Ken Eisenthal brought what is now referred to as the "Eisenthal chi(3) method". This presentation will present the heterodyned version of this technique. Non-resonant second harmonic generation phase and amplitude measurements obtained from the silica:water interface at varying pH and 0.5 M ionic strength point to the existence of a potential-dependent nonlinear susceptibility term, which we call i*chi(3)X, that is associated with a 90° phase shift. Including this contribution in the traditional chi(3) model for the total effective second-order nonlinear susceptibility produces reasonable point estimates for interfacial potentials and second-order nonlinear susceptibilities when i*chi(3)X=1.5 x i*chi(3)water. The model also provides a demonstrated utility for distinguishing apparent differences in the second-order nonlinear susceptibility when the electrolyte is NaCl vs Na<sub>2</sub>SO<sub>4</sub>, pointing to the possibility of using HD-SHG to investigate ion-specificity in interfacial processes.

The Eisenthal chi(3) method, in phase and amplitude space

ACS Fall 2021

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