Investigation of the Second Harmonic Generation at the Water-Vacuum Interface by Using Multi-Scale Modeling Methods

Area of Science:

• Computational chemistry
• Physical chemistry
• Interface science

Background:

• The water-vacuum interface exhibits unique properties.
• Understanding molecular behavior at interfaces is crucial for various scientific disciplines.
• Linear and nonlinear optical responses are sensitive probes of molecular environments.

Purpose of the Study:

• To systematically investigate the polarizability (α) and first hyperpolarizability (β) of water molecules at the water-vacuum interface.
• To quantify the changes in optical responses from bulk water to the interface.
• To elucidate the influence of interfacial effects on molecular polarizability and hyperpolarizability.

Main Methods:

• Sequential Quantum Mechanics/Molecular Mechanics (QM/MM) scheme.
• Classical molecular dynamics (MD) simulations for structural snapshots.
• Time-dependent Density Functional Theory (TD-DFT) for optical response calculations.
• Various embedding schemes (point charges, polarizable point charges, local field effects).

Main Results:

• A modest increase in average polarizability (α) but a doubling of its anisotropy at the interface.
• A ~20% increase in the first hyperpolarizability (β_HRS) response with a slight increase in its depolarization ratio.
• A net increase in the β tensor component normal to the interface (β_zzz) and in-plane (β_//).

Conclusions:

• Interfacial effects significantly alter the polarizability and hyperpolarizability of water molecules.
• Changes in β are primarily localized to the first molecular layer.
• Interfacial effects on α extend up to the fourth molecular layer, indicating a broader influence.