Fluorescence behavior of 7-hydroxycoumarine excited by one-photon and two-photon absorption by means of a tunable dye laser

The fluorescence spectrum of 7-hydroxycoumarine in ethanol excited by a pulsed tunable dye laser reveals different features when excitation proceeds via one-photon and two-photon absorption. In the former case the spectrum shows two peaks delayed in time by approximately 2 ns and characterized by different lifetimes. The relative intensity of these peaks is unaffected by significant changes in the dye concentration and therefore the formation of an “exciplex” species seems to be here confirmed. In the spectrum obtained by two-photon excitation the second peak at longer wavelength is absent. Results are presented for both room and liquid nitrogen temperatures as well as for other solvents such as glycerin and EPA. These results are considered to be important for the evaluation of absolute two-photon cross sections where the quantum efficiencies of one- and two-photon processes are assumed to be the same.