Propagator resolved transverse relaxation exchange spectroscopy

We present in this communication a novel propagator-resolved transverse relaxation exchange experiment. This experiment enhances the previous technique of transverse relaxation exchange by enabling spatial resolution. Hence, we are able to obtain separate, and remarkably different, T2-T2 exchange plots, corresponding to different spatial displacement of the spin bearing water molecules in a porous sand matrix. This experiment is the first to combine two inverse Laplace dimensions with a Fourier dimension, opening the door to a host of new experiments combining Fourier and inverse Laplace spectroscopy.     

Quantitative 2D exchange spectroscopy using time-proportional phase incrementation

We have used a version of 2D exchange spectroscopy which employs amplitude modulation during evolution to obtain pure absorption-mode exchange maps of several multisite systems. Quadrature detection in ω₁ is provided by 90° phase incrementation of the excitation pulse in concert with incrementation of t_i. The matrix A of normalized peak amplitudes is determined by the dynamic processes which occur during the mixing interval τ_m according to the rate matrix R, which contains chemical exchange and longitudinal relaxation terms. The R matrix may be directly calculated from A using an eigenvalue-eigenvector method. In principle all of the dynamic parameters of a spin system of any size may be calculated from a pair of phase-sensitive exchange spectra acquired with and without mixing. This approach distinguishes between direct and indirect (relay) couplings irrespective of mixing time. The principles and practical aspects of exchange spectroscopy with time-proportional phase incrementation are briefly discussed and the method is illustrated with the measurement of chemical exchange rates in three-site and five-site spin systems.     

Multi-exponential relaxation analysis with MR imaging and NMR spectroscopy using fat-water systems

This study was undertaken to evaluate the feasibility of multiexponential relaxation data analysis to MR imaging techniques. The first part of this study contains accurate relaxation time measurements performed on a conventional spectrometer. In the second part, essentially the same measuring techniques were applied but now on standard whole body MR imaging equipment. T2 relaxation was measured using multi-echo techniques, T1 relaxation using multiple inversion recovery measurements. Manganese chloride solutions were used for verification of the single exponential model. Water and fat mixtures were considered for multi-exponentiality. Pure fat showed an intrinsic two-exponentiality in T1 and T2 relaxation. Mixtures of fat and water were analyzed and could at best be characterized by two exponentials, although at least three exponentials were known to be present. From the two-exponential fit the relative amounts of fat and water were calculated and compared with the mixture composition. Statistical criteria are discussed to discriminate between single and double exponential behavior in relaxation curves. It is concluded that the time consuming IR measurements for the determination of multiple T1 relaxation are not applicable in a clinical environment. Multiple T2 relaxation can be determined in a reasonable amount of time using multiple echo measurements in one image acquisition. It is shown that the observed values of T1 and T2 from tissues with intrinsic multiexponential relaxation behavior, measured with MR imaging or MR relaxation techniques on a whole-body imager or a conventional spectrometer, depend strongly on the way the experiments are set up and on the model accepted for data analysis.     

Study of hyperfine interaction and electronic relaxation using coincidence Mössbauer spectroscopy

Coincidence Mössbauer spectroscopy (CMS) in the presence of a hyperfine field and electronic spin relaxation is discussed. The resulting expression for the transmission through a resonant absorber as a function of time is found with the help of the joint solution of the Maxwell equations and the density matrix equations. It is shown that the radiation that reaches the detector behind the absorber is modulated by the hyperfine field. The various polarization schemes for the CMS observation are analyzed. Numerical calculations were made for an⁵⁷ Fe nucleus.     

NMR relaxation spectroscopy: Interference effects

The nuclear magnetic resonance relaxation process and the relaxation spectroscopy is considered in the presence of interference (cross terms) between different spin interactions in a weak-collision approximation. New interference effects are presented.     

Dielectric relaxation spectroscopy of phlogopite mica

An in-depth investigation of the dielectric characteristics of annealed phlogopite mica has been conducted in the frequency range 0.1 Hz–10 MHz and over the temperature range 653–873 K through the framework of dielectric permittivity, electric modulus and conductivity formalisms. These formalisms show qualitative similarities in relaxation processes. The frequency dependence of the M″ and dc conductivity is found to obey an Arrhenius law and the activation energy of the phlogopite mica calculated both from dc conductivity and the modulus spectrum is similar, indicating that same type of charge carriers are involved in the relaxation phenomena. The electric modulus and conductivity data have been fitted with the Havriliak–Negami function. Scaling of M′, M″, ac conductivity has also been performed in order to obtain insight into the relaxation mechanisms. The scaling behaviour indicates that the relaxation describes the same mechanism at different temperatures. The relaxation mechanism was also examined using the Cole–Cole approach. The study elaborates that the investigation regarding the temperature and frequency dependence of dielectric relaxation in the phlogopite mica will be helpful for various cutting edge applications of this material in electrical engineering.     

