The 5f2-->5f16d1 absorption spectrum of Cs2GeF6:U4+ crystals: A quantum chemical and experimental study

Single crystals of U(4+)-doped Cs2GeF6 with 1% U4+ concentration have been obtained by the modified Bridgman-Stockbarger method in spite of the large difference in ionic radii between Ge4+ and U4+ in octahedral coordination. Their UV absorption spectrum has been recorded at 7 K, between 190 and 350 nm; it consists of a first broad and intense band peaking at about 38,000 cm(-1) followed by a number of broad bands of lower intensity from 39,000 to 45,000 cm(-1). None of the bands observed shows appreciable fine vibronic structure, so that the energies of experimental electronic origins cannot be deduced and the assignment of the experimental spectrum using empirical methods based on crystal field theory cannot be attempted. Alternatively, the profile of the absorption spectrum has been obtained theoretically using the U-F bond lengths and totally symmetric vibrational frequencies of the ground 5f2 - 1A(1g) and 5f16d(t(2g))1 - iT(1u) excited states, their energy differences, and their corresponding electric dipole transition moments calculated using the relativistic ab initio model potential embedded cluster method. The calculations suggest that the observed bands are associated with the lowest five 5f2 - 1A(1g)-->5f16d(t(2g))1 - iT(1u) (i = 1-5) dipole allowed electronic origins and their vibrational progressions. In particular, the first broad and intense band peaking at about 38,000 cm(-1) can be safely assigned to the 0-0 and 0-1 members of the a(1g) progression of the 5f2 - 1A(1g)-->5f16d(t(2g))1 - 1T(1u) electronic origin. The electronic structure of all the states with main configurational character 5f16d(t(2g))1 has been calculated as well. The results show that the lowest crystal level of this manifold is 5f16d(t(2g))1 - 1E(u) and lies about 6200 cm(-1) above the 5f2 level closest in energy, which amounts to some 11 vibrational quanta. This large energy gap could result in low nonradiative decay and efficient UV emission, which suggest the interest of investigating further this new material as a potential UV solid state laser.     

Influence of temperature and humidity on NO x removal by corona discharge

The object of this experimental investigation was the influence of temperature and humidity on the efficiency of removal of NO _x by a pulsed corona discharge from a mixture N₂ : O₂ : CO₂ : H₂O : NO simulating a combustion flue gas. The pulsed corona discharge was generated in a wire-to-cylinder reactor. It was found that removal of NO _x was most efficient when H₂O concentration corresponded with the saturated vapour pressure. In the case of the operating gas containing constant H₂O concentration removal of NO _x decreased with increasing temperature of the operating gas. Dedicated to Prof. Jan Janča on the occasion of his 60^th birthday. This work is devoted to the 60^th birthday of Professor Jan Janca, our good colleague, merited teacher, researcher and famous physicist, discussion with whom stimulated this and other our work during years.     

1H NMR studies of solvent and substituent effects on strong intramolecular hydrogen bonds

Chemical shifts of H-bonded protons in tetrabutylammonium hydrogen maleate and 14-substituted picolinic acid N-oxides have been measured in a number of dry solvents, of different activity, in order to distinguish between symmetrical single minimum and asymmetrical hydrogen bonds. In tetrabutylammonium hydrogen maleate the resonance was observed at 20.70 ppm and its was independent of the nature of the solvent used. The chemical shift value of picolinic acid N-oxide varies with the solvent. These observations suggest that the hydrogen bond is symmetrical in tetrabutylammonium hydrogen maleate but that it is asymmetrical in picolinic acid N-oxide. The chemical shifts of substituted picolinic acid N-oxides were correlated with σ_p, σ_m and Δ_pK_a. The substituent and solvent effects are compared and the position of the intramolecular H-bonded protons in picolinic acid N-oxides are estimated and discussed.     

Serial and parallel simulated annealing and tabu search algorithms for the traveling salesman problem

This paper describes serial and parallel implementations of two different search techniques applied to the traveling salesman problem. A novel approach has been taken to parallelize simulated annealing and the results are compared with the traditional annealing algorithm. This approach uses abbreviated cooling schedule and achieves a superlinear speedup. Also a new search technique, called tabu search, has been adapted to execute in a parallel computing environment. Comparison between simulated annealing and tabu search indicate that tabu search consistently outperforms simulated annealing with respect to computation time while giving comparable solutions. Examples include 25, 33, 42, 50, 57, 75 and 100 city problems.     

