Alignment relaxation rate

It is pertinent to comment here on the collapse of the Nuclear spin relaxation rate seen below Tc in NMR experiments. When nuclear spins in a material are aligned by an external magnetic field they can relax to equilibrium by causing... 

If the excited molecules undergo disorienting collisions before emitting light, then the bpQ of various rank K relax at different rates T/c. The anisotropy of polarization % permits us to find the ratio between the population relaxation rate To and that of the alignment relaxation r2 ... 

We can see some important peculiarities in Eqs. (4.29) and (4.30). The intensity difference Ir—Ii is characterized by the parameter uj>/Ti, which contains the relaxation rate of orientation (in the case of linearly polarized excitation the difference I —1 contains the relaxation rate of alignment see (4.21)). At the same time the sum of intensities Ir + Ii in this case still contains transversal components of alignment bP%i and is now magnetic field dependent. 

The dependence of C(B) on two parameters uoj /T1 and ujj>/T2 is of course connected to the fact that, as can be seen from (2.37) and (2.39), alignment is always created along with orientation under circularly polarized excitation. Hence, for the purpose of determining the orientation relaxation rate Ti, one has to register the intensity difference Ir — h... 

T = 50 K, alignment parallel to c-axis. Lower Central transition, alignment perpendicular to c-axis, showing similar second-order effects in the distribution of the EFGs as in the satellite transition spectrum. From Takigawa et al. (1991). B. Relationship between the O relaxation rate and the Cu relaxation rate for the planar sites of YBa2Cu30v with temperature as an implicit parameter. The solid line of unity slope indicates the relationship Cu V O T = 19.3. The data deviate from this relationship above 110 K. From Hammel et al. (1989). Both figures used by permission of the... 

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). 

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