A SINGLE PULSE PICOSECOND LASER STUDY OF EXCITON DYNAMICS IN CHLOROPLASTS

Abstract. Using single picosecond laser pulses at 610 nm, the fluorescence yield (φ) of spinach chloroplasts as a function of intensity ( I ) (10¹²-10¹⁶ photons/pulse/cm²) was studied in the range of 21–300 K. The quantum yield decreases with increasing intensity and the φ vs I curves are identical at the emission maxima of 685 and 735 nm. This result is interpreted in terms of singlet exciton-exciton annihilation on the level of the light-harvesting pigments which occurs before energy is transferred to the Photosystem I pigments which emit at 735 nm.
The yield φ is decreased by factors of 12 and 43 at 300 and 21 K, respectively. The shapes of the φ vs I curves are not well accounted for in terms of a model which is based on a Poisson distribution of photon hits in separate photosynthetic units, but can be satisfactorily described using a one-parameter fit and an exciton-exciton annihilation model. The bimolecular annihilation rate constant is found to be γ= (5–15) times 10^-9cm³s^-1 and to exhibit only a minor temperature dependence. Lower bound values of the singlet exciton diffusion coefficient (≥ 10^-3cm²s^-1), diffusion length (≥ 2 times 10^-6cm) and Förster energy transfer rates (≥ 3 ≥ 10¹⁰s^-1) are estimated from γ using the appropriate theoretical relationships.