Delayed fluorescence ODMR as a probe of triplet-triplet fusion probability

A kinetic scheme is presented to rationalize recent observations of delayed fluorescence ODMR in isotopically mixed naphthalene crystals, in which the sign of the signal changes from negative to positive as the critical concentration for triplet-triplet transport among the perprotonated traps is approached. The effects of chopped optical excitation, prolonged microwave pumping, temperature and light intensity are considered. The observation of positive DF-ODMR in the high concentration case is used to obtain a lower limit for the ratio of the annihilation constantk ₂ to the pair dissociation constantk _?1.