Positronium quenching reactions and diffusion in solutions

The temperature effect on positronium quenching rates for p-benzoquinone and iodine in several solvents was determined. The quenching rate constantk ₁ follows ak ₁∝1/η relationship better than thek ₁∝T/η one, predicted by ordinary theory. A simple theoretical treatment based on diffusion theory shows that the temperatureT of the solute, positronium, may not be the same as the temperature of the solvent if the positronium is not fully thermalized. This temperature is found to be about twice the room temperature for the solvents investigated. The findings further support the theory that positronium reactions are “diffusion controlled” in liquid solutions. The activation energies of all the positronium reactions studied are either negligible or less than 0.1 eV.     

Messungen der Dielektrizitätskonstanten von reinem und NH4F-dotiertem Eis

The dielectric constant of pure and NH₄F doped ice single crystals has been measured at temperatures around — 10°C in the frequency range between 0.07 and 500 kc/s. The values measured on pure ice are in agreement with the Debye theory. At frequencies around 0.1 kc/s the dielectric constant?′ of doped crystals is higher than that of pure ice. Treating the frequency as a parameter, the dielectric constant?′ goes through a maximum and a minimum with rising NH₄F concentration. No dependence of the Debye dispersion frequency on the doping concentration could be detected.     

The maximal randomness principle in d-dimensional turbulence

A set of self-consistent equations in one-loop approximation in a statistical model of fully developed homogeneous isotropic turbulence, which is based on the maximal randomness principle of the incompressible velocity field with stationary energy spectral flux, is obtained. Thanks to the applied principle the model statistics becomes essentially non Gaussian. The set of equations does not possess the infrared and ultraviolet divergences near the obtained Kolmogorov spectral exponents. The solution of these equations leads to the Kolmogorov exponents, but its amplitude proportional to the Kolmogorov constantC _k is negative for Euclidean dimensiond=3. Systematic investigation is made of (inertial) steady state scaling solutions for dimensions 2<d<2.55695,where constantC _k (d) becomes positive. Considered in this way, the model stability is discussed in the context of widely studied fractal aspects of turbulence.We have greatly benefited from discussions with Dr. Altaisky from JINR Dubna. The authors (M.H. and M.S.) are grateful to D. I. Kazakov and to director D. V. Shirkov for hospitality at the Laboratory of Theoretical Physics, JINR, Dubna.This work was supported by Fundamental Research Russian Fund, International Scientific Fund (grant R-63000) and by Slovak Grant Agency for Science (grant 2/550/93).     

Optimal heat kernel estimates for schrödinger operators with magnetic fields in two dimensions

Sharp smoothing estimates are proven for magnetic Schrödinger semigroups in two dimensions under the assumption that the magnetic field is bounded below by some positive constantB ₀. As a consequence theL∞ norm of the associated integral kernel is bounded by theL∞ norm of the Mehler kernel of the Schrödinger semigroup with the constant magnetic fieldB ₀.     

Spin polarization and photoinduced electron transfer between ferrocene and fullerene derivatives containing a nitroxide group

Fulleropyrroliddine derivatives containing a TEMPO group (FPNOs) are examined in toluene solution, between 200 and 270 K, by TR-EPR spectroscopy after LASER excitation. Signals of the ground and excited states are observed and their time profiles are recorded. In presence of ferrocene the time pattern of the polarization is modified, showing the occurrence of intermolecular electron transfer from the donor ferrocene to the excited FPNOs. The rate constantk _E. is obtained from the simulation of the EPR signal time evolution of the ground and excited states.     

NMR and magnetic susceptibility of CeCu6

NMR and magnetic susceptibility of CeCu₆ intermetallic compound were investigated. The most important contributions to the magnetic susceptibility are the Curie-Weiss term, expressing the paramagnetism of the localized f-electrons, and a temperature independent term for which all the contributions were estimated. The phenomenological exchange constantJ _sf between 4f — electron spins and conduction — electron spins is derived to be ?0·012 eV. The Knight shift and Curie temperature are discussed in terms of the Ruderman-Kittel-Kasuya-Yosida theory and in the Rocher's virtual bound states model. The s — f exchange integralΓ and the Fermi wavevectork _F are derived to be — 0·8 eV and 1·32 Å^?1, respectively.     

