We report the results of relative difference absorption measurements for the LH2 antenna of Rhodobacter sphaeroides and the B820 subunit of Rhodospirillum rubrum at room temperature. It is shown that significant differences between shapes and amplitudes of photoinduced, absorption changes reflect a different degree of exciton delocalization in the intact antenna compared with the dimeric subunit. Using the exciton model in the presence of static disorder, we have obtained a consistent and quantitative fit of the amplitudes and shapes of the pump-probe spectra of the LH2 antenna and the B820 subunit. We estimate that the nearest-neighbors interaction energy in the antenna is about 400 cm -1 and the diagonal disorder is about 450 cm -1 . For these values the coherence length (FWHM) of the steady-state exciton wavepacket corresponds to 5 BChl molecules at room temperature. The amplitude of the difference absorption reflects a cooperative behavior within at least 12 BChls of the B850 antenna. The calculations suggest that the dimeric subunit is characterized by a decrease of the interaction energy to 300 cm -1 together with an increase of the disorder value to about 600 cm -1 .

In this paper we explain the wavelength-dependent oscillatory features in the pump−probe kinetics of the core LH1 antenna of Rhodopseudomonas viridis (Monshouwer, R.; Baltuška, A.; van Mourik, F.; van Grondelle, R. J. Phys. Chem. A 1998, 102, 4360). A quantitative fit of the data was obtained using the doorway−window representation of the nonlinear optical response in the vibrational eigenstate basis. In contrast to LH1/LH2 complexes from the BChl a-containing species, the LH1 antenna of Rps. viridis is characterized by a strong coupling of the excitonic states with two underdamped low-frequency modes (58 and 110 cm-1 at 77 K). Following a short femtosecond excitation pulse, this gives rise to the intense oscillations observed in the pump−probe traces, including their time and excitation/detection wavelength dependence. Furthermore, it leads to a pronounced and specific heterogeneity of the major absorption band due to the combined effects of the exciton splitting, disorder, and the presence of vibrational sidebands. The sharp maxima in the second derivative of the low-temperature absorption spectrum (Monshouwer, R.; Visschers, R. W.; van Mourik, F.; Freiberg, A.; van Grondelle, R. Biochim. Biophys. Acta 1995, 1229, 373) were assigned to the lowest exciton−vibrational transitions. The wavelength dependence of the experimentally observed oscillatory pattern suggests a different vibrational coherence decay for the ground- and excited-state wave packets. This can be explained by assuming the (incoherent) migration of the delocalized exciton (polaron) around the ringlike antenna with a characteristic time constant of 0.9−1.5 ps at 77K.