Liquid Water: When Hyperpolarizability Fluctuations Boost and Reshape the Second Harmonic Scattering Intensities

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Second harmonic scattering (SHS) is a powerful technique for probing liquid molecular structures.
• Quantitative interpretation of SHS signals from solvents, like water, is challenging due to complex contributions.
• Existing models often struggle to account for solvent-specific molecular interactions and dynamics.

Purpose of the Study:

• To develop and validate a computational approach for quantitatively modeling polarization-resolved SHS intensity in liquid water.
• To elucidate the contributions of molecular hyperpolarizability fluctuations and intermolecular correlations to the SHS signal.
• To establish a framework for interpreting SHS data in terms of short-range molecular ordering in pure liquids.

Main Methods:

• Implementation of a hybrid quantum mechanics/molecular mechanics (QM/MM) computational approach.
• Modeling of polarization-resolved SHS intensity, explicitly considering molecular hyperpolarizability.
• Analysis of intermolecular orientational and hyperpolarizability correlations up to the third solvation layer.

Main Results:

• The QM/MM model successfully predicts SHS intensity for liquid water without fitting parameters.
• Molecular hyperpolarizability fluctuations and correlations were found to be essential for accurate SHS interpretation.
• Intermolecular correlations significantly enhance scattering intensities and modulate polarization-resolved oscillations.

Conclusions:

• The developed QM/MM approach provides a quantitative interpretation of SHS intensities in liquid water.
• Short-range molecular ordering and correlations play a critical role in SHS phenomena.
• This methodology is generalizable to other pure liquids for detailed structural analysis.