Relaxation (NMR)

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

Characteristic relaxation times and their invariance to thermodynamic conditions

Abstract: The dynamics of molecular liquids and polymers exhibit various “transitions”, associated with characteristic changes in properties. With decreasing temperature or increasing pressure, these transitions include (i) the onset of intermolecular cooperativity with consequent non-Arrhenius and non-Debye behavior; (ii) the dynamic crossover at which derivatives of the relaxation time and strength exhibit breaks; (iii) vitrification, corresponding to cessation of translational and rotational motions; and (iv) for anisotropic molecules the development of liquid crystallinity. At each of these transitions of a liquid, the structural or reorientational relaxation time is constant, independent of thermodynamic conditions; that is, while the temperature of the transition depends on pressure, the relaxation does not.

Nuclear and Electron Relaxation: The Magnetic Nucleus-Unpaired Electron Coupling in Solution

Abstract: Part 1 Introduction: the concept of relaxation in molecular spectrosocpy magnetic resonance the spin Hamiltonian formalism hyperfine coupling effect of hyperfine coupling on relaxation statistical description of spin relaxation correlation time for the nucleus-electron coupling pulsed versus continuous-wave magnetic resonance. Part 2 Relaxation times: definition of a relaxation time at zero magnetic field definition of T1, T2, T1p the Bloch equations nuclear T1 and T2: and instructive picture chemical exchange as a source of relaxation relaxation of a system constituted by spin pairs the analogies between nuclear and electron relaxation. Part 3 Measurement of relaxation times and related experiments: experimental techniques for the measurement of nuclear longitudinal relaxation time T1 selective and non-selective nuclear T1 nuclear overhauser effect experiments experimental techniques for the measurement of nuclear transverse relaxation time T2 measurement of T1p - spin-locking expiments the field-cycling experiment, useful for determing and nuclear relaxation times at low magnetic field effect of short relaxation times on 2D NMR experiments experimental techniques for the measurement of electron relaxation times T1 and T2 the ENDOR experiment measurement of T1 and T2 (Nuclear-Electron Cross-Relaxation). Part 4 Electron Relaxation in dilute systems: physical picture of electron relaxation spin-orbit coupling electron relaxation mechanisms in the solid state - crystal vibrations, electron spin-phonon coupling electron relaxation in solution some numerical values. Part 5 Nuclear relaxation in paramagnetic systems: mechanisms of nuclear relaxation through coupling with unpaired electrons dipolar relaxation a pictorial description of dipolar relaxation diffusion-controlled dipolar relaxation contact relaxation Curie spin relaxation effects of splitting the S Manifold at Zero magnetic field - isotropic hyperfine coupling with the metal nucleus, anisotopic hyperfine coup oing with the metal nucleus g-Anisotropy, zero field splitting Redfield limit and beyond. Part 6 Electron and nuclear relaxation through NMRD: what is NMRD? copper(II) systems cobalt(II) systems nickel(II) systems manganese(II) systems other metal ions field dependence of . Part 7 Magnetic couples systems: effect of magnetic coupling on the electron relaxation times - unlike spin pairs, like spin pairs NMR parameters in magnetic coupled systems - isotropic shift, part contents.

Theory of the semiconductor photon echo.

Abstract: The semiconductor Bloch equations are solved numerically for a two-pulse photon-echo configuration. The time-dependent diffracted signal is computed and the significance of many-body effects, carrier relaxation, and dephasing is investigated in detail. Assuming femtosecond-pulse excitation at various intensities and frequencies, distinctly different results are obtained if the exciton or the continuum electron-hole-pair states are excited. It is shown that pure exciton excitation produces a free-induction decay signal and no photon echo. An echo signal is obtained only if continuum states are excited either directly by choosing the central pulse frequencies appropriately or if the band-gap renormalization is sufficiently strong to shift continuum states into resonance. A continuous transition between free-induction decay and photon-echo signal is obtained with increasing excitation amplitude. A perturbative analytical analysis of the equations allows one to identify the role of the many-body effects in producing the different features.

Anomalous Dispersion and Dielectric Loss in Polar Polymers

Abstract: A theory of dielectric loss of polar polymers at high dilution in a nonpolar plasticizer is developed. The theory leads to a broad distribution in relaxation times, associated with the internal rotatory Brownian motion of the hydrocarbon chain. An approximate relation between the degree of polymerization, the frequency of maximum loss, and the viscosity coefficient of a polar plastic is obtained. The loss factor is calculated from the theoretical relaxation time distribution for a mono‐disperse polymer and for a poly‐disperse polymer with an exponential distribution in chain length. The theoretical loss factor is compared with the experimental loss factor of polyvinyl chloride plasticized with diphenyl. The agreement with experiment is semi‐quantitative.