Relaxation Behavior of Laser-Polarized Xe Gas: Size Dependency and Wall Effect of the T1 Relaxation Time in Glass and Gelatin Bulbs

Size dependency of the relaxation time T₁ was measured for laser-polarized ¹²⁹Xe gas encapsulated in different sized cavities made by glass bulbs or gelatin capsules. The use of laser-polarized gas enhances the sensitivity a great deal, making it possible to measure the longer ¹²⁹Xe relaxation time in quite a short time. The size dependency is analyzed on the basis of the kinetic theory of gases and a relationship is derived in which the relaxation rate is connected with the square inverse of the diameter of the cavity. Such an analysis provides a novel parameter which denotes the wall effect on the relaxation rate when a gas molecule collides with the surface once in a second. The relaxation time of ¹²⁹Xe gas is also dependent on the material which forms the cavity. This dependency is large and the relaxation study using polarized ¹²⁹Xe gas is expected to offer important information about the state of the matter of the cavity wall.     

Dielectric studies in dilute solutions

Dielectric studies in dilute solutions of cyclohexane and benzene have been carried out in the temperature range 294–318°K. The observed data have been utilized to evaluate the relaxation times and thermodynamic parameters of these molecules. The high values ofα for 2-acetyl pyridine indicate the occurrence of more than one relaxation time. In the remaining systems, the observed lowα values indicate their rigid behaviour. The variation in the dielectric relaxation time is mostly correlated with the change in the heterocyclic configuration of the system.     

129Xe and131Xe NMR of xenon on silica surfaces at 77 K

Little is known about¹²⁹Xe NMR spectral features and spin-lattice relaxation behavior, and the dynamics of xenon atoms, for xenon adsorbed on solid surfaces at cryogenic temperatures (≤77 K), where exchange with gas-phase atoms is not a significant complication. We report¹²⁹Xe NMR experiments at 9,4 T that provide such information for xenon adsorbed onto the hydroxylated surface of a number of microporous silica samples at 77 K. A convenient design for these cryogenic experiments is described. Dynamics of surface-adsorbed xenon atoms on the time scale of seconds can be observed by¹²⁹Xe NMR hole-burning experiments; much slower dynamics occurring over hours and days are evidenced from changes with time of the¹²⁹Xe NMR chemical shifts. The peak maxima occur in the region ca. 180–316 ppm, considerably downfield of¹²⁹Xe shifts previously reported on surfaces at higher temperatures, and closer to the shift of xenon bulk solid (316.4±1 ppm). The¹²⁹Xe spin-lattice relaxation timesT ₁ range over five orders of magnitude; possible explanations for both nonexponential relaxation behavior and extremely shortT ₁ values (35 ms) are discussed. Preliminary¹³¹Xe and¹H NMR results are presented, as well as a method for greatly increasing the sensitivity of¹²⁹Xe NMR detection at low temperatures by using closely-spaced trains of rf pulses.     

Quantitative time-resolved EPR CIDEP study of the photodecomposition oftrans-azocumene in solution

The cumyl radical system, which is created after laser flash irradiation oftrans-azocumene in benzene solution at room temperature, is investigated using time-resolved EPR spectroscopy. From the quantitative analysis of EPR time-profiles at different microwave powers the spin relaxation timesT ₁=3.5±0.3 μs andT ₂=2.5±0.1 μs are evaluated as well as the magnitude of the chemically induced electron polarization (CIDEP), which is generated by the radical pair mechanism (RPM). The geminate RPM polarization is found to be considerably smaller than the F-pair one, 32±2 and 48±5 in units of the Boltzmann polarization, respectively. This is attributed to an initial radical separation in the geminate pair, caused by the cleavage reaction. Besides cleavage, the photoexcitedtrans-azocumene also decays via isomerization to the thermally unstablecis-isomer, the lifetime of which is found to be 14±3 μs at 293 K in benzene, three times longer than in cyclohexane. The quantum yield of free radicals, escaping from the primary cage, is determined as 0.28±0.06 for the decay of the excitedtrans-azocumene and 0.18±0.04 for the thermal cleavage of thecis-isomer. The self-termination of cumyl radicals proceeds with a rate constant 2k _t=7±1)·10⁸ M^?1s^?1 in benzene at RT.     

