The principal pigments related to absorption of light and transfer of excitation energy towards reaction centers are chlorophylls (Chls). In addition, Chls play a role in charge separation and electron transfer. These molecules are known to have two electronic Qy and Qx transitions, with almost perpendicular polarization with respect to each other, as well as prominent vibrational progressions. The interplay among these electronic excited states and their vibrational degrees of freedom – vibronic coupling – is known to mix the characters from both states. In practice this means that a vibrational side band of the Qy state can absorb (or emit) in both Qy and Qx polarization directions.
Vibronic coupling can be probed using polarization controlled two-dimensional electronic spectroscopy (2DES), where short coherent laser pulses can excite ladders of states in phase. The coupling is detectable throughout a quantum coherence – a superposition between two levels – and is observed as oscillatory beatings between those levels.
The main aim of the project is to see existence of vibronic coherence signals via polarization controlled 2DES experiments. For this reason, two sets of 2DES experiments at two polarization configurations: all parallel (0/0/0/0) and double-crossed (-45/45/90/0) are performed. Two types of experiments provide us with both, full image of the beating signals within the molecule (all parallel data sets) as well as usually weak electronic/vibronic coherences (double-cross data sets).