Relaxation sink mechanism

However this is not the only source of CIDNP time dependence that can arise from such a reaction. Consider what happens in the limit of very fast nuclear relaxation in A (i.e. no cancellation). Exchange now transfers recombination polarization from A to unpolarized A where it is able to relax very rapidly. This may be called a "relaxation sink" mechanism, and should occur at a maximum rate of k [A ], being most effective when the A radicals are a e to relax completely prior to reconversion to A i.e. 

Figure 5.22 Schemes of possible mechanisms for luminescence concentration quenching (a) energy migration of the excitation along a chain of donors (circles) and a killer (black circle), acting as nonradiative sink (b) cross relaxation (including an illustrative energy-level diagram) between pairs of centers. (Sinusoidal arrows indicate nonradiative decay or radiative decay from another excited level.)...

As an illustration, consider the isothermal, isobaric diffusional mixing of two elemental crystals, A and B, by a vacancy mechanism. Initially, A and B possess different vacancy concentrations Cy(A) and Cy(B). During interdiffusion, these concentrations have to change locally towards the new equilibrium values Cy(A,B), which depend on the local (A, B) composition. Vacancy relaxation will be slow if the external surfaces of the crystal, which act as the only sinks and sources, are far away. This is true for large samples. Although linear transport theory may apply for all structure elements, the (local) vacancy equilibrium is not fully established during the interdiffusion process. Consequently, the (local) transport coefficients (DA,DB), which are proportional to the vacancy concentration, are no longer functions of state (Le., dependent on composition only) but explicitly dependent on the diffusion time and the space coordinate. Non-linear transport equations are the result. 

Spin diffusion to paramagnetic impurity is an important mechanism, not only in solids but in some large molecules which exist in solution. In fact, there can be spin diffusion not only to impurities but to any center which has good coupling to the lattice and this could be a sub-molecular unit with its own motion conducive to rapid relaxation or a quad-rupolar nucleus. Because of the sheer size of the electron moment, spin diffusion to paramagnetic impurities is more common than to these other heat sinks. See the review by Noack (1971) for references to this mechanism. 

---