Showing papers on "Relaxation (NMR) published in 1999"

Magnetic Resonance Imaging: Physical Principles and Sequence Design

Abstract: Magnetic Resonance Imaging: A Preview. Classical of a Single Nucleus to a Magnetic Field. Rotating Reference Frames and Resonance. Magnetization, Relaxation and the Bloch Equation. The Quantum Mechanical Basis of Precession and Excitation. The Quantum Mechanical Basis of Thermal Equilibrium and Longitudinal Relaxation. Signal Detection Concepts. Introductory Signal Acquisition Methods: Free Induction Decay, Spin Echoes, Inversion Recovery and Spectroscopy. One-Dimensional Fourier Imaging, k-Space and Gradient Echoes. Multi-Dimensional Fourier Imaging and Slice Excitation. The Continuous and Discrete Fourier Transforms. Sampling and Aliasing in Image Reconstruction. Filtering and Resolution in Fourier Transform Image Reconstruction. Projection Reconstruction of Images. Signal, Contrast and Noise. A Closer Look at Radiofrequency Pulses. Water/Fat Separation Techniques. Fast Imaging in the Steady State. Segmented k-Space and Echo Planar Imaging. Magnetic Field Inhomogeneity Effects and T-2 Dephasing. Random Walks, Relaxation and Diffusion. Spin Density, T-1 and T-2 Quantification Methods in MR Imaging. Motion Artifacts and Flow Compensation. MR Angiography and Flow Quantification. Magnetic Properties of Tissues: Theory and Measurement. Sequence Design, Artifacts and Nomenclature. Introduction to MRI Coils and Magnets. Appendices. Index.

Electron and hole relaxation pathways in semiconductor quantum dots

Abstract: Femtosecond (fs) broad-band transient absorption (TA) is used to study the intraband relaxation and depopulation dynamics of electron and hole quantized states in CdSe nanocrystals (NC's) with a range of surface properties. Instead of the drastic reduction in the energy relaxation rate expected due to a ``phonon bottleneck,'' we observe a fast subpicosecond $1P$-to-$1S$ electron relaxation, with the rate exceeding that due to phonon emission in bulk semiconductors. The energy relaxation is enhanced with reducing the NC's radius, and does not show any dependence on the NC surface properties (quality of the surface passivation). These data indicate that electron energy relaxation occurs by neither multiphonon emission nor by coupling to surface defects, but is likely meditated by Auger-type electron-hole energy transfer. We use fs infrared TA to probe electron and hole intraband transitions, which allows us to distinguish between electron and hole relaxation pathways leading to the depopulation of NC quantized states. In contrast to the electron relaxation, which is controlled by NC surface passivation, the depopulation of hole quantized states is extremely fast (sub-ps-to-ps time scales) in all types of samples, independent of NC surface treatment (including NC's overcoated with a ZnS layer). Our results indicate that ultrafast hole dynamics are not due to trapping at localized surface defects such as a vacancy, but rather arise from relaxation into intrinsic NC states or intrinsically unpassivated interface states.

Dielectric relaxation of aqueous NaCl solutions

Abstract: The complex dielectric permittivity of aqueous sodium chloride solutions has been determined in the frequency range 0.2 ≤ v(GHz) ≤ 20 with a commercial dielectric measurement system based on a vector network analyzer. NaCl solutions 0.1 ≤ m (mol kg-1) ≤ 5 (mass fraction 0.005 ≤ w ≤ 0.23) were investigated at 5, 20, 25, and 35°C. An improved calibration procedure of the dielectric measurement system for conducting samples was developed. The complex permittivity spectra have been represented by a Cole-Cole relaxation time distribution. Where possible, the obtained fitting parameters, static permittivity ∈ and relaxation time τ, and distribution parameter a, are compared with literature data to assess the performance of the instrument, which was found to be comparable to that of time domain and waveguide systems. Effective solvation numbers were deduced from the effect of NaCl concentration on ∈. The data suggest that in addition to the irrotational bonding of water molecules by Na+ ions, kinetic depolarization under slip boundary conditions determines the solution permittivity. A three-state model is proposed to describe the concentration dependence of τ. © Copyright 1999 by the American Chemical Society.

