The influence of temperature and dynamic quenching on the properties of 3-hydroxyflavone excited states

The influence of temperature and dynamic quenching on the properties of excited states of the normal and tautomeric 3-hydoxyflavone forms was studied. The stationary two-band fluorescence spectra of this luminophore in acetonitrile were recorded and analyzed. The spectra were observed under excitation by electromagnetic radiation in the region of the S ₁ absorption band over the temperature range 20–80°C. TEMPO was used as a quencher of the excited state. Heating caused temperature quenching of luminescence, and the tautomer formed via the excited state of the normal form of the luminophore was quenched more strongly both in pure solvent and in the presence of the quencher. An analysis of two-band fluorescence parameters led us to conclude that solution heating over the temperature range studied increased the rate of proton transfer by 1.25 times. The introduction of the quencher also accelerated proton transfer by 1.16–1.25 times as the temperature increased from room temperature to 80°C.     

Effect of temperature and dynamic fluorescence quenching on proton transfer in 3-hydroxyflavone

The effect of temperature and a fluorescence quencher on the properties of the excited states of 3-hydroxyflavone is considered. The absorption spectra and the spectra of dual fluorescence exited by electromagnetic radiation in the region of the S ₁ absorption band of 3-hydroxyflavone in acetonitrile are measured and analyzed in the temperature range of 20–80°C. The fluorescence lifetimes are also measured at different temperatures. As a quencher of excited states we used the TEMPO spin quencher. The analysis of the fluorescence parameters shows that the heating of the solution to 60°C leads to a considerable (by a factor of 1.24) increase in the proton-transfer rate for the first absorption band. The introduction of a quencher decreases the yield of the two fluorescence bands by the diffusion mechanism and increases the proton-transfer rate with respect to the rate in the pure solvent by a factor of 1.16 at room temperature and 1.65 at T = 80°C.     

Optical absorption in layered MnGaInS<Subscript>4</Subscript> single crystals

Optical absorption in MnGaInS₄ single crystals has been studied. Direct and indirect optical transitions are found to occur in the range of photon energies of 2.37–2.74 eV and in the temperature range of 83–270 K. The temperature dependence of the band gap has been determined; its temperature coefficients E _gd and E _gi are −5.06 × 10⁻⁴ and −5.35 × 10⁻⁴ eV/K, respectively. MnGaInS4 single crystals exhibit anisotropy in polarized light at the absorption edge; the nature of this anisotropy is explained.     

Solvent-free nanocomposite proton-conducting membranes composed of cesium salt of phosphotungstic acid doped PVDF–CTFE/PEO blend

In this research, novel nanocomposite membranes were prepared using polymer blend of polyethylene oxide (PEO) and polyvinylidene fluoride–chloro tetrafluoro ethylene (PVDF–CTFE) copolymer with cesium salt of phosphotungstic acid (Cs_2.5H_0.5PWO₄₀) as proton conductor. Nanocomposite membranes were prepared by solvent-free procedure. The DSC studies show a decrease in crystalinity of polymer matrix with increasing PEO to PVDF–CTFE proportional ratio and the filler. The TGA studies show that membranes are stable up to 180 °C. The TGA also indicates that addition of cesium salt of phosphotungstic acid increases the thermal stability of membranes. The SEMs exhibit that membranes are non-porous and the additive components are homogenously dispersed. Conductivity tests for membranes were carried out in the range of 25–100 °C in dry and hydrated states. Results show that by increasing the temperature, membranes conductivities are increased. In dry state, except at the temperature of 45 °C, membranes which have the highest crystalinity, have the highest conductivity. The alteration of the conductivity in the range of temperatures in dry condition may be attributed to segmental motion of polymer which resulted in proton hopping from one site to another or increasing free volume for proton motion. In fully hydrated state, dynamic equilibrium between different proton moieties determines the mode of proton conductivity which can be described by Grothuss mechanism. In the presence of water molecule, the free proton may be formed. The conductivity for the membrane in hydrated state with the blend ratio of PVDF:PEO = 95:5 w/w and 10% addition of cesium salt of phosphotungstic acid at the temperature of 90 °C is 1.05 × 10⁻⁴ S cm⁻¹.     

