Characteristics of the 4-methylumbelliferone laser dye

The emission characteristics of the coumarin-dye 4-Methyl-umbelliferone (4-MU) are investigated by studying the low-intensity fluorescence spectra and the laser-pumped gain spectra for various solvents and pump powers. A total of five different forms were found to exist, each of which can be isolated in a proper solvent. Of the five species, I is the neutral molecule, two others (II and V) are results of ground-state reactions, while two (III and IV) are produced by excited-state reactions and do not have a stable ground state. By proper adjustment of the solvent a number of combinations of the five forms can coexist and lase together, giving rise to a variety of gain spectra and allowing the possibility of "tailoring" a desired gain curve over most of the visible spectrum. Comparison of the gain and fluorescence spectra indicates the presence of triplet or excited-state losses in some of the forms, yields comparative values of emission parameters, and reveals an acidity-dependent quenching of one of the excited-state species.