Ultrafast 2D and 3D electronic spectroscopy and its applications to the study of Photosynthetic Light Harvesting Complexes

Recently, there has been much interest in the application of ultrafast multi-dimensional electronic spectroscopy in studying various chemical, physical and biological systems. We will review the basic principles of multi-dimensional electronic spectroscopy.  We report on our development and applications of ultrafast coherent 3^rd order two-dimensional (2D) and 5^ththree-dimensional (3D) spectroscopiesbased on a pulse shaper assisted pump-probe setup [1,2].In 5th order 3D optical spectroscopy, we obtain purely absorptive 3D spectra using five ultrashort optical pulses. For the first time, we also directly observe multistep excitation energy transfer (EET) processes in LHCII using the newly developed ultrafast fifth-order three-dimensional electronic spectroscopy (3DES) [3]. Ultrafast 3rd order two-dimensional electronic spectroscopy (2DES) is an important tool to study EET processes in photosynthetic complexes. However, the multistep EET processes can only be indirectly inferred by correlating different cross peaks from a series of 2DES spectra. Here we observe cross peaks in room temperature 3DES spectra of LHCII that directly indicate energy transfer from excitons in the chlorophyll b (Chl b) manifold to the low-energy level chlorophyll a (Chl a) via mid-level Chla energy states. This new spectroscopic technique will allow scientists to move a step towards mapping the complete complex EET processes in photosynthetic systems.