Non‐linear fluorescence quenching in molecular crystals II. Recombination of free excitons

Abstract

AbstractThe decrease of quantum yield and the fluorescence decay time of naphthalene and anthracene crystals with increasing exciton concentration (non‐linear quenching) in the range from 10 to 80 K is measured. Rate constants of bimolecular recombination, γ, found from these data with regard to diffusional escape of excitons from the light absorption layer, are increased by the factor of 1.5 to 2.0 with the increase in temperature from 10 to 30 K. In the same temperature interval the diffusion coefficient of excitons is decreased. Free‐pass model explaining an increase of γ with temperature (by the increase in the mean thermal velocity of free excitons) is suggested. From the measurements of non‐linear quenching for different light absorption coefficients of pumping the diffusion coefficients of excitons along the c′‐axis at 10 K are found to be equal to (1.3 ± 0.5) and (3 ± 1.5) × 10−3 cm2s−1 in anthracene and naphthalene crystals, respectively.