Electronic and Vibrational Coherence in the Core Light-Harvesting Antenna of Rhodopseudomonas viridis

Abstract

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.