Band 28, Heft 8

Phononassisted Augerprocesses are investigated as a mechanism of band-to-band-transitions in III-V-Compounds. It is found that they can explain the temperature dependence in a temperature region, where the normal Auger processes are ineffective.

The trigonal d 2 and d 8 symmetry adapted eigenvectors and energy determinants have been obtained in two different representations, with and without cubic orientation. Spin-orbit perturbation was applied last in both the representations. The complete theory developed here is necessary in a definitive and a meaningful study of the interpretation of the spectral and magnetic properties of trigonally distorted or substituted cubic systems and of trigonal systems in which the axial part of the ligand field is of such a magnitude that the cubic parentage has no meaning.

Paramagnetism in solutions of tertiary aliphatic amines and related compounds in liquid sulphur dioxide is reported. Evidence is presented that the paramagnetic species in triethylamine solutions is a triplet electronic state lying some 0.32 eV (5.1×10 -20 joules) above a diamagnetic groundstate of an n-kπ donor-acceptor complex. The time-dependence of paramagnetism, optical absorption, and electrical conductivity of such solutions has also been investigated and an attempt is made to correlate these phenomena in terms of charge-transfer mechanisms. Finally a few properties of solutions of some similar compounds in liquid SO 2 are noted and discussed briefly.

In radiofrequency and level-crossing experiments the sign of atomic splitting constants (e. g. hfsconstants, g J -values, tensor polarizabilities) can often be measured only under special experimental conditions. Some general features of these conditions are discussed starting from transformation properties of the atomic hamiltonian. In particular, the dependenec of level-crossing signals on the signs of atomic splitting constants is analyzed.

The effects of the added light gases H 2 , He, and D 2 on the separation of the uranium isotopes in the separation nozzle process are compared experimentally. The superiority of H 2 under economically optimum conditions turns out to be caused essentially by its less dissipative nozzle flow characterized by a higher Reynolds number. This advantage is decreased if differences in the flow velocity lose their effects on isotope separation, e. g. at higher expansion ratios. Thus, with He added, the isotope separation effects can get closer to those observed with H 2 ; D 2 even can get better than H 2 , because He and D 2 prevent more efficiently UF 6 from concentrating rapidly at the outer nozzle wall.

The energy consumption of the separation nozzle process for enrichment of U 235 is discussed in terms of the entropy production in the separation system. Experimental results obtained from measurements with Pitot type pressure probes and a numerical Navier-Stokes analysis of the nozzle flow show that ordinary viscous dissipation is the most important entropy production mechanism. The relatively large amount of kinetic energy observed at the end of the nozzle indicates some possibilities of lowering the specific energy consumption of the process.

A high current electric arc in the axis of a supersonic nozzle flow is studied experimentally and theoretically in order to clarify the physical nature of light emission fluctuations which are observed inside the nozzle. The gas flow is produced by discharging a high pressure reservoir of 20 at N 2 through a nozzle of 12 mm throat diameter. The arc is fed with a rectangular current pulse of 1.9 kA amplitude and 5 ms duration. The light emission fluctuations of the arc are observed by photographic and photoelectric methods. The results of the observations are compared to theoretical estimates and lead to the conclusion that the fluctuations are caused by hydrodynamic turbulence. This turbulence is shown to be generated by the combined occurrence of a strong axial pressure gradient and a strong radial density gradient in the boundary layer between the arc and the surrounding cold gas flow. The influence of specific plasma properties on the character of the turbulence is briefly discussed from a theoretical point of view.

Population densities of atomic hydrogen levels have been calculated for homogeneous stationary and diffusion-dominated stationary plasmas. The results show strong overpopulation of the lower lying excited states when the actual number density of ground state atoms is larger than the density obtained for the homogeneous stationary state. At low electron and high ground state densities the so-called Boltzmann slope is not further a characteristic quantity for the determination of electron temperatures, since the slope constant changes with quantum number. - The partial L.T.E. condition for diffusion-dominated plasmas is reexamined.

The variation of electron density and neutral atom density behind incident shock waves in argon, krypton, and xenon are measured using a two wavelength laser interferometer with photoelectric detection of fringe shifts. The accuracy of the measured area densities ∫ Ne dl and ∫ Na dl is better than 1 per cent. Densities measured at the moment of maximum electron density agree well with Rankine-Hugoniot densities. There are no indications to deviations from LTE. Additionally, in the radiation cooling region the decrease of temperature is determined.