Intermolecular Dipole–Dipole Relaxation of Xe Dissolved in Water

Intermolecular ¹²⁹Xe–¹H nuclear Overhauser effects and ¹²⁹Xe longitudinal relaxation time measurements were used to demonstrate that the dipole–dipole coupling is the dominant relaxation mechanism for ¹²⁹Xe in water, at room temperature. ¹²⁹Xe–¹H cross-relaxation rates were derived to be ς_XeH 3.2 ± 0.3 × 10⁻³ s⁻¹, independent of xenon pressure (in the range of 1–10 bar) and of the presence of oxygen. Corresponding xenon–proton internuclear distances were calculated to be 2.69 ± 0.12 Å. Using the magnitude of the dipole–dipole coupling and the spin density ratio between dissolved xenon and bulk water, it is estimated that ¹²⁹Xe–¹H spin polarization-induced nuclear Overhauser effects would yield little net proton signal enhancement in water.     

Wall relaxation of spin polarized 129Xe nuclei

The wall relaxation of spin polarized ¹²⁹Xe nuclei is much longer in silicone coated pyrex cells than in uncoated cells. In contrast to uncoated glass cells where the wall relaxation times are unpredictable and usually only a few tens of seconds, the relaxation time of ¹²⁹Xe in silicone-coated cells is usually 20 minutes or longer.     

Dielectric dispersion studies of aromatic ketones in non-polar solvents

Measurements of dielectric constant (ε′) and loss (ε″) have been made at five different microwave frequencies from 1000 MHz to 67.7 GHz for acetophenone, benzophenone and propiophenone in four different non-polar solvents namely benzene, cyclohexane, 1-4-dioxane and n-heptane. Results are reported at five different temperatures from 25°C to 60°C. The dielectric data of these ketones in different solvents are analysed in terms of the Cole-Cole arc plots and superposition of two Debye-type absorptions. Values of mean relaxation times (τ_o), dipolement (μ), overall relaxation time (τ₁) and group relaxation time (τ₂) have been obtained and presented here. The values of relaxation time and dipolemoment are in reasonable good agreement, at the temperatures, at which there are available known data.     

Effects of Thermal Convection on NMR and Their Elimination by Sample Rotation

It is shown that the presence of thermal convection in a sample tube may lead to a variety of anomalous phenomena in prolonged multiple-pulse NMR experiments. They are investigated by applying inversion-recovery pulse sequences to¹²⁹Xe of xenon gas dissolved in deuterated cyclohexane and acetonitrile, and to¹⁹F in xenon difluoride (XeF₂) dissolved in deuterated acetonitrile. If convection is present, the recovery of the magnetization after the π pulse may be very different from the recovery due to the spin–lattice relaxation alone. It may be much faster, very sensitive to temperature, and nonexponential, exhibiting even oscillatory behavior. In addition, the shape of the spectral lines may be seriously distorted. The results show that convection and the resulting anomalies can be effectively eliminated by rotating the sample tube at a spinning speed on the order of 10 Hz. These phenomena may provide novel methods for investigating thermal convection.     

Nuclear Quadrupole spin-lattice relaxation in Rhombohedral Arsenic

The temperature dependence of the spin-lattice relaxation time T₁ in rhombohedral arsenic has been measured by nuclear quadrupole resonance. The relaxation time is inversely proportional to the temperature and of a magnitude which indicates that the relaxation results from the Fermi contact interaction of the conduction electrons and holes and the arsenic nuclei. The density of electrons and holes at the site of the nucleus, averaged over the Fermi surface is approximately 2.6 × 10²¹ carriers cm^?3.     

