Speaker
Description
Recent advances in attoscience have revealed that electron cloud dynamics, driven by superpositions of electronic states, can influence nuclear motion and, in turn, chemical reactivity [1]. In excited-state processes, conical intersections (CIs) play a central role and are both sensitive to and capable of generating electronic coherences—superpositions where overlapping electronic states interfere within the same spatial region.
Using pyrazine as a model system, we investigate the excited state decay mechanism from the S$_2$ state, which occurs by passing through the S$_2$-S$_1$ CI, through a combination of electronic structure calculations and dynamics simulations (semi-classical Second-Order [2] and fully quantum mechanical Quantum Ehrenfest [3] methods paired with CASSCF). Our results support the historically accepted view that only optically bright states contribute significantly to the decay. However, we also explore the influence of electronic coherences in this process.
Consistent with symmetry-based theoretical predictions [4], we find that coherences do not form between states of different Abelian point group symmetry at the Franck–Condon region. Crucially, we show that this restriction can be bypassed: even a slight initial mixture of states (e.g., a 99:1 population ratio) breaks the symmetry constraint and allows coherence formation—offering a theoretical explanation for recent experimental observations [5].
References
[1] I. C. D. Merritt et al., J. Phys. Chem. Lett., 2021, 12, 8404–8415
[2] M. Vacher et al., Theor. Chem. Acc., 2014, 133, 1505
[3] A. J. Jenkins et al., J. Chem. Phys., 2018, 149, 094108
[4] S. P Neville et al., J. Phys. B: At. Mol. Opt. Phys., 2022, 55, 044004
[5] YP. Chang et al., Nat. Phys., 2025, 21, 137–145