Preparation of sol-gel method P2O5-Al2O3-SiO2 glasses and their characterization

Nanostructures P₂O₅-Al₂O₃-SiO₂ glasses were prepared by sol-gel method. The glasses were characterized by XRD, FTIR and TG/DTA methods. The average pore size of the glass was less than 3 nm as measured by N₂ adsorption — desorption method. The thermal stability was measured as a function of decomposition temperature and weight loss calculations. Proton conductivities of all samples increased with an increase in relative humidity (40–90 %), indicating that continuous paths suitable for proton conduction were developed when glasses heat treated at 300 °C due to the adsorption of water. The temperature dependence of the conductivity for all compositions increases with increasing temperature in the range 30–90 °C with relative humidity 70 %. The overall conductivity was in the range 10⁻⁴–10⁻³ S/cm for compositions.     

Spectral components that form UV absorption spectrum of Ce3+ and Ce(IV) valence states in matrix of photothermorefractive glasses

We have measured the UV absorption spectra of photothermorefractive glasses of the system Na₂O-ZnO-Al₂O₃-NaF-SiO₂ doped by cerium oxide in the range of (2.8–5.0) × 10⁴ cm⁻¹ (360–200 nm). The spectra have been processed by the method of dispersion analysis based on the analytical convolution model for the complex dielectric function of glasses. We show that the absorption band centered at 3.3 × 10⁴ cm⁻¹ (∼303 nm) that is attributed to the transition 2F _5/2 → 5d in the Ce³⁺ ion, is an envelope of three spectral components. The broad absorption range (3.5–4.7) × 10⁴ cm⁻¹ (200–270 nm) that is commonly interpreted as a charge transfer band of the Ce(IV) valence state, is an envelope of at least three spectral components.     

Internal friction in directed-crystallization (Cu-Sn)-Nb alloys

The distinctive features of the low-frequency internal friction Q ⁻¹(T) of (Cu-Sn)-Nb composites at high temperatures (up to 400°C) are investigated for strains in the range 10⁻⁵–10⁻⁴. Considerable hysteresis of Q ⁻¹(T) in the heating-cooling cycle is recorded, including the presence of a minimum at ∼175°C when the sample is heated to 400°C and two peaks P ₂ (at 280°C) and P ₁ (at ∼100°C) when the sample is cooled from 400°C. The activation energy of the anomalous internal friction background (up to 175°C), the oxygen diffusion parameters, and the oxygen concentration in the niobium fibers (all of which govern the peak P ₂) are calculated, and the value and temperature dependence of the yield point of the bronze matrix (which govern the peak P ₁) are estimated. Zh. Tekh. Fiz. 68, 114–117 (November 1998)     

Potentiometric DO detection in water by ceramic sensor based on sub-micron RuO2 sensing electrode

An alumina sensor using sub-micron RuO₂ sensing electrode (SE) was fabricated and examined for potentiometric dissolved oxygen (DO) detection in water at a temperature range of 9–35 °C. The electromotive force (emf) response at these temperatures was linear to the logarithm of DO concentration in the range from 0.6 to 8.0 ppm (log[O₂], −4.71 to −3.59). RuO₂-SE displays a Nernstian slope of −41 mV per decade at pH 8.0. It was also found that the response/recovery time to DO changes were sluggish as the water temperature cools down. Response time T ₉₀ to DO changes increased from 8 min at a temperature of 23 °C to about 30 min at a temperature of 9 °C. The proton conductivity of hydrous RuO₂ appears to be due to the dissociative adsorption of water and the formation of acidic OH groups in Ru (III,IV) cluster ions. In strong alkaline solutions, the sensor’s emf exhibited a mixed potential of fast and slow electrochemical reactions involving DO, RuO₄ ²⁻ and OH⁻ ions. The results also revealed that as pH of the solution increases to pH 10.0–13.0, the response/recovery rate becomes faster, stabilizing more or less quickly depending upon the solution alkalinity. Scanning electron microscopy, energy dispersive X-ray-analysis and impedance spectroscopy techniques were used to examine respectively the morphology, crystalline structure and electrochemical behaviour of sub-micron RuO₂ oxides.     

