STARK HOLE-BURNING SPECTROSCOPY OF A PHOTOSYNTHETIC COMPLEX - LH2 OF PURPLE BACTERIA

Results from the first Stark hole-burning spectroscopic studies of pho tosynthetic complexes are reported. External electric held effects wer e determined as a function of laser burn frequency and polarization fo r the B800 band and the B870 band of Rhodobacter sphaeroides, where B8 70 is the lowest exciton level of the strongly coupled bacteriochlorop hyll a molecules of the B850 ring. For either parallel or perpendicula r polarization of the burn laser relative to the external Stark field, only broadening of the zero-phonon holes which is linear in field str ength was observed. Near the center of the B870 and B800 bands, f(Delt a mu(parallel to), Delta mu(perpendicular to)) = (1.1, 1.4) and (1.1, 0.8) Debye, respectively, where f is the local field correction factor . However, a dependence of the Delta mu-values on location of the hole within the inhomogeneously broadened absorption bands is observed. Fo r B870 they increase by 10-20% from the blue to red edge while for B80 0 an opposite trend is observed. Results for B870 of Rhodopseudomonas acidophila and Rhodospirillum molischianum are similar. A detailed dis cussion of the results and a comparison of them with those from classi cal Stark modulation spectroscopic studies of the entire B800 and B850 absorption bands are given. It is concluded for the B850 ring that mi xing of charge transfer states with the Q(y)-exciton levels is of mino r importance. The hole burning data, together with consideration of th e C-n-symmetry and the effects of energy disorder from structural hete rogeneity, suggest that localization effects from energy disorder are probably important for understanding the effects of external electric field on the B850 band which is comprised of several components. (C) 1 998 Elsevier Science B.V. All rights reserved.