Luminescence Decay Times: Concentration Effects

Abstract

The optical properties (e.g., absorption spectrum, fluorescence spectrum, luminescence decay) of solutions of 2,5-diphenyloxazole (PPO) and p-terphenyl in benzene and cyclohexane are reported as functions of solute concentration. The absorption spectra of the solutions, when plotted as extinction coefficient vs wavelength, are unaffected by scintillator concentration, but the fluorescence spectrum of PPO solutions is affected in both intensity and shape. The luminescence decay of all solutions studied is nonexponential and can be resolved into the sum of two exponential decays. The fluorescence spectra and decay curves are affected by oxygen. The interactions between excited and unexcited molecules and between unexcited molecules themselves and the effect which these interactions have on the optical properties of luminescent solutions are examined. The experimental results can be adequately explained by a mechanism which involves formation of transient dimers by an interaction between excited and unexcited molecules in addition to excitation of solute domains of unexcited solute molecules. It is shown that cage effects by themselves cannot account for the luminescent properties of the systems studied.