Decomposition of large uniform hypergraphs

We determine a minimum cardinality family _{n, k} (resp. _{n, k} ) ofn-uniform,k-edge hypergraphs satisfying the following property: all, except for finitely many,n-uniform hypergraphs satisfying the divisibility condition have an _{n, k} -decomposition (resp. vertex _{n, k} -decomposition).     

Some sources of defects in α-HgI2, the room temperature γ-ray detector

In the course of a systematic optimization of the materials properties of -HgI₂ crystals for room temperature - and X-ray detectors, we have investigated possible sources of defects and discuss briefly the possibility to supress them. Due to the particular structure of -HgI₂, large amount of impurities, particularly hydrocarbons, can be absorbed invan der Waals layers and lattice channels. Purification by sublimation does not work due to the affinity of hydrocarbons to iodine and their easy re-absorption in the sublimate. Lattice filtering of the large concentration of hydrocarbons contained even in suprapure iodine has been performed using the close spaced lattice of CuI. Oxidation of hydrocarbons by reaction of oxygen with HgI₂ is another possibility for their removal.Mass spectrometric investigations of a molecular beam of -HgI₂ has solved the long disputed problem of the existence of nonstoichiometry: both excess of Hg or excess of I are possible. The removal of nonstoichiometry can be achieved by suitable thermal treatment only in the case of pure crystals. In the presence of hydrocarbons, the non-stoichiometric defects are fixed showing the predominant importance of hydrocarbons for -HgI₂.Investigation of the evaporation of -HgI₂ with mass spectrometry at low temperatures (150>T>40 °C) shows a strong change of the enthalpy of evalporation at 67 °C whereas DSC does not show any peak at this temperature. It seems probable that this is due to a surface reconstruction which influences the evaporation but not the thermal bulk lattice effects which are detected by DSC.Dedicated to Prof. Dr.K. L. Komarek at the occasion his 60th birthday.     

Determination of selenium in infant foods using electrothermal atomic absorption spectrometry with direct slurry sample introduction

Slurry sampling electrothermal atomic absorption (SS ETAAS) was applied to the development of a sensitive and precise method for selenium determination in infant foods without sample pretreatment. Suspensions prepared in a medium containing 0.1% Triton X-100, 0.5 or 5% v/v concentrated HNO₃ were directly introduced into the furnace. The accuracy of the procedure was confirmed by analysis of a standard reference material and comparison with hydride generation atomic absorption spectrometric (HGAAS) procedure. The characteristic mass is 44 pg and detection limit 0.43 μg·l⁻¹.     

o-Phenylenediamine as a New Catalyst in the Highly Regioselective Conversion of Epoxides to Halohydrins with Elemental Halogens – A Reinvestigation

Summary. In contrast to a previous report, o-phenylenediamine is not a catalyst in the ring opening reaction of epoxides by means of bromine or iodine. The o-phenylenediamine is just a reactant which reacts with iodine to give phenazine-2,3-diamine and hydrogen iodide, or with bromine to give a mixture of brominated and polymerized products as well as hydrogen bromide. The hydrogen halogenides are in fact the real epoxide ring opening reactants.     

Solvation of bivalent transition metalcations. II. Heats of transfer of metal perchlorates between water and several amides

Heats of solution of acetonitrile, solvated or hydrated perchlorates, Mn(ClO₄)₂·6AN, Co(ClO₄)₂·6AN, Ni(ClO₄)₂·6AN, Cu(ClO₄)₂·4AN, Cu(ClO₄)₂·6H₂O, Zn(ClO₄)₂·6AN, and Zn(ClO₄)₂·6H₂O have been determined in NMF, DMF, DMA, and water. Complete or almost complete exchange of AN and water molecules in amides is inferred from the visible spectra determinations. The heats of transfer of perchlorate anion from water to DMA and NMF have been obtained from separately determined heats of solution of NaBPh₄, AsPH₄Cl·H₂O, NaClO₄, NaCl, and SiPh₄ in the respective solvents. The heats of transfer of cations from water to amides have been determined from the above data and the heats of solvation of cations using literature data for the heats of hydration.     

The Smearing Out of the Thermal Initial Conditions Created in a Planck Era

In this paper the quantum heat transport in a Planck gas in the presence of the potential (other than the thermal one) is investigated. The new quantum heat transport equation which generalizes our potential-free QHT is developed. The thermal wave solution of QHT for a Planck gas is obtained and a condition for distortionless propagation of thermal wave is formulated. It is argued that the initial conditions of the Beginning (i.e., at t=0) are smeared in the time scale of the Planck time.