Real-Time Vibronic Coupling Dynamics in a Prototypical Conjugated Oligomer

Abstract

Phonon modes which are strongly coupled to the lowest optical transition of $\ensuremath{\alpha}$-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 ${\ensuremath{\omega}}_{1}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}112{\mathrm{cm}}^{\ensuremath{-}1}$, ${\ensuremath{\omega}}_{2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}299{\mathrm{cm}}^{\ensuremath{-}1}$, ${\ensuremath{\omega}}_{3}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}702{\mathrm{cm}}^{\ensuremath{-}1}$, ${\ensuremath{\omega}}_{4}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1040{\mathrm{cm}}^{\ensuremath{-}1}$, and ${\ensuremath{\omega}}_{5}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1454{\mathrm{cm}}^{\ensuremath{-}1}$. 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.