Abstract
A vibronic-exciton model is applied to investigate the recently proposed mechanism of enhancement of coherent oscillations due to mixing of electronic and nuclear degrees of freedom. We study a dimer system to elucidate the role of resonance coupling, site energies, vibrational frequency 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 two coupled bachteriochloropylls of the FMO complex, long-lived vibronic coherences are found to be generated only when the frequency of the mode is in the vicinity of the electronic energy difference. Although the vibronic-exciton coherences exhibit a larger initial amplitude compared to the ground-state vibrational coherences, we conclude that, due to the dephasing of the former, both type of coherences have a similar magnitude at longer population time.
Highlights
Enhancement of Vibronic and Ground-State Vibrational Coherences in Aurelia Chenu[1], Niklas Christensson[2], Harald F
Experimental confirmation of correlated distributions of pigment energies has been claimed for FMO12, but neither dynamic nor static correlation of the pigment energies have been found in molecular dynamics (MD) simulations of the FMO protein environment[13,14]
The detailed investigation of the interaction between nuclear and electronic degrees of freedom (DOF) in a model dimer provides more insight into the mechanism of enhancement of the amplitude and life time of the oscillations seen in 2D experiments
Summary
Correspondence and requests for materials should be addressed to T.M. The vibronic-exciton states involved in the long-lived coherences are to a large extent composed of different vibrational states on the same pigment This automatically leads to a correlation in the fluctuations of the involved transitions even for a random distribution of pigment energies. Tiwari et al.[30] used a model similar to the vibronic-exciton model used here to show that the mixing of electronic and vibrational DOF leads to an enhancement of the excitation of vibrational coherences in the electronic ground state as well, and it was argued that this effect can explain the long-lived oscillations in FMO. We turn to a model system (an FMO-inspired molecular dimer) in order to systematically investigate how the mixing of vibrational and electronic DOF leads to long-lived oscillatory signal in non-linear optical spectroscopy. After determining the basic properties of the enhancement for vibronic (excitonically mixed electronic and vibrational) and the ground-state vibrational coherences on an FMOinspired heterodimer, we discuss the results in context of FMO and the low frequency vibrational spectrum of BChl-a
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