Resolving the Nature of the Lowest-Frequency Raman Mode of Liquid Water

TL;DR

This study uses ab initio simulations to clarify the microscopic origin of the lowest-frequency Raman mode in water, revealing that both rotational and translational motions contribute equally and are influenced by complex correlations.

Contribution

It provides a detailed computational analysis that resolves the debate on the mode's origin, highlighting the roles of rotational, translational, and correlation effects in pure water.

Findings

Rotational and translational motions contribute equally to the Raman mode.

Negative orientational cross-correlations significantly affect the rotational component.

The mode's characteristics differ from dielectric spectroscopy expectations.

Abstract

The lowest-frequency Raman mode of water, observed through depolarized light scattering or optical Kerr effect techniques, is routinely used to track dynamic changes in water molecules near ions or biomolecules. Yet, the microscopic origin of this mode and its relation to dielectric relaxation still remains debated for pure water with conflicting interpretations attributing it to either translational or rotational molecular motions. In this study, we compute the low-frequency Raman spectrum in the GHz to THz range using ab initio simulations, achieving excellent agreement with experimental data. Detailed decomposition analysis reveals that the rotational and translational contributions are equally important, while strong negative orientational cross-correlations as well as internal field effects significantly modify the rotational component, making it distinct from expectations inferred from dielectric spectroscopy.