Volume 37, Issue 9

The Kratzer-Fues-Varshni-V-potential, applied to ionic dissociation energies, is shown to yield rather accurate potential energy curves in the bonding region for H 2 , HF, LiH, Li 2 and LiF. Vibrational levels, calculated by this ionic approximation to the ground state of widely differing molecules, nearly coincide with RKR-data. At the repulsive side of the curve and up to 2re, the agreement with RKR-curves is even better than for Morse's curve, also for the "covalent" molecules H 2 and Li 2 . Calculated spectroscopical constants α e and ω e χ e are far better than those calculated with Morse's function. Even the existence of a maximum in the potential curve at larger r-values seems not in confict with an ionic approximation. From the universal character of the function used, it is concluded that a reasonable approximation for the ground state of all molecules considered is one in terms of ionic structures, even for H 2 and especially for Li 2 . According to the present results, the term “covalent bonding” seems to be definitely superfluous, as the usually made distinction between ionic and covalent bonding is more appearant than real.

Irradiation of benzene with 15 ns flashes of 265 nm light caused ionization as evidenced by optical absorption measurements. In the case of dilute methanol solutions solvated electrons were formed. The yield depends on the intensity of the incident light I as ϕ (e s̄olv ) ∝ I 1.8 . In the case of cyclohexane solution dimer radical cations were detected by their absorption between 600 and 1100 nm. O 2 enhances the yield of dimer cations because it scavenges electrons, thus impeding ion recombination.

Various guiding-center drift theories are presented that are optimized in respect of consistency. They satisfy exact energy conservation theorems (in time-independent fields), Liouville’s theorems and appropriate power balance equations. A theoretical framework is given that allows direct and exact derivation of associated drift-kinetic equations from the respective guiding-center drift-orbit theories. These drift-kinetic equations are listed. Northrop's non-optimized theory is dis-cussed for reference, and internal consistency relations of G.C. drift theories are presented.

The dielectric properties of water-in-oil (W/O) suspensions have been studied by means of the depolarization thermocurrent (DTC) method in the temperature range of 85-250 K. Two predominant peaks have been observed at about 140 and 225 K. Evidence has been obtained that the peak at 140 K and the dielectric absorption observed by many investigators at sub-zero temperatures in the kHz frequency range are due to the same relaxation mechanism.

In EPR studies on Fe(III) ions in CdGa 2 S 4 crystals, in accordance with the two different Ga sites two paramagnetic centers could be detected which differ widely in their axial parameter D. An investigation of the crystal field allowed a unique site assignment. A comparison with the Fe(III) resonances in CuGaS 2 and AgGaS 2 does show that the superposition model of Newman and Urban cannot be applied to CdGa 2 S 4 with similar parameters as in the chalcopyrite structures. - A characteristic twinning could be revealed, similar to the “reversed cubic structure” in sphalerite.

The time-resolved sensitized fluorescence of tetracene-doped anthracene crystals following a two-photon picosecond-pulse excitation was investigated experimentally as a function of dopant concentration and temperature. Even at the optimum time resolution achieved in these experiments (about 5ps) the time dependence of the host fluorescence was always purely exponential, that of the guest fluorescence a superposition of two exponentials. Thus no time dependence of the energy-transfer rate could be identified. The temperature dependence of the energy-transfer rate is explained tentatively in terms of a "hopping model" (hopping time t h ≈ 5.8·10 -14 s) and thermal reactivation from shallow exciton traps (trap depth 43 cm -1 ) in the vicinity of the guest molecules. In pure anthracene crystals the intrinsic exciton lifetime (observed at 20 K) is (5.0 ± 0.5) ns. At higher temperatures it increases to about 15.5 ns at 300 K due to reabsorption processes. Below 20 K it decreases to about 1.5 ns at 1.6 K because of exciton trapping at shallow X-traps (trap depth 23 cm -1). When using conventional one-photon excitation exciton-exciton annihilation was observed with an annihilation constant γ ss ≈ 1.0 × 10 -8 cm 3 s -1 .

Absorption and emission spectra of methine dyes in polar and nonpolar solvents were measured. From the Stokes shift as a function of solvent polarity the dipole moments of excited singlet states have been estimated and compared with the quantum chemical predictions. Also the π-electronic charge distribution in the ground and first excited singlet electronic states and bond orders of the dyes were calculated. All investigated methines showed an increase in dipole moment upon excitation to the first excited emitting state.

