Quadrupole interactions relaxation effect

KFeFs, like RbFeFj, has a cubic perovskite lattice above the Neel temperature of 115 K, but below this temperature there is a trigonal distortion along the [111] axis which is expected to be the spin-axis. A quadrupole interaction is seen in the ordered phase [15] which is positive as required by a trigonal distortion. Relaxation narrowing effects are seen close to the critical point. 

Fig. 17.29 Yb spectra at 4 2 K showing the effect of a long paramagnetic relaxation time (Yb(N03)3.6H20 and Yb2(S04)3.8H20), short (YbCls and 6203), and intermediate (Yb acetate). Note that the splitting in 6203 is due to two sites with quadrupole interactions. [Ref. 178, Fig. 1]...

Later Wallach [53] made a more general treatment of the effect of internal rotations on quadrupole relaxation rates. His results, which are important for the interpretation of quadrupole relaxation data in macromolecular solutions (Chapter 8), are that every rapid internal rotation will reduce the quadrupole interaction by a factor (3 cos 3 - 1) (see also Ref. [64]). 

Since quadrupole interaction constitutes such an effective relaxation mechanism it is only rarely that other types of interactions have to be considered for relaxation of covalent chlorine, bromine or iodine. However, it can be estimated that in paramagnetic molecules the interactions between the halogen nuclear spin and the unpaired electron briefly mentioned in sub-Section 1.3.2 may give significant contributions to the relaxation rate. For VCl modulation of the electron-... 

It should be noted that for anisotropic liquid crystals the NMR signal is split into 21 component signals due to first-order static quadrupole interactions but that, in the absence of second-order quadrupole effects, the width of the central line gives the transverse relaxation rate both for powder samples and for macroscopically aligned samples (see further Chapter 7). 

Simulations reported in this and other reviews (Singel, 1989) demonstrate that, based on the spin-Hamiltonian model, effective zero-field nuclear quadrupole interaction parameters can be obtained by invoking a condition known as exact cancellation. On the basis of observations made concerning what experimental conditions yield optimal performance in ENDOR and ESEEM experiments, it has been suggested in this review that level crossing and the associated cross-relaxation is responsible for the deep modulation and corresponding narrow lines in the ESEEM spectrum. If level crossing and the resultant cross-relaxation processes are, in fact, the requisite condition for deep ESEEM, then the techniques described here are... 

In the La-H system, the proton diffusive motion also has significant effects on the linewidth, relaxation time T, and Knight shift of the La resonance via the nuclear quadrupole interaction of the La with the fluctuating electrostatic environment resulting from the proton motion. All of these parameters of the La spectrum are strongly temperature and concentration dependent. Schreiber and Cotts (1963) show in detail that the La data confirm and support the model of proton diffusion based on the proton NMR results alone. 

Since the magnetic ordering temperatures of these salts are either very low or non-existent, the Mdssbauer effect spectrum of the lanthanide nucleus reflects the temperature dependence of the quadrupole interaction, except that in cases where the ionic relaxation rate is sufficiently slow, magnetic hyperfine structure may also appear at low temperatures. 

Now Retails is the transition probability from the spin state j3 to the spin state a and Raa/sis = R/3/3aa- The diagonal part of Eq. (5.25) is a second-rank equation of motion for evolution of the density matrix under the effect of a random perturbation. There are two important second-rank relaxation mechanisms the dipole-dipole and the quadrupole interactions. Chapter 2 showed that these interactions and the anisotropic chemical shift can all be written as a scalar product of two irreducible spherical tensors of rank two, that is. 

The characteristic shift of the center of the Mossbauer spectrum from zero velocity is called the isomer (chemical) shift and reflects the differences in electronic environment of Fe in source and absorber. When there is a quadrupole splitting of the I = 3/2 14.4 keV level, a two-line spectrum results. Six lines are found when there is no quadrupole interaction and when either an external magnetic field is applied or a hyperfine interaction exists, thai produces an effective field at the nucleus. Relaxation effects and the different charge states of Fe lead to a wide variety of spectra which have been analyzed (Cammack et al, 1977). 

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