EPR studies on aqueous sucrose solutions using (Cu(H2O)6)2+ spin probe

EPR investigations using Cu²⁺ ion as a probe have been performed on supersaturated sucrose solution with percent concentration c = 66 as a function of temperature T, and at room temperature as a function of c. The motionally averaged spectrum of [Cu(H₂O)₆]²⁺ was used to monitor the changes in intermolecular interactions that occur as a function of [c, T]. A drastic increase in the line width, symptomatic of increase in the rotational correlation time of [Cu(H₂O)₆]²⁺, is observed between 293 and 288 K. The motionally averaged spectrum disappears below 281 K. The motionally averaged spectrum is also absent in the room temperature spectra of the solution with c= 85. Even in the [c, T] range where [Cu(H₂O)₆]² is found to be nearly static, these molecules appear to have an orientational fluctuation manifesting in the m ₁ dependence of the line width of the parallel component.     

On the isotope effect of Tc in squaric acid (H2C4O4)

The phase transition mechanism in squaric acid is reviewed from the viewpoint of the pressure dependence of the O—H—O bond length (2R). The two-dimensional pressure effect on T_c is extracted from the hydrostatic pressure behaviour by taking account of the uniaxial stress applied perpendicular to the layer planes. The result shows that if 2R(H₂SQ) is stretched to 2R(D₂SQ) the phase transition temperature 7^H _c of H₂SQ coincides with T^D _c of D₂SQ. This result supports the suggestion that the phase transition mechanism is of the order-disorder type.     

Influence of Vibrational Excitation of Triplet Molecules of Anthraquinone on the Photochemical Processes in Gas Mixtures with Amines

By the quenching of the delayed fluorescence (DF) of anthraquinone vapors by aliphatic amines (diethylamine, dibutylamine, cyclohexylamine) and pyridine the photoinduced processes proceeding with the participation of vibrationally excited triplet molecules of anthraquinone have been investigated. The DF quenchingrate constants K _q varying from 1·10⁶ sec^–1·torr^–1 in mixtures with diethylamine to 7·10³ sec^–1·torr^–1 in mixtures with pyridine have been estimated. A correlation between the values of K _q and the ionization potentials of foreign gases confirming the important role of interactions with charge transfer in the quenching of triplet molecules in the gas phase has been established. The influence of other relaxation processes on the DF quenching is considered. It is shown that the intermolecular vibrational relaxation in the T ₁ triplet state leading to the establishment of relaxation equilibrium at a vibrational temperature T _vib considerably increasing the medium temperature is the fastest process among the biomolecular processes (rate constants K _col ^V > 10⁶ sec^–1·torr^–1 > K _q). The values of T _vib and the vibrational energies E _vib of the triplet molecules after the energy exchange in the collisional complex have been estimated. It has been concluded that the photochemical reaction yield is determined by the intermolecular processes proceeding in the T ₁ state at a vibrational equilibrium characterized by high values of T _vib. The influence of E _vib of triplet molecules on the DF quenching rates at a photoinduced electron transfer is considered.     

Vibrational non-equilibrium in the hydrogen–oxygen reaction. Comparison with experiment

A theoretical model is proposed for the chemical and vibrational kinetics of hydrogen oxidation based on consistent accounting of the vibrational non-equilibrium of the HO₂ radical that forms as a result of the bimolecular recombination H+O₂ → HO₂. In the proposed model, the chain branching H+O₂ = O+OH and inhibiting H+O₂+M = HO₂+M formal reactions are treated (in the terms of elementary processes) as a single multi-channel process of forming, intramolecular energy redistribution between modes, relaxation, and unimolecular decay of the comparatively long-lived vibrationally excited HO₂ radical, which is able to react and exchange energy with the other components of the mixture. The model takes into account the vibrational non-equilibrium of the starting (primary) H₂ and O₂ molecules, as well as the most important molecular intermediates HO₂, OH, O₂(¹Δ), and the main reaction product H₂O. It is shown that the hydrogen–oxygen reaction proceeds in the absence of vibrational equilibrium, and the vibrationally excited HO₂(v) radical acts as a key intermediate in a fundamentally important chain branching process and in the generation of electronically excited species O₂(¹Δ), O(¹D), and OH(²Σ⁺). The calculated results are compared with the shock tube experimental data for strongly diluted H₂–O₂ mixtures at 1000 < T < 2500 K, 0.5 < p < 4 atm. It is demonstrated that this approach is promising from the standpoint of reconciling the predictions of the theoretical model with experimental data obtained by different authors for various compositions and conditions using different methods. For T < 1500 K, the nature of the hydrogen–oxygen reaction is especially non-equilibrium, and the vibrational non-equilibrium of the HO₂ radical is the essence of this process. The quantitative estimation of the vibrational relaxation characteristic time of the HO₂ radical in its collisions with H₂ molecules has been obtained as a result of the comparison of different experimental data on induction time measurements with the relevant calculations.     

