol (1), 9,10-di(l-naphthyl)phenanthrene (2), 9,10-di(2-naphthyl) phenanthrene (3), 10,10-di(1 -naphthyI)- 9,10-dihydro-9-phenanthrone (5 a) and 10,10-di(2-naphthyl)-9,10-dihydro-9-phenanthrone (5 b) have been studied. In contrast to l,4-di(l-naphthyl)butane compound 1 shows strong (intramolecular) excimer fluorescence in dilute solution at room temperature. As a result of intramolecular triplettriplet energy transfer the phosphorescence of 2, 3, 5 a and 5 b stems from the naphthalene moieties. The increase (compared to naphthalene) of the rate constant of the radiative deactivation of the lowest triplet state is probably due in the case of 2 and 3 to the non-planarity of the molecules, which is expected to increase spin-orbit coupling, while in compounds 5 a and 5 b intramolecular chargetransfer interaction between the carbonyl group and the naphthalene moieties may be responsible for this effect. Delayed fluorescence (from the phenanthrene moiety) of 2 has been observed in fluid solution at room temperature, and an excitation mechanism is proposed

The Γ 7 ( 2 T 2t ) Γ 8 ( 4 A 2g ) luminescence spectra of mixed Cs 2 [TcCl n Br 6-n , n= 1-5, in various host crystals have been measured between 14 000 and 12 800 cm -1 at 10 K. For the first time host lattices have been doped with purely isolated chloro-bromo-technetates(IV), including the geometrical isomers with n = 2, 3, 4. The assignments of the electronic transitions and their extensive vibronic structure are discussed.

The 79,81 Br and 127 I NQR spectra of several hydrobromides, respectively hydroiodides, of amino acides and dipeptides were studied, mostly as functions of temperature in the range 77 < T/K <420. The investigated compounds are: L-Arg • HBr • H 2 O, L-Cys • HBr • H 2 O , L - Cys - S - S - L - Cys • 2HBr, ethanolamine • HBr, L-Glu • HBr, L-His • HBr, L-His • 2HBr, L-Ile HBr • H 2 O , Sar • HBr, (Sar)2 • HBr, L-Val • HBr • H 2 O , Gly • LiBr, Gly-Gly • LiBr, ethanolamine HI, Sar • HI, (Sar) 2 • HI, (Gly) 2 • HI, (L-Val) 2 • HI, Gly-L-Leu • HI • H 2 O . A phase transition with hysteresis was observed for L-Val • HBr • H 2 O (T c.up = 318 K, T c.down = 242 K). Two solid phases of Sar • HI have been studied by NQR, one crystallized from melt, the other one from aqueous solution. For three of the title compounds the crystal structure was determined at room temperature: L-His - 2HBr, P2 1 2 1 2 1 , Z = 4, aj pm = 1652, b/pm = 916, c/pm = 721; L-Cys HBr H 2 O , P2 1 2 1 2 1 , Z = 4, a/pm = 1955, b/pm = 746, c/pm = 550; Gly-L-Leu • HI • H 2 O , P2X, Z = 2, a / p m = 1289, b/pm = 914, c/pm = 615, ß/° = 99. In most cases the halogen ion in the studied hydrohalides is polycoordinated by hydrogen bonds of the type N - H • • • X ⊖ and O - H • • • X ⊖ , X = Br, I. The NQR frequencies and, for iodine, the nuclear quadrupole coupling constants depend on this coordination. A low frequency (coupling constant) region is found for pure N - H • • • X ⊖ coordination. Replacing one N - H • • • X ⊖ bond by O - H • • • X ⊖ rises the electric field gradient, EFG, respectively the resonance frequencies. The dependence of the EFG on the hydrogen bond coordination N - H • • • X ⊖ plus O - H • • • X ⊖ is discussed for the title compounds including information from literature

This supplement to Z. Naturforsch. 46 a, 1026 (1991) provides additional material that should be combined with the contents of that paper. The combined content then provides a model of gravitation which is complete in essential details. Its main features are that it (i) predicts all the local GR tests accurately but (ii) does not exhibit Schwarzschild singularity, (iii) allows the wellknown Birkhoff's theorem to hold and (iv) has well defined conservation laws in virtue of the maximal group of motions admissible on the flat background spacetime

Reflection of planewaves at the planar interface of two natural optically active media has been examined in order to obtain the Brewster condition