Real-time vibronic coupling dynamics in a prototypical conjugated oligomer

Phonon modes which are strongly coupled to the lowest optical transition of alpha-sexithienyl are directly probed in real time by coherent vibrational spectroscopy. The resonant excitation source is a tunable 10-fs optical pa rametric amplifier. We identify five strongly coupled phonon modes at omega (1) = 112 cm(-1) omega(2) = 299 cm(-1), omega(3) = 702 cm(-1), omega(4) = 1 040 cm(-1), and omega(5) = 1454 cm(-1). Vibrational dephasing times are mea sured to be between 0.4 and 3 ps. We gain direct hints on potential energy anharmonicity by measuring the transient vibrational frequency within the f irst ps.