Fluorescence Spectra of Organic Crystals

Relative quantum intensity measurements have been made of the fluorescence spectra, excited by 254 mμ radiation, of crystalline anthracene, trans -stilbene, para -terphenyl and diphenylacetylene at room temperature, using a calibrated spectrophotometer and photomultipher. Observations were made of the reflection and transmission spectra from 1 cm thick crystals of each compound, and also of the reflection spectrum from a thin microcrystalline layer of anthracene. The latter corresponds closely to the true molecular emission. Due to self-absorption, this has only a probability k e = 0.24 of direct transmission through a thick crystal. This value confirms the difference in technical and molecular photo-fluorescence decay times, reported by Birks and Little (1953). For the other materials, k e is larger, and for para -terphenyl it is practically unity. Vibrational bands have been resolved, which have been assigned to the 1-0 and 0- n ( n = 0 to 3) transitions in anthracene, and to the 0- n ( n = 0 to 5) transitions in para -terphenyl. The 1-0 transition in anthracene is attributed to fluorescence prior to the establishment of thermal equilibrium.