Exciton relaxation and polaron formation in LH2 at low temperature

Excited state dynamics of antenna complexes in an LH2-only mutant of the pu rple bacterium Rb, sphaeroides have been investigated by means of two-color femtosecond spectroscopy combined with computer simulations. Measurements of transient absorption spectra revealed a new spectral band in the stimula ted emission region around 880 nm at 6 K. To simulate this new dynamic feat ure that appears within the first 2 ps after light absorption. we have used a simplified polaron model via a decoupled red pigment within the B850 rin g. Comparison of simulations and experiment suggests the following kinetic scheme: (1) initial ultrafast (similar to 100 fs) dynamics correspond to ex citon relaxation to the lowest exciton state; (2) mixing of the lowest exci ton state with charge-transfer states results in a relaxation to these stat es in about 600 fs; (3) picosecond dielectric relaxation in the charge-tran sfer states corresponding to polaron formation follows.