Transient absorption study of two-photon excitation mechanism in the LH2 complex from purple bacterium Rhodobacter sphaeroides

Ilya Stepanenko, Viktor Kompanetz, Zoya Makhneva, Sergey Chekalin, Andrei Moskalenko, Andrei Razjivin
Journal of Physical Chemistry. B 2012 March 8, 116 (9): 2886-90
The mechanism of two-photon excitation of a peripheral light-harvesting complex LH2 (B800-850) from purple bacterium Rhodobacter sphaeroides was explained on the basis of femtosecond transient absorption data. Fast bleaching of the B850 absorption band was measured under two-photon excitation by 1350 nm femtosecond pulses, showing fast subpicosecond arrival of excitation energy to B850 circular aggregates. Any spectral changes connected with the B800 absorption band of B800-BChl molecules were absent. A similar picture was observed under one-photon excitation of the LH2 complex by 675 nm femtosecond pulses. We believe these effects may be attributed to direct excitation of high-energy excitonic states of a B850 circular aggregate or its vibrational manifold in accordance with the model of Abe [Chem. Phys. 2001, 264, 355-363].

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