Theoretical calculation of femtosecond time-resolved spectra of initial electron transfer in photosynthetic reaction centers

Abstract

In this paper, we propose to study the dynamics and spectroscopy of photosynthetic reaction centers (RCs). We shall analyze the experimental femtosecond spectra of Rb. sphaeroides (R-26) which usually consist of contributions from populations and coherences. We shall extract information on vibrational relaxation, electron transfer (ET), and the vibrational modes involved in ET from the analysis of the femtosecond time-resolved spectra with pronounced coherence contributions. Conventional ET theory assumes that vibrational relaxation (VR) is much faster than ET so that the vibrational equilibrium is established before ET takes place. However, the primary ET in RCs occurs in 1-4 ps. This implies that this assumption should be examined. We shall study theoretically the effect of the excitation wavelength; that is, we shall study how the ET in RCs is affected by exciting to different vibronic states of RCs. In the dynamic aspect of primary ET in RCs, these experiments will provide us not only with the mechanism of ET but also with the vibrational relaxation of various modes involved in ET. In this case, it is necessary to know the expressions of single-vibronic level ET rate constants. This will also be presented in this paper. 45 refs., 6 figs., 1 tab.