  We report two-dimensional electronic spectroscopy (2DES) experiments on the
bacterial reaction center (BRC) from purple bacteria, revealing hidden
excitonic and vibronic structure. Through analysis of the coherent dynamics of
the BRC we resolve specific coherent signatures that allow us to definitively
assign the upper exciton energy of the "special pair." This assignment is
supported by simulations of coherent dynamics of a reduced excitonic model of
the BRC. The simulations also identify nonsecular vibronic coherence transfer
processes neglected in standard models of photosynthetic energy transfer and
charge separation. In addition, the coherent dynamics reveal multiple
quasi-resonances between key intramolecular pigment vibrations and excited
state energy gaps in the BRC. The functional significance of such
electronic-vibrational resonances for photosynthetic energy transfer and charge
separation remains an open question.
