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

The mechanisms of scintillation of organic crystals bombarded by 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 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 (LP) and delayed (LD) components of the total scintillation yield L=LP+LD. 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, LD appears to be strongly temperature dependent, while LP is not. This is explained in terms of the high density of transient singlet exciton quenchers in the particle track. The density of these transient quenchers is estimated to be in the range of 3 × 1017 -5 × 1018 cm-3, 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 particle track. The delayed scintillation LD which arises from the fusion of two triplet excitons is proportional to radtot (where rad is the radiative and tot the total rate constant for the fusion of two triplets), whereas under conditions of weak photoexcitation, the delayed fluorescence is proportional to rad. It is shown how the contribution of LD 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 LD 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 LP in tetracene because this quenching of singlets by triplets dominates over the fission term (the singlet exciton lifetime in the particle track is estimated to be about 10-11 sec in tetracene). Irradiation of tetracene for prolonged periods of time (equivalent to a dose of 106 rad) changes the magnetic field dependence at room temperature from the small positive fissionlike dependence to a negative (-2%) fusionlike dependence. This is due to the introduction of permanent singlet exciton quenching centers which effectively compete with fission and whose density is estimated to be of the order of 1018 cm-3.