  Time-resolved X-ray absorption spectroscopy is emerging as a uniquely
powerful tool to probe coupled electronic-nuclear dynamics in photo-excited
molecules. Theoretical studies to date have established that time-resolved
X-ray absorption spectroscopy is an atom-specific probe of excited-state wave
packet passage through a seam of conical intersections (CIs). However, in many
molecular systems, there are competing dynamical pathways involving CIs of
different electronic and nuclear character. Discerning these pathways remains
an important challenge. Here, we demonstrate that time-resolved X-ray
absorption spectroscopy (TRXAS) has the potential to resolve competing channels
in excited-state non-adiabatic dynamics. Using the example of 1,3-butadiene, we
show how TRXAS discerns the different electronic structures associated with
passage through multiple conical intersections. Trans 1,3-butadiene exhibits a
branching between polarized and radicaloid pathways associated with ethylenic
"twisted-pyramidalized" and excited-state cis-trans isomerization dynamics,
respectively. The differing electronic structures along these pathways give
rise to different XAS signals, indicating the possibility of resolving them.
Furthermore, this indicates that XAS, and other core-level spectroscopic
techniques, offer the appealing prospect of directly probing the effects of
selective chemical substitution and its ability to affect chemical control over
excited-state molecular dynamics.
