Manifestation of specific features of T-exciton migration in magnetic field effects on TT-annihilation in molecular crystals: Analysis of low-field resonances.

The manifestation of specific features of T-exciton migration in the shape of low field resonances (LFRs) in the magnetic field effects on the TT-annihilation in molecular crystals is studied in detail. The LFRs are shown to be caused by avoided crossing of spin-levels of T-excitons in magnetic fields nearly parallel to the axis of the zero field splitting interaction tensor. Simple and accurate formulas for the shape of the LFR-line are derived within the hopping model of T-exciton migration. With these formulas, we demonstrate that the LFR-line shape is fairly sensitive to the anisotropy of T-exciton migration, in particular, in quasi-one-dimensional (quasi-1D) and quasi-two-dimensional (quasi-2D) limits of exciton migration. The analysis of the shape is shown to allow for obtaining the magnitude of the small rate of jumps out of 1D and 2D spaces of fast migration in the cases quasi-1D and quasi-2D migration, respectively. In addition, this analysis enables one to obtain the spin relaxation rate of T-excitons.