Dynamics of different molecules adsorbed in porous media

We present in this paper a comparative study on the dynamics of benzene, cyclohexane, and methanol molecules, confined in the pores of MCM-41 molecular sieve and HZSM-5 zeolite. The quasi-elastic neutron scattering (QENS) measurements revealed that the physical state of these adsorbed molecules depended not only on the structural characteristics of the host matrix but also on the chemical properties, such as dipole moment, of the guest molecules. Thus, while no motion was observed in the time-scale of 10⁻¹⁰−10⁻¹² s in the case of methanol, the larger size benzene and cyclohexane molecules are found to perform six-fold and three-fold jump rotation, respectively, when adsorbed inside the cages of HZSM-5 at room temperature. At the same time, all the three molecules are found to undergo a translational motion inside the pores of MCM-41 molecular sieves, the value of diffusion constant being the lowest in case of methanol because of its higher polarity. Translationl motion of the guest molecules inside the pores of MCM-41 can be satisfactorily described by Chudley-Eliott fixed jump length diffusion and accordingly the residence time, jump length and diffusion constant are estimated.     

Solvent effect on the correlation between dipole increment and complexation enthalpy in charge transfer complexes

The dipole moments μ_AB and the complexation enthalpies ?ΔH_ab of a series of complexes between iodine and substituted pyridines were determined in CCl₄ and in C₆H₆ at 298.16 K. Within the limits of experimental errors the moments are the same in CCl₄ as in cyclohexane, whereas in benzene they are systematically larger by 0.48±0.04 Debye. The transfer enthalpies of the complexes from cyclohexane towards CCl₄ are of the order of ?5.9±1.0 kJmole^?1 but reach ?10.4±1.0 kJmole^?1 from cyclohexane towards C₆H₆. A part of the stabilization is due to the increase of the dielectric constant. For benzene however the major part can be ascribed to specific interaction between the solvent molecules and the π-electrons of the pyridine-iodine complexes. The complexation enthalpies (?ΔH_ab) are smaller in benzene and in CCl₄ than in cyclohexane due to the specific bonds between the separate partners and the last solvents, which have to be broken before the iodine-pyridine bond can be formed. The dipole increment Δμ brought about by the complexation is in both CCl₄ and C₆H₆ directly proportional to ?ΔH_ab, the proportionality coefficients being respectively 0.097±0.004 and 0.098±0.004 Debye/kJmole^?1. The similitude between these coefficients and that of 0.094±0.002 found in cyclohexane can be interpreted by the fact that the weak interactions between the separate partners and the active solvents exhibit a Δμ/ΔH coefficient of the same order of magnitude.     

Hole spin relaxation in Ge quantum dots

Hole spin relaxation in an isolated Ge quantum dot due to interaction with phonons is investigated. Spin relaxation in this case occurs through the mechanism of the modulation of the spin-orbit interaction by lattice vibrations. According to the calculations performed, the spin relaxation time due to direct single-phonon processes for the hole ground state equals 1.4 ms in the magnetic field H = 1 T at the temperature T = 4 K. The dependence of the relaxation time on the magnetic field is described by the power function H^?5. At higher temperatures, a substantial contribution to spin relaxation is made by two-phonon (Raman) processes. Because of this, the spin relaxation time decreases to nanoseconds as the temperature is raised to T = 20 K. Analysis of transition probabilities shows that the third and twelfth excited hole states, which are intermediate in two-step relaxation processes, play the main part in Raman processes.     

199Hg and63,65Cu NMR studies of mercury based high-T c superconductors

Hg-oxide ceramic high temperature superconductors were studied by¹⁹⁹Hg and^63,65Cu NMR spectroscopy. Room temperature spectra, spin-spin and spin-lattice relaxation times of samples with different superconducting transition temperatures are presented. A spin-lattice relaxation time ofT ₁=35 msec and a spin-spin relaxation time ofT ₂=1.6 msec were found for the¹⁹⁹Hg NMR. All samples exhibit similar characteristic powder spectra caused by an axially symmetric¹⁹⁹Hg spin interaction. The isotropic value and the anisotropy of the tensor relative to solid HgCl₂ as a standard substance is estimated. Furthermore, results of^63,65Cu NMR measurements at a temperature of 4.2 K which exhibit a typical powder line shape (forI=3/2) are presented.     

