# Nonlinear Spectroscopy in Chlorophyll Dimers Embedded in an Asymmetric Phonon Bath

## TL;DR

This paper studies how nonlinear spectroscopy can reveal the effects of an asymmetric phonon environment on chlorophyll dimers in photosynthetic complexes.

## Contribution

A new method for modeling exciton–phonon coupling using asymmetric phonon spectral densities in photosynthetic systems.

## Key findings

- Asymmetric phonon spectral densities influence the 1-phonon profile and absorption spectra.
- The method enables fine-tuning of electron–phonon coupling in multimode photosynthetic systems.
- Nonlinear optical signals reveal insights into exciton–exciton and exciton–phonon interactions.

## Abstract

The electronic transition dipole moment 4-point time
correlation
function for a dimeric photosynthetic complex, from which nonlinear
optical time-domain signals may be obtained. This 4-point time correlation
function draws on an experimentally fit spectral density of the surrounding
phonons of the photosynthetic protein. The spectral density of the
photosynthetic phonons renders a phonon-sideband characterized by
its asymmetry, caused by the unequal contribution
from the photosynthetic phonons (bath) to the low- and high-energy
sides of the optical signals. This spectral density manifests its
asymmetry explicitly in the 1-phonon profile, due to the intimate
spectral connection between them, which will in turn reflect in the
entire phononic part of the absorption spectrum. The asymmetry plays
an important role in characterizing the exciton–phonon coupling
strength and the phonon relaxation mechanism, thereby providing flexibility
in modeling the degree of symmetry needed for the bath and imparting
the capability of fine-tuning the nature of electron–phonon
coupling caused by pigment–protein interaction. To this end,
the obtained nonlinear optical electronic transition dipole moment
time correlation functions (Liouville space pathways) whereby both
excitonic and exciton–phonon couplings are accounted for are
deemed convenient, more tractable, and computationally expedient,
a unique advantageous feature in the case of a multimode system, which
is often the case in photosynthetic complexes. Linear spectra and
photon echo signals to probe pigment–protein complexes, in
which pure electronic dephasing, vibrational relaxation effects, 1-phonon
profile asymmetry, exciton–exciton coupling, and exciton–phonon
coupling in bacterial reaction centers and photosynthetic complexes
are provided.