Single-Molecule Magnet Behavior of a Tetranuclear Iron(III) Complex. The Origin of Slow Magnetic Relaxation in Iron(III) Clusters

Abstract: The synthesis, crystal structure, and magnetic characterization of a novel tetranuclear iron(III) methoxo-bridged cluster of formula Fe4(OCH3)6(dpm)6 (where Hdpm = dipivaloylmethane) is reported. The cluster has a ground spin state of S = 5, which is selectively populated below 20 K. High-field EPR spectra revealed that the system has a uniaxial magnetic anisotropy, corresponding to a zero field splitting parameter D = −0.2 cm-1 of the S = 5. Such anisotropy below 1 K gives rise to the slow relaxation of the magnetization similar to that of super-paramagnets. To investigate the origin of the magnetic anisotropy we have evaluated the projection of the single-ion and dipolar contributions to the zfs of the ground state. The zfs tensors of the three structurally independent iron(III) centers have been calculated from the coordination geometry and spectroscopic data using the angular overlap model. To test the reliability of the approach high-field EPR spectra of the parent monomer Fe(dpm)3 have been recorded...

Theory of proton relaxation induced by superparamagnetic particles

Abstract: Evaluating and understanding the performances of magnetic colloids as contrast agents for MRI requires a theory describing their magnetic interactions with water protons. The field dependence of the proton longitudinal relaxation rate (nuclear magnetic relaxation dispersion profiles) in aqueous colloidal suspensions of superparamagnetic particles is based on the so-called Curie relaxation, which essentially accounts for the high field part of the NMRD profiles (B0>0.02 T). The low-field part of the NMRD profiles can only be explained by the crystal’s internal anisotropy energy, a concept which clarifies the important difference between superpara- and paramagnetic compounds: the anisotropy energy modifies both the electronic precession frequencies and the thermodynamic probability of occupation of the crystal magnetic states. Our theory clearly explains why a low-field dispersion exists for suspensions of small size crystals, and why it does not for large crystals’ suspensions. This important effect is due...

Linear combination steady-state free precession MRI

Abstract: A fast, spectrally-selective steady-state free precession (SSFP) imaging method is presented. Combining k-space data from SSFP sequences with certain phase schedules of radiofrequency excitation pulses permits manipulation of the spectral selectivity of the image. For example, lipid and water can be rapidly resolved. The contrast of each image depends on both T 1 and T 2 , and the relative contribution of the two relaxation mechanisms to image contrast can be controlled by adjusting the flip angle. Several applications of the technique are presented, including fast musculoskeletal imaging, brain imaging, and angiography. The technique is referred to herein as linear combination steady-state free precession (LCSSFP) and fluctuating equilibrium magnetic resonance (FEMR).

Relaxational dynamics of supercooled water in porous glass

Abstract: We have made a high-resolution quasielastic incoherent neutron scattering (QENS) study of the translational dynamics of supercooled water contained in micropores of Vycor glass at different hydration levels. QENS spectra from the confined H{sub 2}O are analyzed in terms of the {alpha}-relaxation dynamics predicted by mode-coupling theory of supercooled liquids and by a recent computer molecular-dynamics simulation of extended simple point charge model water. We verify that the stretched exponential relaxation description of the long-time test-particle dynamics is consistent with the measured QENS spectral line shape. We are thus able to determine the wave-number dependence of magnitudes of the structural relaxation rate 1/{tau} and the stretch exponent {beta} as functions of temperature and coverage. A power-law dependence of the average relaxation time on the magnitude of the scattering vector {ital Q} is observed. In the {ital Q} range studied, the exponent starts out with nearly {minus}2.0, at room temperature, indicating a continuous diffusion, and gradually becomes less negative as the temperature is decreased to below the freezing temperature. thinsp {copyright} {ital 1999} {ital The American Physical Society}

Analytical model of susceptibility-induced MR signal dephasing: effect of diffusion in a microvascular network.

Abstract: A deterministic analytical model that describes the time course of magnetic resonance signal relaxation due to magnetic field inhomogeneity induced by a vascular network is developed. Both static and diffusion dephasing are taken into account. The contribution of the diffusion dephasing is calculated for relatively large vessels (R>10 microm) or short measurement times when the diffusion length is smaller than the vessel radius. The signal is found to possess the following features: a) an initial deviation from the monoexponential relaxation which is more pronounced for the imaginary part of the signal; b) a deviation from monoexponential relaxation at short echo times for the spin-echo (SE) signal measured as a function of the echo time; c) the echo maximum of the SE signal shifted from the nominal echo time to a shorter time; and d) a diffusion effect much stronger for the SE than for the free induction decay experiment. The model presented agrees within its validity domain with a known Monte Carlo simulation.

