Exciton delocalization probed by excitation annihilation in the light-harvesting antenna LH2

Singlet-singlet annihilation is used to study exciton delocalization in the light harvesting antenna complex LH2 (B800-B850) from the photosynthetic p urple bacterium Rhodobacter sphaeroides. The characteristic femtosecond dec ay constants of the high intensity isotropic and the low intensity anisotro py kinetics of the B850 ring are related to the hopping time tau (h) and th e coherence length N-coh Of the exciton. Our analysis yields Ncoh = 2.8 +/- 0.4 and tau (h) = 0.27 +/- 0.05 ps. This approach can be seen as an extens ion to the concept of the spectroscopic ruler.