UC Berkeley

Photosynthetic light harvesting begins with collecting ambient photons via excitation of pigment molecules. The sophisticated design of the pigment-protein complexes is based on the cooperative network among structural motifs of pigment molecules and amino acid residues in proteins to achieve the remarkable efficiency and speed in light harvesting.

The insight from nature's photosynthetic nano-machinery can be translated to an optimal material design. The principle behind such inherent excitation energy transfer dynamics were revealed with the advance of the femtosecond laser technique. One of the examples is the quantum mechanical coherence excitation energy transfer observed by nonlinear photon echo technique.

This dissertation presents the investigation of the bacterial reaction center complexes using electronic coherence photon echo spectroscopy. The study focused on elucidating the origin of the coherence signals and identifying quantum mechanical components relevant to the excitation energy transfer. Experimental variables affecting the coherence signals were explored for the excitation energy transfer of photosynthetic bacterial reaction center. The result showed that the vibrational coherence mimics the behavior of the electronic coherence, and the lifetime is near 2 ps. A mutant bacterial reaction center was also examined to confirm the vibrational contribution. A new polarization control experiment indicates that majority of the observed vibrational coherence is from the electronic ground state.