α-Helix Peptide Folding and Unfolding Activation Barriers: A Nanosecond UV Resonance Raman Study

Abstract: We used UV resonance Raman spectroscopy to characterize the equilibrium conformation and the kinetics of thermal denaturation of a 21 amino acid, mainly alanine, R-helical peptide (AP). The 204-nm UV resonance Raman spectra show selective enhancements of the amide vibrations, whose intensities and frequencies strongly depend on the peptide secondary structure. These AP Raman spectra were accurately modeled by a linear combination of the temperature-dependent Raman spectra of the pure random coil and the pure R-helix conformations; this demonstrates that the AP helix-coil equilibrium is well-described by a two-state model. We constructed a new transient UV resonance Raman spectrometer and developed the necessary methodologies to measure the nanosecond relaxation of AP following a 3-ns T-jump. We obtained the T-jump by using a 1.9-μm IR pulse that heats the solvent water. We probed the AP relaxation using delayed 204-nm excitation pulses which excite the Raman spectra of the amide backbone vibrations. We observe little AP structural changes within the first 40 ns, after which the R-helix starts unfolding. We determined the temperature dependence of the folding and unfolding rates and found that the unfolding rate constants show Arrhenius- type behavior with an apparent ∼8 kcal/mol activation barrier and a reciprocal rate constant of 240 ( 60 ns at 37 °C. However, the folding rate constants show a negative activation barrier, indicating a failure of transition- state theory in the simple two-state modeling of AP thermal unfolding, which assumes a temperature-independent potential energy profile along the reaction coordinate. Our measurements of the initial steps in the R-helical structure evolution support recent protein folding landscape and funnel theories; our temperature-dependent rate constants sense the energy landscape complexity at the earliest stages of folding and unfolding.

A pictorial description of steady-states in rapid magnetic resonance imaging

Abstract: Magnetic resonance imaging in biochemical and clinical research requires rapid imaging sequences. Time-resolved imaging of heart movement and the acquisition of a three-dimensional image block within the circulation time of a contrast agent bolus are two typical examples. Rapid imaging sequences are characterized by a very fast train of radiofre- quency (rf) and gradient pulses. Between these rf pulses, the excited magnetization is unable to return to its thermal equilibrium. As a consequence, further rf pulses will influence both the remaining transversal and the remaining equilibrium state. The steady-state magnetization of a multi-rf pulse and gradient pulse experiment is thus a mixture or superposition of different transversal and longitudinal states and the acquired image amplitude becomes a complex func- tion of the investigated tissue's relaxation properties. Based on the works of Woessner, Kaiser, and Hennig, this article intends to give a pictorial description of rapid multipulse imaging ex- periments. It also provides an extension of this theory applied to modern imaging sequences such as TRUE FISP and rf-spoiled techniques. © 1999 John Wiley & Sons, Inc. Concepts Magn Reson 11: 291-304, 1999.

Femtosecond Dynamics of Double Proton Transfer in a Model DNA Base Pair: 7-Azaindole Dimers in the Condensed Phase

Abstract: The dynamics of double proton transfer in 7-azaindole (7-AI) dimers, a model DNA base pair, are investigated in real time using femtosecond transient absorption and fluorescence upconversion techniques. In nonpolar solvents we examine the isotope effect, the excitation energy dependence, and the structure analogue of the tautomer (7-MeAI). A detailed molecular picture of the nuclear dynamics in the condensed phase emerges with the relationship to the dynamics observed in molecular beams: Following the femtosecond excitation there are three distinct time scales for structural relaxation in the initial pair, proton (hydrogen) transfers, and vibrational relaxation or cooling of the tautomer. The molecular basis of tunneling and concertedness are elucidated by careful examination of the isotope effect and the time resolution. Comparison with the results in the isolated pair indicates the critical role of the N−H and N···N nuclear motions in determining the effective potential, and the thermal excitation in solution. Because the barrier is small, ∼1.3 kcal/mol, both are important factors and experiments at much higher energies will be unable to test either tunneling or concertedness. Finally, we compare the experimental results and the dynamical picture with detailed ab initio and molecular dynamics simulations.

