Exciton dynamics inα-particle tracks in organic crystals: Magnetic field study of the scintillation in tetracene crystals

Abstract

The mechanisms of scintillation of organic crystals bombarded by $\alpha$ particles are discussed in terms of the current knowledge of exciton dynamics, which has been derived from a study of the photofluorescence of crystals such as anthracene and tetracene. The scintillation of tetracene excited by 4.4-MeV $\alpha$ particles incident in a direction perpendicular to the ab plane has been studied in the presence of external magnetic fields (0-4000 G) and compared to the scintillation of crystalline anthracene. At 298 °K, the magnetic field effect on the total scintillation yield is (+2.5 ± 0.5)% in tetracene and displays a typical fissionlike (fission of one singlet exciton into two triplets) dependence. At low temperatures when fission is suppressed, a fusionlike dependence (reverse of fission) appears with a (-4 to -5)% effect at 4000 G at 148 °K. In anthracene, the fusionlike dependence is observed at all temperatures in the range studied (148-298 °K). Using appropriate kinetic equations, expressions are derived for the prompt ($L_P$) and delayed ($L_D$) components of the total scintillation yield $L=L_P+L_D$. These expressions describe the temperature and magnetic field dependence of $L$, which arises because of the temperature and magnetic field dependence of the exciton fission and fusion rate constants in tetracene. In tetracene, $L_D$ appears to be strongly temperature dependent, while $L_P$ is not. This is explained in terms of the high density of transient singlet exciton quenchers in the α-particle track. The density of these transient quenchers is estimated to be in the range of 3 × ${10}^{17}$ -5 × ${10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$, and they are identified, in accord with a previous suggestion by Schott, as triplet excitons which are created by random recombination of electrons and holes in the $\alpha$-particle track. The delayed scintillation $L_D$ which arises from the fusion of two triplet excitons is proportional to $\frac{{\gamma }_{\mathrm{rad}}}{{\gamma }_{\mathrm{tot}}}$ (where ${\gamma }_{\mathrm{rad}}$ is the radiative and ${\gamma }_{\mathrm{tot}}$ the total rate constant for the fusion of two triplets), whereas under conditions of weak photoexcitation, the delayed fluorescence is proportional to ${\gamma }_{\mathrm{rad}}$. It is shown how the contribution of $L_D$ to $L$ can be estimated from the magnetic field dependence of $L$. In tetracene, this contribution of the delayed component is ∼ 10% at 298°K, and ∼ 50% at 150°K. whereas in anthracene the contribution of $L_D$ is ∼(50-70)%. The ratio of the $L$ values for anthracene/tetracene was found to be 6 ± 2 at 298°K and to be of the order of unity at 148°. This is in contrast to the photofluorescence efficiency which at 298 °K is 50 to 100 times lower in tetracene because of fission. This behavior is attributed to a lack of a temperature dependence of $L_P$ in...