Dipolar Relaxation Time Research Articles

A Measurement of Relaxation Time of Dipole by Thermally Stimulated Current

A Measurement of Relaxation Time of Dipole by Thermally Stimulated Current

Research Project for Undergraduates: Ionic Thermoconductivity in Dielectrics

Research projects for undergraduates should satisfy several conditions with regards to the physical insight, experimental manipulation, data analyses, and time for the experimental run that are required of the students. These conditions are satisfied by the study of dipole relaxations in dielectric materials using the method of ionic thermocurrents (ITC). ITC can be used to determine the concentration, activation energy, and relaxation time of dipoles in a dielectric. The necessary equipment and some possible research projects are discussed.

Isotope effects in paraelectric relaxation

Through measurements of the complex dielectric susceptibility in the temperature range between 0.3°K and a few °K we have studied the influence of the isotopic substitutions H→D and16O→18O on the relaxation time of the hydroxyl dipole in weakly doped KBr and RbCl. The H→D effect is present in both host lattices. The oxygen effect, however, is only for RbCl outside experimental error, indicating that in this host lattice the reorientation of the hydroxyl ion involves a considerable displacement of its center of mass.

Fluorescence induced by coherent optical pulses

A coherent electromagnetic field, resonant with an optical transition, drives the molecules of a gas into the upper and lower states alternately, unless relaxation is too fast. The observation of this coherent excitation of optical levels by fluorescence measurement is discussed and experimental results are presented. The 4.3μ fluorescence of CO 2 gas excited by a 10.6μ laser is studied; values for the dipole moment and relaxation times are obtained in good agreement with those obtained by other methods.

Many-Electron Effects on the Enhancements of the Korringa Constants and Spin-Lattice Relaxation Rates in Alkali Metals

A quantitative explanation of the observed enhancements of the Korringa product and the ratio of Zeeman to dipolar spin-lattice relaxation times in sodium is given. The explanation is based on an extension of the theories of Moriya and Wolff, using for the generalized paramagnetic susceptibility the expression given by the self-consistent theory of Singwi et al. of spin correlations in a low-density interacting electron gas. The theory also predicts that the Korringa constant in the alkali metals is almost constant - a result in agreement with experiment.

Dielectric Relaxation of ZnF2:LiF and Its Crystallographic Orientation Dependence

The thermally activated release of the dielectric polarization of ZnF2:LiF crystals has been studied using the ionic thermoconductivity method. Measurements of the current discharge are made in single crystal samples oriented for polarization in the [001], [100], and [110] directions in order to study the crystallographic orientation dependence of the dielectric relaxation process. Strong discharge currents are observed only for [001]-oriented crystals. The important parameters (including the activation energy, relaxation time, and number of dipoles) for the process are determined; the change in dipole orientation causing the discharge is examined in relation to possible anion vacancy movements in the lithium-doped zinc fluoride crystal structure. The results confirm that the dielectric relaxation of lithium-doped ZnF2 is dependent upon the crystallographic orientation of the dielectric material.

Electric dipole moment and relaxation time of some ester molecules in benzene. Part II

Electric dipole moments of ethyl phenylacetate, 2-naphthyl acetate, benzyl cinnamate, methyl cinnamate, and diethyl bromomalonate at a frequency of 1 MHz and relaxation times at 9·80 GHz have been determined in benzene at 30 °C. A comparison of µobs and µcalc shows the absence of free rotation from these molecules except for diethyl bromomalonate and restricted rotation of the –OR2 group about the position of minimum potential energy is more probable.

Effects of Electron-Electron Interactions on the Nuclear Spin-Lattice Relaxation Times in Aluminum

