Temperature dependence of femtosecond time-resolved fluorescence intensity of a dye molecule in solution

We have measured femtosecond time-resolved fluorescence intensity for coumarin in an ethanol/methanol mixture from 150 K to room temperature using an up-conversion technique. A liquid nitrogen flow-type cryostat was vibrated up and down during the measurement so that the same molecule in the sample might not be kept irradiated with a laser pulse. At 150 K, no fading of the fluorescence intensity due to the damage of the dye molecule under light irradiation was observed when the cryostat was moved, while the fluorescence intensity faded when it was not. The time profile of fluorescence intensity depends on temperature and wavelength of the detection on a time scale up to several picoseconds in the temperature range examined. This is considered to be ascribed to the temperature dependence of the energy relaxation in the excited state of the dye molecule due to solvation process of the surroundings of it.