The emission anisotropics of the fluorescence of the prolate molecules 2[4(N,N-dimethyloamino)-α styrylo]benzothiazol and 2[4(N,N-dimethyloamino)α styrylojbenzoksazol in solvents of different viscosities were measured, yielding information about the rotational motion of the molecules. The non-linear dependence of 1/r on T/η was confirmed. On the basis of a comparison of experimental and quantum chemical calculations the orientation of the transition moments of the dye molecules is proposed.

91 Zr-and 63 Cu-NMR measurements of the amorphous alloys Cu x Zr 100-x (40≦x≦60) are reported. By comparison of the experimental quadrupole disturbed spectra with simulated spectra of the amorphous alloys a short range order can be deduced, which is similar to the local lattice symmetry of the corresponding crystalline compounds. Furthermore the Knight shift of both nuclei in the alloys could be determined.

The barrier hindering internal rotation of the methyl group was determined by analysing the splittings of rotational lines in the ground state. So model errors are minimized. The assignment was checked by double resonances and a centrifugal distortion analysis.

A molecular dynamics simulation of a 1.1 molal aqueous MgCl 2 solution has been performed employing the central force model for water. The effective pair potentials for ion-water have been derived from ab initio calculations. The basic box with a sidelength of 18.30 Å contained 200 water molecules, 8 anions and 4 cations, corresponding to an experimental density of 1.079 g/cm 3 . The simulation extended over about 3.3 picoseconds at an average temperature of 309 K. The structure of the solution is described by radial distribution functions and the orientation of the water molecules in respect to physically meaningful directions. Values for the dielectric constant and hydration energies have been calculated. The strong influences of the twofold charged magnesium ion on the geometry of the water molecules and the structure of the hydration shell is discussed in comparison with the results of a previous simulation of a 2.2 molal NaCl solution.

The results of a molecular dynamics simulation of a 1.1 molal aqueous MgCl 2 solution are com-pared with newly performed x-ray measurements. The structural properties of the solution are evaluated from the scattering data by a model fit to the experimental structure function. The comparison on the basis of fit parameters and partial structure functions shows an overall good agreement between experiment and simulation. Detailed information on the structure of the hydration shells is deduced from the simulation and shown in form of density maps and angular distributions. It is demonstrated that the octahedral arrangement of the water molecules in the first hydration shell of Mg ++ is strongly pronounced while it is only indicated in the case of Cl - . A preferential arrangement in tetrahedral directions has been found for the nearest neighbor water molecules around a central water molecule with an asymmetry in respect to lone pair and hydrogen atom directions. In addition, the probability of finding water molecules at a given number of symmetry sites at the same time has been calculated.

The He (I) photoelectron spectrum of gaseous S 4 N 2 is reported, and an assignment is made on the basis of the correlation with ab initio molecular orbital calculations. The latter, based upon the recently optimised geometry of C ѕ symmetry, used both a large basis set of greater than double zeta quality, together with configuration for both the ground state and the ionised states. The Koopmans’ Theorem order gives a reasonable prediction of the order of states for most of the low energy ionised states; beyond IP 5 additional states involving excitation processes in conjunction with ionisation were computed. The numerical agreement between the observed IP’s and the CI value is reasonable.

The salt families formed with silver halides and lithium or sodium halides were studied in order to investigate demixing in the liquid state. In particular, measurements were carried out on the stable diagonals of the 12 pertinent reciprocal ternaries. Liquid-liquid equilibria were found to occur in the 10 mixtures containing LiF, NaF, LiCl, LiBr and NaCl (the last one only in the presence of Agl). The miscibility gap, however, could be fully measured only on the four mixtures AgBr+LiCl, Agl + LiCl, AgI+LiBr and Agl+NaCl: for these and for the mixture AgI + NaBr an “a priori” prediction of the gap was possible on the basis of current thermodynamic theories.

