Femtosecond time-resolved fluorescence spectra of a coumarin dye in glycerol

In order to obtain information on the structural relaxation of glass-forming materials, we have performed femtosecond time-resolved fluorescence spectroscopy for a coumarin dye in glycerol at room temperature, where the time window of observation was similar to 1 ns. A sum-frequency generation technique was used by means of a mode-locked Ti:Sapphire laser. The time-resolved fluorescence spectrum has shown a time-dependent red shift up to 1 ns. The analysis of the time evolution of the peak energy of the spectrum reveals that the energy relaxation in the electronic excited state of the dye molecule has two components of the order of 100 Es and 100 ps. It is considered that the fast and slow components are attributed to the vibrational relaxation and alpha -relaxation of the matrix, respectively. Further, the existence of a component on a time scale of a few ps, perhaps due to the so-called fast beta -process, is suggested. (C) 2000 Elsevier Science B.V. All rights reserved.