Coherent External and Internal Phonons in Quasi-One-Dimensional Organic Molecular Crystals

We have directly time resolved coherent phonon oscillations in quasi-one-dimensional organic crystals of MePTCDI ( $N-N^{\prime }$-dimethylperylene-3,4,9,10-dicarboximide), using femtosecond pump-probe experiments. We observe both higher-energy oscillations caused by intramolecular vibrations (internal phonons) and, for 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 time of about 2 ps is almost independent of the mode. In contrast, the damping time of the external phonons increases strongly with decreasing energy.