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The successful implementation of ultrasensitive fluorescence spectroscopy of biological and chemical species depends upon certain photophysical parameters associated with the fluorescent dye used in the investigation. These parameters include the fluorescence quantum efficiency, photodestruction quantum efficiency, absorption cross section and fluorescence lifetime. These photophysical constants were measured for several fluorescent dyes that are used for the tagging of biological species. Three different solvents, ethanol, water and a cationic surfactant used above its critical micelle concentration, were studied. The effective photon yield (ratio of the fluorescence quantum yield to the photodestruction quantum efficiency) for the dyes is nearly 100 times greater in ethanol than it is in water because of the superior photostabilities of these dyes in ethanol solvents. The implications of these parameters for the …