INTERACTIONS AMONG PHOTOSYNTHETIC ANTENNA EXCITED STATES

Abstract. The data of Kung and DeVault (1978) showing high‐order fluorescence from chromatophores of photosynthetic bacteria are analyzed in relation to other data on first‐order fluorescence of photosynthetic systems, particularly that of Monger and Parson (1977). The wavelengths of emission observed (down to 445 nm) require energy equivalent to two lowest singlet‐excited states. The dependence on excitation intensity is best explained by any of the following third‐order processes: (a) 3S₁→3S₀; (b) 2S₁+T, → 2S₀+T₁; (c) S₁+ 2T₁→ 3S₀. However, (c) is ruled out because it predicts heavy T₁‐destruction which is not observed. Contribution from the second order process: 2 S₁→S₀ is probable, but even the data of Monger and Parson show that it is insufficient by itself. Two‐photon absorption: S₀+hv₁→S₁; S₁+hv₁→S_n; S_nS₀+hv₂ could also account for the high‐order fluorescence and its dependence on excitation intensity. [S₀, S₁S_n are ground, first excited and a higher excited singlet states, respectively, of antenna bacteriochlorophyll, T_t is the lowest triplet state, c/v, is the exciting wavelength (694 or 868 nm) and c/v₂ the wavelength of the high‐order fluorescence (445, 535. or 600 nm), where c= velocity of light.] Maximum values are estimated for some of the rate constants.