On singlet exciton fission in anthracene and tetracene at 77°K

Relative variations of prompt flourescence yield, on application of a magnetic field are studied at 77°K for anthracene and tetracene crystals excited by light with wavenumbers up to 50,000 cm^-1. The results obtained for anthacene, as compared to those previously reported at 300°K, indicate a very small temperature dependence. The variations observed for tetracene at 77°K are comparable in magnitude to those for anthracene. The singlet exciton fission process, responsible for the experimental observations, does not involve a thermally relaxed lowest bound or charge transfer singlet exciton. The possible role of upper excited vibronic states is discussed.