Electronic Coherence Dephasing in Excitonic Molecular Complexes: Role of Markov and Secular Approximations

TL;DR

This paper compares four second-order equations of motion for excitonic molecular complexes, analyzing how different approximations affect the simulation of electronic coherence dynamics and their spectral signatures.

Contribution

It provides a systematic comparison of time-nonlocal and time-local equations with secular approximations for modeling excitonic coherence dephasing.

Findings

Secular approximation impacts coherence dynamics significantly.

Time-nonlocal effects can be distinguished via spectral features.

Different equations vary in accuracy for population and coherence simulations.

Abstract

We compare four different types of equations of motion for reduced density matrix of a system of molecular excitons interacting with thermodynamic bath. All four equations are of second order in the linear system-bath interaction Hamiltonian, with different approximations applied in their derivation. In particular we compare time-nonlocal equations obtained from so-called Nakajima-Zwanzig identity and the time-local equations resulting from the partial ordering prescription of the cummulant expansion. In each of these equations we alternatively apply secular approximation to decouple population and coherence dynamics from each other. We focus on the dynamics of intraband electronic coherences of the excitonic system which can be traced by coherent two-dimensional spectroscopy. We discuss the applicability of the four relaxation theories to simulations of population and coherence dynamics, and identify features of the two-dimensional coherent spectrum that allow us to distinguish time-nonlocal effects.