Effects of Magnetic Fields on the Mutual Annihilation of Triplet Excitons in Molecular Crystals

The intensity of triplet-exciton annihilation luminescence in anthracene crystals at room temperature increases in weak magnetic fields up to a maximum increase of 5% at 350 Oe. For stronger fields the intensity decreases, finally leveling off at 80% of the zero-field value for $H\gtrsim 5$ kOe. Sharp dips are found in the high-field intensity for field orientations at which a particular kind of level crossing occurs for pairs of triplet excitons. These effects result solely from field-induced changes in the rate constant for triplet-triplet annihilation.