A FEMTOSECOND FLUORESCENCE UP-CONVERSION STUDY OF THE DYNAMIC STOKES SHIFT OF THE DCM DYE MOLECULE IN POLAR AND NONPOLAR-SOLVENTS

Time-resolved fluorescence spectra of the DCM dye molecule in methanol and chloroform have been studied using the fluorescence up-conversion technique with femtosecond time resolution. There is only one fluorescent excited state for all times and the dynamic Stokes shift observed is only due to solvent relaxation. In methanol, the mean position of the fluorescence spectrum shifts towards the red bi-exponentially (175 fs and 3.2 ps) while in chloroform it remains practically unchanged for all times. A spectral narrowing with a 10 ps time constant in methanol (7 ps in chloroform), due to vibrational energy dissipation from the excited solute to the surrounding solvent, is also observed.