Long-Lived Nuclear Coherences inside Helium Nanodroplets

Abstract

Much of our knowledge about dynamics and functionality of molecular systems has been achieved with femtosecond time-resolved spectroscopy. Despite extensive technical developments over the past decades, some classes of systems have eluded dynamical studies so far. Here, we demonstrate that superfluid helium nanodroplets, acting as thermal bath of 0.4 K temperature to stabilize weakly bound or reactive systems, are well suited for time-resolved studies of single molecules solvated in the droplet interior. By observing vibrational wavepacket motion of indium dimers (In2) for over 50 ps, we demonstrate that the perturbation imposed by this quantum liquid can be lower by a factor of 10-100 compared to any other solvent, which uniquely allows to study processes depending on long nuclear coherence in a dissipative environment. Furthermore, tailor-made microsolvation environments inside droplets will enable to investigate the solvent influence on intramolecular dynamics in a wide tuning range from molecular isolation to strong molecule-solvent coupling. Introduction.A comprehensive understanding of mechanisms to convert solar energy into other energy forms is a prime objective for many research fields, with potential impact on light harvesting applications or the modelling of photoprotection in biomolecules. Despite extensive investigations, the primary processes triggered by photoexcitation of molecular systems remain insufficiently understood, as they proceed on pico-to sub-femtosecond time scales and involve concerted motion of electrons and nuclei in a complex manner. Insight into the functionality and dynamics of photoactive systems can be obtained in a unique way with femtosecond laser spectroscopy (1), revealing information about, for example, photofragmentation dynamics (2), molecular chirality (3), non-adiabatic coupling dynamics of 2 electrons and nuclei (4), charge transfer (5), or electron dynamics (6). Recent experiments on chemically and biophysically relevant molecules suggest that nuclear motions and in particular their coherences have strong influence on the electronic evolution of the system (7); examples include prototypical molecules for photosynthesis (8-10) and photovoltaics (11).The evolution of a molecular system after photoexcitation strongly depends on its immediate environment, with significant differences between isolated systems, where photodynamics can be precisely studied (12, 13), and the system in its real-world environment in condensed phase. Isolated molecules can be produced in a seeded supersonic expansion (14), where investigations are, however, often prevented by fragmentation resulting from excess energy during photoexcitation, or simply by the fact that weakly bound systems cannot be produced.This harmful vibrational energy can be dissipated to a thermal bath by embedding molecules in a high-pressure buffer gas (15) or a cryogenic matrix (16). As a disadvantage, influences of the environment on intrinsic dynamics can be severe and disentangling intra-and intermolecular dynamics i...

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