VIBRONIC EXCITATIONS OF LARGE MOLECULES IN SOLUTION STUDIED BY 2-COLOR PUMP-PROBE EXPERIMENTS ON THE 20 FS TIME-SCALE

The ultrafast vibronic response of organic dye molecules in solution i s studied in pump-probe experiments with 30 fs excitation pulses reson ant to S-0-S-n, transitions. The molecular dynamics is probed either b y pulses at the same spectral position or by 20 fs pulses overlapping with both the S-0-S-1 absorption and emission bands. Three contributio ns on distinctly different time scales are observed in the temporally and spectrally resolved two-color measurements. In the regime below 50 fs, a strong coherent coupling of the S-0-S-1 and the S-0-S-1 transit ions occurs that is due to coherent vibrational motions in the electro nic ground state. This signal is superimposed on the fast bleaching of the electronic ground state, resulting in a steplike increase of tran smission. In the range of the S-0-S-1 emission band, one finds a subse quent `picosecond rise of transmission that is due to stimulated emiss ion from vibronic Si states. The data demonstrate that the relaxation of S-n, states directly populated by the pump pulses is much faster th an the buildup of stimulated emission. This gives insight into differe nt steps of intramolecular vibronic redistribution and is compared to the S-n-S-1 relaxation in other molecules. (C) 1996 American Institute of Physics.