Influence of post-grown Li-rich and Li-poor vapor transport equilibration on composition, OH− absorption and optical-damage threshold of Mg (5 mol%) : LiNbO3 crystals

Li-rich (Li-poor) vapor transport equilibration (VTE) treatments on a number of Z-cut 0.47 mm thick congruent MgO (5 mol% in melt) : LiNbO₃ crystals were carried out at 1100°C over different durations ranging in 1–172 (40–395) h. Neutron activation analysis shows that neither Li-rich nor Li-poor VTE-induced Mg and Nb loss from the crystal occurred. The Li₂O content in the crystal was measured as a function of VTE duration by the gravimetric method. The Li-rich/Li-poor VTE effects on OH⁻ absorption were studied in comparison with the as-grown crystal. The study shows that the Li-rich VTE results in OH⁻ absorption band annihilation. After further oxidation treatment the band reemerges and peaks at the same wavenumber as that of the as-grown crystal (∼3535.6 cm⁻¹), showing that the MgO concentration in the Li-rich VTE crystal is still above the optical-damage threshold. The Li-poor VTE causes OH⁻ band shift to 3486.3–3491.6 cm⁻¹, indicating that the MgO concentration in all Li-poor VTE crystals is all below the optical-damage threshold. Further successive Li-rich VTE and oxidation treatments on the Li-poor VTE-treated crystal lead the band to shift back to 3535.6 cm⁻¹, showing that the post Li-rich VTE brought the Li-poor VTE-treated crystal above the optical-damage threshold again. It is found that the peaking position, band width, peaking absorption and band area of the absorption at ∼3486 cm⁻¹ all increase monotonously with the decrease of the Li₂O content arising from prolonged Li-poor VTE, and quantitative relationships to the Li₂O content are established for the latter two parameters. The VTE effects on the OH⁻ absorption are conducted with the VTE-induced OH⁻ content alteration and charge redistribution.     

NMR studies on hydrogen bonding and proton transfer in Mannich bases

A review of studies on the ortho Mannich bases containing various substituents in the phenyl ring on the basis of¹H,¹³C and¹⁵N nuclear magnetic resonance (NMR) spectra in various solvents over the temperature range 110–298 K is presented. Some new results are also included. The data gathered so far show that there is some critical (inversion) range of ΔpK _a (= pK _a(NH⁺) − pK _a(OH)) in which the proton transfer equilibrium appears. This inversion range is well reflected in the behaviour of secondary deuterium isotope effect in¹³C NMR spectra. A strong temperature effect on the strength of hydrogen bonding should be emphasized. The¹H chemical shift for trichloroderivative increases from 13.5 at room temperature up to 17 ppm at 130 K when the proton is equally shared between the bridging atoms (¹ J(¹H,¹⁵N) = 30–40 Hz). The potential for the proton motion in such bridges is discussed taking into account the behaviour in the ultraviolet and infrared spectra. The role of dimerization in proton transfer equilibria is shown. In addition the rotation of OH groups involved in hydrogen bond formation and nitrogen pyramidal inversion was studied by the¹H dynamical NMR spectra.     

Luminescence of oxygen molecular ion in neodymium-doped yttria

Pulsed cathodoluminescence spectra of Nd³⁺:Y₂O₃ compacts registered after their annealing in air at a temperature above 950°C exhibit a structured band in the range 610–660 nm with four well-resolved components located at wavelengths of 620.6, 630.6, 645.3, and 655.6 nm. At the same time, the lattice parameter of the cubic yttria increases, and the color of samples changes from light blue to yellowish. In addition, the nearly complete absorption of the intrinsic luminescence band of yttria is observed in the range of 380–600 nm with a center at λ = 485 nm. We assume that these effects are caused by oxygen molecular ions O₂⁻, which are formed in cubic yttria due to the penetration of oxygen into natural anion vacancies. The frequencies of vibrations of the ground and excited states of the oxygen molecular ion has been determined.     

