Origin of Long Lived Coherences in Light-Harvesting Complexes

TL;DR

This paper develops a vibronic exciton model to explain the long-lived quantum coherences observed in light-harvesting complexes, attributing them mainly to vibronic superpositions rather than purely electronic states.

Contribution

It introduces a model using experimental parameters to predict long-lived vibronic coherences in the FMO complex, clarifying their origin and decay times.

Findings

Vibronic exciton states cause observed oscillations with 1.3 ps dephasing time at 77 K.

Pure electronic coherences decay within 200 fs.

Model accurately reproduces experimental nonlinear spectra.

Abstract

A vibronic exciton model is developed to investigate the origin of long lived coherences in light-harvesting complexes. Using experimentally determined parameters and uncorrelated site energy fluctuations, the model predicts oscillations in the nonlinear spectra of the Fenna-Matthews-Olson (FMO) complex with a dephasing time of 1.3 ps at 77 K. These oscillations correspond to the coherent superposition of vibronic exciton states with dominant contributions from vibrational excitations on the same pigment. Purely electronic coherences are found to decay on a 200 fs timescale.