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

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.

Figures (4)

• Figure 1: a) Comparison of the experimental depolarized light scattering spectrum hansen2016identification with the one calculated from the AIMD simulations using Eq. (\ref{['eq:chi']}). Inset: Total, self- and cross-correlation part of the polarizability correlation function underlying the calculated spectrum in the main panel. b) Same for the dielectric loss spectrum adapted from Ref. holzl2021dielectric and compared to the experimental dielectric spectrum lunkenheimer2017electromagnetic.
• Figure 2: Comparison of the DID and intrinsic contribution to the total Raman spectrum. Spectra are calculated by inserting the respective polarizability tensor as indicated in the legend into Eq. (\ref{['eq:chi']}); see also text. Inset: Different contributions to the polarizability correlation functions underlying the spectra in the main panel.
• Figure 3: Comparison of the DID, IDID and intrinsic contribution to the total Raman spectrum for the calculations "in vacuum", see text. Spectra are calculated by inserting the respective polarizability tensor as indicated in the legend into Eq. (\ref{['eq:chi']}). Inset: Total, self and cross contributions to the intrinsic correlation function.
• Figure 4: Comparison of dielectric and Raman spectra, calculated by inserting the respective dipole moment into Eq. (\ref{['eq:eps']}) or polarizability tensor into Eq. (\ref{['eq:chi']}) as indicated in the legend, see text.