Octave spanning pulses for 2D-electronic spectroscopy

From bio-monomers to dyads to photosynthetic complexes During the past decade, coherent multidimensional spectroscopy has emerged as a comprehensive method for studying condensed phase electronic and nuclear dynamics with femtosecond time resolution. Two-dimensional electronic spectroscopy (2D-ES) is a frequency-correlation technique derived from NMR but bears the advantage of different possible phase matching conditions: by choosing the appropriate excitation pulse vector geometry, 2D- ES allows one to isolate specific polarization components induced in the sample. Typically, 2D-ES experiments employ three interactions with pulsed electromagnetic fields in the phasematching direction kI,II = -/+ k1 + k2 + k3 . The excitation frequency integrated sum of the two signals kI and kII, also called rephasing and non-rephasing terms, equals the well-established pump-probe signal. In this proposal, another phasematching condition is at the center of attention, namely kIII = k1 + k2 - k3 . This so-called double-quantum signal is only present in systems with at least three electronics levels and is therefore predestined for probing electron correlation effects. In systems like semi-conductors or molecular aggregates, 2Q-2D shows great potential. Theoretical studies in a pigment-protein complex (PPC) have shown that the 2Q-2D can be used as a direct measure for correlations between excitonic excited states. Other techniques cannot give such an unbiased view on the electronic level structure of a PPC. Based on these considerations, the following research plan is proposed: 1. A novel experimental design for the combined measurement of 1Q- and 2Q-2D signals is implemented based on an octave-spanning supercontinuum from filamentation in a gas. 2. Starting at bio-relevant monomers, the electronic level structure of species like chlorophylls or carotenoids are determined in displacement, Franck-Condon point transition energies and correlated spectral motion by a combination of 1Q- and 2Q-2D. 3. Next, coupled dimer systems like an artificial bio-mimetic caroteno-purpurin dyad are investigated to elucidate effects like ultrafast population transfer and the role of vibrational wavepacket sigantures in electronic 2D-spectra. 4. Photosynthetic PPCs are studied. The Hamiltonian at the heart of the resulting structural model will be based on the electronic level structures obtained from the monomers and donor-acceptor systems. The obtained model of energy flow in a PPC will contain an experimentally validated treatment of excitonic and intramolecular doubly excited states. The results will explain phenomena like exciton-exciton annihilation rates and will elucidate the structure-function relationship in photosynthetic complexes.

In photosynthesis, plants and bacteria utilize sunlight with remarkable efficiency. Nine out of ten excitations reach the reaction centre, where they are turned into chemical energy. For several years the scientific community argued whether or not quantum effects are responsible for this high yield. The experimental observation at the heart of this discussion showed that photosynthetic molecules spend a remarkably long time in a state, which can only be explained by the means of quantum physics. As part of a START project, a team lead by Jürgen Hauer examined this effect in natural and artificial systems. It was revealed that the hotly debated quantum states are a side-product of another, highly interesting effect. It is the coupling between vibrations and electrons that turned out to play a crucial role. This effect fully describes the experimental results. Chlorophylls and molecules capable of harvesting and utilizing sunlight are not randomly oriented in the photosynthetic cell but arranged within well-defined groups and structures. Within them, the molecules are in such close proximity that they vibrate against each other. As one of the key findings within the START project, it was found that these vibrations play a decisive role in energy transfer between the molecules. The vibrations couple molecular electronic states. Light absorption then leads to what is referred to as vibronic excitation, in which vibrations and electronic states are inseparable and indistinguishable. Vibronic coupling enables the fast and nearly loss-free transfer of energy within light harvesting complexes. Such molecular assemblies are first excited by light and promoted to a state of higher energy. Similar to a ball falling down a stairway, the energy has to be lowered in a stepwise fashion to be made available for the photosynthetic cell. It is within the important initial steps of this energy cascade that vibrations play their decisive role. A better understanding of natural photosynthesis can pave the way to a future generation of solar cells with bio-inspired designs; designs which were optimized in billions of years of evolution. First steps in such applications were already made in the START-project: vibronic effects were measured not only in natural but also in artificial light harvesting systems.