Ultrafast molecular events are determined by conical intersections. There, an approaching vibrational wave packet branches and explores specific quantum pathways. The emerging vibrational and electronic coherences that are unique to these events contain information about the fundamental properties of the conical intersection. They also determine the vibrational motion of molecules during and after the non-adiabatic passage.<br/>We present spectroscopic signals that monitor these coherences. On the example of azobenzene photoisomerization, we simulate and disentangle all contributions to the time-resolved diffraction signal [1]. Vibronic coherences contribute as weak signatures in the total diffraction pattern by mixed elastic/inelastic photon scattering, in addition to the usual elastic scattering from electronic state densities. A real-space image of the electron transition density at the conical intersection is provided. This can be recovered by, e.g., employing covariance-based measurements with inherently stochastic and noisy X-ray pulses. In contrast to photons, electron beams scatter from the nuclear charge density as well. This causes nuclear and mixed nuclear/electronic terms in the ultrafast electron diffraction signal that monitor vibrational structures during the conical intersection passage.<br/>In a second project, we simulate the molecular dynamics of a large bichromophoric heterodimer including all nuclear degrees of freedom [2]. Our formalism naturally includes coherence and decoherence behavior and tracks the vibrational molecular evolution during several conical intersection passages. The vibronic coherence is persisting through several hundred femtoseconds and across the large molecular phase space. Their spectroscopic detection allows to extract the distribution of states in the vibronic coherence, and the average energy splitting around the conical intersection.

Spectroscopic detection of vibronic coherences at conical intersections

ACS Fall 2021

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