Dynamics of the transformation of bistable centers and lattice in CdF<Subscript>2</Subscript> crystals under femtosecond pulsed photoexcitation

The kinetics of photoinduced absorption spectra of CdF₂ crystals with bistable indium and gallium centers under femtosecond pulsed excitation has been experimentally investigated. Based on the example of indium ions, it is shown that the transmission band in the absorption spectrum of deep centers is formed for 0.8–1 ps, which significantly exceeds the photon absorption time. This process is interpreted as a result of displacement of indium ion from the initial interstitial position to a site of neighboring unit cell; the displacement velocity is estimated to be 200–250 m/s, a value close to the thermal velocity of this ion at room temperature. The times characteristic of the formation of free polarons as a result of the displacement of neighboring lattice ions have been experimentally estimated for the first time at a level of 0.8–1.2 ps. The capture times of free polarons by trivalent gallium and indium ions are estimated (5 and 10 ps, respectively), as well as the corresponding cross sections (2 × 10⁻¹⁶ and 8 × 10⁻¹⁶ cm²).     

Magnetoresistivity and acoustoelectronic effects in a tilted magnetic field in <Emphasis Type="Italic">p</Emphasis>-Si/SiGe/Si structures with an anisotropic <Emphasis Type="Italic">g</Emphasis> factor

Magnetoresistivity ρ _xx and ρ _xy and the acoustoelectronic effects are measured in p-Si/SiGe/Si with an impurity concentration p = 1.99 × 10¹¹ cm⁻² in the temperature range 0.3–2.0 K and an tilted magnetic field up to 18 T. The dependence of the effective g factor on the angle of magnetic field tilt θ to the normal to the plane of a two-dimensional p-Si/SiGe/Si channel is determined. A first-order ferromagnet-paramagnet phase transition is observed in the magnetic fields corresponding to a filling factor ν = 2 at θ ≈ 59°–60°.     

Effect of mixed glass formers on the crystallization kinetics in AgI–Ag<Subscript>2</Subscript>O–V<Subscript>2</Subscript>O<Subscript>5</Subscript>–MoO<Subscript>3</Subscript> glassy superionic system

The crystallization and glass transition kinetics using differential scanning calorimetry (DSC) in 50AgI–33.33Ag₂O–16.67[(V₂O₅)_1−x –(MoO₃) _x ] superionic glassy system is discussed. Thermal stability of glass, studied using various criteria, does not vary significantly with glass former variation. However, the activation energies for structural relaxation (E _s) at glass transition temperature and crystallization (E _c) obtained using Moynihan and Kissinger, Matusita-Sakka formulations found to exhibit interesting trends with MoO₃ substitution in the glass matrix. It is noticed that the electrical conductivity (σ)–temperature (T) cycles obtained at a typical heating rate of 1 °C/min do exhibit significant thermal events. The conductivity after first heating cycle at room temperature is found to be increasing with MoO₃ content and maximum for x = 0.3 (~10⁻³ Ω⁻¹ cm⁻¹ at 30 °C) which is comparable to that of the host 50AgI–33.33Ag₂O–16.67V₂O₅ glassy system. The parameters obtained from σ–T plots and DSC scans do complement each other in a particular range of composition.     

X-ray determination of thermal expansion of cadmium fluoride and lead fluoride

The lattice parameters of CdF₂ andβ-PbF₂ have been determined over the temperature range 300–670 K. The coefficient of expansion at room temperature is 21·3 × 10⁻⁶ K⁻¹ and 25·4 × 10⁻⁶ K⁻¹ for CdF₂ and PbF₂ respectively and it increases linearly with temperature over the range of temperature covered. The Grüneisen parameter decreases with temperature in both the crystals.     

Fundamental absorption edge of evaporated amorphous WO3 films

The fundamental absorption edge of evaporated WO₃ films is investigated. The optical gap of the virgin film is estimated to be 3.41 eV at room temperature and it decreases with increase of annealing temperature up to 200°C. Annealing at 300°C leads to change in the spectral shape, which is caused by crystallization. For the films annealed at 200°C, temperature coefficient of the optical gap is estimated to be −2×10⁻⁴ eV/K and the slope of Urbach's tail is found to be independent of measuring temperature up to 200°C. With electrolytic coloration, shift of the optical gap toward higher energy is observed. Magnitude of this shift is estimated to be 0.05 eV at the color center concentration of 7.5×10²¹ cm⁻³ when H⁺ electrolyte is used. If Li⁺ electrolyte is used, the magnitude of this shift is about three times larger than in the case of H⁺ electrolyte. This fact is interpreted by a small change in the host matrix structure owing to the injection of proton or Li⁺ during coloration.     

