Low magnetic field effects on triplet pair annihilations at canonical orientations

Using the density operator formalism, a simple analytical model is developed to study low magnetic field effects on triplet pair annihilations in organic solids. Analysis is restricted to canonical orientations where two identical triplet molecules have the same orientation and the direction of the external magnetic field is parallel to one of the principle axes of the dipolar coupling tensor for a triplet. The analytical solution reveals that the low magnetic field effect in the triplet pair arises from the anisotropic dipole-dipole coupling in a triplet. In the presence of the dipole-dipole coupling, the spin quantization axis for each triplet gradually changes with the increase of the external magnetic field from zero field to high field. The low magnetic field effect reaches a maximum when the Zeeman splitting between the spin states matches a dipole-dipole coupling component orthogonal to the external magnetic field direction. The result is also discussed with the low magnetic field effect in the radical pair with one isotropic hyperfine coupling. Published under license by AIP Publishing.