Mechanisms of creation and destruction of polyhalide V centers in irradiated AHC

The mechanisms of creation and destruction of polyhalide V centers in AHC were modeled by the MNDO semiempirical quantum-chemical method. It is shown that X ₃ ⁻ molecules dissociate from the ground state into X ₂ ⁰ and X⁻ and from the triplet state into X ₂ ⁻ and X⁰, whereas X ₅ ⁻ and X ₇ ⁻ molecules dissociate into X ₃ ⁻ and X ₂ ⁰ and X ₅ ⁻ and X ₂ ⁰ , respectively. At room temperatures, X ₃ ⁻ or larger aggregates, which consist only of X ₃ ⁻ centers, form stable polyhalide V centers. It is shown that the thermal stability decreases when going along the series X ₃ ⁻ , X ₅ ⁻ , and X ₇ ⁻ . The radiation-induced instability of stable F centers in AHC at room temperatures is associated with the destruction of X ₃ ⁻ centers and the thermal instability of X ₅ ⁻ and X ₇ ⁻ centers. E. A. Buketov Karaganda State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 50–54, March, 2000.     

Fluorescent Method for the Determination of Sulfide Anion with ZnS:Mn Quantum Dots

Water-soluble Mn²⁺-doped ZnS quantum dots (QDs) were prepared using mercaptoacetic acid as the stabilizer. The optical properties and structure features were characterized by X-Ray, absorption spectrum, IR spectrum and fluorescence spectrum. In pH 7.8 Tris-HCl buffer, the QDs emitted strong fluorescence peaked at 590 nm with excitation wavelength at 300 nm. The presence of sulfide anion resulted in the quenching of fluorescence and the intensity decrease was proportional to the S²⁻ concentration. The linear range was from 2.5 × 10⁻⁶ to 3.8 × 10⁻⁵ mol L⁻¹ with detection limit as 1.5 × 10⁻⁷ mol L⁻¹. Most anions such as F⁻, Cl⁻, Br⁻, I⁻, CH₃CO₂ ⁻, ClO₄ ⁻, CO₃ ²⁻, NO₂ ⁻, NO₃ ⁻, S₂O₃ ²⁻, SO₃ ²⁻ and SO₄ ²⁻ did not interfere with the determination. Thus a highly selective assay was proposed and applied to the determination of S²⁻ in discharged water with the recovery of ca. 103%.     

Highly Selective Fluorescent Recognition of Pyrophosphate in Water by a New Chemosensing Ensemble

A new chemosensing ensemble that displays sensitive and selective fluorescent recognition of pyrophosphate in water at pH 7.4 has been developed. The ensemble is constructed by a copper complex (receptor) and eosin Y (indicator), the constructed ensemble is capable of highly selectively discriminate pyrophosphate from other common existing anions such as CH₃COO⁻, HSO₄⁻, NO₃⁻, H₂PO₄⁻, HPO₄²⁻, PO₄³⁻, NCS⁻, I⁻, Cl⁻, Br⁻, F⁻as well as some structurally similar carboxylates such as citrate, tartrate, oxalate, malonate, succinate and glutarate.     

One-pot fabrication and enhanced thermoelectric properties of poly(3,4-ethylenedioxythiophene)-Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> nanocomposites

Bi₂S₃ nanotubes and de-doped poly(3,4-ethylenedioxythiophene) (PEDOT) composite nanopowders were synchronously synthesized by a one-pot self-assembly method. The powders were characterized by X-ray powder diffraction, infrared spectroscopy, and transmission electron microscopy, respectively. Thermoelectric properties of the Bi₂S₃–PEDOT composite nanopowders with different Bi₂S₃ contents after being cold pressed into pellets were measured at room temperature. The sample with 36.1 wt% Bi₂S₃ has a highest power factor of 2.3 μWm⁻¹K⁻², which is higher than that of both pure PEDOT (0.445 μWm⁻¹K⁻²) and Bi₂S₃ (1.94 μWm⁻¹K⁻²).     

