Ultrafast dynamics of the S1 excited state of benzene

We investigate the ultrafast intramolecular dynamics of electronically and vibrationally excited benzene using time-resolved photoelectron spectroscopy and quantum dynamics simulations. In addition to an ultrafast initial decay, we observe an oscillation between two states. We interpret this data in terms of excited state population moving away from the Franck-Condon region towards the singlet-singlet conical intersection with the ground-state, where ultrafast intersystem crossing from the initially populated singlet state to an optically dark triplet state is enhanced. Our results challenge the currently accepted view that intramolecular processes in hydrocarbons which involve a change of spin are negligibly slow. (C) 2008 Elsevier B.V. All rights reserved.