Volume 31, Issue 8

Well-developed phonon-structures are observed on Ta/Ta-Oxid/Ag and Nb/Nb-Oxid/Ag tunneling junctions, if the conditions of oxidation are suitably choosen. These structures are analyzed ac-cording to McMillan's numerical program and the results are discussed. From the nonlinear current-voltage characteristics in the normal state, informations are obtained on the electronic structure of the semiconducting oxid presenting the barrier. Impurity concentration leading to a nonideal semiconductor gap in the oxid, seems to be largely reduced if oxidation is performed with low oxygen pressure.

Instead of emphasizing the ground state as is done in Green's function method, we take a finite-dimensional subspace of the Hilbert space: the space of the "intermediate states". A systematic introduction of intermediate states is effected by an extension of the method of generating functionals: we combine the generating functionals of the n-point Green's functions to a "matrix functional" T, and form new matrix functionals, which are matrix functions of T. The aim of this paper is to develop the functional calculus in such a way that the transition from scalar functionals to matrix functionals is straightforward, and the method of obtaining further results becomes clear. Following the lines of Dürr and Wagner we get u η-and ζ-rules with intermediate states". Using them we define a truncation procedure for the equations of motion of the n-point Green's functions, the "New Tamm-Dancoff method with intermediate states". This extension makes it possible to treat the effect of nearby levels in many body systems with Green's functions. In ad-dition to well-known approximations, such as the Hartree-Fock and the Hartree-Bogoliubov theory, the RPA and the quasiparticle RPA, we obtain a series of new approximations. Among these are the "Hartree-Fock theory with intermediate states" and the "random-phase approximation with intermediate states", which we already applied with great success to some exactly soluble models.

A method is proposed, which primarily determines screening constants for incomplete atomic shells from the experimental ionization potentials of spin-paired Pauli-type orbitals. The method is complimentary and in addition to the standard Sommerfeld approach, which derives screening constants from X-ray spin doublet term differences. Atomic photoelectric absorption cross sections are computed from screened hydrogen-like eigenfunctions for low atomic number elements Z = 6 to 15, using a range of incident photon energies between 1 and 30 keV. Comparison with experiment and alternate more exact theories shows, that improved cross sections are obtained for the more important incomplete L-shell contributions. X-ray photoelectric mass absorption coefficients for low Z elements (Z= 6 to 15), employing 33 characteristic X-ray energies {NiKα (0.852 keV) to AgKβ (24.942 keV)}, are given.

For any given potential, linear response theory tells us the size that the von Weizsäcker coefficient must assume in order to obtain the correct energy in the weak coupling limit. With this necessarily imposed choice, the quality of results is investigated as the coupling is strengthened. From the study of a number of model problems (the harmonic oscillator, the Yukawa well and the Mathieu potential) it is concluded that good results can be maintained to high couplings.

The solution of the T-t fundamental combinatorial problem is given in a purely analytic form as a sum over binomials. This simplifies other calculations considerably. For an illustration the moments of the optical absorption line shape in the strong coupling limit are derived in a closed form. Furtheron, the optical zero phonon line is calculated. An application to a T - t phase transition chain is indicated.

A study of the spin-pair Hamiltonian with regard to the effective g-factor in the strong isotropic exchange limit is presented. The case of two different spins with anisotropic g (i) and D (i) tensors is considered. Effective g s and D s tensors for the various total spin S values are deduced in terms of the corresponding single-ion parameters. The characteristics of the resultant g s -tensor, in the case of an ion-pair with C σ symmetry, are treated in detail. The principal values are linear expressions of the single-ion principal values and the principal directions relative to different 5 values result to be correlated in a characteristic manner. The limits of the present strong exchange treatment are also discussed.

For the transport of electrons in a non-thermal, weakly ionized plasma an analytical model and a detailed numerical calculation is presented. The following transport coefficients: mean energy, characteristic energy, mobility, diffusion coefficient, excitation and ionization coefficient for He, Ne, Ar, Kr, Xe are calculated in the region of reduced electric field 0.01 V/cm Torr ≦ Elp ≦ 100 V/cm Torr. The parameters of the model are developed using the existing experimental and our numerical data. With increasing E/p the dominant energy loss of the electrons changes from elastic to inelastic collisions and here from constant to increasing effective excitation energy. Three model regions of E/p result, in which the transport coefficients can be described as a power of E/p. The data presented here provide a starting point for further investigations of similarity rules and models of non-thermal plasmas.

The propagation and absorption of waves in the vicinity of the lower hybrid resonance are investigated under linear conditions in a cesium plasma generated by a Q-machine. The results of the measurements are compared with linear theories based on the fluid model. It is shown that in the case of high ion mass plasma only a theory including a finite k || -spectrum yields an appropriate description of wave absorption near the lower hybrid resonance.

