Main components of the EFG

The value of is given by the component of the EFG tensor along the main quantization axis. Therefore, in this example where the EFG is axial (77 = 0) with the main component the quadrupole shift is eQVzJ - This is just half the quadrupole splitting that would be observed in an unperturbed quadrupole spectrum without a magnetic field at the nucleus. 

Fig. 4.13 Combined magnetic hyperfine interaction for Fe with strong electric quadrupole interaction. Top left, electric quadrupole splitting of the ground (g) and excited state (e). Top right first-order perturbation by magnetic dipole interaction arising from a weak field along the main component > 0 of the EFG fq = 0). Bottom the resultant Mossbauer spectrum is shown for a single-crystal type measurement with B fixed perpendicular to the y-rays and B oriented along...

The shape of the ellipsoid was on average non-cylindrical. In fact since the distribution of eigenvalues was fairly symmetric, it is possible to describe with simply a rhombic component. The TCF of the form of the EFG shows a rapid initial decay to a plateau with a order parameter of 0.8 of the time zero value. The reorientational motion is multi exponential and is the main cause of the decay of the EFG-TCF. 

In the low-field condition, the quantization axis is defined by the EFG main component In this situation, and rj can both be determined from powder spectra when recorded in an externally applied field. Figure 4.14 shows simulated spectra as is often encountered in practice such as in applied-field measurements of diamagnetic compounds or fast-relaxing paramagnetic compounds at high temperatures. The simulated traces differ in detail from a single-crystal spectrum as shown in Fig. 4.13, but their features still correlate in a unique manner with rj and the sign of... 

Recent developments in solid-state 170 NMR techniques make it possible to analyse 170 NMR spectra of biological solids, from which 170 EFG and CS tensors can be obtained. Instead of isotropic values such as r)iso, which are mainly obtained from solution 170 NMR, the use of tensor components enables understanding of the 170 NMR parameters in more detail. This section describes a few concrete examples of the analysis of solid-state 170 NMR spectra of amino acids and the current understanding of 170 NMR tensors. 

If the direction of the main component V z of the electric field gradient (EFG) fluctuates rapidly as a result of diffusion, the Mbssbauer spectrum may be changed in shape. Two atomic sites have very different isomer shift values and the jump between these two states also may show the time-dependent Mbssbauer spectra as a function of the jump firequency. The time scales over which the Mbssbauer effect can be used to observe the dynamical effect is determined by the characteristic times associated with the resonance the natural lifetime and the Larmor precession times of the hyperfine interactions. 

---