NMR study of fluorine and proton motion in the ionic conductor NH4Sn2F5

The nuclear spin relaxation times of protons and fluorine atoms in the ionic conductor NH₄ Sn₂ F₅ show a particular temperature behaviour: they decrease as the temperature decreases below 200 K and have the same temperature and frequency dependences.Cross relaxation experiments show an important coupling between fluorine atoms and protons. NMR results as well as crystallographic and conductivity data on hydrogenated and deuterated compounds, allow to determine two dominant kinds of motion: at low temperature the reorientation of the ammonium ion is observed and above 250 K the diffusion of the fluorine atoms is responsible for the relaxation. In the high temperature range, the activation energies of the motion deduced from conductivity and NMR measurements are in good agreement.     

电离效率对激光电离团簇的高价离子产物的影响

纳秒激光与团簇相互作用产生高价离子逐渐成为分子物理界的热点之一, 为了深入研究团簇电离的本质, 本文以分子结构相似、元素组成相同的苯、环己烯和环己烷的分子团簇为对象, 利用飞行时间质谱研究了其与5 ns的532 nm激光相互作用时电离产物的价态和强度分布. 结果表明: 这三种化合物多光子电离效率苯>环己烯>环己烷, 但其高价离子的价态和比值苯是最低的, 环己烷的碳离子最高价态为4价, C³⁺和C²⁺的比值为1.1; 环己烯电离产物C³⁺和C²⁺ 的比值降低为0.6; 苯团簇的最高价态只有3价, C³⁺和C²⁺的比值约为0.4. 引起这种现象的原因可以归结于高的多光子电离效率会导致团簇多位点的电离, 引起团簇在电子加热到发生碰撞电离之前发生解离, 减小了团簇的尺寸, 进而减少了离子发生碰撞电离产生高价离子的反应时间, 最终阻碍了高价态离子的产生.     

Experimental and kinetic modeling study of sooting atmospheric-pressure cyclohexane flame

Experimental data and modelling results of the main products and intermediates from a fuel-rich sooting premixed cyclohexane flame were presented in this work. Model predictions well agree with experimental data both in sooting and non-sooting flames. Major and minor species are properly predicted, together with the soot yield. The initial benzene peak was demonstrated to be due to the fast dehydrogenation reactions of the cycloalkane, which gives rise to cyclohexene and cyclohexadiene both via molecular and radical pathways. Once formed cyclohexadiene quickly forms benzene whereas in the postflame zone, benzene comes from the recombination and addition reactions of small radicals, with C₃H₃ + C₃H₃ playing the most important role in these conditions. An earlier soot inception was detected in the cyclohexane flame with respect to a n-hexane flame and this feature is not reproduced by the model that foresees soot formation significant only in the second part of the flame. The model insensitivity of soot to the reactant hydrocarbon was also observed comparing the predictions of three flames of cyclohexane, 1-hexene and n-hexane with the same temperature profile. A sensitivity analysis revealed that soot primarily comes from the HACA mechanism for the three flames, acetylene being the key species in the nucleation. Experimental data on soot inception seem to indicate the importance of the early formation of benzene, that depends on the fuel structure. It is thus important to further investigate the role of benzene and aromatics in order to explain this discrepancy.     

Temperature dependence of21Ne,83Kr,and131Xe quadrupole relaxation and of xenon diffusion in polyatomic solvents

