Nuclear dynamics in electronic ground and excited states probed by spectrally resolved four wave mixing

Abstract

Time-resolved ground-state bleach and excited-state stimulated emission spectra have been measured for indocyanine green dissolved in methanol by employing spectrally resolved four wave mixing (SRFWM). The separation of the SRFWM signals into the ground-state bleach and excited-state stimulated emission contributions allows observation of intramolecular vibrational wave packet motions and intermolecular solvation dynamics upon impulsive excitation, while the molecule resides either in the ground or in the excited state. Frequencies of the indocyanine green intramolecular vibrational modes in the ground and excited states are practically the same. Vibrational dephasing times in the excited state range from a few hundred fs to similar to2 ps, and they are consistently shorter than those in the ground state. When excitation frequency is centered near the 0-0 transition, center frequencies of the stimulated emission redshift due to solvation of the excited state in nonequilibrium solvent configuration, whereas those of the ground-state bleach blueshift due to equilibrium fluctuation of the solvent molecules around the chromophore in the ground state. At early times, the solvation function obtained from the time-resolved ground-state bleach spectra is slower than the solvation function obtained from the time-resolved excited-state stimulated emission spectra.(C) 2002 American Institute of Physics.