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A vibronic-exciton model is applied to investigate the mechanism ofenhancement of coherent oscillations due to mixing of electronic and nucleardegrees of freedom recently proposed as the origin of the long-livedoscillations in 2D spectra of the FMO complex [Christensson et al. J. Phys.Chem. B 116 (2012) 7449]. We reduce the problem to a model BChl dimer toelucidate the role of resonance coupling, site energies, nuclear mode andenergy disorder in the enhancement of vibronic-exciton and ground-statevibrational coherences, and to identify regimes where this enhancement issignificant. For a heterodimer representing the two coupled BChls 3 and 4 ofthe FMO complex, the initial amplitude of the vibronic-exciton and vibrationalcoherences are enhanced by up to 15 and 5 times, respectively, compared to thevibrational coherences in the isolated monomer. This maximum initial amplitudeenhancement occurs when there is a resonance between the electronic energy gapand the frequency of the vibrational mode. The bandwidth of this enhancement isabout 100 cm-1 for both mechanisms. The excitonic mixing of electronic andvibrational DOF leads to additional dephasing relative to the vibrationalcoherences. We evaluate the dephasing dynamics by solving the quantum masterequation in Markovian approximation and observe a strong dependence of thelife-time enhancement on the mode frequency. Long-lived vibronic-excitoncoherences are found to be generated only when the frequency of the mode is inthe vicinity of the electronic resonance. Although the vibronic-excitoncoherences exhibit a larger initial amplitude compared to the ground-statevibrational coherences, we conclude that both type have a similar magnitude atlong time for the present model. The ability to distinguish betweenvibronic-exciton and ground-state vibrational coherences in the general case ofmolecular aggregate is discussed.

Enhancement of Vibronic and Ground-State Vibrational Coherences in 2D Spectra of Photosynthetic Complexes