Effect of aggregation on the radiative (T1 → S0) and nonradiative (T1⇝S0 and S1⇝T1) transitions in xanthene dyes

The measurement of triplet decay time (τ_p) and the relative yields of phosphorescence to fluorescence (φ_p/φ_f) have been made in solutions of xanthene dyes. The value of φ_p/φ_f increases while that of τ_p decreases as the concentration of the solution is increased. The rate constants k_p, k_qp and k_is have been estimated. It is found that the values of k_qp and k_is increase while that of k_p remains substantially constant for the entire range of concentration. The deuterium solvent effect is observed in τ_p and φ_p.     

Numerical calculations of emission probabilities of primarily excited molecules in dye solutions

The “active sphere” concentration depolarization theory of Jabloński involves the probability, W_Dk, that a photon is emitted by the donor. Up till now, the mean value of W_Dk, has been exactly calculated only for k = 2 (1 donor and 1 acceptor in the active sphere). For k > 2 the approximate method of Knox was employed. In this paper the use of the Monte Carlo integration method to find <W_Dk > for k up to ten is described. The more accurate values thus obtained coincide closely with the ones found using the simplified method. This finding validates the use of the latter method without reservation up to high dye concentration.     

Low temperature polarization effects in ice

When an initially unpolarized HF doped ice specimen is warmed at a constant rate in an applied electric field two peaks are observed in the current. The low temperature peak occurs near 100°K and the temperature at which this peak occurs is seen to increase as the HF concentration decreases. The second peak appears to occur randomly in the temperature range 125–135°K. These peaks are also observed if the sample is cooled in an applied electric field and then warmed at a constant rate with the field removed. It is suggested that the first peak is due to a dielectric relaxation process which is governed by the L defects released from the HF molecules. This release of L defects is shown to obey the law of mass action with an activation energy for liberation of an L defect of 0.12±0.06 eV and a dissociation constantk _o ^L≈10²⁹ m^?3. A simple theoretical model of ice is also developed which predicts the current reversal phenomenon observed by Dengelet al. [11] suggesting that it is due to dipole relaxation and not to ferroelectric ordering.     

Regularities of energy transfer in aqueous solutions of Ln(III) ions upon formation of their bridged complexes via the CO 3 2− and OH− anions

The influence of the solvent pH and of the presence of carbonic acid anions on the energy transfer from the Eu(III) and Tb(III) ions to a large namber of Ln(III) ions, as well as on the concentration quenching of the Dy(III) ions, is studied. It is shown that, when the anions are present in solution at a concentration by 2–4 orders of magnitude lower than that of the lanthanide ions, the energy transfer rates increase by 2–4 orders of magnitude, but the luminescence decay remains exponential. It is established that the rate constant for energy transfer (k _t ) via the hydroxyl bridge increases with decreasing concentration of lanthanide ions in solution. In an alkalinized solution, (k _t ) depends weakly on the initial water pH, because the concentration of hydroxo groups is governed by hydrolysis of water under the action of the lanthanide ions introduced into it. It is found that, at the 10^?2 M concentration of lanthanide ions in solution, the values of (k _t ) change by almost two orders of magnitude depending on the choice of an ion pair; however, these changes in no way correlate with the overlap integrals of spectra, calculated by adopting the Forster mechanism, and the inclusion of an interaction of higher multipoles does not improve the agreement with the experimental data. It is shown that, when the energy is transferred via the OH^? bridge in solutions with a total concentration of Ln(III) ions of 10^?4 M, the value of (k _t ) increases up to 10⁷ M^?1 s^?1 and becomes independent of the choice of pairs of Ln(III) ions. The dependence of (k _t ) on the ratio of the dissociation constant for a binuclear complex and the probability of energy transfer between the ions within this complex are discussed. It is concluded that the change in this ratio explains the disappearance of the dependence of (k _t ) on the choice of ions as their concentration in solution decreases, but does not explain all the observed changes in (k _t ) if only the dipole-dipole mechanism of energy transfer between the ions in bridged complexes is considered.     

