Luminescence of Free and Self-Trapped Excitons in α- and β-Perylene Crystals

Luminescence spectra and decaytimes have been measured over a wide temperature range. Thermal equilibrium population between free and self-trapped excitons has been confirmed in both dimeric (α) and monomeric (β) forms of perylene crystals. Two-center type self-trapped exciton in α-perylene, and one-center type and two-center type self-trapped excitons in β-perylene are stable and play prominent roles in exciton radiative decay. The decaytimes of free and self-trapped exciton luminescence are discussed in terms of exciton-lattice interaction. The exciton band width is briefly discussed.