Study on the phase transitions by nuclear magnetic resonance of α-type RbAl(SO4)2·12H2O and β-type CsAl(SO4)2·12H2O single crystals

The thermodynamic properties, spin–lattice relaxation times, T₁, and spin–spin relaxation times, T₂, of the ²⁷Al, ⁸⁷Rb, and ¹³³Cs nuclei in MAl(SO₄)₂·12H₂O (M=Rb and Cs) crystals were investigated, and the two crystals were found to lose H₂O with increases in temperature. From our results for T₁ and T₂, we conclude that the discontinuities near T_d in the T₁ curves of the two crystals correspond to structural changes. In both crystals, below T_d the water molecules surrounding the Al³⁺ and M⁺ nuclei form distorted octahedra, whereas above T_d the water molecules around the Al³⁺ and M⁺ nuclei form regular octahedra and the environment of the Al³⁺ and M⁺ nuclei has cubic symmetry. Further, the T₁ for the ²⁷Al and ⁸⁷Rb nuclei in RbAl(SO₄)₂·12H₂O below T_d were found to increase with increasing temperature, whereas the T₁ for the ²⁷Al and ¹³³Cs nuclei in CsAl(SO₄)₂·12H₂O were found to decrease. It is possible that this difference is due to the different characteristics of α- and β-type crystals.     

Polar relaxation mode in PbZrO3 crystals

Abstract

In antiferroelectric lead zirconate crystal with one phase transition a nearly monodispersive dipolar relaxation has been found in the paraelectric phase and temperature range 20 K below T _c in the frequency region 20—3 ′ 10⁵ Hz. This relaxation has a dominating influence on the temperature dependence of dielectric susceptibility. Relaxation time obeys the Arrhenius law increasing up to 1.5 ′ 10^?2s (11 Hz) at T _c and then exhibiting a distinct jump.     

Photoacoustic measurements of Lorentz broadening in CO2 between 25° C and 450° C

A variable temperature photoacoustic cell has been constructed and tested by studying the interplay of CO₂, H₂O, and NH₃ in synthetic smoke. Saturation effects for CO₂ and NH₃ have been modeled and compared with experiments, and results are obtained for the vibrational relaxation rate associated with NH₃-H₂O collisions. The cell has been used for studying the temperature dependence of self-broadening and N₂ broadening of CO₂ lines. The temperature dependence of the scattering rate is well described by a T ^–n law with n=0.77 in both cases. This result agrees with previous results obtained by tunable diode laser spectroscopy, but disagrees with results obtained by indirect methods.Supported by the Danish Science Research Council under grant no. 11-7777, and by FLS-airloq A/S     

Properties of (Bi1/9Na2/3)(Mn1/3Nb2/3)O3 analysed within dielectric permittivity,conductivity, electric modulus and derivative techniques approach

The (Bi_1/9Na_2/3)(Mn_1/3Nb_2/3)O₃ ceramics with perovskite structure were sintered. The XRD test proved that the samples are cubic (a?=?3.920?±?0.001?Å). Microstructure and atomic composition were determined with a SEM (JSM-5410) equipped with energy dispersion X-ray analyser (ISIS-300). The fluctuation in the chemical composition was found indicating on local disorder. Broadband dielectric spectroscopy in the range 10^?1–3?·?10⁷?Hz was applied within the range of 100–650?K. The real, ?′(f,?T), and imaginary, ?″(f,?T), parts of complex dielectric permitivity characteristics, both in the temperature and frequency domain, show relaxation processes partially covered by electric conductivity. At high temperatures the electric conductivity exhibits a thermally activated behaviour σ(f,?T)?∝?exp(?E _a/kT) but the variable range hopping (VRH) dependence σ?∝?exp[?(T ₀/T)^1/4] is manifested at low temperatures. The derivatives technique in the frequency (??log??/??log?ω) and temperature (??log??/?T) domain enabled various relaxation processes to be distinguished. The data converted to electric modulus representation, M*(f,?T)?=?1/?*, exhibited clearly resolved relaxation peaks. The relaxation times obtained from the peaks position show a slightly non-Arrhenius temperature behaviour with the activation energy varying in 0.4–0.6?eV range and characteristic time of the electric conductivity relaxation of the order of 10^?12?s. The relaxation times can be fitted at better accuracy with the VRH dependence where T ₀ are of the order of 10⁸?K. It is shown that the low frequency ac-conductivity converges to dc-conductivity and the relation σ(0)?～?ω_m?～?τ_m ^?1 typical for the disordered solids applies. The conduction current relaxation relationship behaves in accord with the VRH system: σ_dc?∝?(T/T ₀)^q (e ²/kT) ω_c, where ω_c?=?ν_ph exp[?(T ₀/T)^1/4] is valid for the locally disordered (Bi_1/9Na_2/3)(Mn_1/3Nb_2/3)O₃ compound.     