The main aim of the present work is to contribute, by an extension of the experimental data base, to the understanding of quadrupole relaxation of nuclei of noble gases dissolved in molecular liquids. We have performed temperature dependent spin-lattice relaxation rate measurements of²¹Ne,⁸³Kr, and¹³¹Xe in various non-aqueous solvents (e.g. in methanol, ethanol, n-butanol, acetone, acetonitrile, benzene, carbon tetrachloride, N,N-dimethylformamide, dodecane, tetradecane, p-xylene, and hexafluorobenzene). In nine liquids we also measured translational diffusion coefficients of dissolved xenon as a function of temperature by the NMR spin-echo technique, obtaining partly the very first diffusion data for a number of systems. The comparison of the remarkable low activation energies for the noble gas nuclear quadrupole relaxation with that of the noble gas diffusion reveals that both seem to be closely connected. There are strong hints that the nuclear relaxation process follows an empirical “Dη ^γ =A correlation” found previously by Evans and co-workers for the tracer diffusion of noble gases in polyatomic liquids. For almost all solvents γ decreases from¹³¹Xe to²¹Ne parallel with a decrease of the corresponding activation energy for the quadrupolar relaxation. Possible physical reasons for this behavior are briefly discussed. Essential qualitative results in this paper were found to agree with two MD computer simulations for¹³¹Xe relaxation in benzene and methanol. Further MD simulations are proposed which are obviously required for the deeper understanding of the experimental results found in the present paper.     

13C NMR relaxation and computational study of anisole and derivatives in the solution state

¹³C NMR spin–lattice relaxation times and nuclear Overhauser effects were measured at several temperatures for the methoxyl methyl carbon and the phenyl ring carbons in neat samples and in dilute cyclohexane solution for anisole, 4‐methylanisole, and 4‐chloroanisole. Similar measurements were made for 2‐methylanisole, 2‐methyl‐4‐bromoanisole, and 2,4,6‐trimethylanisole in dilute cyclohexane solution. Density functional theory (DFT) computations were performed on anisole, 4‐chloroanisole and 2,4,6‐trimethylanisole to obtain the minimum energy structures and the potential energy barriers to the internal rotations of the methoxyl group. The shortest distance between a methoxyl methyl hydrogen and the ortho hydrogen in anisole is 1.920 Å. The DFT results point to steric interactions that arise thereof as the principal source of the energy barriers to the internal rotation of the methyl or of the methoxyl group. The carbon relaxation data are consistent with the existence of noncovalent intermolecular interaction, especially π ? π stacking interaction. The nuclear magnetic resonance and DFT results are discussed with reference to the rotational characteristics of the methoxyl methyl and the anisotropy in the reorientational motion of anisole and its derivatives in dilute cyclohexane solution. Copyright © 2012 John Wiley & Sons, Ltd.     

EPR Study of Sc<Subscript>2</Subscript>SiO<Subscript>5</Subscript>:Nd<Superscript>143</Superscript> Isotopically Pure Impurity Crystals

The Sc₂SiO₅ single crystals doped with 0.001 at.% of the ¹⁴³Nd³⁺ ion were studied by continuous-wave and pulse electron paramagnetic resonance methods. The g-tensors and hyperfine structure tensors for two magnetically non-equivalent Nd ions were obtained. The spin–spin and spin–lattice relaxation times were measured at 9.82 GHz in the temperature range from 4 to 10 K. It was established that three relaxation processes contribute to the spin–lattice relaxation processes. There are one-phonon spin–phonon interaction, two-phonon Raman interaction and two-phonon Orbach–Aminov relaxation processes. It was established that spin–spin relaxation time is of the same magnitude for neodymium ion doped in Sc₂SiO₅ and in Y₂SiO₅.     

半导体量子阱中电子自旋弛豫和动量弛豫

根据电子自旋轨道耦合对自旋极化弛豫影响的DP机理进一步导出了半导体中电子自旋弛豫与动量弛豫及载流子浓度的关系，并采用飞秒抽运探测技术在室温下测量AlGaAs/GaAs 多量子阱中载流子浓度在 1×10¹⁷—1×10¹⁸cm^-3范围内，电子自旋弛豫时间由58ps增加至82 ps的变化情况，与理论计算值符合，说明了随着载流子浓度的增加，载流子间的频繁散射加速了电子动量驰豫，减弱了电子自旋轨道耦合作用，从而延长了电子自旋寿命. 关键词： 电子自旋轨道耦合 电子自旋弛豫和动量弛豫 飞秒光谱技术