System–bath coupling dynamics by femtosecond pump–probe spectroscopy

Femtosecond pump–probe spectroscopy of a laser dye IR132 in the solvent DMSO has been performed. It is shown that the experimental spectra generated following excitation at two different wavelengths can be reasonably reproduced by simulations starting from an assumed system–bath energy autocorrelation function. The correlation times and coupling strengths compare favorably with results from previous studies. A novel oscillation of the spectral width is observed in the experimental spectra that can be traced to out of phase oscillations of the vibrational wavepacket between the ground and excited electronic states.