1. Understanding electronic and geometric structural changes during solar energy conversion processes using ultrafast lasers and x-rays

Molecular structures, both electronic and geometric, determine how and when a chemical reaction could occur. We use laser pulses as short as tens of femtoseconds (1fs = 10 -15s) to trigger chemical reactions where chemical bond formation and breakage could take place, and use either laser pulses or X-ray pulses to probe responses of the molecules and materials as a function of evolving time after the reaction starts. The laser probe pulses detect optical responses induced by the trigger to provide excited state reaction rates, lifetimes, reaction and coherence between different states. The X-ray probe pulses detect the time evolution of electronic and geometric structures during the reaction. Based on transient laser and x-ray spectroscopie results, we gain insight into fundamental chemical reaction mechanisms that would not otherwise available with either technique. This combined approach has been applied to metal complexes in solution and metal coated semiconductor nanoparticles, both of which represent key reactions in solar energy conversion and storage. In collaboration with organic, inorganic, polymer and biological chemists, we characterize molecular structural dynamics and provide feedback to them on rational molecular design. In collaborating with theoretical chemists and physicists, we conduct calculations and simulations to model the systems and further our understanding of different processes in inorganic photochemistry, metalloprotein enzymology, and semiconductor photocatalysis.

2. Fundamental studies on photon and electron flow in self-assembled organic and hybrid materials for photovoltaic and molecular device applications