Femtosecond Differential Transmission Spectroscopy of a-Sexithienyl Thin Film at Low Temperature

We have reassessed the influence of the charge-transfer state on the photogeneration of free charges in ?-sexithienyl. We performed femtosecond differential transmission spectroscopy on an ?-sexithienyl film at low temperature, covering a broad range of excitation energies. In the probed spectral region from 2.0 to 2.8 eV we found a bleaching of the first excited singlet manifold with an excited-state absorption superimposed on the bleaching. During the first few picoseconds the dynamics of the photoexcitations in the range of the first excited singlet manifold are independent of the excitation photon energy, pointing to a negligible influence of the charge-transfer state. For longer delay times the spectra are dominated by thermal effects.