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

Area of Science:

• Nonlinear Optics
• Molecular Spectroscopy
• Computational Chemistry

Background:

• Second-harmonic scattering (SHS) experiments on molecular liquids traditionally assume incoherent scattering from individual molecules.
• Previous interpretations overlooked the contribution of coherent scattering due to intermolecular interactions.
• The sensitivity of SHS to molecular interactions in bulk liquids has been a long-standing question.

Discussion:

• This study introduces a novel computational method to calculate the SHS pattern of molecular liquids directly from atomistic simulations, incorporating coherent terms.
• Application to large-scale molecular dynamics simulations of liquid water reveals a significant coherent contribution in nanosecond SHS experiments.
• This coherent signal arises from radial and angular correlations within approximately 1 nm, challenging previous hypotheses about the length scale of these interactions.

Key Insights:

• SHS experiments on liquids are sensitive to intermolecular correlations on nanometer length scales.
• Atomistic simulations coupled with experimental data can reveal hidden structural information in liquids.
• The effective molecular hyperpolarizability in the liquid phase can be determined by combining simulation and experimental data.

Outlook:

• This synergistic approach using SHS and atomistic simulations opens new avenues for studying the structure of complex liquids, solutions, and biomembranes.
• It provides a powerful tool to probe intrinsic intermolecular correlations in various condensed phases.
• Future work can extend this method to investigate dynamic correlations and phase transitions in molecular liquids.