By using sub-10-fs optical pulses we excite and probe vibrational coherence in isolated polydiacetylene chains in benzene solution. The ultrafast relaxation process following photoexcitation allows selecting ground state coherence only, as demonstrated by measurements carried out with chirped pulses and further supported by quantum mechanical simulations. Vibrational ground state dephasing and dynamics are studied in the time domain. Typically $T_2$ is about 600 fs, but there is evidence of faster dephasing for higher vibrational levels. Vibrational coherence in the excited state is short lived, but still detectable at the red tail of the broad pulse spectrum, providing hints on the corresponding mode frequencies, which appear lower than the ground state ones.

• Received 3 December 2003

DOI:https://doi.org/10.1103/PhysRevB.69.134302