Self-absorption of molecular fluorescence, the design of equipment for measurement of fluorescence decay, and the decay times of some laser dyesa)

Abstract

At moderate concentrations (10−3 to 10−2 M), self-absorption of the fluorescence of a dye is a major obstacle to the accurate measurement of its decay time and quantum yield. Equations are derived for this self-absorption correction for the experimental condition where the correction is minimized namely for a thin sheet of dye medium betwen parallel plates. Simple equipment is described for the determination of decay times at such concentrations. An order of magnitude estimation relates the largest exciting flux density that may be employed for a laser dye so that the measured decay time is not distorted by amplified fluorescence. Tunable, short-pulse excitation has been achieved by a nitrogen laser pumped oscillator–amplifier dye-laser configuration in which a saturable absorber is used both to shorten the oscillator pulses and to prevent feedback from the amplifier. Decay times are reported for systems containing rhodamine 6G, rhodamine B, and kiton red S for a range of concentrations between 10−7 M to 10−2 M. Self-quenching of fluorescence is minor and is not evident at concentrations below 10−3 M.