The reaction 115 In (n,e) 116 In has been measured with the beta spectrometer at the FRM reactor in Munich. Multipolarities of 44 transitions up to 600 keV have been determined. The spin and parity assignments in the level scheme of Rabenstein et al. are verified by this measurement. New spins and parities could be assigned to 6 levels.

Effective electron numbers are proposed for the evaluation of the dispersion forces potentials between polyatomic molecules. Using these potentials improved formulas are derived for the calculation of the surface tension of nonassociated fluids in terms of molar volume and molecular refraction (polarizability). - Critical comments are made on certain connections between polarizability and diamagnetic susceptibility supposed in the literature.

The helix instability of a convection stabilized high current arc burning in a high pressure nitrogen gas flow is investigated theoretically and experimentally. Equations for the electric and magnetic fields and the current density are solved in helix coordinates for small displacements. The resulting electromagnetic force on a helix segment is calculated and compared to the gasdynamic stabilizing forces. A formula is derived for the growth rate as a function of helix pitch and the physical parameters current density and gas density. The comparison with experimental data yields good qualitative agreement for the onset and the growth rate of the helix instability.

Twelve systems formed with lithium fluoride and alkali halides were studied in order to put into evidence possible demixing phenomena. Liquiddiquid equilibra were found in the seven mixtures containing KBr, KI, RbBr, Rbl, CsCl, CsBr, and Csl; however, the miscibility gaps could be fully detected only for LiF + CsCl, LiF + KBr and LiF + RbBr. The tendency to demix in these systems, formed by typically ionic components, agrees with the reciprocal Coulomb effect rule. On the basis of the solid liquid and liquiddiquid equilibria, the LiF excess potentials were evaluated and compared with those calculated according to current theories.

The systems (Ag+Me)I and (Cu+Me)I were investigated by measuring their initial thermoelectric power; mixtures containing AgI show a behaviour much closer to the ideality than mixtures containing CuI. Interesting conclusions are suggested about the heats of transport of silver and copper ions in the pure molten iodides.

The 23 Na and the 7 Li NMR spectra from several binary- and ternary alkali silicate and alkali borate glasses have been examined as a function of composition and temperature. At temperatures well below room temperature both the magnetic dipolar interactions between various nuclei and the electric quadrupolar interactions with the surroundings remain more or less the same in all the glasses. Upon increasing the temperature the width of the central transition of the 7 Li absorption decreases, the 23 Na dispersion changes from a quadrupolar to a more dipolar like shape, and important spin lattice relaxation is produced. Substitution of other alkali ions for lithium and sodium, respectively, affect the dynamic rather than the static behaviour of the glasses. The results are discussed with the assumption that the main contribution to the NMR signal is from nuclei belonging to alkali-rich regions which look similar in silicate and borate glasses, and for different alkali ions. The phenomena of motional narrowing, temperature dependent relaxation and drastic change of dispersion spectra are attributed to the diffusion of the alkali cations. From the relaxation data activation energies between 14 and 22 kcal/Mol have been obtained. Particular attention has been paid to the mixed alkali effect.

The analysis, carried out in the work, of the emergence of the effect of suppression of the nuclear reaction (ES) during resonance Bragg scattering of Mössbauer gamma-quanto from nuclei under conditions of a hyperfine splitting allows to clearly reveal the fundamental features characteristic of this phenomenon. An is shown in sections 3 and 4, in a general case of realization of ES neither electric nor magnetic vectors of the wave field in the crystal turn to zero at the points of equilibrium positions of the nuclei. For the reaction to be suppressed, it is necessary that the total amplitude of the excited nucleus formation be zero. In this case, a rather peculiar picture of the electromagnetic field within the crystal sets in which depends on the character of the hyperfine splitting and on the multiplicity of the transition. In this work, conditions have been found of the coexistence of ES for both polarizations, and with these conditions being fulfilled, the strongly resonantly absorbing crystal becomes transparent for the whole of radiation. Special attention has been paid to the proof that lattice vibrations do not restore the nuclear absorption of gamma-quanta. It is interesting that all this takes place during resonant interaction with a separate nucleus when the cross-section of the inelastic process may be large as compared with the elastic scattering cross-section. Although all these features (and a number of others discussed in detail in the text) essentially distinguish ES from the Borrmann effect for X-rays, there is a circumstance which makes them related. In both cases it is necessary to produce a coherent superposition of waves within the crystal owning to the Bragg diffraction of the incident quanta, and in the both cases the crystal becomes more transparent for the radiation that comes through.