2p3/2?13x?1?3x?13d?1 X-ray satellites spectra in the Lα1 region of 4d transition elements

The X-ray satellite spectra arising due to 2p _3/2 ⁻¹3x ⁻¹−3x ⁻¹3d ⁻¹ (x ≡ s, p, d) transition array, in elements with Z = 40 to 48, have been calculated, using available Hartree-Fock-Slater (HFS) data on 1s ⁻¹−2p ⁻¹ 3x ⁻ and 2p _3/2 ⁻¹−3x ⁻¹,3x ⁻¹ Auger transition energies. The relative intensities of all the possible transitions have been estimated by considering cross — sections for the Auger transitions simultaneous to a hole creation and then distributing statistically the total cross sections for initial two hole states 2p _3/2 ⁻¹−3x ⁻¹ amongst various allowed transitions from these initial states to 3x ⁻¹ 3d ⁻¹ final states by Coster-Kronig (CK) and shake off processes. In both these processes initial single hole creation is the prime phenomenon. Each transition has been assumed to give rise to a Gaussian line and the overall spectrum has been computed as the sum of these Gaussian curves. The calculated spectra have been compared with the measured satellite energies in Lα₁ spectra. Their intense peaks have been identified as the observed satellite lines. The peaks in the theoretical satellite spectra were identified as the experimentally reported satellites α₃, α₄ and α₅, which lie on the high-energy side of the Lα₁ dipole line.     

Laser oscillation by the F3− and F2− color centers in LiF crystal at room temperature

A stable, free-running LiF:F₃ ⁻ and LiF:F₂ ⁻ color center laser oscillation is achieved at room temperature by pumping with the 930-nm laser radiation. The LiF:F₃ ⁻ laser radiation has a peak at 1100 nm and shifts from the peak wavelength of the F₃ ⁻ luminescence band because of the absorption of the F₃ ⁻ luminescence by the F₂ ⁻ center, which co-exists with the F₃ ⁻ center. Received: 2 March 1999 / Revised version: 16 March 1999 / Published online: 12 May 1999     

Negative muon spin precession in ferromagnetic iron and the hyperfine anomaly

The hyperfine field (B _μ ^hf ) at the negative muon μ⁻ in ferromagnetic iron was investigated by means of the zero-field μ⁻ spin precession technique. In the temperature range 320–690 K,B _μ ^hf for μ⁻ Fe departs from the magnetization curve of pure iron in the same way as the hyperfine field seen by a⁵⁵Mn impurity in dilute MnFe measured by NMR. The hyperfine anomaly for μ⁻ Fe relative to dilute (1.5 at.%)⁵⁵Mn in iron is found to be −0.9(3)% and temperature independent over the temperature range investigated.     

Measurement and simulation of specific heat for metastable liquid Ni80Fe10Cu10 alloy

The specific heat of undercooled liquid Ni₈₀Fe₁₀Cu₁₀ alloy was experimentally measured by electromagnetic levitation drop calorimeter, and also numerically simulated by the molecular dynamics method. The achieved maximum undercooling is up to 252 K (0.15 T _L) in the experiments, and the measured result is 41.67 J mol⁻¹ K⁻¹. The simulation provides calculated data within 0∼702 K undercooling range, which is much broader than the experimental regime. The simulated value is 37.02 J mol⁻¹ K⁻¹. Although there exists a difference of 4.65 J mol⁻¹ K⁻¹ between them, the result is quite acceptable for simulation. Furthermore, the liquid structure of undercooled Ni₈₀Fe₁₀Cu₁₀ alloy is studied in terms of the total and partial pair distribution functions, which display that the ordered degree of atoms enhances from a normal liquid to metastable state.     

