Picosecond exciton annihilation in photosynthetic systems

Abstract

We have previously reported measurements of fluorescence lifetimes for both photosynthetic systems and chlorophyll solutions using a streak camera technique and picosecond excitation. Our lifetime measurements on antenna systems, however, as well as earlier results obtained using a picosecond resolution optical gate, indicate somewhat shorter decay times than measured with previous techniques. Our present experimental investigations of a possible intensity-dependent effect as the cause of these differences are prompted by the recent results of Mauzerall. He has shown that the quantum efficiency for the fluorescence emission from Chlorella decreases at higher pumping intensities for 7-ns duration excitation pulses, and he has interpreted the decrease in terms of a multitrap model of the photosynthetic unit and exciton-exciton collisions. In order to assess the respective roles that singlet and triplet excitons play in the kinetics of these interactions, it is desirable to repeat these measurements using a much shorter excitation pulse and under conditions comparable to the previously mentioned picosecond experiments. In this letter we report results for the quantum efficiency of Chlorella as a function of intensity, but with picosecond excitation. A single 20 ps pulse has been selected from a mode-locked laser pulse train for these measurements. Results show amore » drop of quantum efficiency with intensity in agreement with results of Mauzerall. Previous picosecond fluorescence lifetime measurements must be reinterpreted in view of this nonlinear optical effect.« less