Accuracy in determination of the temperature and partial pressure of CO<Subscript>2</Subscript> in CO<Subscript>2</Subscript>:N<Subscript>2</Subscript>:H<Subscript>2</Subscript>O:NO<Subscript>2</Subscript> mixtures by multiple-frequency laser probing

We present the results of analysis of the errors introduced by hot-band transitions 11¹0-01¹1, 03¹0-01¹1, 12⁰0-12⁰1 of the CO₂ molecule and the absorption lines of the H₂O and NO₂ molecules in determination of the temperature and partial pressure of CO₂, included in the gas mixture CO₂: N₂:H₂O: NO₂ at atmospheric pressure, by multiple-frequency laser probing using a CO₂ laser tunable over the lines of the 00⁰1-[10⁰0,02⁰0]_I,II ground-state laser transitions. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 810–815, November–December, 2007.     

NQR Study of Phase Transition and Cationic Motion in 4-NH2C5H4NHBiBr4·H2O

Four ⁸¹Br NQR lines in 4-NH₂C₅H₄NHBiBr₄·H₂O were observed in the temperature range between 77 and ca. 380 K; with increasing temperatures the respective sets of higher and lower two resonance lines coalesced into single lines discontinuously at 274 K, showing the occurrence of a first-order type phase transition of this crystal. The transition was confirmed with heat anomaly on a DTA curve. Each higher and lower line of high-temperature phase is assignable to the terminal Br atoms and the bridging ones of one-dimensional poly anions (BiBr₄ ⁻) _n in the crystal structure (C2/c), which was investigated by a X-ray structure analysis at room temperature. The 1/T ₁ temperature dependence of ⁸¹Br NQR follows the usual T ² law in the temperature range between 77 and ca. 140 K, being explained by fluctuation of the EFG at Br nucleus due to lattice vibrations. The T ₁ vs. 1/T curve in the temperature range between about 160 and 190 K was describable by the exponential curves, allowing us the estimation of activation energies. These exponential behaviors of T ₁ of ⁸¹Br NQR are attributable to the fluctuations caused by the thermal motion of 4-NH₂C₅H₄H⁺ ions. Echo signals of the ⁸¹Br NQR could not be detected above 190 K owing to poor S/N with very short T ₂.     

Features of Propene Oxidation in Argon,Carbon Dioxide and Water Vapor Media at a High Density of Reagents

The features of propene oxidation in high-density mixtures of C₃H₆/O₂ ([C₃H₆]₀ = 0.23–0.25 mol/dm³, [O₂]₀ = 0.76–0.82 mol/dm³), diluted with argon, carbon dioxide and water vapor at uniform heating (1 K/min) to T ≤ 620 K are investigated for the first time. From the time dependences of reaction mixtures temperature it is found that propene self-ignition occurs at 465 K and does not depend on the nature of the diluent. Using mass spectrometry analysis it is demonstrated that in the composition of the products of propene oxidation in the Ar and CO₂ medium predominate methanol, acetaldehyde, acetone, acetic acid and formaldehyde; in the oxidation in the H₂O medium, only small amof O₂ in the oxidation of propene increases in ounts of these substances were registered. Degree of consumption the following order: CO₂ ? Ar < H₂O, which is a consequence of the involvement of CO₂ and H₂O molecules in chemical transformations. Mechanisms of the observed processes are discussed.     

