DISORDER-INDUCED EXCITON SCATTERING IN THE LIGHT-HARVESTING SYSTEMS OF PURPLE BACTERIA - INFLUENCE ON THE ANISOTROPY OF EMISSION AND BAND-]BAND TRANSITIONS

A time-domain analysis of the effects of pigment inhomogeneity upon th e dynamics of optical excitations within bacterial light-harvesting co mplexes is presented. We focus upon examining the manifestation of dis order scattering in polarized femtosecond spectroscopy and the degree to which exciton delocalization is revealed in emission and transient absorption anisotropy measurements. The time evolution of states prepa red by impulsive excitation of a statically disordered circular aggreg ate model for LH2 antenna complexes have been calculated exactly for v arying degrees of pigment inhomogeneity. For a Gaussian distribution o f site energies, the dynamics of coherence-loss (scattering) is explor ed as a function of the ratio of the standard deviation (sigma) of the distribution to the intersite interaction energy (beta). It is found that modest degrees of disorder (sigma/beta-0.4) are sufficient to cau se scattering on a sub-100 fs time scale. Results from model calculati ons of the pump-probe anisotropy strongly suggest that the initial ult rafast emission depolarization component reported for LH1 and LH2 ante nna complexes by several groups represents the decay of an initially d elocalized exciton, prepared by coherent excitation of eigenstates tha t become partially localized due to inhomogeneity. A novel approach to studying exciton coherence is proposed based on measurement of the an isotropy of band-->band transient absorption transitions, which exhibi t a pronounced sensitivity to exciton delocalization. (C) 1998 America n Institute of Physics.