Time-dependent photoionization of azulene: Optically induced anistropy on the femtosecond scale

We measure the photoionization cross-section of vibrationally excited levels in the S-2 state of azulene by femtosecond pump-probe spectroscopy. At the wavelengths studied (349-265 nm in the pump) the transient signals exhibit two distinct and well-defined behaviours: (i) short-term (on the order of a picosecond) polarization dependent transients and (ii) longer (10 ps-1 ns) timescale decays. This Letter focuses on the short-time transient. In contrast to an earlier study by Diau et al. [E. G. Diau, S. De Feyter, A. H. Zewail, J. Chem. Phys. 110 (1999) 9785.] we unambiguously assign the fast initial decay signal to rotational dephasing of the initial alignment created by the pump transition. (C) 2008 Elsevier B. V. All rights reserved.