Dilution effects analysis of opposed-jet H2/CO syngas diffusion flames

This paper reported the analysis of dilution effects on the opposed-jet H₂/CO syngas diffusion flames. A computational model, OPPDIF coupled with narrowband radiation calculation, was used to study one-dimensional counterflow syngas diffusion flames with fuel side dilution from CO₂, H₂O and N₂. To distinguish the contributing effects from inert, thermal/diffusion, chemical, and radiation effects, five artificial and chemically inert species XH₂, XCO, XCO₂, XH₂O and XN₂ with the same physical properties as their counterparts were assumed. By comparing the realistic and hypothetical flames, the individual dilution effects on the syngas flames were revealed. Results show, for equal-molar syngas (H₂/CO = 1) at strain rate of 10 s^?1, the maximum flame temperature decreases the most by CO₂ dilution, followed by H₂O and N₂. The inert effect, which reduces the chemical reaction rates by behaving as the inert part of mixtures, drops flame temperature the most. The thermal/diffusion effect of N₂ and the chemical effect of H₂O actually contribute the increase of flame temperature. However, the chemical effect of CO₂ and the radiation effect always decreases flame temperature. For flame extinction by adding diluents, CO₂ dilution favours flame extinction from all contributing effects, while thermal/diffusion effects of H₂O and N₂ extend the flammability. Therefore, extinction dilution percentage is the least for CO₂. The dilution effects on chemical kinetics are also examined. Due to the inert effect, the reaction rate of R84 (OH+H₂ = H+H₂O) is decreasing greatly with increasing dilution percentage while R99 (CO+OH→CO₂+H) is less affected. When the diluents participate chemically, reaction R99 is promoted and R84 is inhibited with H₂O addition, but the trend reverses with CO₂ dilution. Besides, the main chain-branching reaction of R38 (H+O₂→O+OH) is enhanced by the chemical effect of H₂O dilution, but suppressed by CO₂ dilution. Relatively, the influences of thermal/diffusion and radiation effects on the reaction kinetics are then small.     

Intermolecular Vibrational Relaxation in Mixtures of Excited Benzophenone and Anthraquinone Molecules with Water Vapor

The intermolecular vibrational energy transfer from triplet molecules of benzophenone and anthraquinone to H₂O molecules has been investigated. To determine the rates of establishment of vibrational (V-V) and thermal (V-T) equilibrium in a vibrational quasi-continuum of mixed singlet-triplet levels, the dependences of the decay rates and intensities of the fast and slow components of delayed fluorescence on the H₂O vapor pressure have been investigated. For V-V relaxation, the efficiencies ₁ and the mean energies E transferred per collision in mixtures with H₂O and other polyatomic foreign gases have been compared. It has been established that the efficiencies ₁ for quasi-resonant vibrational energy transfer (V-V) from benzophenone and anthraquinone to H₂O are an order of magnitude lower than the gas-kinetic ones and lower than those obtained under the same experimental conditions for such foreign gases as C₅H₁₂, SF₆, and CCl₄, and decrease with increasing temperature in the 433–513 K range. It has been concluded that the mechanism of V-V relaxation in mixtures with H₂O are determined by long-range attractive forces. In mixtures with H₂O, no acceleration of V-T relaxation due to the formation of hydrogen bonds has been revealed. The low-efficiency thermalization process (V-T relaxation) is controlled by short-range repulsive forces, and the differences between the intensities ₂ for mixtures of benzophenone and anthraquinone with H₂O and other polyatomic gases are determined by the change in the reduced mass of interacting molecules.     

Nuclear Magnetic Resonance Relaxation Study of the Phase Transitions of Rb2CuCl4·2H2O and Cs2MnCl4·2H2O Single Crystals

The physical properties and phase transitions of Rb₂CuCl₄·2H₂O and Cs₂MnCl₄·2H₂O crystals were investigated by performing ⁸⁵Rb, ⁸⁷Rb, and ¹³³Cs nuclear magnetic resonance relaxation analyses. The temperature dependences of the spectra and the spin–lattice relaxation times T ₁ near T _C are related to changes in the symmetry of the octahedrons consisting of four Cl⁻ ions and two water molecules around Rb⁺ or Cs⁺; the forms of these octahedrons are disrupted by the loss of H₂O. The difference between the relaxation times of the two crystals is possibly due to the difference between the electron structures of the Cu and Mn ions. Cu²⁺ has nine valence electrons in its 3d orbital, whereas Mn²⁺ has five valence electrons in its 3d orbital.     

Nuclear spin-lattice and nuclear spin-spin relaxation time measurements in EuB6 at low temperatures using the spin-echo technique

Experimental results on the nuclear spin-lattice and nuclear spin-spin relaxation times in the ferromagnetic EuB₆ at temperatures below 4·2 K are presented using the external magnetic field,H _ext, in the range of 0 ⩽H _ext ⩽ 10 kG. Nuclear spin-spin relaxation time computed on the basis of the Suhl-Nakamura process turns out to be 3·2μs, which compares well with the experimental value 11·1μs obtained with the 10 kG magnetic field at 1·7 K. It is found that in the ferromagnetic EuB₆,T ₁ is approximately 5 × 10³ times larger thanT ₂ at 1·7 K with the 10 kG magnetic field. Thus the effect ofT ₁ onT ₂ can be neglected. From the experimental value ofT ₂, the value of the homogeneous line broadening is found to be 14 kHz. The corresponding value obtained from the cw method is 175 kHz. This evidently shows the presence of the inhomogeneous line broadening in the cw NMR.     

