The rates and outcomes of virtually all photophysical and photochemical
processes are determined by Conical Intersections. These are regions of
degeneracy between electronic states on the nuclear landscape of molecules
where electrons and nuclei evolve on comparable timescales and become strongly
coupled, enabling radiationless relaxation channels upon optical excitation.
Due to their ultrafast nature and vast complexity, monitoring Conical
Intersections experimentally is an open challenge. We present a simulation
study on the ultrafast photorelaxation of uracil, which demonstrates a new
window into Conical Intersections obtained by recording the transient
wavepacket coherence during this passage with an x-ray free electron laser
pulse. We report two major findings. First, we find that the vibronic coherence
at the conical intersection lives for several hundred femtoseconds and can be
measured during this entire time. Second, the time-dependent energy splitting
landscape of the participating vibrational and electronic states is directly
extracted from Wigner spectrograms of the signal. These offer a novel physical
picture of the quantum Conical Intersection pathways through visualizing their
transient vibronic coherence distributions. The path of a nuclear wavepacket
around the Conical Intersection is directly mapped by the proposed experiment.Comment: 7 pages, 5 Figures, to be published in PNA
