Light Absorption by Pyrene Single Crystals Between 4 and 300 °K

Abstract

Pyrene single crystals have been grown from zone refined material by sublimation and from the melt. The absorption edge has been measured for thick and thin crystals at room temperature. Thin crystals have been successfully cooled below the phase transition temperature (123 °K) without cracking. The sharp absorption peak at 376 nm vanishes at this temperature while the fluorescence spectrum remains unchanged. Using polarized light below 50 °K, a band with a maximum at 410 nm appears whose intensity increases with decreasing temperature. It is concluded that this band is due to ground-to-excimer state absorption in the ground state equilibrium distance of the pyrene molecules. The consequences of this finding for excimer-exciton transport is discussed.