Evidence for Exciplex Laser Action in Coumarin Dyes by Measurements of Stimulated Fluorescence

Experiments have been performed that measure the time dependence of stimulated fluorescence from acidified solutions of 4-methylumbelliferone and 7-hydroxycoumarin. The results show that laser action takes place from excited state complexes. It was found that exciplex emission is delayed after the neutral molecule emission. The delay gives a measure of the exciplex formation time. The measured dependence of exciplex formation time on hydrogen ion concentration, temperature, and pumping power was found to agree qualitatively with a simple model. Evidence for multiple exciplex formation is also presented.