A vibronic-exciton model is applied to investigate the mechanism of
enhancement of coherent oscillations due to mixing of electronic and nuclear
degrees of freedom recently proposed as the origin of the long-lived
oscillations 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 to
elucidate the role of resonance coupling, site energies, nuclear mode and
energy disorder in the enhancement of vibronic-exciton and ground-state
vibrational coherences, and to identify regimes where this enhancement is
significant. For a heterodimer representing the two coupled BChls 3 and 4 of
the FMO complex, the initial amplitude of the vibronic-exciton and vibrational
coherences are enhanced by up to 15 and 5 times, respectively, compared to the
vibrational coherences in the isolated monomer. This maximum initial amplitude
enhancement occurs when there is a resonance between the electronic energy gap
and the frequency of the vibrational mode. The bandwidth of this enhancement is
about 100 cm-1 for both mechanisms. The excitonic mixing of electronic and
vibrational DOF leads to additional dephasing relative to the vibrational
coherences. We evaluate the dephasing dynamics by solving the quantum master
equation in Markovian approximation and observe a strong dependence of the
life-time enhancement on the mode frequency. Long-lived vibronic-exciton
coherences are found to be generated only when the frequency of the mode is in
the vicinity of the electronic resonance. Although the vibronic-exciton
coherences exhibit a larger initial amplitude compared to the ground-state
vibrational coherences, we conclude that both type have a similar magnitude at
long time for the present model. The ability to distinguish between
vibronic-exciton and ground-state vibrational coherences in the general case of
molecular aggregate is discussed.Comment: 16 pages, 6 figure
