Polarized Absorption and Fluorescence Spectra of Crystalline Anthracene at 4°K: Spectral Evidence for Trapped Excitons

The long-wavelength absorption and fluorescence spectra of anthracene crystal have been examined in the ab (cleavage) plane at 4^∘{}K. The spectra are compared with those reported previously, and are related to the spectra of the solid solution. The spectra of the crystal and the mechanism of the fluorescence process are interpreted in terms of Frenkel's theory of free and trapped excitons. The relative shift of the electronic transition in the vapor, mixed crystal, and pure crystal is related to the possibility of detecting trapped exciton states in molecular crystals.