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 $\ensuremath{\alpha}$ 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 $\ensuremath{\alpha}$ 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 \ifmmode^\circ\else\textdegree\fi{}K, the magnetic field effect on the total scintillation yield is (+2.5 \ifmmode\pm\else\textpm\fi{} 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 \ifmmode^\circ\else\textdegree\fi{}K. In anthracene, the fusionlike dependence is observed at all temperatures in the range studied (148-298 \ifmmode^\circ\else\textdegree\fi{}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 \ensuremath{\alpha}-particle track. The density of these transient quenchers is estimated to be in the range of 3 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ -5 \ifmmode\times\else\texttimes\fi{} ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$, 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 $\ensuremath{\alpha}$-particle track. The delayed scintillation ${L}_{D}$ which arises from the fusion of two triplet excitons is proportional to $\frac{{\ensuremath{\gamma}}_{\mathrm{rad}}}{{\ensuremath{\gamma}}_{\mathrm{tot}}}$ (where ${\ensuremath{\gamma}}_{\mathrm{rad}}$ is the radiative and ${\ensuremath{\gamma}}_{\mathrm{tot}}$ the total rate constant for the fusion of two triplets), whereas under conditions of weak photoexcitation, the delayed fluorescence is proportional to ${\ensuremath{\gamma}}_{\mathrm{rad}}$. 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 \ensuremath{\sim} 10% at 298\ifmmode^\circ\else\textdegree\fi{}K, and \ensuremath{\sim} 50% at 150\ifmmode^\circ\else\textdegree\fi{}K. whereas in anthracene the contribution of ${L}_{D}$ is \ensuremath{\sim}(50-70)%. The ratio of the $L$ values for anthracene/tetracene was found to be 6 \ifmmode\pm\else\textpm\fi{} 2 at 298\ifmmode^\circ\else\textdegree\fi{}K and to be of the order of unity at 148\ifmmode^\circ\else\textdegree\fi{}. This is in contrast to the photofluorescence efficiency which at 298 \ifmmode^\circ\else\textdegree\fi{}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 tetracene because this quenching of singlets by triplets dominates over the fission term (the singlet exciton lifetime in the $\ensuremath{\alpha}$-particle track is estimated to be about ${10}^{\ensuremath{-}11}$ sec in tetracene). Irradiation of tetracene for prolonged periods of time (equivalent to a dose of ${10}^{6}$ 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 ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$.