PNNL

Abstract

Probing the polarization of water molecules at charged interfaces by second harmonic generation (SHG) spectroscopy1 has been heretofore limited to isotropic solids. The signal intensity follows the interfacial potential, fo, according to I2? ?|?(2)+ ?(3).fo|2, where I2? is the SHG signal intensity oscillating at frequency 2?, and ?(2) and ?(3) are the second- and third-order susceptibilities (?(2) and ?(3)) of the interface probed. Here, we report the first phase-referenced SHG measurements under non-resonant conditions at the interface of z-cut a-quartz and water under conditions of dynamically changing ionic strength and bulk solution pH. Comparison to non-referenced SHG measurements obtained from the fused silica/water interface reveals that the ?(3).fo term takes the form of ( ?(3)±i?(3)).fo, and that the interference between the ?(3).fo term and the bulk quartz ?(2) term depends on the rotation angle of a-quartz around the z-axis. This newly identified term, i?(3).fo, which is out of phase from the surface terms, is of bulk origin. The experiment expands the scope of SHG spectroscopy to probe solid/liquid interfaces beyond amorphous and centrosymmetric materials towards crystal classes that lack centrosymmetry. The possibility of internally phase referencing the interfacial SHG response for the interfacial orientation analysis of species or materials in contact with a-quartz are discussed along with the implications for conditions of resonance enhancement.