Study of spatial characteristics of the incomplete surface discharge in atmospheric-pressure air

Spectra of plasma of the incomplete surface discharge in atmospheric-pressure air were measured. Bands of the systems 2⁺, 1⁺, and 1⁻ of nitrogen were identified. It was shown that periodic excitation at the C ³Π _u nitrogen level results in local equilibrium characterized by a vibrational temperature of 2250 K. For the 1⁺ band system of nitrogen, the structure of rotational transitions was resolved; it was found that the distribution in the 1⁺ band system is significantly nonequilibrium due to the A ³Σ _u ⁺ level metastability. Transverse and longitudinal sections of the plasma band were scanned. It was found that the transverse distribution of the emission intensity has a maximum at a distance of 1 mm from the electrode edge followed by an exponential decay. The plasma band width was ∼5 mm. The longitudinal discharge structure consisted of a system of microchannels (with a characteristic diameter of 0.4 mm) with diffuse overlapping. The average channel density was 10–15 cm⁻¹. It was shown that the spatial distribution of incomplete discharge plasma is characterized by high stability against wide-range variations of discharge electrical characteristics. For example, as the excitation voltage varies within 2–6 kV, the plasma band width changed by no more than 1 mm, and the vibrational temperature varied within 10–12%.     

Mass measurements in the endpoint region of the rp-process at SHIPTRAP

The Penning-trap mass spectrometer SHIPTRAP was designed for precision mass measurements of radionuclides produced in fusion–evaporation reactions. The latest measurement campaign covered heavy nuclei (A>90) related to the astrophysical rapid proton capture process. The masses of 34 neutron-deficient radionuclides have been measured since February 2006 with relative uncertainties between 5×10⁻⁸ and 1×10⁻⁷. Furthermore, the use of an octupolar RF excitation for the time-of-flight ion-cyclotron-resonance technique was investigated and an increase of the resolving power by a factor of ten was observed in agreement with simulations. This will allow to resolve isomeric states with excitation energies of a few 10 keV only.      

Solid polymeric electrolyte of PVC–ENR–LiClO<Subscript>4</Subscript>

The ionic conductivity of PVC–ENR–LiClO₄ (PVC, polyvinyl chloride; ENR, epoxidized natural rubber) as a function of LiClO₄ concentration, ENR concentration, temperature, and radiation dose of electron beam cross-linking has been studied. The electrolyte samples were prepared by solution casting technique. Their ionic conductivities were measured using the impedance spectroscopy technique. It was observed that the relationship between the concentration of salt, as well as temperature, and conductivity were linear. The electrolyte conductivity increases with ENR concentration. This relationship was discussed using the number of charge carrier theory. The conductivity–temperature behaviour of the electrolyte is Arrhenian. The conductivity also varies with the radiation dose of the electron beam cross-linking. The highest room temperature conductivity of the electrolyte of 8.5 × 10⁻⁷ S/cm was obtained at 30% by weight of LiClO₄. The activation energy, E _a and pre-exponential factor, σ _o, are 1.4 × 10⁻² eV and 1.5 × 10⁻¹¹ S/cm, respectively.     

An unique method of determining the excitation energy migration coefficient in organic liquid scintillators

An experimental method is described to determine the excitation energy migration coefficient Λ in an organic liquid scintillator by measuring the energy transfer rate parameterk ₃ and the quenching rate parameterk _7b as a function of temperature in the range of 20°C–70°C. In this experiment we have used toluene as donor, ethyl-1-n-butyl-2-methyl-5-hydroxyindole-3-carboxylate (EBMHC) as acceptor and bromobenzene as quencher. The values of Λ are found to be in good agreement with literature values. Hence this method can be used to determine the migration coefficient in organic liquid scintillators.