Disentangling electronic and vibronic coherences in two-dimensional echo spectra

TL;DR

This paper develops a method to distinguish between electronic and vibronic contributions in 2D echo spectra of light-harvesting complexes, revealing that spectra may overestimate electronic coherence lifetimes.

Contribution

It introduces a time-windowed Fourier transform technique to separate vibronic and electronic signals in 2D spectra, clarifying their individual roles.

Findings

2D spectra can be dominated by vibronic pathways absent in energy transport

Electronic coherence lifetimes are often underestimated in 2D spectra

The method improves understanding of energy transfer mechanisms in light-harvesting complexes

Abstract

The prevalence of long-lasting oscillatory signals in the 2d echo-spectroscopy of light-harvesting complexes has led to a search for possible mechanisms. We investigate how two causes of oscillatory signals are intertwined: (i) electronic coherences supporting delocalized wave-like motion, and (ii) narrow bands in the vibronic spectral density. To disentangle the vibronic and electronic contributions we introduce a time-windowed Fourier transform of the signal amplitude. We find that 2d spectra can be dominated by excitations of pathways which are absent in excitonic energy transport. This leads to an underestimation of the life-time of electronic coherences by 2d spectra.