Relaxation (NMR)

About: Relaxation (NMR) is a research topic. Over the lifetime, 29342 publications have been published within this topic receiving 689851 citations.

Nuclear Spin–Lattice Relaxation in Axially Symmetric Ellipsoids with Internal Motion

Abstract: The calculation of the nuclear‐magnetic‐resonance spin–lattice relaxation time has been extended to include molecules which approximate an axially symmetric ellipsoid and have internal motion about an axis at any constant angle to the symmetry axis of the ellipsoid. The calculation is carried out explicitly for two models of internal motion. These are, random reorientation among three equivalent positions 120° apart, and rotational diffusion.

Water−Ethanol Mixtures at Different Compositions and Temperatures. A Dieletric Relaxation Study

Abstract: At eight temperatures T between 0 and 60 °C and at five mole fractions xe of ethanol (0 < xe ≤ 1) the complex (electric) permittivity of ethanol/water mixtures has been measured as a function of frequency ν between 1 MHz and 24 GHz. At 25 °C the ethanol permittivities are completed by literature data for the frequency range 200 MHz to 90 GHz. The spectra for ethanol and for the ethanol/water mixtures are compared to permittivity spectra for water which, at some temperatures, are available up to 900 GHz. All spectra of the ethanol/water system can be well represented by the assumption of two relaxation regions. The relaxation time τ1 of the dominating relaxation process varies between 4 ps (xe = 0, 60 °C) and 310 ps (xe = 1, 0 °C). The relaxation time τ2 of the second relaxation process is smaller. Evaluation of the extrapolated low frequency (“static”) permittivity yields a minium in the effective dipole orientation correlation of the ethanol/water system at 0.2 ≤ xe ≤ 0.4. In this composition range, othe...

Influence of vibrational motion on solid state line shapes and NMR relaxation

Abstract: The influence of vibrational motion on bond lengths and quadrupole constants obtained from dipolar and quadrupolar solid state line shapes is considered. It is shown that such motions average both the magnitude and the orientation of the intrinsic interaction tensor. Explicit expressions for the effective coupling constants that can be conveniently evaluated using the results of a normal mode analysis are derived. When the vibrationally averaged interaction tensor is axially symmetric, it is shown that the effect of vibrational motion on relaxation can be rigorously incorporated into an effective coupling constant which is formally identical to the one that determines the line shape. Illustrative calculations for several alkanes, in both the gas and solid phases, are presented. The relative contributions of stretching and bending vibrations and the effect of anharmonicity on the effective C–H bond lengths and deuterium quadrupole constants are examined. The influence of vibrational averaging on the magnitude of the dipolar coupling is shown to be essentially independent of the nature of the molecule and its environment and to be quite small (i.e., for C–H bonds, the coupling constant decreases by 3% and hence the effective bond length increases by 1%). Vibrational averaging of the orientation of the dipolar interaction vector, on the other hand, depends on the size of the molecule and its environment because of the predominant role played by low frequency bending and torsional modes. The implication of these results for the value of the effective internuclear distance that should be used for the interpretation of the dipolar relaxation experiments is considered.

Hard-core bosons on optical superlattices: Dynamics and relaxation in the superfluid and insulating regimes

Abstract: We study the ground-state properties and nonequilibrium dynamics of hard-core bosons confined in one-dimensional lattices in the presence of an additional periodic potential (superlattice) and a harmonic trap. The dynamics is analyzed after a sudden switch-on or switch-off of the superlattice potential, which can bring the system into insulating or superfluid phases, respectively. A collapse and revival of the zero-momentum peak can be seen in the first case. We study in detail the relaxation of these integrable systems towards equilibrium. We show how after relaxation time averages of physical observables, like the momentum distribution function, can be predicted by means of a generalization of the Gibbs distribution.

Ultrafast intersubband relaxation (⩽150 fs) in AlGaN/GaN multiple quantum wells

Abstract: The ultrafast intersubband relaxation in GaN quantum wells has been verified. Al0.65Ga0.35N/GaN multiple quantum wells, with as many as 200 wells, were grown by optimizing the barrier thickness and introducing GaN intermediate layers. The intersubband absorption is sufficiently strong for the relaxation time to be measured. A pump–probe measurement is performed to investigate the relaxation. An ultrashort relaxation time of less than 150 fs is obtained at a wavelength of 4.5 μm. The transient time is shorter than that of InGaAs quantum wells by approximately an order of magnitude. This result is promising for realizing ultrafast optical switches.