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 parametric amplifier. We identify five strongly coupled phonon modes at ${\omega }_1=112\mathrm{}{\mathrm{cm}}^{-1\mathrm{}}$, ${\omega }_2=299\mathrm{}{\mathrm{cm}}^{-1\mathrm{}}$, ${\omega }_3=702\mathrm{}{\mathrm{cm}}^{-1\mathrm{}}$, ${\omega }_4=1040\mathrm{}{\mathrm{cm}}^{-1\mathrm{}}$, and ${\omega }_5=1454\mathrm{}{\mathrm{cm}}^{-1\mathrm{}}$. Vibrational dephasing times are measured to be between 0.4 and 3 ps. We gain direct hints on potential energy anharmonicity by measuring the transient vibrational frequency within the first ps.