The "dipolar" spin-lattice relaxation time in aluminum has been measured for temperatures between $1.3\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}<T<295\ifmmode^\circ\else\textdegree\fi{}\mathrm{K}$. In contrast to the Zeeman spin-lattice relaxation time ${T}_{1z}$, the dipolar time does not vary linearly with $\frac{1}{T}$. This is interpreted in terms of cross relaxation between different groups of nuclear spins, some of which experience quadrupole interactions as a result of defects in the lattice, and others which are well removed from such defects. Both cross-relaxation and spin-lattice effects have been measured by our technique; a three-bath model of the nuclear-spin system permits a separation of these effects, with a true dipolar relaxation time ${T}_{1dd}$ related to ${T}_{1z}$ by $\ensuremath{\delta}=\frac{{T}_{1z}}{{T}_{1dd}}=2.15\ifmmode\pm\else\textpm\fi{}0.07$, $\ensuremath{\delta}$ being independent of temperature. This enhancement of $\ensuremath{\delta}$ over the value 2.00 and the enhancement of the Korringa relation between ${T}_{1z}$ and $K$, the Knight shift, are discussed and compared with the predictions of the theory of Wolff in which the effects of electron-electron interactions are considered. A similar analysis in sodium is included for comparison, sodium being the metal to which the theory is best applicable.

Electron spin correlations in copper

Values of the dipolar spin-lattice relaxation times, T 1dd, of the nuclei in copper metal between 4.2°K and 170°K are reported. The results are discussed in terms of the presence of perturbing quadrupole effects associated with defects in the lattice structure of the metal and a temperature independent value of T 1dd T is deduced.

Electric dipole moment and relaxation time of some ester molecules in benzene. Part I

The electric dipole moments of benzyl benzoate, 2-naphthyl benzoate, o-hydroxybenzyl benzoate, and methyl bromoacetate at a frequency of 1 MHz and relaxation times at a frequency of 9·80 GHz have been determined in benzene at 30 °C. A comparison of µcalc and µobs shows that there is no free rotation in the ester molecules. However the results indicate the possibility of a restricted rotation of the –OR2 group about the position of minimum potential energy.

Molecular characterisation by electric field light‐scattering

AbstractExamination of the intensity of the light scattered from a solution of randomly distributed and oriented molecules, whose major dimensions are of the order of the wavelength of visible light, leads to information about the molecular size. When subjected to electric fields, the molecules become orientated and aligned, thereby changing the polar intensity scattering diagram. Measurements of the intensity changes lead to a host of additional molecular parameters. An outline of the method is given, with particular emphasis being laid on the basic physical principles. Details of the apparatus, the handling of the experimental data and the type of molecular information that can be obtained using direct, alternating and pulsed electric fields are given. Measurements of the equivalent changes in scattered intensity due to d.c. using a.c. fields are probably the most useful as they lead to information on the dipole moments, relaxation time, flexibility and polydispersity of the sample and yet involve little modification to commercial scattering photometers.

Absorption of microwave radiation by solutions. Part 1.—Determination of electric dipole moments and relaxation times

An apparatus and experimental procedure are described for obtaining the complex electric permittivity of a liquid at microwave frequencies. Equations are developed by means of which the electric dipole moment and relaxation time of a polar molecule in a non-polar solvent may be evaluated from these measurements. Results are presented and discussed for five typical solutes in benzene solution at 25.0°.

Electron spin correlations in aluminum

Further values of nuclear dipolar relaxation times in aluminum are reported, and their compatibility with previously measured values is discussed.

Dielectric studies. Part XVIII. Dipole moments and relaxation times of some symmetrically substituted alkylbenzenes

The dielectric absorption at four microwave frequencies of pure liquid benzene and p-cymene at 25 °C, p-xylene and mesitylene at 25, 40, 50, and 60 °C, and solutions of durene and hexamethylbenzene in mesitylene at 60 °C has been examined. All show measurable loss factors and apparent dipole moments of about 0.1 to 0.2 D. These moments are less in magnitude than those associated with the short relaxation time (τ2) process for the polar monoalkylbenzenes. o-xylene and m-xylene. Their relaxation times are too short for molecular reorientation and there is a rough correlation between the number of collisions/molecule s and the reciprocal relaxation time.

Nuclear Magnetic Resonance

A survey is presented of some of the properties of nuclear resonances with reference to possible use by chemists. The principles of nuclear magnetic and electron spin resonance are described briefly. Chemical shift s are considered, and the various ways in which interactions between the nuclear magnetic moments of a sample may affect the spectrum are discussed for the following: static dipole interactions, spin-spin coupling, relaxation times, and electric quadrupole interactions. (D.L.C.)

