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.