Control and study of energy flow in light harvesting systems

Coherent control as a field of modern research has diversified greatly in recent years.1-3 Besides the core competence of steering photochemical events into a desired product channel, exciting new branches of control applications have emerged. Examples range from pulse compression4 and optimization of high-harmonics5 to isotope selective mass spectrometry. 6 In the field of ultrafast spectroscopy, coherent control has also proven to be of great value. The behaviour of a molecule after Fourier-limited excitation can reveal insightful differences to the trajectory after tailored excitation. This idea forms the conceptual basis of Quantum Control Spectroscopy (QCS) and has already yielded important results on molecular dynamics and biological function unattainable by conventional spectroscopic techniques. 7-13 This proposal focuses on the application of QCS on four-wave mixing (FWM) processes in the time domain. The general aim is to understand and control molecular dynamics with special emphasis on photosynthetic systems, both artificial and naturally occurring. Specifically, the following experiments are proposed: Coherent control of exciton dynamics in multidimensional electronic spectra by polarization pulse shaping. Two dimensional electronic spectroscopy14 in the visible offers new aspects on fundamental processes like electronic or vibrational coupling in light harvesting complexes. By coherent control, the set up can be greatly simplified. Additionally, the polarization of the excitation pulses becomes a freely tuneable parameter. This offers an effective and robust method to highlight the essential cross peaks in a 2D-spectrum. Unravelling Franck-Condon integrals by coherent control of a multicolour four-wave-mixing process. The overlap between vibronic wavefunctions on electronic ground- and excited state determines the strength of an optical transition. By selectively exciting ground state modes,15 their specific contribution to population transfer probability is quantified. Coherent control of an excited state photoreaction via non-resonant preparation of ground state modes. In an extension of the previous experiment, the effects of selectively prepared ground state modes on excited state photo-reactions will be investigated. Coherent control of a photoreaction on the excited state via pump-DFWM. 16 Phase-modulation of an initial pump pulse preceding a FWM-sequence is employed for studying and controlling photo- reactions directly on the excited state. Comparison to the method outlined in the previous point will lead to valuable new insights into reaction control mechanisms. The cooperation between the group around Prof. Motzkus at the Philipps-Universität Marburg, specializing in coherent control, and the group around Prof. Kauffmann at the University of Vienna with an expertise in 2D- and FWM-techniques is an essential part of this proposal.