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.

• Received 13 June 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.4675