Femtosecond differential transmission spectroscopy of α-sexithienyl thin film

We have investigated the nature of primary excitations in alpha-sexithienyl thin film by comparing excitations above and below the charge transfer exciton. We performed femtosecond differential transmission spectroscopy at low temperature. In the probed spectral region from 2 to 2.8 eV, we found a bleaching of the first excited singlet with a superimposed absorption into higher excited gerade states. 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. When exciting below the origin of the first excited singlet, we observe an optical Stark effect. Thermal effects dominate the change in transmission for longer delay times for all excitation energies. (C) 2000 Elsevier Science B.V. All rights reserved.