Observation of the Distribution of Molecular Spin States by Resonant Quantum Tunneling of the Magnetization

Abstract: Below 360 mK, Fe magnetic molecular clusters are in the pure quantum relaxation regime and we show that the predicted square-root time relaxation is obeyed, allowing us to develop a new method for watching the evolution of the distribution of molecular spin states in the sample. We measure as a function of applied field H the statistical distribution P(\xi_H) of magnetic energy bias \xi_H$ acting on the molecules. Tunneling initially causes rapid transitions of molecules, thereby digging a hole in P(\xi_H) (around the resonant condition \xi_H = 0). For small initial magnetization values, the hole width shows an intrinsic broadening which may be due to nuclear spins.

Theory of exciton dephasing in semiconductor quantum dots

Abstract: A resonant optical excitation creates an excited state population and also induces an optical polarization. Dynamics of this optical excitation is characterized by relaxation of the population as well as decay of the induced optical polarization. In lower dimensional semiconductors, electronic confinement leads to qualitative changes in population relaxation, including spontaneous emission and exciton-phonon scattering, as shown in extensive recent studies [1]. These population relaxation processes are expected to contribute to dephasing with a dephasing rate given by half the population decay rate. Pure dephasing processes that do not involve population or energy relaxation of excitons can also contribute to dephasing. Pure dephasing, which is a well-established concept for atomic systems, remains yet to be investigated in lower-dimensional semiconductors due to a lack of direct comparison between dephasing and population relaxation and between theory and experiment. Studies of pure dephasing processes in lower-dimensional semiconductors will renew and deepen our understanding of dephasing of collective excitations in solids, although several seminal studies were done on the exciton dephasing in quantum well (QW) structures [2–6].

Slow dynamics and ergodicity breaking in a lanthanum-modified lead zirconate titanate relaxor system

Abstract: The freezing of the dynamic process in a 9/65/35 lanthanum lead zirconate-titanate (PLZT) ceramics has been investigated by measurements of the frequency-dependent complex dielectric constant and the quasistatic field-cooled (FC) and zero-field-cooled (ZFC) dielectric susceptibilities. It was found that the aging process is responsible for the difference in temperature variations of the FC static dielectric constant and the static dielectric constant determined in the dynamic ZFC experiment. Analysis of the complex dielectric susceptibility by a temperature-frequency plot has revealed that for an aged PLZT sample the ergodicity is broken due to the divergence of the longest relaxation time in the vicinity of 249 K, i.e., the temperature where the ferroelectric phase can also be induced by applying sufficiently high electric field. However, the bulk of the distribution of relaxation times was found to remain finite even below the freezing temperature. It is shown that the behavior of the relaxation spectrum and the splitting between the field-cooled and zero-field-cooled dielectric constants in PLZT relaxor is qualitatively similar to what was observed in the lead magnesium niobate (PMN) relaxor and is reminiscent of the nonergodic behavior reported in various spin glasses. Moreover, the temperature dependence of the third order nonlinear susceptibility indicates a glassy rather than ferroelectric multidomain nature of the nonergodic relaxor state in both PMN and PLZT systems.

Structure and conformation of complex carbohydrates of glycoproteins, glycolipids, and bacterial polysaccharides

Abstract: For nuclear magnetic resonance determinations of the conformation of oligosaccharides in solution, simple molecular mechanics calculations and nuclear Overhauser enhancement measurements are adequate for small oligosaccharides that adopt single, relatively rigid conformations. Polysaccharides and larger or more flexible oligosaccharides generally require additional types of data, such as scalar and dipolar coupling constants, which are most conveniently measured in 13C-enriched samples. Nuclear magnetic resonance relaxation data provide information on the dynamics of oligosaccharides, which involves several different types of internal motion. Oligosaccharides complexed with lectins and antibodies have been successfully studied both by X-ray crystallography and by nuclear magnetic resonance spectroscopy. The complexes have been shown to be stabilized by a combination of polar hydrogen bonding interactions and van der Waals attractions. Although theoretical calculations of the conformation and stability of free oligosaccharides and of complexes with proteins can be carried out by molecular mechanics methods, the role of solvent water for these highly polar molecules continues to present computational problems.

