Coherent external and internal phonons in quasi-one-dimensional organic molecular crystals

We have directly time resolved coherent phonon oscillations in quasi-one-di mensional organic crystals of MePTCDI (N-N ' -dimethvlperylene-3,4,9,10-dic arboximide), using femtosecond pump-probe experiments. We observe both high er-energy oscillations caused by intramolecular vibrations (internal phonon s) and, fur the first time in a quasi-one-dimensional organic system, lower -energy modulations which are related to coherent lattice phonons (external phonons). For internal A(g) vibrations, the coherence decay rime of about 2 ps is almost independent of the mode. In contrast, the damping time of th e external phonons increases strongly with decreasing energy.