TRIPLET-SINGLET EMISSION SPECTRA OF BENZENE AND NAPHTHALENE CRYSTALS AT LOW $TEMPERATURES^{*}$

Abstract

The triplet-singlet transition of benzene at 3400 \AA sharpens at liquid helium temperature to a spectrum with remarkably well-defined structure. With naphthalene crystals the known lowest triplet-singlet emission was observed at 5200 \AA as a fairly sharp spectrum at liquid nitrogen and at liquid helium temperatures. In addition, evidence of another T-S transition at 3600 \AA was obtained from plates taken at liquid helium temperature. The results in the experiments with naphthalene are very sensitive to the purity of the sample. For example, with a commercial sample of naphthalene studied at liquid helium temperature the T.S emission of thianapthene is present, and in addition a spectrum at shorter wavelengths which proved to be identical with the fluorescence spectrum of naphthalene described by Obreimov and Shabaldas at liquid hydrogen temperature, usually not observable with a phosphoroscope. We consider that a second triplet state of thianaphthene may be so located as to sensitize the flourescence emission of naphthalene.