Abstract

Absolute quantum yield measurements of fluorescence and phosphorescence have been made for a number of organic molecules dissolved in a transparent solid solution (E.P.A.) at 77°K. The quantum efficiencies range from nearly zero for bromobenzene to 0.6 for benzophenone. The measurements are believed to be accurate to about 10 percent. The nonradiative processes, which account for the quanta not re-emitted, are discussed. If the radiationless processes all originate in the triplet state, the triplet state lifetimes measured in solid solution can be corrected to give the natural lifetime of the triplet (phosphorescent) state. Since one cannot tell a priori that this assumption is justified, other experiments are necessary. If the singlet-triplet absorption strength is known, one can tell whether this correction should be applied or not, and infer the origin of the radiationless transition. Finally, a comparison of quantum yields in solid solution with quantum yields in other phases suggests that the mechanism of radiationless transitions, in the absence of concentration quenching and specific solvent and quencher action, is intramolecular potential surface crossing, followed by the transfer of excess vibrational energy to the solvent.