Subpicosecond Pump−Supercontinuum Probe Spectroscopy of LH2 Photosynthetic Antenna Proteins at Low Temperature

Exciton relaxation and energy-transfer processes in the B850 circular aggregate of bacteriochlorophyll a molecules from the purple nonsulfur photosynthetic bacterium Rhodobacter sphaeroides were studied at temperatures below 18 K. Excitons were selectively excited by a 7 nm spectral bandwidth pump pulse resonant with the inhomogeneously broadened long-wavelength side of the B850 ground-state absorption spectrum (between 860 and 879 nm). The transient spectra were measured over the 786−924 nm spectral range using a white light continuum probe pulse. Characteristic changes of transient spectra were observed over 4 decades of time, from about 10-13 to about 10-9 s. The spectral evolution was pump wavelength-dependent, changes being least notable at far-red excitation. A simple model was put forward to interpret the data, assuming that the sample consists of an ensemble of spectrally disorded excitons, each representing a separate B850 ring. It was found that the exciton coupling and diagonal disorder play al...