Femtosecond differential transmission spectroscopy of alpha-sexithienyl thin film at low temperature

We have reassessed the influence of the charge-transfer state on the photog eneration of free charges in alpha-sexithienyl. We performed femtosecond di fferential transmission spectroscopy on an a-sexithienyl film at low temper ature, covering a broad range of excitation energies. In the probed spectra l region from 2.0 to 2.8 eV we found a bleaching of the first excited singl et manifold with an excited-state absorption superimposed on the bleaching. During the first few picoseconds the dynamics of the photoexcitations in t he range of the first excited singlet manifold are independent of the excit ation photon energy, pointing to a negligible influence of the charge-trans fer state. For longer delay times the spectra are dominated by thermal effe cts.