Demixing was investigated in the molten salt families formed with thallium (I) halides and lithium or sodium halides. Measurements were carried out on the twelve stable diagonals of the corresponding reciprocal ternary mixtures. Liquid-liquid equilibria were found in the three systems TlCl+LiF, TlBr+LiF and TlI+LiCl while in the two systems formed with Til and LiF or NaF measurements could not be carried out owing to the thermal instability of Til. The reciprocal ternary Li, Tl/Cl, I exhibits demixing in a temperature range which is convenient for investigation: therefore the whole composition square was studied. The upper critical solution point of the gap lies along the stable diagonal at 734°C and x LiCl = 0.58. The projection of the stratification lens occupies 38.8% of the composition square. Moreover an “a priori” prediction of the liquid-liquid equilibria was carried out by means of the conformal ionic solutiontheory.

The semireduced transients of proflavin were studied in airfree acid aqueous solutions by a combined pulse radiolysis-computer optimization procedure. The main product, semiquinone (C-radical; 75%), formed with k=6.4 × 10 9 dm 3 mol -1 s -1 , decays by dimerisation (2k=2 × 10 9 dm 3 mol -1 s -1 ) and dismutation (2k=0.5 x 10 9 dm 3 mol -1 s -1 ). It possesses several absorption bands (ε 270 = 1900, ε 360 = 1530, ε 460 = 2250 and ε 530 = 200 m 2 mol -1 ). The second transient, R·-species (N-radical; 16.5%), is produced with k=1.4 × 10 9 dm 3 mol -1 s -1 and decays preferentially by transformation into semiquinone with k = 2 × 10 5 s -1 ; its absorption bands are: ε 295 = 2300, ε 380 = 1000, ε 440 = 5000 and ε 680 = 530m 2 mol -1 . Finally H-adducts on various positions of the aromatic ring (8.5%) are arising with k = 0.7 × 10 9 dm 3 mol -1 s -1 , decay according to a pseudo first order reaction, k = 1.1 × 10 4 s -1 , and have absorption bands at ε 270 = 2100 and ε 435 = 1580m 2 mol -1 . The obtained results are of particular interest for the photochemical and photoelectrochemical solar energy utilization.

The molar magnetic susceptibilities (χ м ) of spherical balls of rare-earth iron garnets REIG's, with RE = Y, Gd, Dy, Ho, Er and Yb, prepared from pellets pressed at pressures ranging from 2 to 8 × 107 kg m -2 , have been measured in the temperature range 300-900 K. It is found that χ м α exp( - f), where f is the pore fraction of the pressed balls. It has also been observed that χ м for a particular garnet at a fixed temperature increases from powder through pressed material to single crystal. REIG's reveal typical ferrimagnetic behaviour with the ferrimagnetic Curie temperature (T c ) lying in the range 550-570 K for single crystals. In general, for pressed material T c increases linearly with density. The χ м -1 vs, T variation has been analysed using molecular field theory. It has been found that for T ≫ T c the magnetic ions behave as if they were almost free and yield a magneton number very close to their free ion value.

The mechanoluminescence of coumarin crystals is studied. It is found that mechanoluminescence decreases sharply with temperature near the melting point and that it may be a useful probe for studying the behaviour of crystals near the melting points.

The electron paramagnetic resonance of Cu 2+ in ferroelectric LiH 3 (SeO 3 ) 2 has been studied at 298 K in X-band. The Cu 2+ appears to substitute Li + in the lattice with the z-axis nearly along the Li-O(6) bond direction. The spectra have been analysed using the spin-Hamiltonian appropriate for Cu 2+ in orthorhombic symmetry.

The effect of molecular oxygen on the broadening and shift of the 535.0 nm Tl line resulting from the photodissociation of thallium iodide vapour was studied at low densities of O 2 . The broadening and shift coefficients were determined.

Matrix elements for interelectronic repulsion, including differential orbital expansion, have been calculated for d3 complexes with tetragonal symmetry. The expansion parameter e influences the overall positions of the five components of the 2 E g and 2 T 1g states and the separation between 2 E g and 2 T 1g , but is relatively unimportant with respect to the splittings within those states.

At a certain temperature T n (nematic temperature) a mixture of two cholesteric components which separately form helices of opposite senses behaves like a nematic. An inversion of the helical structure occurs when passing T n , from a right-handed to a left-handed structure or vice versa. In the temperature-pressure space such compensated mixtures therefore will exhibit an inversion line which - on the basis of optical rotatory power measurements under high pressure - is presented for the first time.