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.

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