Exponential intermolecular dynamics in optical Kerr effect spectroscopy of small-molecule liquids

Abstract: Optical Kerr effect spectroscopy has been employed to study the behavior of six symmetric-top liquids (acetonitrile, acetonitrile-d3, benzene, carbon disulfide, chloroform, and methyl iodide) over a broad range of temperatures. In all of the liquids, an exponential intermolecular response is observed on a time scale of a few hundreds of femtoseconds. Comparison of the temperature dependence of the time scale of this relaxation with the viscosity and single-molecule and collective orientational times in the liquids suggests that the exponential relaxation arises from motional narrowing.

Superdiffusion and out-of-equilibrium chaotic dynamics with many degrees of freedoms

Abstract: We study the link between relaxation to the equilibrium and anomalous superdiffusive motion in a classical $N$-body Hamiltonian system with long-range interaction showing a second-order phase transition in the canonical ensemble. Anomalous diffusion is observed only in a transient out-of-equilibrium regime and for a small range of energy, below the critical one. Superdiffusion is due to L\'evy walks of single particles and is checked independently through the second moment of the distribution, power spectra, trapping, and walking time probabilities. Diffusion becomes normal at equilibrium, after a relaxation time which diverges with $N$.

Effect of blending on the PVME dynamics. A dielectric, NMR, and QENS investigation

Abstract: In this work we have investigated how the dynamics of poly(vinyl methyl ether), PVME, changes by blending with deuterated polystyrene. The experimental techniques used were dielectric spectroscopy, quasielastic neutron scattering, and 13C nuclear magnetic resonance. By means of these techniques, the dynamics of the poly(vinyl methyl ether) units in the blends can be selectively investigated in a huge time range (101−10-11 s). Two different blend compositions have been investigated. The main relaxation processes observed in this range are the secondary β-process and the segmental α-relaxation. It turns out that the β-relaxation is not affected by blending. The data analysis procedure followed by us in the case of the α-process is based on the assumption that the dynamics of the PVME segments in the blends is a superposition of dynamical processes with the same shape as that in pure PVME, but with the relaxation times distributed due to the presence of concentration fluctuations. From this analysis we found...

Three-body decay of a rubidium Bose–Einstein condensate

Abstract: We have measured the three-body decay of a Bose–Einstein condensate of rubidium (87Rb) atoms prepared in the doubly polarized ground state F=mF=2. Our data are taken for a peak atomic density in the condensate varying between 2×1014 cm-3 at initial time and 7×1013 cm-3, 16 s later. Taking into account the influence of the uncondensed atoms on the decay of the condensate, we deduce a rate constant for condensed atoms L=1.8 (±0.5) ×10-29 cm6 s-1. For these densities we did not find a significant contribution of two-body processes such as spin dipole relaxation.

Photoinduced electron transfer at liquid/liquid interfaces Part II. A study of the electron transfer and recombination dynamics by intensity modulated photocurrent spectroscopy (IMPS)

Abstract: The dynamics of photoresponses associated with heterogeneous quenching of zinc tetrakis(carboxphenyl)porphyrin (ZnTPPC) and ferrocene derivatives at the water/1,2-dichloroethane interface were studied by intensity modulated photocurrent spectroscopy (IMPS) The contribution arising from the electron injection, recombination–product separation competition and the attenuation associated with the uncompensated resistance and interfacial capacitance (RC) time constant of the cell were deconvoluted in the frequency domain The flux of electron injection was described in terms of a competition between the relaxation of the porphyrin excited state and the electron transfer step Experimental results in the presence of ferrocene and diferrocenylethane confirmed that as the Galvani potential difference is increased, the phenomenological electron transfer rate constant increases and the ZnTPPC coverage at the liquid/liquid junction decreases Furthermore, the recombination rate constant decreases with increasing potentials, while the product separation rate constant did not show a clear potential dependence Photocurrent studies were extended to the electron donors dimethylferrocene and trianisylamine, as well as to the electron acceptor tetracyanoquinodimethane The results obtained clearly indicate that the Gibbs energy of activation for the charge transfer process is affected by the Galvani potential difference It is suggested that the electron transfer dynamics are dependent on the local electric field generation by the specifically adsorbed ZnTPPC The general expressions for the frequency dependent photocurrents at liquid/liquid interfaces are also introduced

