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

We measure the photoionization cross-section of vibrationally excited levels in the S2 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.