The structure and the plasma parameters of a rotating plasma showing Alfvén's critical velocity were investigated for the first time by means of Langmuir probes. It was demonstrated that the occurrence of the critical velocity is causally connected with the formation of a spoke structure. The electron temperature and the plasma density were determined with local resolution by which a detailed balance concerning the plasma density and the current transport became possible. The importance of turbulent electron heating for rapid ionization at the critical velocity is emphasized. In this process the modified two-stream-instability obviously plays an outstanding role.

Calculations for constrained (one-dimensional) diffusion of current carriers in an external field are made, which concern the spectral density of fluctuations resulting from the relaxation of spatial concentration perturbations. Two different model calculations are presented: First diffusion in an infinite medium. In this case the spectral density of concentration fluctuations shows a 1/f behaviour, but the spectral density of current fluctuations does not. Second the diffusion in a semi-infinite medium is treated. The flux through the boundary (e. g. pore end) is assumed to be pro-portional to the concentration of current carriers at this boundary. In this case concentration as well as current exhibit 1/f behaviour. The results indicate that the existence of flux controlling boundaries (contacts) may be decisive for the occurence of 1/f noise.

The behaviour of energetic heavy ions in thin films is simulated by means of a Monte Carlo method taking especially into account the energy loss of the scattered particles. Based on a variably matched screened Coulomb potential the calculations yield the angular and energy distributions of the transmitted ions. The results show a good agreement with the theory of Meyer. Furthermore for keV He ions scattering distributions have been measured in C, Al and Ag, which agree fairly well with the computations, if a suitable fit of the screening parameter is used in the Monte Carlo simulations.

A modified method for measuring absolute values of the refractive index and its temperature dependence is presented. A direct comparison is made between the refractive index of a liquid sample and ambient air. Values for LiNO 3 , RbNO 3 , and CsNO 3 within a temperature interval of about 80 K above the melting point for several different wavelengths are presented as well as the calculated values of the molar refractivity and the absorption lines of the NO 3 - ion.

The structure of a υ component liquid or amorphous alloy is analyzed in terms of υ(υ+1)/2 independent partial structure factors which are associated to the different correlations between the numbers of particles of all chemical species. This formalism, which is specially adapted to the study of alloys of more than two components, is related to Fournet-Faber-Ziman and extended Bhatia-Thornton theory. Sum rules, inequality relations and limiting values are calculated in the static approximation and Debye pair expressions of the structure factors are derived in the isotropic approximation.

A single component hard sphere description molten halides and nitrates, applied to self-diffusion and viscosity data, has led to a hard sphere size which for a given cation depends linearly upon the anionic radius. This correlation has been used to predict viscosity coefficients of fused alkali fluorides and to estimate the effective size of the NO 3 - -anion in molten nitrates. The Stokes-Einstein relationship between η and D̅ is satisfied, where D̅= (D + +D - )/2, and this relationship in combination with the observed linear dependence of T½/η] and D̅/T½ on the molar volume can in principle be used as a criterion for data selection.

DTA and powder x-ray diffraction measurements allowed to draw the phase diagrams of the systems AgI-Ag 2 XO 4 (X = Cr, Mo, W). A more detailed investigation was devoted to the composition range around X AgI = 0.8, where in preliminary tests it was observed that the room temperature ionic conductivity strongly depends on the thermal treatment of the sample. By quenching mixtures at X AgI = 0.8, glass-like phases with high conductivity were obtained; in the systems AgI-Ag 2 CrO 4 and AgI-Ag 2 MO 4 the subsequent crystallization gave rise to non con-ducting phases, whereas the occurrence of the highly conducting compound 4AgI·Ag 2 WO 4 was recognized in the third system investigated.

Fourier Transform NMR measurements of 17 O and 33 S are reported for several aqueous sulfate, thiosulfate, molybdate and thinftnolybdate solutions. Using informations from 17 O signals, 33 S NMR lines are assigned. Experimental parameters, chemical shifts and linewidths are given.

The efficiency of hole injection from electrolytic contacts into organic insulator crystals is con-trolled by hole generation and hole destruction at the crystal surface through oxidized and reduced redox ions respectively and by hole escape into the crystal bulk which competes with hole destruction. By increasing the concentration of reduced redox ions from virtually zero in a suitable system the transition has been studied from the one extreme case where all the generated holes contribute to the injection current to the other extreme case where the majority of the generated holes recombines with reduced redox ions before they can escape into the crystal bulk. From the experiments one can deduce that the generated hole spends less than 10 -10 s within the reaction distance of its reduced parent redox ion, at external field strengths ≧ 10 4 V/cm, but returns to the crystal surface during a much longer time, e.g. during about 10 -8 s at an external field strength of 10 5 V/cm, before it can escape finally into the crystal bulk.

From the fluorescence and phosphorescence spectra of several aromatic amines it is concluded that the conjugation between nitrogen and aryl groups in the excited state is restricted. With this result a sterical explanation for the lack of the expected photochemical carbazole ring closure of 1-and 2-anilino naphthalene is given.