Gluon exchange in meson spectroscopy

We discuss the spin structure ofN=L=0 andN=L=1 mesons in terms of the one-gluon exchange model with scalar confinement of quarks. SU(3)_flavour is broken by taking different masses for the quarks and the SU (3) breaking of the nonperturbed hamiltonian is shown to be very important in understanding the hyperfine splittings in the ground state mesons, including the charmed states. A fit to the experimental mass spectrum is performed for theN=L=0 andN=L=1 levels simultaneously. The spin-orbit splittings of the spectrum cannot be explained by the convential terms from gluon exchange and scalar confinement. We argue that additional terms, which break the ideal mixing pattern, have to be present. The inclusion of such a term gives a dramatic improvement in χ² and leads to an almost perfect fit to the data for masses and mixing angles.     

Ultrafast relaxation of excited Dirac fermions in epitaxial graphene using optical differential transmission spectroscopy

We investigate the ultrafast relaxation dynamics of hot Dirac fermionic quasiparticles in multilayer epitaxial graphene using ultrafast optical differential transmission spectroscopy. We observe differential transmission spectra which are well described by interband transitions with no electron-hole interaction. Following the initial thermalization and emission of high-energy phonons, the electron cooling is determined by electron-acoustic phonon scattering, found to occur on the time scale of 1 ps for highly doped layers, and 4-11 ps in undoped layers. The spectra also provide strong evidence for the multilayer structure and doping profile of thermally grown epitaxial graphene on SiC.     

Study of heterogeneous interaction in binary mixtures of 2-methoxyethanol-water using dielectric relaxation spectroscopy

The dielectric relaxation measurements on binary mixtures of 2-methoxyethanol with water have been carried out over entire concentrations and at temperature range of 0 °C to 25 °C using a picosecond time domain reflectometry technique. The complex dielectric permittivity spectra of 2-methoxyethanol/water mixtures were fitted using Havriliak-Negami equation. The static dielectric constant and relaxation time for all concentrations were obtained using least square fit method. The principal relaxation time is small if compared to that of corresponding alcohol/water mixtures this may be due to the hydrogen bonding ether oxygen in the 2-ME-water system. Excess dielectric properties, Kirkwood correlation factor, thermodynamic properties and Bruggeman factor are also determined and the results are interpreted in terms of heterogeneous interactions among the unlike molecules due to hydrogen bonding.     

Allowance for irreversible relaxation in echo spectroscopy

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 48, No. 2, pp. 296–302, February, 1988.     

Paramagnetic relaxation in Mössbauer spectroscopy

The theory of calculating Mössbauer spectra by anab initio method is shortly reviewed. The relaxation supermatrix is calculated in an electronic basis and its dimension is reduced. Spectra are fitted using a dynamical spin-Hamiltonian. A procedure for fitting the spectra in the whole temperature range is suggested.     

Strangeness exchange reactions and hypernuclear spectroscopy

Recent data obtained by the strangeness exchange reactions on p and s-shell nuclei are analyzed. Using the shell-model to describe the structure of the hypernuclei information about the Λ-nuclues interaction is extracted and compared to previous knowledge. In particular, spin-orbit coupling is found to be much smaller than in the nucleon-nucleus case.     

Spin relaxation and chemical exchange in NMR simulations

Theory for describing the density matrix of a spin system experiencing chemical exchange and relaxation during the steps of an NMR experiment is presented in a form suitable for computation. Features in the theory that arise from exchange are discussed in detail, and comparisons to the exchange-free situation are made. A general computer program to carry out simulations of NMR experiments is described, and several examples of its performance are presented.     

