Phase-referenced Nonlinear Spectroscopy of the alpha-Quartz/Water Interface

TL;DR

This study introduces a nonlinear spectroscopy method to analyze water molecules at the alpha-quartz/water interface, revealing phase-dependent interfacial responses influenced by bulk properties and quartz orientation.

Contribution

It reports a novel phase-referenced nonlinear optical measurement technique for anisotropic interfaces, highlighting a bulk-origin term affecting interfacial spectroscopy.

Findings

Identification of a new phase-dependent interfacial term, iχ(3).Φ(0)

Demonstration of interference effects depending on quartz rotation angle

Implications for phase referencing in interfacial material analysis

Abstract

Probing the polarization of water molecules at charged interfaces by second harmonic generation spectroscopy has been heretofore limited to isotropic materials. Here, we report non-resonant nonlinear optical measurements at the interface of anisotropic z-cut {\alpha}-quartz and water under conditions of dynamically changing ionic strength and bulk solution pH. We find that the product of the third-order susceptibility and the interfacial potential, \c{hi}(3). {\Phi}(0), is given by ( \c{hi}(3)-i\c{hi}(3)). {\Phi}(0), and that the interference between this product and the second-order susceptibility of bulk quartz depends on the rotation angle of {\alpha}-quartz around the z-axis. Our experiments show that this newly identified term, i\c{hi}(3). {\Phi}(0), which is out of phase from the surface terms, is of bulk origin. The possibility of internally phase referencing the interfacial response for the interfacial orientation analysis of species or materials in contact with {\alpha}-quartz is discussed along with the implications for conditions of resonance enhancement.