Triplet exciton annihilation in naphthalene at high triplet densities

Triplet exciton annihilation in naphthalene has been investigated as a function of triplet excition density. With increasing triplet density, a change of the magnetic field dependence of the delayed fluorescence as well as of the phosphorescence is observed, which is not expected on grounds of current theoretical models for triplet exciton annihilation. These density effects are studied in detail by varying the triplet exciton density by a factor of ${10}^5$. Analogous effects have been observed also in $p$-terphenyl. To explain the experimental findings a theoretical model which includes electron-hole pair production by triplet annihilation is presented.