  Nonlinear spectroscopy has revealed long-lasting oscillations in the optical
response of a variety of photosynthetic complexes. Different theoretical models
which involve the coherent coupling of electronic (excitonic) or
electronic-vibrational (vibronic) degrees of freedom have been put forward to
explain these observations. The ensuing debate concerning the relevance of
either one or the other mechanism may have obscured their potential synergy. To
illustrate this synergy, we quantify how the excitonic delocalization in the
LH2 unit of Rhodopseudomonas Acidophila purple bacterium, leads to correlations
of excitonic energy fluctuations, relevant coherent vibronic coupling and,
importantly, a decrease in the excitonic dephasing rates. Combining these
effects, we identify a feasible origin for the long-lasting oscillations
observed in fluorescent traces from time-delayed two-pulse single molecule
experiments performed on this photosynthetic complex.
