  Ongoing developments in ultrafast X-ray sources offer powerful new means of
probing the com- plex non-adiabatically coupled structural and electronic
dynamics of photoexcited molecules. These non-Born-Oppenheimer effects are
governed by general electronic degeneracies termed conical in- tersections
which play a key role, analogous to that of a transition state, in the
electronic-nuclear dynamics of excited molecules. Using high level ab initio
quantum dynamics simulations, we studied time-resolved X-ray absorption and
photoelectron spectroscopy (TRXAS and TRXPS, respectively) of the prototypical
unsaturated organic chromophore, ethylene, following excitation to its S2
state. The TRXAS in particular is highly sensitive to all aspects of the
ensuing dynamics. These X-ray spectroscopies provide a clear signature of the
wavepacket dynamics near conical intersections, related to charge localization
effects driven by the nuclear dynamics. Given the ubiquity of charge
localization in excited state dynamics, we believe that ultrafast X-ray
spectroscopies offer a unique and powerful route to the direct observation of
dynamics around conical intersections.
