Two-dimensional infrared-Raman spectroscopy as a probe of water's tetrahedrality

TL;DR

This paper explores how two-dimensional IR-Raman spectroscopy can reveal the tetrahedral structure of water by linking spectral features to molecular arrangements, using molecular dynamics simulations.

Contribution

It introduces a novel spectrum decomposition method connecting water's tetrahedrality with its 2D IR-Raman spectra, advancing understanding of water's local structure.

Findings

Spectral features correlate with water's tetrahedral order.

Temperature affects vibrational coupling in water.

Proposed experimental approaches to probe water structure.

Abstract

Two-dimensional spectroscopic techniques combining terahertz (THz), infrared (IR), and visible pulses offer a wealth of information about coupling among vibrational modes in molecular liquids, thus providing a promising probe of their local structure. However, the capabilities of these spectroscopies are still largely unexplored due to experimental limitations and inherently weak nonlinear signals. Here, through a combination of equilibrium-nonequilibrium molecular dynamics (MD) and a tailored spectrum decomposition scheme, we identify a relationship between the tetrahedral order of liquid water and its two-dimensional IR-IR-Raman (IIR) spectrum. The structure-spectrum relationship can explain the temperature dependence of the spectral features corresponding to the anharmonic coupling between low-frequency intermolecular and high-frequency intramolecular vibrational modes of water. In light of these results, we propose new experiments and discuss the implications for the study of tetrahedrality of liquid water.