Analysis of 2D THz-Raman spectroscopy using a non-Markovian Brownian oscillator model with nonlinear system-bath interactions

TL;DR

This study employs a non-Markovian Brownian oscillator model with nonlinear interactions to analyze 2D THz-Raman spectra, successfully reproducing key features of molecular inter-molecular vibrations observed in MD simulations.

Contribution

The paper introduces a non-Markovian Brownian oscillator model with nonlinear system-bath interactions to interpret 2D THz-Raman spectra, bridging MD simulations and theoretical analysis.

Findings

Model captures essential features of inter-molecular motion

Reproduces qualitative 2D spectral profiles

Elucidates origins of echo peaks and peak elongation

Abstract

We explore and describe the roles of inter-molecular vibrations employing a Brownian oscillator (BO) model with linear-linear (LL) and square-linear (SL) system-bath interactions, which we use to analyze two-dimensional (2D) THz-Raman spectra obtained by means of molecular dynamics (MD) simulations. In addition to linear absorption (1D IR), we calculated 2D Raman-THz-THz, THz-Raman-THz, and THz-THz-Raman signals for liquid formamide, water, and methanol using an equilibrium non-equilibrium hybrid MD simulation. The calculated 1D IR and 2D THz-Raman signals are compared with results obtained from the LL+SL BO model applied through use of hierarchal Fokker-Planck equations with non-perturbative and non-Markovian noise. We find that all of the qualitative features of the 2D profiles of the signals obtained from the MD simulations are reproduced with the LL+SL BO model, indicating that this model captures the essential features of the inter-molecular motion. We analyze the fitted 2D profiles in terms of anharmonicity, nonlinear polarizability, and dephasing time. The origins of the echo peaks of the librational motion and the elongated peaks parallel to the probe direction are elucidated using optical Liouville paths.