OSCILLATING ANISOTROPIES IN A BACTERIOCHLOROPHYLL PROTEIN - EVIDENCE FOR QUANTUM BEATING BETWEEN EXCITON LEVELS

Optical anisotropies of bacteriochlorophyll a Q(y) electronic transiti ons in Fenna-Mabhews-Olson trimers from the green photosynthetic bacte rium Chlorobium tepidum exhibit strong oscillations with similar to 22 0 fs period for certain wavelengths in one-color absorption difference experiments. The oscillations in the polarized pump-probe signals Del ta A(parallel to)(t), Delta A(perpendicular to)(t) have opposite phase , and they nearly cancel in the isotropic signals. This, combined with the fact that they only appear when the laser pulse spectrum overlaps both of the lowest-energy groups of exciton levels clustered near 815 and 825 nm, suggests that the oscillations stem from quantum beating between exciton levels, rather than from coherent nuclear motion. In s ome cases, the anisotropy r(t) exhibits values > 0.4 at times well bey ond the laser autocorrelation function. (C) 1997 Published by Elsevier Science B.V.