钛离子辐照对MgB2超导薄膜的载流能力和磁通钉扎能力的影响

利用混合物理化学气相沉积法(hybrid physical-chemical vapor deposition, HPCVD)可以制备出高性能的MgB₂超导薄膜, 再对薄膜进行钛(Ti)离子辐照处理.经过辐照处理后的样品被掺入了Ti元素, 与未处理的干净MgB₂样品相比,其超导转变温度没有出现大幅度的下降, 而在外加磁场下的临界电流密度得到了明显的提高,同时样品的上临界磁场也得到了提高. 在温度5 K, 外加垂直磁场为4 T的情况下, Ti离子辐照剂量为1× 10¹³/cm²的样品的临界电流密度达到了1.72× 10⁵ A/cm², 比干净的MgB₂要高出许多,而其超导转变温度仍能维持在39.9 K的较高水平.     

电子型掺杂高温超导体La2-xCexCuO4飞秒时间分辨动力学研究

利用飞秒时间分辨光抽运探测技术研究了电子型掺杂La_2-xCe_xCuO₄(LCCO)高温超导材料的准粒子超快动力学过程.得到低温(T<0.7T_c)、转变温度附近(0.7T_c≤T≤T_c)和高温(T>T_c)三个温区内的动力学行为.研究发 关键词： 电子型掺杂高温超导体 飞秒时间分辨 准粒子 声子瓶颈     

Homogeneous ignition of CH4/air and H2O and CO2-diluted CH4/O2 mixtures over Pt; an experimental and numerical investigation at pressures up to 16 bar

The homogeneous ignition of CH₄/air, CH₄/O₂/H₂O/N₂, and CH₄/O₂/CO₂/N₂ mixtures over platinum was investigated experimentally and numerically at pressures 4 bar p 16 bar, temperatures 1120 K T 1420 K, and fuel-to-oxygen equivalence ratios 0.30 0.40. Experiments have been performed in an optically accessible catalytic channel-flow reactor and included planar laser induced fluorescence (LIF) of the OH radical for the determination of homogeneous (gas-phase) ignition and one-dimensional Raman measurements of major species concentrations across the reactor boundary layer for the assessment of the heterogeneous (catalytic) processes preceding homogeneous ignition. Numerical predictions were carried out with a 2D elliptic CFD code that included elementary heterogeneous and homogeneous chemical reaction schemes and detailed transport. The employed heterogeneous reaction scheme accurately captured the catalytic methane conversion upstream of the gaseous combustion zone. Two well-known gas-phase reaction mechanisms were tested for their capacity to reproduce measured homogeneous ignition characteristics. There were substantial differences in the performance of the two schemes, which were ascribed to their ability to correctly capture the p–T– parameter range of the self-inhibited ignition behavior of methane. Comparisons between measured and predicted homogeneous ignition distances have led to the validation of a gaseous reaction scheme at 6 bar p 16 bar, a pressure range of particular interest to gas-turbine catalytically stabilized combustion (CST) applications. The presence of heterogeneously produced water chemically promoted the onset of homogeneous ignition. Experiments and predictions with CH₄/O₂/H₂O/N₂ mixtures containing 57% per volume H₂O have shown that the validated gaseous scheme was able to capture the chemical impact of water in the induction zone. Experiments with CO₂ addition (30% per volume) were in good agreement with the numerical simulations and have indicated that CO₂ had only a minor chemical impact on homogeneous ignition.     

The oxidation of H2CO on a copper(110) surface

The oxidation of H₂C¹⁶O by adsorbed ¹⁸O was studied on an Cu(110) sample by temperature programmed reaction spectroscopy. Formaldehyde exchanged its oxygen with surface ¹⁸O upon adsorption to yield H₂C¹⁸O_(a) and ¹⁶O_(a). Formaldehyde was also oxidized by surface ¹⁶O and ¹⁸O atoms to H₂COO which subsequently released one of the hydrogen atoms to form HCOO. The evolution of H₂ from the Cu(110) surface was desorption limited, and the low pre-exponential factor for the recombination of the surface hydrogen atoms suggested stringent requirement on the trajectories of the colliding partners. The formate was very stable and dissociated at elevated temperatures to simultaneously yield H₂ and CO₂. The surface concentration of ¹⁸O exerted a pronounced affect on the activity of the oxidation of formaldehyde on Cu(110).