Size Enhancement of Transition Dipoles to One- and Two-Exciton Bands in a Photosynthetic Antenna

Measurement of nonlinear absorption and differential optical density spectrum as well as fluorescence behavior of the light harvesting antenna LH2 from purple bacterium Rhodobacter sphaeroides indicates exciton delocalization over $16\pm 4$ bacteriochlorophyll-a molecules, corresponding to probably the full physical length of the circular aggregate responsible for the 850 nm absorption band. The giant dipole moments for the $\mathrm{consecutive}\mathrm{transitions}\mathrm{ground}\mathrm{state}\to \mathrm{o}\mathrm{n}\mathrm{e}-\mathrm{e}\mathrm{x}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{d}\to \mathrm{t}\mathrm{w}\mathrm{o}-\mathrm{e}\mathrm{x}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{b}\mathrm{a}\mathrm{n}\mathrm{d}$ are $25.5\pm 2.5\mathrm{D}$ and $21.5\pm 3.5\mathrm{D}$, respectively.