Stimulated-echo technique

Thus, the stimulated echo technique allows one to estimate the elementary angular jump angle a/. For selected aj, the crossover in the stimulated echo correlation function, observed between the tp 0 to tp > oo limits in a simulation, is displayed in Fig. 8. 

While the advantages of the stimulated-echo technique lie in the resolution of small jump angles, measurement of two-dimensional (2D) NMR spectra is best suited to the study of large angular displacements, as typically found in crystalline rotator phases, for example. They lead to characteristic off-diagonal patterns [cf. 

The stimulated-echo technique is well-suited not only to investigate the primary relaxation of glass-forming liquids above Tg (cf. Section IV.C.3) but also secondary relaxations [96,97]. Figure 57 shows correlation functions Ccos(tm tp) for three type B glasses at T/Tg 0.83. It was demonstrated that the... 

In view of these results for binary glasses composed of molecules with different masses respectively different Tg, one may speculate that the isotropic reorientation of the small molecules results from translational diffusion in an essentially rigid glassy matrix formed by the large molecules, i.e., the diffusional process may be probed by 2H NMR via rotational-translational coupling.183 Finally, we note that the stimulated-echo technique was also applied to study the main relaxation in... 

To determine permeability distributions, experiments are conducted whereby fluid is injected into saturated samples. A pulsed-field-gradient stimulated-echo technique is used to resolve the velocity field within the sample. An inverse problem is formulated and solved to determine the permeability distribution. 

Complementary information can be obtained from surfactant self-dilfusion. In a typical NMR self-dilfusion experiment, the value of the surfactant diffusion coefficient is obtained along with the values of the solvents. However, for two reasons the accuracy in self-diffusion coefficient of a surfactant is lower than that of a solvent. First, surfactant is typically present at a considerably lower concentration, and second, the transverse relaxation rate is higher and thus more unfavorable. Depending on the experimental conditions and the system, it may turn out to be impossible, to measure surfactant diffusion. However, by using stimulated echo techniques this problem can be diminished. 

Fig. 12. Sequences for volume selective single voxel spectroscopy. Both techniques work with three slice-selective RF-pulses. (a) The Point RESolved Spectroscopy (PRESS) sequence generates a volume selective double spin-echo. The entire time delay between the initial 90° excitation and the echo is sensitive to transverse relaxation, (b) The Stimulated Echo Acquisition Mode (STEAM) sequence generates a stimulated echo. Maximal signal intensity (without relaxation effects) is only half the signal intensity of PRESS under comparable conditions, but slice profiles are often better (only 90° pulses instead of 180° pulses) and the TM interval is not susceptible to transverse relaxation, (c) The recorded echo signal is only generated in a volume corresponding to the intersection of all three slices.

Users of any NMR instrument are well aware of the extensive employment of what is known as pulse sequences. The roots of the term go back to the early days of pulsed NMR when multiple, precisely spaced RF excitation pulses had been invented (17,98-110) and employed to overcome instrumental imperfections such as magnetic field inhomogeneity (Hahn echo) or receiver dead time (solid echo), monitor relaxation phenomena (saturationrrecovery, inversion recovery, CPMG), excite and/or isolate specific components of NMR signals (stimulated echo, quadrupole echo), etc. Later on, employment of pulse sequences of increasing complexity, combined with the so-called phase-cycling technique, has revolutionized FT-NMR spectroscopy, a field where hundreds of useful excitation and detection sequences (111,112) are at present routinely used to acquire qualitatively distinct ID, 2D, and 3D NMR... 

Tanner [49] measured diffusion coefficients of water in three different types of frog muscle cells. He used a variety of magnetic field gradient techniques so as to cover a wide range of diffusion times A= 1 ms to 1 s. The time dependence of the diffusion coefficient was analyzed to obtain the intracellular diffusion coefficients and estimates of the permeability of the cell membranes. In restricted diffusion studies three 90 degree r.f. pulse sequences (stimulated echo) are often used which provides PG-NMR experiments with long diffusion times to explore the dependence of diffusion time on the echo attenuation [49]. 

Many NMR techniques rely on the formation of magnetization echoes [Bagl, Blu5]. Examples are (1) the measurement of T2 relaxation times by the Hahn echo (Section 2.2), (2) the analysis of slow motion by stimulated echoes for spins with / = 1/2, and for nuclei with spin / = 1 or pairs of coupled spins with / = 1/2 by the solid echo and the alignment echo (Section 3.2), (3) MAS, where echoes are generated by mechanical... 

The dephasing time, T2, can be measured by the photon echo technique or determined from the homogeneous width of saturation spectroscopy, which is a Fourier transform of the former, as easily seen from Eq. (5.35). When a sample is irradiated with three consecutive laser pulses at times, 0, t2, and f3, an echo pulse is emitted at time, t2 + t. This is called stimulated photon echo. Several additional echo techniques have been proposed. 

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