Improved pore space structure characterization by fusion of relaxation tomography maps

Quantitative Relaxation Tomography in porous media furnishes maps of internal sections where each pixel represents T1 or T2 of water 1H in the corresponding voxel, so that quantitative information on the pore space structure can be obtained. The porosity can be determined at different length scales by correcting pixel by pixel the signal intensity for T2 decay. Moreover, on the basis of the distribution of T1, the microporosity fraction can be computed, as well as several voxel-average porosities. Since T1 and T2 encode different pieces of information, fusion image techniques can improve the characterization of the pore space, showing simultaneously, on the same image, maps of the two parameters. Examples are given of application to a water-saturated travertine core and to a pig femur. Different kinds of look-up tables were tried by varying two of the three dimensions of the HSV color space in such a way as to optimize both the T1 and T2 contrasts simultaneously.     

19F spectroscopy and relaxation behavior of trifluorovinyldichloroborane

The 19F NMR spectra and spin-lattice relaxation rate, R1, of trifluorovinyldichloroborane as shown in were studied as a function of temperature, T, and magnetic field, B. All logR1 vs 1/T plots show a minimum at 299K indicating the presence if dipolar relaxation at lower T and spin-rotation relaxation at higher T. The R1 values increase with increasing B due to chemical shift anisotropy relaxation. Estimates of the fluorine chemical shift values for F3 (cf. Fig. 1) suggest that there is pi character in the F-C bond. The other two C-F bonds are largely single bonded. No evidence was found for intermolecular exchange of the trifluorovinyl group. Two of the three fluorine atoms show large increases in their NMR linewidth with increasing temperature while the third shows only a small increase but the activation energy for the process is the same for all. The increase in linewidths is due to scalar coupling to the boron atoms. The boron linewidths were measured between 253 and 363K and decreased with increasing temperature. A plot of logR2, where R2 is the linewidth of the boron as a function of 1/T shows some curvature indicating a second relaxation mechanism. This is ascribed to spin-rotation but not enough data are available to be conclusive. In all cases there is a second small set of fluorine peaks that are due to 10B interactions separated from the 11B peaks by amounts varying from 1 to 4 ppm depending on the field and fluorine atom.     

Point-contact spectroscopy of electron relaxation mechanisms in semiconductors

The I–V characteristic of a tiny semiconducting channel connecting bulk electrodes is shown to have singularities arising due to phonon emission by hot electrons at energies eV = n?ω₀, where ω₀ is the optical phonon frequency and n = 1, 2, 3,…. The nonlinear part of the I–V curve provides direct information concerning the energy dependence of the elastic-scattering time of charge carriers.     

Single-scan longitudinal relaxation measurements in high-resolution NMR spectroscopy

Several single-scan experiments for the measurement of the longitudinal relaxation time (T1) are proposed. These experiments result in fast and accurate determinations of the relaxation rate, are relatively robust to pulse imperfections, and preserve information about the chemical shift. The method used in these experiments is to first encode the T1 values as a spatial variation of the magnetization and then to read out this variation either by applying a weak gradient during acquisition or by sequentially observing different slices of the sample. As a result, it is possible to reduce the time necessary to determine the T1 values by one or two orders of magnitude. This time saving comes at the expense of the signal-to-noise level of the resulting spectrum and some chemical shift resolution.     

Probing anomalous relaxation by coherent multidimensional optical spectroscopy

We propose to study the origin of algebraic decay of two-point correlation functions observed in glasses, proteins, and quantum dots by their nonlinear response to sequences of ultrafast laser pulses. Power-law spectral singularities and temporal relaxation in two-dimensional correlation spectroscopy signals are predicted for a continuous time random walk model of stochastic spectral jumps in a two-level system with a power-law distribution of waiting times psi(t) approximately t;{-alpha-1}. Spectroscopic signatures of stationary ensembles for 1相似文献   

Thermal relaxation of exchange bias field in an exchange coupled CoFe/IrMn bilayer

This paper reports that a CoFe/IrMn bilayer was deposited by high vacuum magnetron sputtering on silicon wafer substrate; the thermal relaxation of the CoFe/IrMn bilayer is investigated by means of holding the film in a negative saturation field at various temperatures. The exchange bias decreases with increasing period of time while holding the film in a negative saturation field at a given temperature. Increasing the temperature accelerates the decrease of exchange field. The results can be explained by the quantitative model of the nucleation and growth of antiferromagnetic domains suggested by Xi H W et al. [2007 Phys. Rev. B 75 014434], and it is believed that two energy barriers exist in the investigated temperature range.