On the delay mechanism of Cl<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack> diatomic anion dissociation up to the microsecond timescale

The dissociative capture of slow electrons by tetrachlorethylene (C₂Cl₄) has been investigated by resonant electron capture negative ion mass spectrometry. Metastable ions with fractional mass numbers 7.5, 17.5, and 19 corresponding to the C₂Cl₄⁻ → Cl⁻ + C₂Cl₃ and Cl₂⁻ → Cl⁻ + Cl decays occurring at the microsecond timescale have been detected. It has been revealed that Cl₂⁻ anions, which are fragment ions, can dissociate at the microsecond timescale, which is very surprising for a system with one internal degree of freedom. This process is assumingly attributed to the rotational excitation of Cl₂⁻ anions. Thus, the experimental estimate of the time of rovibronic relaxation in the Cl₂⁻ anion has been obtained.     

Change of membrane fluidity and protein conformation involved in [Ca2+]i overload in neuronal cells induced by ONOO−

Considerable evidence indicates that the formation of peroxynitrite (ONOO⁻) with superoxide anion (O₂^−⊙ot) may be involved in the neuronal toxicity of NO. Here, the effects of ONOO⁻ on intracellular free calcium concentration ([Ca²⁺]_i) in single MN9D cells was studied by the Fura-2 microfluorometric technique. The results showed that [Ca²⁺]_i was increased dose-dependently with the addition of ONOO⁻ (0–40 μmol/l) after 5 s and then decreased rapidly back to the basal level after ONOO⁻ was removed. [Ca²⁺]_i response to ONOO⁻ can be blocked by removing Ca²⁺ from the bath or adding L-type calcium channel antagonist nifedipine (10 μmol/l) to the bath. [Ca²⁺]_i transients by ONOO⁻ were substantially inhibited by dithiothreitol (DTT), which indicated ONOO⁻ may alter the L-type calcium channel on neuronal cell by thiol oxidation. To elucidate the mechanism of ONOO⁻ on [Ca²⁺]_i, the electron spin resonance spin-labeling technique was used to study the effects of ONOO⁻ on the membrane fluidity and the membrane protein conformation on freshly dissociated neurons. The results indicate that ONOO⁻ decreases membrane fluidity both near the surface and deep in the membrane and affects protein conformation. The fact that DTT effectively inhibits the deterioration supports the conclusion that the change of membrane fluidity and protein conformation is involved in [Ca²⁺]_i overload in neuronal cells induced by ONOO⁻.     

LSP spectral changes correlating with SERS activation and quenching for R6G on immobilized Ag nanoparticles

In terms of chemical enhancement in Surface Enhanced Raman Scattering (SERS), we investigated the effect of halide and other anions to rhodamine 6G (R6G) adsorbed Ag particles that were immobilized on the substrates. The residual species on chemically prepared Ag particles such as citrate or a-carbon were thoroughly substituted by various anions, e.g., Cl⁻, Br⁻, I⁻, SCN⁻, CN⁻, or S₂O₃ ²⁻ anions, whose adsorption features are elucidated by the formation constants for AgX₂ ^(m−1)−, here X denotes the above anions. In particular, Cl⁻, Br⁻, or SCN⁻ ions activated SERS of R6G via intrinsic electronic interaction with Ag, whereas CN⁻, S₂O₃ ²⁻, or I⁻ anions quenched it due to their exclusive adsorption onto the Ag surfaces. We found that the activation process with the anions commonly yields a marked blue-shift of the coupled plasmon peak from ca. 650–700 to 500–550 nm in elastic scattering. It is rationalized by slight increase of the gap size between adjacent Ag nanoparticles by only ca. 1 nm based on theoretical simulations. This is probably caused by slight dissolution, oxidative etching, of the particles according to large formation constants of the complexes. Consequently, partly remaining negative charges on the Ag surface, and a slight increase in the gap size, providing huge electric field, facilitated R6G cations to adsorb on the nanoparticles, especially at the junction.     

