: Proc Natl Acad Sci U S A 1991 Oct 15;88(20):8885-9
Direct Observation of Vibrational Coherence in Bacterial Reaction
Centers Using Femtosecond Absorption Spectroscopy
Vos M.H., Lambry J.C., Robles S.J., Youvan D.C., Breton J., Martin J.L.
Laboratoire d'Optique Appliquee, Institut National de la Sante et de la Recherche Medicale
Unite 275, Ecole Polytechnique-Ecole Nationale Superieure de Techniques Avancees,
Palaiseau, France.
It is shown that vibrational coherence modulates the femtosecond kinetics of stimulated
emission and absorption of reaction centers of purple bacteria. In the DLL mutant of Rhodobacter
capsulatus, which lacks the bacteriopheophytin electron acceptor, oscillations with
periods of approximately 500 fs and possibly also of approximately 2 ps were observed,
which are associated with formation of the excited state. The kinetics, which reflect
primary processes in Rhodobacter sphaeroides R-26, were modulated by oscillations
with a period of approximately 700 fs at 796 nm and approximately 2 ps at 930 nm. In the
latter case, at 930 nm, where the stimulated emission of the excited state, P*, is probed,
oscillations could only be resolved when a sufficiently narrow (10 nm) and concomitantly
long pump pulse was used. This may indicate that the potential energy surface of the
excited state is anharmonic or that low-frequency oscillations are masked when higher
frequency modes are also coherently excited, or both. The possibility is discussed that
the primary charge separation may be a coherent and adiabatic process coupled to
low-frequency vibrational modes.