Femtosecond pump-probe study of molecular vibronic structures and dynamics of a cyanine dye in solution

Time-resolved pump–probe spectra of 1,18,3,3,38,38-hexamethyl-4,48,5,58-dibenzo-2,28indotricarbocyanine ~HDITC!, a cyanine dye, in ethylene glycol are obtained using 11 fs and 90 fs duration pulses and analyzed in order to study its potential energy surfaces and vibrational dynamics. Ten oscillatory frequencies ranging from 30 cm to 1400 cm are observed in the 11 fs duration wavelength-resolved pump–probe measurements. They are assigned as fundamental vibrational frequencies of HDITC. The relative displacements of the equilibrium position between electronic excited and ground states along the resolved ten vibrational modes are determined through the wavelength dependence of the oscillatory amplitude. After considering the contributions of the ten vibrational modes, it is found that most of the Stokes shift and the early fast decays of the pump–probe signals are due to relaxation along the low frequency overdamped modes of the chromophore. The overdamped modes are characterized by the 90 fs pump–probe signals with the excitation at the red edge of the absorption band. © 1999 American Institute of Physics. @S0021-9606~99!50124-4#