Vibrational relaxation of dye molecules in solution studied by femtosecond time-resolved stimulated emission pumping fluorescence depletion

A new method, femtosecond time-resolved stimulated emission pumping fluorescence depletion (FS TR SEP FD), has been developed to study the vibrational relaxation of electronic excited states of molecules. Two relaxation rates of dye molecules in different solvents have been observed: (i) the intramolecular redistribution of energy, with a short time constant of less than 500 fs; (ii) the subsequent cooling of the vibrationally hot molecules on the picosecond time scale. The rates of these processes strongly depend on the solvent. This may result from a solvent-induced structural modification of the dye molecules.