Second-Harmonic Scattering as a Probe of Structural Correlations in Liquids

TL;DR

This paper presents a method combining atomistic simulations and second-harmonic scattering experiments to reveal short-range structural correlations in liquids, challenging previous assumptions of incoherence.

Contribution

It introduces a computational approach to account for coherent effects in second-harmonic scattering, enabling detailed analysis of molecular correlations in liquids.

Findings

Coherent contributions in second-harmonic scattering arise from correlations within <1 nm.

The method allows extraction of effective molecular hyperpolarizability from experimental data.

Simulations and experiments together reveal intrinsic intermolecular correlations in liquids.

Abstract

Second-harmonic scattering experiments of water and other bulk molecular liquids have long been assumed to be insensitive to interactions between the molecules. The measured intensity is generally thought to arise from incoherent scattering due to individual molecules. We introduce a method to compute the second-harmonic scattering pattern of molecular liquids directly from atomistic computer simulations, which takes into account the coherent terms. We apply this approach to large-scale molecular dynamics simulations of liquid water, where we show that nanosecond second-harmonic scattering experiments contain a coherent contribution arising from radial and angular correlations on a length scale of < 1 nm, much shorter than had been recently hypothesized (Shelton, D. P. J. Chem. Phys. 2014, 141). By combining structural correlations from simulations with experimental data (Shelton, D. P. J. Chem. Phys. 2014, 141), we can also extract an effective molecular hyperpolarizability in the liquid phase. This work demonstrates that second-harmonic scattering experiments and atomistic simulations can be used in synergy to investigate the structure of complex liquids, solutions, and biomembranes, including the intrinsic intermolecular correlations.