Dielectric relaxation of water and heavy water in the whole fluid phase

Abstract: Recently we developed a new microwave spectroscopy technique in the frequency range up to 40 GHz, and measured the static dielectric constant and the dielectric relaxation time for supercritical water. In the present work we report the dielectric properties of heavy water at temperatures and pressures up to 770 K and 59 MPa, respectively. The static dielectric constant of D2O as well as H2O are well described by the Uematsu–Franck formula when the number density instead of the mass density is used as the input parameter. The dielectric relaxation time decreases rapidly with increasing temperature in liquid H2O and D2O and jumps to a large value at the liquid–gas transition. The relaxation time of D2O is longer than that of H2O in the liquid state, and the difference becomes smaller with decreasing density in the gaseous state. For both H2O and D2O the most relevant parameter determining the relaxation time is the temperature at high densities or at low temperatures, and it is the density at low densities ...

Influence of cholesterol on dynamics of dimyristoylphosphatidylcholine bilayers as studied by deuterium NMR relaxation

Abstract: Investigation of the deuterium (2H) nuclear magnetic resonance (NMR) relaxation rates of lipid bilayers containing cholesterol can yield new insights regarding its role in membrane function and dynamics. Spin-lattice (R1Z) and quadrupolar order (R1Q) 2H NMR relaxation rates were measured at 46.1 and 76.8 MHz for macroscopically oriented bilayers of 1,2-diperdeuteriomyristoyl-sn-glycero-3-phosphocholine (DMPC-d54) containing cholesterol (1/1 molar ratio) in the liquid-ordered phase at 40 °C. The data for various segmental positions along the DMPC-d54 acyl chain were simultaneously fitted to a composite membrane deformation model, including fast segmental motions which preaverage the coupling tensor along the lipid acyl chain, slow molecular reorientations, and small-amplitude collective fluctuations. In contrast to pure DMPC-d54 in the liquid-crystalline (Lα) phase, for the DMPC-d54:cholesterol (1/1) system a linear square-law functional dependence of the relaxation rates on the order parameter (quadrupolar splitting) does not appear evident. Moreover, for acyl segments closer to the top of the chain, the angular anisotropy of the 2H R1Z and R1Q relaxation rates is more pronounced than toward the chain terminus. The residual (preaveraged) coupling tensor has its greatest effective asymmetry parameter near the polar groups, decreasing for the groups closest to the end of the chain. The results suggest that axial rotations of the phospholipid molecules occur at a somewhat higher rate than in pure bilayers, as a consequence of the higher ordering and reduction of chain entanglement. On the other hand, the rigid cholesterol molecule appears to undergo somewhat slower axial rotation, possibly due to its noncylindrical shape. Collective motions are found to be less predominant in the case of DMPC-d54:cholesterol than for pure DMPC-d54, which may indicate an increased dynamical rigidity of lipid bilayers containing cholesterol versus pure lipid systems.

Carrier energy relaxation by means of Auger processes in InAs/GaAs self-assembled quantum dots

Abstract: Carrier relaxation processes are investigated in self-assembled InAs/GaAs quantum dots using time-resolved photoluminescence spectroscopy. The quantum-dot photoluminescence rise time has been measured as functions of carrier excitation density and excitation wavelengths. The measured relaxation time is about 32 ps at low excitation density and decreases by 1 over the excitation density from about 3 W/cm2, under nonresonant laser excitation. The threshold of this density-dependent regime occurs at a slightly higher density as the excitation wavelength increases and it disappears when the photon pumping energy is below the wetting layer barrier energy. These results clearly establish the regime where Auger processes become the dominant carrier relaxation mechanism in these self-assembled quantum dots.

Are mono-exponential fits to a few echoes sufficient to determine T2 relaxation for in vivo human brain?

Abstract: T2 relaxation decay curves from in vivo human brain tissue are rarely mono-exponential. Partial volume averaging further reduces the chance of mono-exponential decay. Moreover, the parameters derived from few-echo mono-exponential fits change with the measurement echo times and have the largest possible variance. In this note, multi-exponential fits to 32-echo relaxation decay curves from in vivo human brain are used to design simulations (where the truth is known) to demonstrate the pitfalls of few-echo mono-exponential interpretations.