Second Harmonic Scattering (SHS) is a method of choice to investigate the molecular structure of liquids. While a clear interpretation of SHS intensity exists for diluted solutions of dyes, the scattering due to solvents remains difficult to interpret quantitatively. Here, we report a quantum mechanics/molecular mechanics (QM/MM) approach to model the polarization-resolved SHS intensity of liquid water, quantifying different contributions to the signal. We point out that the molecular hyperpolarizability fluctuations and correlations cannot be neglected. The intermolecular orientational and hyperpolarizability correlations up to the third solvation layer strongly increase the scattering intensities, and modulate the polarization-resolved oscillation that is predicted here by QM/MM without fitting parameters. Our approach can be generalized to other pure liquids to provide a quantitative interpretation of SHS intensities in terms of short-range molecular ordering.
Liquid Water : When Hyperpolarizability Fluctuations Boost and Reshape the Second Harmonic Scattering Intensities , Supplementary Materials
Experimental details on polarization-resolved SHS intensities. Summary of the SHS formalism. Methodological details (QM/MM Method, Numerical analysis of hyperpolarizability corre-lations). Details on Results (First hyperpolarizability in the molecular frame, First hyperpolarizability in the laboratory frame, Details of the contributions of fluctuations or correlations to the incoherent part of the tensor Y.