Addition and spin exchange rate constants by longitudinal field μSR: The Mu+NO reaction

The addition reaction Mu+NO+M→MuNO+M and the spin exchange reaction Mu(↑) +MO(↓)→Mu(↓)+NO(↑) have been measured by longitudinal field μSR at room temperature in the presence of up to 58 atm of N₂ as inert collider. The pressure dependence of the longitudinal relaxation rate due to the addition reaction (λ_c) demostrates that the system is still in the low pressure regime in this pressure range. The corresponding termolecular rate constant has been determined ask _0,Mu =(1.10±0.25)×10⁻³² cm⁶ molecules⁻² s⁻¹, almost 4 times smaller than the corresponding H atom reactionk _0,H=3.90×10⁻³² cm⁶ molecules⁻² s⁻¹ [I.M. Campbell et al., J. Chem. Soc. Faraday Trans. 1.71 (1975) 2097]. The average value of the spin exchange rate constants in the 2.5–58 atm pressure range,k _SE=(3.16±0.06)×10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, is in good agreement with previous values obtained by transverse field μSR [D.G. Fleming et al., J. Chem. Phys. 73 (1980) 2751].     

Total electron scattering cross-sections for O atoms: O2 and O3 molecules

Total (elastic+inelastic) cross sections fore ⁻-O,e ⁻-O₂ ande ⁻-O₃ scattering have been calculated at sample energies between 100 and 1000eV. The basice ⁻-O atom scattering amplitudes are obtained from the partial wave analysis with a complex optical potential. Thee ⁻-O₂ ande ⁻-O₃ cross-sections are obtained through the independent atom model. Oure ⁻-O₂ cross-sections reproduce the experimental data quite well. Adequate comparisons are made in all the three cases.     

The superconducting properties of YBa2(Cu1−xMx)3O7 for M=Zn and Ni

Transverse and zero-field μSR measurements were made on YBa₂(Cu_1−xNi_x)₃O_7−y withx=0.1 and 0.2, and YBa₂(Cu_1−x Zn _x )₃O_7−y withx=0.03, 0.06, 0.1, and 0.16, wherey≈0.1. Since doping may lead to magnetic ordering this was searched for with both zero and transverse field μSR, but no evidence was found over the temperature range studied: 10–100 K. However, depolarization rates as functions of temperature were obtained, and the low temperature values of these are σ=3.2 μs⁻¹.1.6μs⁻¹, and 1 μs⁻¹ forx=0.01, and 0.2 Ni, respectively, and σ=0.8 μs⁻¹, 0.75 μs⁻¹, 0.65 μs⁻¹, and 0.4 μs⁻¹ forx=0.03, 0.06, 0.1, and 0.16 Zn, respectively. Estimates for the effect of decreasing electron concentration for Zn are made, but these alone do not account for the drop in σ. Estimates for the effect of scattering on λ and hence σ are made. The reduction in σ for Ni dopant is in surprisingly good agreement with these estimates. For Zn the order of magnitude is correct, but the relative lack of further change in σ after the effect of the first 0.03 addition seems to imply a saturation of the effect of scattering.     

Thermomechanical and conductivity studies of doped niobium titanates as possible current collector material in the SOFC anode

In the reducing atmosphere of the SOFC anode at operating temperatures of 800 °C and above Nb₂TiO₇ is reduced to Nb_1.33Ti_0.67O₄. This material displays very high electronic conductivity of >100 Scm⁻¹, suitable for use in such applications as a current collector. It has a low thermal expansion coefficient of 3 × 10⁻⁶ K⁻¹, however, which may cause problems due to mismatch with other SOFC components, e.g. YSZ. Doping with Fe₂O₃ successfully increased the thermal expansion to a maximum of 6 × 10⁻⁶ K⁻¹. A conductivity of 140 Scm⁻¹ at 900 °C in dry 5% H₂/Ar, with an activation energy of 0.18 eV, was achieved for the Nb_1.344Ti_0.642Fe_0.014O₄, making it suitable for the use as a current collector. Conductivity runs in wet 5%H₂/Ar showed lower conductivities of 15–18 Scm⁻¹ and lower activation energies of 0.08 − 0.09 eV. Single cell tests of Nb_1.33Ti_0.67O₄ showed power outputs of 5.5 − 7.2 mW·cm⁻² at 850 °C, lower than for Ni with 150 − 200 mW·cm⁻² at 850 °C, however, this material displayed much better stability at high temperatures than Ni. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