The effect of deuteration on all six photophysical and kinetic constants of pyrene excimer formation is investigated. The rate constants of monomer and excimer emission and the rate of dissociation of the excimer are not affected by deuteration. The increase of the radiationless deactivation of the monomer (anomalous effect) agrees with the theoretical expectation. The radiationless deactivation of the excimer at various temperatures shows no effect and therefore does not take place via the excimer triplet. The decrease of the excimer formation rate can be explained by the effect of deuteration on the partition function and hence on the entropy.

The molecular electric dipole moment of each of the two planar thiol rotamers of monothio-formic acid, in its ground state, has been determined from Stark effect measurements. The results are for cis-HC(:O)SH: μ a =1.805(1) D, μ b = 2.228(1) D and μ=2.868(1) D; whereas for trans-HC(:O)SH: μ a = 1.366(1) D, μ b = 0.702(1) D and μ = 1.536(1) D. Additional Stark effect measurements on HC(:O)SD and DC(:O)SH have yielded the direction of the dipole moment in each rotamer. The energy difference between the ground vibrational states of cis-and trans-HC(:O)SH has also been determined by relative intensity measurements. The cis rotamer is found to lie at higher energy than the trans rotamerb y 2767(7) J·mol -1 (231.3(6) cm -1 or 661.3(17) cal·mol -1 ).

Deuterated benzene mixed crystals are irradiated by intensity modulated i.r. at low temperatures. The i.r. causes a modulation of crystal temperature which in turn causes a modulation of fluorescence intensity of the crystal. Phase sensitive detection of fluorescence allows the assignment of bands to different trap emissions due to their different temperature dependences. A simple kinetic model gives qualitative agreement with experiments.

The effect of inertial terms in the Navier-Stokes equation of viscous drainage on a flat plate is investigated. It is found that they give rise to a constant correction term to the Jeffreys' parabola which persists in the steady state.

Interesting linear relationships (one for hydrides and another for nonhydrides) are found to exist between a certain parameter c characterising the vibrational eigenvector matrix L and the mass ratio m y/ m x in the case of bent symmetric XY 2 type molecules with small mass coupling.

The thermal instability of a Hall plasma taking compressibility into account is studied. The effect of compressibility is found to be stabilizing while the Hall currents have a destabilizing effect. The system is stable for (c p /g)β<1. Oscillatory modes are introduced due to the presence of a magnetic field which are missing in the absence of a magnetic field (and hence in the absence of Hall currents) for (c p /g)β > 1. The case of overstability is also discussed.

Recent measurements of electromotive forces in electrochemical cells have shown the feasibility of measuring relative chemical potentials of alloy components. The excess chemical potential is formulated from the excess Gibbs free energy ⊿G E = ⊿U E +P ⊿V E -T ⊿S E (at constant P and T). The excess internal energy ⊿U E is evaluated from the structure-dependent internal energy of the metals. T ⊿S E is taken as zero (to a good approximation in lithium-magnesium). P ⊿VE is found to be negligible at atmospheric pressure. A calculation is made for the alloy Li-Mg at about 600 °C for the excess chemical potential where N i is the number of lithium ions, which is found to be of the right magnitude compared to two sets of experimental results.

Polymethine oxonols show temperature dependent 1 H-NMR-spectra because of restricted rotation of the end groups. The dependence of the value of the corresponding free enthalpy of activation AGt on the length of the poly-methine chain can be explained by the change of the π-electron contribution to ⊿G ≠.

Absolute radical yields, as determined by flash spectro-scopy, are reported for the reaction of the thionine triplet 3 TH + and its protonated form 3 TH 2 ++ with a variety of electron donors in methanolic solution. The different yields, extrapolated to infinite donor concentrations, are due to partial physical quenching of the triplets in the electron transfer reaction. The results are indicative of a triplet exciplex as an intermediate in the electron transfer reaction.

An LCAO approach to the Slater SCF-Xα method is described. The equilibrium distances and Hellman-Feynman forces are determined.

By the shear cell technique, the self-diffusion coefficient (D Sn ) of tin in a liquid Sn - 38.1 wt% Pb alloy has been measured in the temperature range 466 to 786 K. The slope of the D Sn vs. T curve decreases up to about 573 K, where it gets constant. This behaviour may be explained by the cluster hypothesis.

We discuss the dependence of the diffusion coefficient and Abstractof the viscosity on the temperature and on the composition for ideal binary liquid mixtures, in particular for the system chlorobenzene + bromobenzene. The activation energy for dif-fusion is independent of the composition and exceeds the activation energy for viscous flow which is a linear function of the mole fractions.

We report an analysis of the torsional fine structure of rotational lines of (CH 3 ) 2 O in excited torsional states leading to the potential parameters V 3 and V′ 12 of a Hamiltonian for the overall-and internal rotation.