We present a general approach for establishing correlations between the optical second-harmonic (SH) response generated from a metal/electrolyte interface and the interface structure. Our approach entails the construction of a response function for optical second-harmonic generation (SHG) from the metal surface in the presence of an electrolyte and an applied electrochemical field. The response function approach, a powerful and general method, is developed here for the first time for SHG data. Here, the response function describes the nonlinear optical response of a mesoscopic region of the surface to an applied static mesoscopic electric field and is a characterization of how the electrostatic nature of the surface responds to changes in the concentration and composition of the electrolyte. We construct the response function from experimental measurements of the SH response and from models representing known interface structure. A significant aspect of our approach is that it combines, through the modification of the response function, existing models of metal-interface structure with models for mechanisms of SHG response. Our approach provides, therefore, a framework for correlating existing and emerging models of the double layer with optical experimental measurements. Case study analyses of prototype interface systems are presented here, demonstrating applications of our approach.
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