The absolute integrated intensities diffracted in anomalous transmission through thick, nearly perfect crystals of silicon were measured for AgKα and MoKα wavelengths and for the {220} reflection, at room and liquid nitrogen temperatures. There is good agreement between experimental values and those calculated by using the formulas of the dynamical theory of diffraction, provided the contribution of Compton scattering μ C * is included in the effective absorption coefficient μ*. μ C * is a considerable fraction of μ* (from 23 to 55 per cent) for the two wavelengths and temperatures used in the present work. The experimental values of μ C * agree well with those calculated by using the theory of the Compton contribution to the dynamical absorption coefficient of X-rays. A simple formula which is a good approximation of the rigorous expression of μ C * is also given. The Debye temperature Θ of Si was derived from the experimental dependence of the intensities on crystal thickness for the {220} reflection at room and liquid nitrogen temperatures by using CuKα radiation. It was found that Θ = 521+5 and 543 + 5 °K at 295 and 77 °K, respectively, in agreement with the results of other authors.

Changes in absolute and relative intensities of Pseudo-Kossel lines from crystals containing lattice vibrations were observed. Since the relation between vibration amplitude and intensity modification is known theoretically, vector amplitudes and gain factors of the acoustoelectric amplification can be determined. Advantages of this method are discussed.

The ratio of the Larmor frequencies of 33 S and 85 Rb has been measured and a magnetic moment for 33 S of μ( 33 S in SO 4 2- in D 2 O) =0.643 1028(14)/μ N has been calculated. 33 S chemical shifts have been determined and the SO 4 2- ion in aqueous alkali sulfate solutions is proposed as 33 S NMR reference compound.

The g J -factor and the magnetic susceptibility anisotropy ξ T have been measured for 120 Sn 18 O and 208 Pb 16 O in the vibrational state υ=0: 120 Sn 16 O g J = -0,14631 (38), ξ T /h = 1844(18) Hz/kG 2 , 208 Pb 16 O g J = -0,16233 (39), ξ T /h = 2598(18) Hz/kG 2 . The molecular quadrupole moment Θ and the asymmetry of the electronic charge distribution 〈Σ(z i 2 -x i 2 )〉 are derived from the values of g J , ξt , and the rotational constant B 0 .

The g J -factor and the molecular magnetic susceptibility anisotropy ξ T have been measured for 74 Ge 16 O and 28 Si 32 S in the vibrational states υ=0, 1, 2 and υ= 0, 1, 2, 3 respectively: Having measured g J and ξ T as functions of υ we were able to derive the molecular and electronic quadrupole moment equally as functions of the vibrational state according to the method of Newell and Ramsey.

Mean amplitudes of vibration for the ions SF 5 - , SeF 5 - and TeF 5 - have been calculated from spectroscopic data using an approximation method. The results are briefly discussed, especially in relation with those of isoelectronic species.

The emission spectra, thermoluminescence glow curves and thermoluminescence spectra of cubic and hexagonal ZnS/(10 -4 )Cu phosphors were measured comparated to these of ZnS/ (5·10 -4 ) Cu phosphors by excitation with light. The shape of the glow curves is correlated with the crystal structure of the materials.

The interdiffusion coefficient of silver ions in molten thallous nitrate has been determined with the gravimetric technique. The observed data can be "least-squares" fitted into an Arrhenius equation D 12 = 1.07 × 10 -3 exp-(4130 ± 400)/R T where D 12 is expressed in cm2s-1, R in calories equivalent-1, and T in degrees K. Friction coefficients r AgTl calculated according to the Klemm 1 model indicate association between the cations, as has been indicated in mixtures of silver nitrate with heavy alkali nitrates also.

Fluorescence, phosphorescence and E-type delayed fluorescence of circobiphenyl (II) has been studied. Besides coronene (I) circobiphenyl as yet is the only known polycyclic aromatic hydrocarbon which displays E-type delayed fluorescence.

The kinetic behavior of OH radicals was studied in the presence of a large excess of n-butane. Hydroxyl radicals were produced by pulsed vacuum-uv photolysis of H 2 O and were monitored directly by a resonance fluorescence method. The rate constant for the reaction OH + n-butane was determined to be 2.35 (±0.35) × 10 -12 cm 3 molecule -1 sec -1 at 298 °K.