Selected recent applications

Vibronically driven ultrafast singlet fission in pentacene crystals (2015)
Self-analysis of coherent beatings in time-resolved signals (2014)
Coherent energy transfer in photosynthetic antennae complexes (2010)
Vibronic effects in two-dimensional photon-echo signals (2008, 2014)

Vibronically driven ultrafast singlet fission in pentacene crystals

in collaboration with A. Bakulin, A. Rao, A. Chin and S. Morgan at the Univerity of Camridge and D. Zigmantas at Lund University
Press releases:
University of Kiel (in German)
University of Cambridge

Singlet fission - the ability of molecular materials to split one singlet excitation into two trip lets - has gained especially much attention in the recent years due to its potential to improve efficiency of molecular photovoltaics: it promises to generate two electron-hole pairs per one photon absorbed. Pentacene films are particularly interesting candidates, since the decay of the excited singlet occurs at ultrafast timescale of about 80 fs and results in high fission efficiency. Two-dimensional (2D) photon echo (PE) of pentacene films has been recorded in Lund in summer 2013 (Bakulin, Rao, Zigmantas). We have developed a model explaining the physical processes and ultrafast dynamics behind the experimental data. We manage to tell apart the ground-state and to the excited-state coherent motion in the experimentally detected oscillations. The excited-state dynamics is in focus, since it is responsible for the initial steps of fission process: the fission efficiency is governed by the interaction between the lowest optically allowed singlet state with the close-lying optically dark multiexciton triplet state. The formation of the multiexciton triplet state is vibronically driven, i.e. it can be understood as the result of the mixing of vibrational and electronic degrees of freedom.

Self-analysis of coherent beatings in time-resolved signals

The specific origin of oscillations in time-resolved optical signals, in particular for complex systems with nontrivial interstate couplings and non-separable electron-nuclear motion, is often difficult to assign. However, coherent oscillations in two-dimensional photon echo are capable of self-analysis: their beating maps provide a tool to tell apart ground-state bleach (GSB), stimulated emission (SE) and excited-state absorption (ESA) contributions to the oscillatory signal component. Since GSB carries information on ground-state coherence, while SE and ESA reflect the excited-state coherence, the observed oscillations can be unambiguously assigned to ground-state or excited-state coherent motion. The findings prove especially advantageous for systems with dense detectable manifolds of states pertaining to each electronic state.

Coherent energy transfer in photosynthetic antennae complexes

Plants and photosynthetic bacteria use complexes built of pigments and proteins to streamline the transfer of energy from light-harvesting antenna systems, which efficiently capture sunlight, to the reaction center, in which energy is stored for later use in biochemical processes. Recently, oscillations in two-dimensional electronic photon-echo spectra of the FMO (Fenna-Matthews-Olson) photosynthetic complex have been recorded. In contrast to the previous findings and assumptions, these new experimental results suggest a strongly coherent nature of the energy-transfer dynamics which persists at room temperature as well. Similar oscillative signals have been reported for algae complexes at room temperatures. Although these results are extremely intriguing they remain slightly contradictorily and lack an unambiguous and convincing interpretation. Our methods allow us to simulate the experimental signals without any assumptions and approximations with respect to parameters of the involved laser fields. Therefore, a comparison to the experiment can reveal the validity of a model employed for the description of the underlying system dynamics. So far, our calculations demonstrate that the commonly accepted model of the FMO subunit renders a poor agreement with the first experiment, but is supported by the more recent experimental results. To resolve this contradiction we presently develop more sophisticated models for the system dynamics, as well as theoretically search for the most suitable schemes for more precise experiments.

Vibronic effects in two-dimensional photon-echo signals

Two-dimensional (2D) photon-echo (PE) technique in optical domain is based on the same ideas as the 2D nuclear magnetic resonance (NMR), but in contrast to their NMR counterparts, the recorded signals are usually very broad and hard to interpret. The reason for this is that 2D PE in optical domain involves ultrafast transitions between electronic states. In polyatomic molecules the electronic excitations are usually coupled to many vibrational degrees of freedom. Upon the action of a short optical pulse vibrational excitations are almost inevitable. This coupling of electronic and vibrational motion can result in strong vibronic effects on 2D PE signals. We are currently systemising our findings on these effects to establish ultimate criteria for identifying vibrational and vibronic features in electronic 2D PE spectroscopy. Since electronic 2D PE is designed to provide information on electronic interactions, such a tool for ``ruling out'' the vibrations is highly relevant and important.

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