The transient difference absorption spectrum of a substituted α-linked oligothiophene 13-mer was measured by sub 5-fs pump-probe spectroscopy. This molecule is the longest oligothiophene studied under time-resolved investigation. The spectrum observed in the sub-ps range exhibited a feature of the dynamic Stokes shift. A simple numerical calculation based on the Franck-Condon (FC) overlaps induced by molecular vibration reproduced the modulation on the difference absorption spectrum. These results verify the time-dependent FC overlap in this molecule.
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