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 thus 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, based on a quantum description of the nuclei. We demonstrate an additional window into conical intersections obtained by recording the transient wavepacket coherence during this passage with an X-ray free-electron laser pulse. Two major findings are reported. 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 physical picture of the quantum conical intersection pathways through visualizing their transient vibronic coherence distributions. The path of a nuclear wavepacket in the vicinity of the conical intersection is directly mapped by the proposed experiment.
Sensing ultrashort electronic coherent beating at conical intersections by single-electron pulses
Consolidation of ultrafast optics in electron spectroscopies based on free electron energy exchange with matter has matured significantly over the past two decades, offering an attractive toolbox for the exploration of elementary events with unprecedented spatial and temporal resolution. Here, we propose a technique for monitoring electronic and nuclear molecular dynamics that is based on self-heterodyne coherent beating of a broadband pulse rather than incoherent population transport by a narrowband pulse. This exploits the strong exchange of coherence between the free electron and the sample. An optical pulse initiates matter dynamics, which is followed by inelastic scattering of a delayed high-energy broadband single-electron beam. The interacting and noninteracting beams then interfere to produce a heterodyne-detected signal, which reveals snapshots of the sample charge density by scanning a variable delay T . The spectral interference of the electron probe introduces high-contrast phase information, which makes it possible to record the electronic coherence in the sample. Quantum dynamical simulations of the ultrafast nonradiative conical intersection passage in uracil reveal a strong electronic beating signal imprinted onto the zero-loss peak of the electronic probe in a background-free manner.
- Award ID(s):
- 1953045
- Publication Date:
- NSF-PAR ID:
- 10340564
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 119
- Issue:
- 22
- ISSN:
- 0027-8424
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Publisher's Version of Record
- https://doi.org/10.1073/pnas.2205510119
