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Efficient energy transfer in light harvesting (LH) complexes forms an important part of the photosynthetic process that results in the conversion of light energy into chemical free energy. A number of factors are important for understanding the process that directs the optical excitation to the reaction center. Included are the nature of relevant excited states of chlorophylls (e.g. localised or delocalised), bath induced mechanisms for homogeneous broadening of transitions etc. The Q sub y-absorption (S sub 1) of chlorophyllic molecules in protein complexes appears as inhomogeneously broadened bands with Gamma sub I approx. 50 - 200/cm at liquid helium temperatures., This article is from 'Organization of the Optical Society of America Photonic Science Topical Meeting Series (1991). Volume 16. Conference Edition: Summaries of papers presented at the Persistent Spectral Hole-Burning: Science and Applications Topical Meeting Held in Monterey, California on 26-28 September 1991', AD-A258 663, p125-128.

Novel nonphotochemical hole burning action spectra are persented that yield the low-temperature absorption profiles of B896 and B870 and their underlying structures (linear electron-phonon coupling and site inhomogeneous broadening). The results establish that B896 and B870 are associated with the far more intense B875 and B850 bacteriochlorophyll a absorption bands, respectively, of the light harvesting I and II complexes. The homogeneous widths of the B896 and B870 zero-phonon holes are the same within experimental uncertainty, 3.2 cm-1 at 4.2 K, which corresponds to a total optical dephasing time of 6.6 ps. A number of interpretations for B870 and B896 are considered. Favored is one in which they are due to the lowest energy levels of the B850* and B875* exciton bands (asterisk denoting the S1(Q(y)) state). Based on studies of the dephasing of excitons in organic crystals, the 6.6-ps dephasing of B896* is attributed to exciton scattering with energetic inequivalent neighboring unit cells. Such scattering and B870 to B875 energy transfer are suggested to be contributors to the dephasing of B870*. The effect of glasslike structural heterogeneity on the optical selection rules for unit cells of cyclic symmetry is also considered.

Hole burned spectra for the B800—B850 bacteriochlorophyll a antenna complex of Rb. sphaeroides chromatophores have been obtained for burn wavelengths ranging from the high-energy side of the B800 absorption profile to the low-energy side of B850 profile. The results indicate that B800—B800 energy transfer at 1.6 K is slow relative to B800 to B850 transfer which occurs in 2.4 ps. Whereas B800 is largely site inhomogeneously broadened, B850 is found to exhibit a homogeneous linewidth of about 220 cm−1 at low temperatures. The latter results is used to explain the temperature independence of the B800 to B850 energy transfer rate. The homogeneous linewidth of 220 cm−1 is argued to be a measure of the exciton bandwidth of B850. It is concluded that B800 and B850 are weakly exciton-coupled and that the mechanism for B800 to B850 energy transfer is most likely of the Förstertype. The B850 vibronic hole structure allows for an identification of bacteriochlorophyll a modes that participate in Förster transfer.