A New Imaginary Term in the Second-Order Nonlinear Susceptibility from Charged Interfaces.

Area of Science:

• Physical Chemistry
• Surface Science
• Nonlinear Optics

Background:

• Interfacial phenomena are crucial in various chemical and physical processes.
• Second Harmonic Generation (SHG) is a powerful surface-sensitive nonlinear optical technique.
• Accurate modeling of interfacial nonlinear optical responses is essential for understanding surface properties.

Purpose of the Study:

• To investigate the nonlinear optical response of the silica-water interface.
• To identify and characterize novel nonlinear susceptibility terms contributing to interfacial phenomena.
• To develop an improved model for interfacial potentials and nonlinear susceptibilities.

Main Methods:

• Performed nonresonant second harmonic generation (SHG) phase and amplitude measurements.
• Studied the silica-water interface across a range of pH values.
• Maintained a constant ionic strength of 0.5 M and varied electrolyte composition (NaCl vs. MgSO₄).

Main Results:

• Identified a new nonlinear susceptibility term, χ_X⁽³⁾, associated with a 90° phase shift.
• A model incorporating χ_X⁽³⁾ (≈1.5χ_water⁽³⁾) accurately estimated interfacial potentials and susceptibilities.
• Models lacking χ_X⁽³⁾ failed to reproduce experimental data; the new model distinguished between NaCl and MgSO₄ electrolytes.

Conclusions:

• The χ_X⁽³⁾ term is necessary for accurately modeling the silica-water interface's nonlinear response.
• The developed model enhances the understanding of interfacial potentials and nonlinear susceptibilities.
• Heterodyne-detected SHG shows potential for investigating ion specificity in interfacial processes.