Dynamics in Isolated Bacterial Light Harvesting Antenna (LH2) of Rhodobacter sphaeroides at Room Temperature

The underlying dynamics of the B800 absorption band in isolated LH2 of Rb. sphaeroides at room temperature is studied by transient absorption, transient grating, and photon echoes using 30 fs pulses. The energy transfer time from B800 to B850 is determined to be 800 fs, similar to the value reported previously. The three pulse stimulated photon echo identifies several important contributions to the B800 absorption line shape and thereby the dynamics of the system involved: several low frequency intramolecular vibrations; ultrafast bath (solvent and protein) responses, and static inhomogeneity longer than the time scale of B800 to B850 energy transfer make significant contributions. Transient absorption decay is nonexponential as found previously. It is argued that the fast component in the two-exponential analysis of the transient absorption signal originates from vibrational relaxation within the B800 absorption band. Calculations of the nonlinear signals based on the optical transition frequency correlation function, M(t), including all three contributions (intramolecular vibrations, ultrafast bath (solvent and protein) responses, and static inhomogeneity) are presented, and a form for M(t) is obtained by fitting the three pulse photon echo experimental data.