Nuclear-Magnetic-Resonance Spin-Lattice Relaxation in High and Low Fields

An experimental and theoretical study is presented of the magnetic-field dependence of nuclear-magnetic-resonance (NMR) spin-lattice relaxation in solids. Theoretically, density-matrix methods are used along with the spin-temperature assumption to derive an expression for the field dependence of the relaxation time ${T}_{1}$ in the laboratory and rotating reference frames. It is shown that with the assumptions of extreme narrowing and cubic symmetry, the ratio of the Zeeman relaxation time to the dipolar relaxation time should be two for a one-ingredient crystal in both the laboratory and rotating reference frames. The ratio of the Zeeman ${T}_{1}$ to the dipolar ${T}_{1}$ is also calculated for two-ingredient crystals. To test the theory, measurements were made of the field dependence of the laboratory- and rotating-frame spin-lattice relaxation times of the sodium nuclei in NaCl. There is a large discrepancy between the calculated and measured ratios of high-field to low-field relaxation times in both frames. The origin of the discrepancy is not fully understood.

Relation of the Relaxation Time of Electric Dipoles in Condensed Phase to Molecular Structure

Relation of the Relaxation Time of Electric Dipoles in Condensed Phase to Molecular Structure

Proton spin-lattice relaxation in liquid water and liquid ammonia

The proton spin-lattice relaxation time has been measured at 20·8 Mc/s for a series of solutions of water in heavy water and solutions of ammonia in heavy ammonia for the temperature range from the melting point to the liquid-vapour critical temperature. Measurements have also been made for water over limited temperature ranges at several fixed densities. The contributions to the spin-lattice relaxation time from direct dipolar and spin-rotation interactions have been separated. The spin-rotation interaction contribution appears to be the same for H2O as for HDO and also as between NH3, NH2D and NHD2 and this result is justified. The correlation times for molecular re-orientation, τd, and for molecular angular velocity, τsr, are derived from the results and in so doing some support for the Hubbard [12] relation betweent τsr and τd is adduced. It is found that at the critical temperature τsr≪τd which contrasts with other liquids for which it is usually found that τsr≪⃒τd. The spin-rotation interaction constants in the water and ammonia molecules are found to be approximately 120 kc/s and 80 kc/s, respectively. An attempt to separate the inter- and intra-molecular contributions to the dipolar spin-lattice relaxation time is possible in principle, in spite of the rapid proton exchange, but is frustrated by the fact that the equilibrium constants are little different from their statistical values. Nevertheless there is evidence that the two interactions vary in much the same way with temperature. The correlation times deduced from the dipolar relaxation time show close relationship with dielectric, self diffusion and deuteron relaxation time data. It is suggested that the re-orientation of both water and ammonia molecules may be by a small angle Brownian diffusion even near the critical temperature.

Paraelectric heating and cooling with OH--dipoles in alkali halides

Abstract It is quantitatively demonstrated, that adiabatic polarization and depolarization of OH--dipoles in KCl can be used for paraelectric heating and cooling at low temperatures. A reversible temperature change was measured which depends on T-4 and E2 in complete agreement with theory. A variation in therise-and decay-time of the electric field shows that the dipole relaxation time is smaller than 10-7 sec.

New microwave procedure for determining dipole moments and relaxation times

An improved bridge technique for dielectric absorption work at microwave frequencies has been evolved which yields electric dipole moments of polar solutes in non-polar solvents in excellent agreement with those derived by the Halverstadt and Kumler 1 approach in which the dielectric constant measurement are carried out at 1 Mc/s. An equation—based on Debye theory—has been developed to evalute dipole moments, from microwave results for molecules with a single relaxation time. This equation does not involve atomic polarization approximations, and hence, might be expected to yield slightly lower moments than the Halverstadt-Kumler approach. This has proved to be so for bromobenzene in cyclohexane and pyridine in carbon tetracloride. The relaxation time ' values are similar to those obtained by some of the other methods. On the whole, the dielectric data resulting from these measurements would seem not to be inferior to the other methods in current practice. In constrast to the impedance methods this approach should have certain appeal to physical-organic chemists because it involves more straightforward measurement and readily appreciated theory. The potential of dielectric absorption work for studying molecular interaction is considered briefly, particularly the use in examining the nature of intermolecular complexes and dipolar interaction at different concentrations. It would seem that such work has a unique approach to certain problems but has, so far, been surprisingly neglected.