The symmetry selection rules for the mixing of states by spin orbit interaction are known to cause unequal population of the three electronic levels of excited triplet states in aromatic molecules. It is shown here that the same selection rules, when combined with the hyperfine interaction, can also produce a selective population and depopulation of the nuclear sublevels. This mechanism is proposed to be responsible for the large optical nuclear polarisation (ONP) in low fields observed experimentally.
The correlation times, τ, of the magnetic interactions in solutions of organic free radicals are determined by measuring the frequncy dependence of the relaxation of the protons of the solvent. The measured frequency dependence agrees well with theoretical expectations based on a model of translational diffusion. It depends on the diffusion constant of the solvent and on the temperature, but is independent, to first approximation, of the type and concentration of the dissolved radical.