Dephase, spins

Various ID and 2D high-resolution solid-state NMR techniques have been applied to hydrous silicate and aluminosilicate glasses simple acquisition, CPMAS, HETCOR, dipolar dephasing, spin counting, DQ correlation, and rotational echo double resonance (REDOR). ... 

FID does not die away before the deadtime has elapsed. In die case of inliomogeneously broadened EPR lines (as typical for free radicals in solids) the dephasing of the magnetizations of the individual spin packets (which all possess slightly different resonance frequencies) will be complete within the detection deadtime and, therefore, the FID signal will usually be undetectable. 

As the spins precess in the equatorial plane, they also undergo random relaxation processes that disturb their movement and prevent them from coming together fiilly realigned. The longer the time i between the pulses the more spins lose coherence and consequently the weaker the echo. The decay rate of the two-pulse echo amplitude is described by the phase memory time, which is the time span during which a spin can remember its position in the dephased pattern after the first MW pulse. Tyy is related to the homogeneous linewidth of the individual spin packets and is usually only a few microseconds, even at low temperatures. 

Q-branch non-broadening 117 rotational cross-sections 193 spin-lattice relaxation 47 weak dephasing, collisional narrrowing 93... 

When, however, phonons of appropriate energy are available, transitions between the various electronic states are induced (spin-lattice relaxation). If the relaxation rate is of the same order of magnitude as the magnetic hyperfine frequency, dephasing of the original coherently forward-scattered waves occurs and a breakdown of the quantum-beat pattern is observed in the NFS spectrum. 

Fig. 1.22 RARE sequence. Here the formation of the first spin echo is conventional. The CPMG form of spin echo is used to avoid the accumulation of flip angle errors over the echo train. However, before the second echo can be acquired, the phase-encoding has to be rewound to undo the dephasing of the spins. Therefore, a phase encoding step of equal...

The reference scan is to measure the decay due to spin-lattice relaxation. Compared with the corresponding stimulated echo sequence, the reference scan includes a jt pulse between the first two jt/2 pulses to refocus the dephasing due to the internal field and the second jt/2 pulse stores the magnetization at the point of echo formation. Following the diffusion period tD, the signal is read out with a final detection pulse. The phase cycling table for this sequence, including 2-step variation for the first three pulses, is shown in Table 3.7.2. The output from this pair of experiments are two sets of transients. A peak amplitude is extracted from each, and these two sets of amplitudes are analyzed as described below. 

Notice that dephasing of the transverse magnetization does not affect Mz a T2 process involves no energy transfer but, being a spontaneous process, does involve an increase in the entropy of the spin system. 

It should be emphasized that the approach to equilibrium by a Tx process, in which Mz approaches M0, also causes Mx and My to approach zero. Thus, the T2 of eqn (5.5) must include both the effects of spin-lattice relaxation as well as the dephasing of the transverse magnetization. Transverse relaxation is often much faster than spin-lattice relaxation and T2 is then determined mostly by spin dephasing. In general, however, we should write ... 

In order to obtain kinetic parameters for the electron transfer of [11] /K +, the dephasing time tm of the electron-spin echo near the phase-transition temperature Tt was measured. These experiments gave a correlation time tc of 100 ns for the electron transfer at Tg = 170 K. From the assumption of an exponential decrease of c in solution, a value of 100 ps was estimated for tc at room temperature (Rautter, 1989 Rautter et al., 1992). 

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