Analysis of fluorescence quenching of pyronin B and pyronin Y by molecular oxygen in aqueous solution

The fluorescence quenching of pyronin B and pyronin Y molecules by molecular oxygen in aqueous solution was studied by using steady-state and time-resolved fluorescence and UV-Vis absorption spectroscopy techniques. In order to understand the quenching mechanism, fluorescence decays, absorption and fluorescence spectra of the probes were recorded as a function of the oxygen concentration and temperature. The quenching was found to be appreciable and shows positive deviation in the Stern-Volmer representation obtained from the fluorescence intensity ratio. Fluorescence quenching constants (k_q) were calculated from the τ_o/τ vs. [Q] plots having linear correlation and compared with calculated diffusion-controlled rate constants (k_diff) values. Experimental results were in good agreement with the simultaneous dynamic and static quenching model.     

Transverse instability of a plane front of fast impact ionization waves

The transverse instability of a plane front of fast impact ionization waves in p ⁺-n-n ⁺ semiconductor structures with a finite concentration of donors N in the n layer has been theoretically analyzed. It is assumed that the high velocity u of impact ionization waves is ensured owing to the avalanche multiplication of the uniform background of electrons and holes whose concentration ??_b ahead of the front is high enough for the continuum approximation to be applicable. The problem of the calculation of the growth rate s of a small harmonic perturbation with wavenumber k is reduced to the eigenvalue problem for a specific homogeneous Volterra equation of the second kind containing the sum of double and triple integrals of an unknown eigenfunction. This problem has been solved by the method of successive approximations. It has been shown that the function s(k) for small k values increases monotonically in agreement with the analytical theory reported in Thermal Engineering 58 (13), 1119 (2011), reaches a maximum s _M at k = k _M, then decreases, and becomes negative at k > k ₀₁. This behavior of the function s(k) for short-wavelength perturbations is due to a decrease in the distortion of the field owing to a finite thickness of the space charge region of the front and ??smearing?? of perturbation of concentrations owing to the transverse transport of charge carriers. The similarity laws for perturbations with k ? k _M have been established: at fixed ??_b values and the maximum field strength on the front E _0M, the growth rate s depends only on the ratio k/N and the boundary wavenumber k ₀₁ ?? N. The parameters s _M, k _M, and k ₀₁, which determine the perturbation growth dynamics and the upper boundary of the instability region for impact ionization waves, have been presented as functions of E _0M. These dependences indicate that the model of a plane impact ionization wave is insufficient for describing the operation of avalanche voltage sharpers and that fronts of fast streamers in the continuum approximation should be stable with respect to transverse perturbations in agreement with the previously reported numerical simulation results. The results have been confirmed by the numerical simulation of the evolution of small harmonic perturbations of the steady-state plane impact ionization wave.     

Gammaübergangswahrscheinlichkeit und Quasiteilchenapproximation

The transition probability of the isometricM4-transitions 1g _9/2-2p _1/2, 1h _11/2-2d _3/2 and 1i _13/2-2f _5/2 in near magic nuclei is calculated on the basis of the theory of finite Fermi systems. It is shown, that the constantg ₀ for the spin-spin interaction of quasiparticles, depends strongly on the special kind of representation for the single particle states. Using oscillator functions the valueg ₀=3.3 has to be taken, in order to get hindrance factors, which are approximately equal to one, whereas in the case of square well functions the constantg ₀=1.4 is necessary, in order to get agreement with the experimental transition probabilities.     

A re-evaluation of isotope shift constants

The field isotope shift constantC _unif for a uniform nuclear charge distribution has been re-evaluated fors-electrons for elements with 10≦Z≦ 95. A table is given which permitsC _unif to be found for any isotope pair.     

