Two-dimensional IR-Raman spectroscopy of vibrational polaritons: Role of dipole surfaces

TL;DR

This study uses cavity molecular dynamics simulations to analyze 2D IR-Raman spectra of water under vibrational strong coupling, emphasizing the importance of consistent dipole surface models for accurate spectral predictions.

Contribution

It demonstrates the critical role of employing a consistent dipole surface model in simulations and spectroscopic analysis of vibrational polaritons, improving the accuracy of 2D spectra under VSC.

Findings

Consistent dipole models are essential for accurate 2D spectra.

Cavity 2D-IIR spectrum shows splitting of the OH stretch into polariton branches.

Inconsistent models distort the spectral features significantly.

Abstract

Nonlinear spectroscopy provides a unique perspective to understand time-resolved molecular dynamics under vibrational strong coupling (VSC). Herein, equilibrium-nonequilibrium cavity molecular dynamics simulations are performed to compute the two-dimensional (2D) infrared-infrared-Raman (IIR) spectroscopy of liquid water under VSC. In conventional computational chemistry practices, accurate molecular spectra are often constructed by using an advanced molecular dipole or polarizability model to post-process molecular dynamics trajectories evolved under a computationally efficient potential. By contrast, this work highlights the necessity of employing a consistent dipole surface model in both CavMD simulations and spectroscopic post-processing. While utilizing inconsistent dipole models only mildly influences the linear polariton spectrum, it severely distorts 2D spectra in wide frequency regions. With a consistent dipole-induced-dipole model, compared to the outside-cavity molecular 2D-IIR spectrum, the cavity 2D-IIR spectrum splits the OH stretch band to a pair of polariton branches along only the IR (not Raman) axis, while fading molecular signals at other frequency regions. This work provides the foundation for employing direct CavMD simulations to construct 2D spectra of realistic molecules under VSC.