Abstract
Transient absorption kinetics of bacteriochlorophylls in the chlorosomes from Cf. aurantiacus were measured at room temperature and 77 K using low- intensity near-infrared femtosecond pulses with a duration as short as 50 fs. For accurate determination of the isotropic and anisotropic decays, simultaneous two channel detection of both parallel and perpendicular polarizations in the one-color pump-probe setup was employed. A typical fast isotropic component was found within the Qy band of the BChl c absorption, which has a decay lifetime varying from 620 fs to 1.5 ps at room temperature, and between 400 fs and 1.6 ps at 77 K. Both exhibit a general trend of increasing decay time with detection wavelength. Very similar time constants were also observed for the corresponding single anisotropy decays. The origin of these fast decays is suggested to be due to either multiple excitonic relaxation or a sequential intra-chlorosomal energy transfer between different spectral forms of BChl c pigments. These spectral forms may be organized in such a way that there is a gradual decrease of polarization from the short-wavelength absorbing form to the long-wavelength absorbing one, accounting for the wavelength-dependent residual anisotropies with values of 0.34 at 720 nm, 0.30 at 730. nm and 0.22 between 734 - 760 nm. The 2 ps isotropic component resolved at 780 and 796 nm may be related to vibrational relaxation or to energy transfer among the baseplate BChl a molecules. Further energy transfer processes such as that from chlorosomal BChl c to the baseplate BChl a, are also discussed.
Original language | English |
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Pages (from-to) | 530-542 |
Number of pages | 13 |
Journal | Brazilian Journal of Physics |
Volume | 26 |
Issue number | 2 |
State | Published - Jun 1996 |
Externally published | Yes |