Time-Resolved Absorption Difference Spectroscopy of the LH-1 Antenna of Rhodopseudomonas viridis

The LH-1 antenna of the purple bacterium Rhodopseudomonas viridis has been investigated using femtosecond pump−probe spectroscopy. We find that qualitatively the relaxation processes are comparable to those in Rhodobacter sphaeroides and Rhodospirillum rubrum . Both the spectral relaxation and depolarization take place in approximately 150 fs, which suggests that the excitations in the LH-1 ring are delocalized over two to three pigments. Strong oscillations of the pump−probe signal are observed at all temperatures. We conclude that these must be of a vibrational origin and could be related to the observed fine structure seen in the low-temperature absorption spectrum of Rps. viridis membranes. The main frequencies of the oscillations are 65 and 105 cm -1, and the damping time is ∼700 fs. From the amplitude of the oscillations and the slow damping time, we conclude that excitations are transferred in a vibrationally coherent manner from site to site.