Organic Liquid Scintillators. III. On the Quantum Yield of Fluorescence and the Quenching of Fluorescence by Oxygen

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Abstract
Expressions for the quantum yield of fluorescence of aromatic hydrocarbons in benzene and alkylbenzenes have been derived assuming zero internal quenching. These expressions indicate that the molecular quantum yield of fluorescence increases with (a) the lifetime of the first excited singlet state of the solvent, (b) the transition probabilities from the levels of the solute that correspond to the levels involved in the excitation of the solvent [A. Heller, J. Chem. Phys. 35, 1980 (1961)], and (c) the increase in the degree of degeneracy in the solute levels corresponding to those involved in the excitation of the solvent. Minimum values of the molecular quantum yield of fluorescence of a group of aromatic hydrocarbons have been computed at different concentrations of the solutes. The compounds with the highest values are p‐divinylanthracene, perylene, p‐quaterphenyl and p‐terphenyl. The dimensions of the excited aggregates that are formed by the exchange interaction between the molecules in liquid or plastic scintillators are estimated from Basile's data for solutions of PBD in polyvinyltoluene [L. S. Basile, J. Chem. Phys. 27, 803 (1957)]. The number of monomer units per aggregate is about 1.2×103 and the radius of interaction about 18 A. The quenching of fluorescence by oxygen is explained by the existence of π‐electron level pairs of almost similar energy difference in the solvent and the solvent—oxygen complex [H. Tsubormura and R. S. Mulliken, J. Am. Chem. Soc. 82, 5966 (1960)], and by the short lifetime of the complex compared with that of the solvent. The latter results from the lower order of symmetry of the complex in comparison with that of the π‐electron system in benzene.