Fluorescence reabsorption in anthracene single crystals: Lifetime variations with emission wavelength and temperature

Abstract

Fluorescence spectra and lifetimes of anthracene melt-grown single crystals and sublimation flakes have been examined at 298 and 77°K, using a mono-photon counting technique for the lifetime measurements. The observed emission decay times were nearly independent of the excitation wavelength, though a small dependence of the fluorescence spectrum on the excitation wavelength was noted. By contrast, large variations of fluorescence lifetimes in thick crystals were found as a function of emission wavelength. For thick melt-grown single crystals at 298°K the lifetime was found to increase from 9.8 nsec at 405 nm to 20.4 nsec at 445 nm. For sublimation flakes at 77°K and at 298°K and for thick melt-grown crystals at 77°K, the lifetimes were less than 10 nsec and were nearly independent of emission wavelength. Despite these relatively large variations in lifetimes, the decay rates at each separate wavelength remained exponential, within experimental error. Theoretical calculations were made of emission lifetimes based on a model with one reabsorbing state. The calculations are in substantial agreement with the experimental results.