Vibrational and vibronic dynamics of large molecules in solution studied on a 20 fs timescale

The ultrafast dynamics of organic dye molecules in solution are studied in temporally and spectrally resolved experiments with 30 fs pump pulses resonant to short-wavelength S0-Sn transitions and 20 fs probe pulses overlapping with both the S0-S1 absorption and emission bands. Around zero delay, a pronounced coherent signal occurs with sign and amplitude depending on the spectral position within the probe pulses. The coherent coupling of the type electronic transitions is due to the coherent vibrational motion induced by an impulsive Raman-type excitation in the electronic ground state. The molecules promoted to Sn states give rise to a fast ground state bleaching on a 50 fs timescale that is followed by the picosecond onset of stimulated emission after accumulation of the excited molecules at the bottom of the S1 state.