Steady State and Time-Resolved Fluorescence Studies of a Hemagglutinin from Moringa oleifera

The saccharide binding and conformational characterization of a hemagglutinin, a low molecular weight protein from the seeds of Moringa oleifera was studied using steady state and time resolved fluorescence. The lectin binds sugars LacNAc (K _a = 1380 M⁻¹) and fructose (K _a = 975 M⁻¹), as determined by the fluorescence spectroscopy. It has a single tryptophan per monomer which is exposed on the surface and is in a strong electropositive environment as revealed by quenching with iodide. Quenching of the fluorescence by acrylamide involved both static (K _s = 0.216 M⁻¹) and collisional (K _sv = 8.19 M⁻¹) components. The native protein showed two different lifetimes, τ ₁ (1.6 ns) and τ ₂ (4.36 ns) which decrease and get converted into a single one, (2.21 ns) after quenching with 0.15 M acrylamide. The bimolecular quenching constant, k _q was 7.55 × 10¹¹ M⁻¹ s⁻¹. ANS binding studies showed that the native protein has exposed hydrophobic patches which get further exposed at extreme acidic or alkaline pH. However, they get buried in the interior of the protein in presence of 1 M GdnHCl or urea.     

Electrical transport properties of CuWO4

The temperature dependence of the electrical conductivity, thermoelectric power and dielectric constant of the antiferromagnetic CuWO₄ have been studied in the temperature range 300–1000 K. The conductivity results can be summarised by the equations σ_I=6.31 × 10⁻³ exp (−0.29 eV/kT) ohm⁻¹ cm⁻¹ in the temperature range 300–600 K and σ_II=3.16 × 10⁵ exp (−1.48 eV/kT) ohm⁻¹ cm⁻¹ between 600 K and 1000 K. The thermoelectric power can be expressed byθ=[− 1.25 (10³/T) + 3.9] mV/K. Initially dielectric constant increases slowly but for high temperatures its increase is fast.     

Low-temperature mobility of surface electrons and ripplon-phonon interaction in liquid helium

The low-temperature dc mobility of the two-dimensional electron system localized above the surface of superfluid helium is determined by the slowest stage of the longitudinal momentum transfer to the bulk liquid, namely, by the interaction between the surface and volume excitations of liquid helium, which decreases rapidly with the temperature. Thus, the temperature dependence of the low-frequency mobility is μ_dc ≈ 8.4 × 10⁻¹¹ n _e T ^−20/3 cm⁴ K^20/3/(V s), where n _e is the surface electron density. The relation T ^20/3 E_⊥⁻³ ≪ 2 × 10⁻⁷ between the pressing electric field (in kilovolts per centimeter) and temperature (in Kelvins) and the value ω ≲ 10⁸ T ⁵ K⁻⁵ s⁻¹ of the driving-field frequency have been obtained, at which the above effect can be observed. In particular, E _⊥ ≃ 1 kV/cm corresponds to T ≲ 70 mK and ω/2π ≲ 30 Hz.     

O<Subscript>2</Subscript> photodesorption from AuO<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack> and Au<Subscript>2</Subscript>O<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack>

Using time-resolved photoelectron spectroscopy, the decay channels of AuO₂⁻ and Au₂O₂⁻ following photoexcitation with 3.1-eV photons have been studied. For AuO₂⁻, a state with a rather long lifetime of 30 ps has been identified. Its decay path could not be determined but photodesorption can be excluded. For Au₂O₂⁻, the spectra indicate O₂ desorption after 3.1-eV photoexcitation on a time scale of 1 ps. While comparing these results on Au _n O₂⁻ with analogous data on Ag _n O₂⁻ clusters, a discernible pattern emerges: for dissociatively bound O₂(AuO₂⁻, Ag₃O₂⁻), there are long-living excited states which do not decay by oxygen desorption, while for molecular chemisorption (Au₂O₂⁻, Ag₂O₂⁻, Ag₄O₂⁻, Ag₈O₂⁻), the 3.1-eV photoexcitation triggers fast O₂ desorption with a high quantum yield.