Evidence for a very early intermediate in bacterial photosynthesis. A photon-echo and hole-burning study of the primary donor band in Rhodopseudomonas sphaeroides

Abstract

Two coherent spectroscopic methods, accumulated photon echo and population bottleneck hole-burning, have been employed in a study of the decay rate of the primary donor (P) of Rhodopseudomonas sphaeroides at 1.5 K. The decay rate is instrument-limited in the photon-echo experiment, implying a population relaxation time <100 fs. The hole-burning study revealed the P absorption at 900 nm to be largely homogeneously broadened, from which a decay time of = 25 fs was inferred. Comparison of these data with a photon-echo study of the bacteriochlorophyll a monomer suggests that this ultrafast process is not due to vibrational relaxation within P *, but to an excited state electronic decay mechanism. It is suggested that the initial event after excitation in P is a very rapid charge separation within the dimer pair, prior to the electron-transfer process, which occurs on a much longer timescale.