Theory of stimulated nuclear polarization in high magnetic fields

Abstract

A theory of stimulated nuclear polarization (SNP) is proposed for the first time. The calculations are performed by the method of summation of radical pair (RP) re-encounter contributions to the recombination of neutral radicals in high magnetic fields in the framework of a continuous diffusion model. Some rules for the sign of the SNP effect are formulated. Based on the calculations for model systems, we have studied the effect of different parameters of RPs on the SNP effects: the rate constants of RP recombination from singlet and triplet states, the rates of RP decay, e.g. due to the reaction of radicals with acceptors, and the effective in-cage RP lifetime. It is shown that in SNP the phenomenon of spin-looking manifests itself: when the amplitude of an applied microwave field exceeds the hf splitting in the ESR spectrum, the sign of polarization changes. The mutual effect of nuclei on the magnitude and sign of SNP is studied. For non-viscous liquids (η ≈ 1 cP), the analytical expressions for the SNP effects are obtained by perturbation theory. It is demonstrated that over a wide range of RP parameters, the results for SNP obtained on the basis of these analytical expressions are in good agreement with numerical results. The comparison of theoretical and experimental results exhibits their full conformity.