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Vibrational coherence as a quantum probe for ultrafast molecular dynamics

Final Report Summary - VIBCOH (Vibrational coherence as a quantum probe for ultrafast molecular dynamics)

The sunlight carries a huge amount of energy in form of photons. The absorption of photon energy by molecules represents a primary step in many processes relevant in biology, chemistry and physics. Light-induced processes such as vision or photosynthesis are extremely efficient, i.e. the absorbed photon energy is rapidly converted into a specific action. The latter typically occurs on timescales faster than one thousand billionth of one second (1/1000000000000 s). The rapid energy transfer is essential to avoid energy losses to the environment which would render a process less efficient. An important question is how the molecular structure affects the efficiency of such processes. The goal of the work is to use advanced spectroscopic techniques to map, with unprecedented accuracy, how photon energy moves throughout a molecule after photoexcitation, thereby generating fundamental knowledge about the design principles behind efficient light-matter interactions, one of the premier challenges of contemporary science in the context of solar energy conversion.

We have developed an experimental setup that uses a sequence of short laser pulses that enables us to initiate a photochemical reaction in a molecular system and to follow how the absorbed photon energy is transferred and dissipated during the induced process. The setup is particularly sensitive to the nuclear motion of the molecule and allows us to resolve even the fastest motion in terms of vibrational spectra. Such spectrum represents a fingerprint of the molecular structure, and thus, yields crucial information about structural changes during the reaction. Based on such measurements, we aim to unravel the specific nuclear motions that promote a rapid and efficient energy transfer. Although extensively studied, the exact underlying mechanisms are still poorly understood.

The molecules studied in this project range from biologically active photoreceptors, such as carotenoids and opsin proteins, to materials proposed as next generation devices for solar energy conversion. A particular highlight of the work is the investigation of the nuclear motion during the process of singlet fission in TIPS-pentacene and its derivatives. The latter represent promising materials for organic solar cells devices which have the ability to convert photon energy into electrical energy much more efficiently compared to traditional silica-based solar cells. Organic materials such as pentacene, use the process of singlet fission to divide the energy of one photon between two molecules, and thereby, generate twice as much charge carriers than traditional solar cells. Using our setup, we were able to reveal the underlying molecular dynamics behind this phenomenon.