The transient difference absorption spectrum of a substituted alpha-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. (c) 2005 Elsevier B.V. All rights reserved.