Intermediate electronic relaxation betweenS=3/2 andS=1/2 states in Fe(J-mph)NO with a jäger-type ligand J-mph

The incomplete spin transition between the low-spin (LS) (S=1/2) and the intermediate-spin (IS) (S=3/2) states in the iron (III) complex Fe(J-mph)NO (mph=4-methyl-o-phenylene), centered atT _c≈212 K, has been studied with⁵⁷Fe Mössbauer spectroscopy and magnetic susceptibility measurements between 80 K and 320 K. The lineshape of the Mössbauer spectra is well reproduced by a two state stochastic relaxation model resulting in values of about 2·10⁶ s^?1 to 7·10⁶ s^?1 for the IS→LS transition rate constantk _IL. The kinetics of this spin transition can be described by an Arrhenius equation yielding activation energiesE _IL=1.1 (2) kJ/mol andE _LI=6.1 (2) kJ/mol for the IS→LS and LS→IS conversion, respectively.     

The energy tensor of matter in the projectve theory of relativity

In the projective theory of relativity the 5-dimensional field equation \(_{\mu \nu } \) and the resulting equation of motion Tμυ_;μ = 0 are investigated. There Tμυ stands for the 5-dimensional tensor of macroscopic matter. The 4-dimensional field equations and equation of motion obtained by projection are a generalization of Einstein's theory of general relativity and Maxwell's electrodynamics, involving a scalar field φ.They contain a single constant φ₀.The weak field approximation is investigated for the case of an ideal fluid and leads to Newton's mechanics, including Newton's gravitational law, and to Maxwell's electrodynamics. For the constant φ₀ one obtains the approximate value φ₀c⁴/γ_N with Newton's gravitational constant γ_N.For homogeneous and isotropic cosmological models consisting of matter only the general solution for the radius K of curvature is given. This solution is independent of the equation of state of matter For a pure dust universe the general solution for the scalar field φ is given. For a closed universe a power law φ ?K^?1 is valid which leads to Mach's principle. The calculation of the age of a closed universe yields over 7×10⁹y,if one uses mean values of the present cosmological data.     

The surfactant influence on the surface magnetic properties of Fe3O4 microcrystals

Mössbauer measurements, electron microscopy and infrared spectroscopy, have been used for investigating the Fe₃O₄ microcrystals coated with different surfactants. The experimental results show that the surface condition of the system changes and the anisotropy energy constantK of Fe₃O₄ microcrystals increases effectively when the surfactants being added. The spin pinning effect on a sample surface was observed and the pinning depth and the pinning direction were estimated. Besides, it has been shown that the anisotropy energy constantK of the coated Fe₃O₄ has an abrupt change nearbyT _v.     

Behavior of silver substitution in single-grain TSMG YBCO bulk superconductor

Ag concentration in the single-grain TSMG Y₁Ba₂(Cu_1?xAg_x)₃O₇ bulk superconductor with nominal Ag concentration for x = 0.05 was measured by WDX microprobe. The partition coefficient of Ag between Y123 crystal and the melt, k_Ag, was estimated to be k_Ag = 0.1. It is also shown that the solubility of Ag in the Y₂BaCuO₅ phase is below the WDX detection limit. As the measured Ag concentration is about four times higher than the concentration of Ag for optimum chemical pinning, the clustering of Ag atoms is supposed and a mechanism of Ag clustering is proposed.     

Negative Virial Coefficients and the Dominance of Loose Packed Diagrams for D-Dimensional Hard Spheres

We study the virial coefficients B _k of hard spheres in D dimensions by means of Monte-Carlo integration. We find that B ₅ is positive in all dimensions but that B ₆ is negative for all D≥6. For 7≤k≤17 we compute sets of Ree–Hoover diagrams and find that either for large D or large k the dominant diagrams are “loose packed.” We use these results to study the radius of convergence and the validity of the many approximations used for the equations of state for hard spheres.     

The ground state of the neutral Hubbard model

The ground state energy of the neutral Hubbard model is calculated by BCS methods for all values of total spinS _z . Numerical results are given for the simple cubic and for the body centred cubic lattice. Antiferromagnetic ordering and a finite paramagnetic susceptibility is found for all values of the coupling constantV ₀.