High-order fluorescence and exciton interaction in photosynthetic bacteria

We have observed fluorescence at visible wavelengths from chromatophores of photosynthetic bacteria excited with infrared radiation which we attribute to bacteriochlorophyll of the antenna system. The fluorescence is prompt (no delay greater than 5 ns). Its spectrum shows peaks at 445, 530 (broad) and 600 nm when excited with either 694 or 868 nm. Quantum yield is of the order of 10^?9. The dependence on intensity indicates generation by mainly third-order processes which could involve triplet states in combination with excited singlets. Second-order single-singlet fusion could also contribute. The high-order fluorescence can also be explained as arising from absorption of a second photon by singlet excited states.