Ultrafast energy transfer in LHC-II revealed by three-pulse photon echo peak shift measurements

We report the results of three-pulse photon echo peak shift (3PEPS) measure ments on the light-harvesting complex II (LHC-II) of the green algae Chlamy domonas reinhardtii. Experiments were performed at two different excitation wavelengths, 670 and 650 nm. corresponding to Chi-a and Chl-b excitation, respectively, The 3PEPS data are analyzed using a new theory that incorpora tes the effect of energy transfer on third-order response functions. Our th eoretical model separates energy transfer dynamics from the solvation dynam ics arising from coupling of the electronic transitions to the protein envi ronment. We suggest that the protein fluctuations can be described by an ul trafast solvation on a sub-100 fs time scale and a long time correlation (s tatic disorder). Decay of the 670 nm peak shift reveals spectral equilibrat ion time scales for Chl-rr molecules that range from 300 fs to 6 ps and agr ee well with other experiments. 3PEPS data at 650 nm (Chl-b excitation) rev eal rapid Chl-b to Chl-b energy transfer (<1 ps), which suggests excitation hopping between a pair of Chls-b, and slow energy transfer from these Chls -b to Chls-a. Also, a 60 cm(-1) oscillatory mode is observed for Chl-b whic h we attribute to the first observation of coherent nuclear dynamics in LHC -II. Calculating the energy transfer dynamics based on recently proposed as signments of chromophores by solving the master equation reveals Chl-b intr a- and interband energy transfer dynamics that are in qualitative agreement with the simulation model of the peak shift data.