Conical intersections are ubiquitous in chemical systems but, nevertheless, extraordinary points on the molecular potential energy landscape. They provide ultra-fast radiationless relaxation channels, their topography influences the product branching, and they equalize the timescales of the electron and nuclear dynamics. These properties reveal optical control possibilities in the few femtosecond regime. In this theoretical study, we aim to explore control options that rely on the carrier envelope phase of a few-cycle IR pulse. The laser interaction creates an electronic superposition just before the wave packet reaches the conical intersection. The imprinted phase information is varied by the carrier envelope phase to influence the branching ratio after the conical intersection. We test and analyze this scenario in detail for a model system and show to what extent it is possible to transfer this type of control to a realistic system like uracil.
Photo-initiated processes in molecules often involve complex situations where the induced dynamics is characterized by the interplay of nuclear and electronic degrees of freedom. The interaction of the molecule with an ultrashort laser pulse or the coupling at a conical intersection (CoIn) induces coherent electron dynamics which is subsequently modified by the nuclear motion. The nuclear dynamics typically leads to a fast electronic decoherence but also, depending on the system, enables the reappearance of the coherent electron dynamics. We study this situation for the photo-induced nuclear and electron dynamics in the nucleobase uracil. The simulations are performed with our ansatz for the coupled description of the nuclear and electron dynamics in molecular systems (NEMol). After photo-excitation uracil exhibits an ultrafast relaxation mechanism mediated by CoIn's. Both processes, the excitation by a laser pulse and the non-adiabatic relaxation, are explicitly simulated and the coherent electron dynamics is monitored using our quantum mechanical NEMol approach. The electronic coherence induced by the CoIn is observable for a long time scale due to the delocalized nature of the nuclear wavepacket.
The ultrafast relaxation within the Q-bands of chlorophyll plays a crucial role in photosynthetic light-harvesting. Yet, despite being the focus of many experimental and theoretical studies, it is still not...
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