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 studie d in temporally and spectrally resolved experiments with 30 fs pump pu lses resonant to short-wavelength S-o-S-n transitions and 20 fs probe pulses overlapping with both the S-o-S-n 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 two electronic transitions is due to the coherent vibrational motion induced by an impulsive Raman-type excita tion in the electronic ground state. The molecules promoted to S-n sta tes give rise to a fast ground state bleaching on a 50 fs timescale th at is followed by the picosecond onset of stimulated emission after ac cumulation of the excited molecules at the bottom of the S-1 state.