We present an analysis of second-harmonic (SH) response from metal surfaces that is based on a model representation whose associated parameters are defined in terms of nonlinear polarizabilities originating from charge displacement occurring on a molecular level. Our analysis suggests that the general functional dependence of SH response at a metal/electrolyte interface on the time-independent (dc) electric field is typically different from that implied by previous models. The analysis shows that our model representation is sufficiently general for extracting features from second-harmonic generation (SHG) data in order to analyze such data by methods based on system identification and pattern recognition. Our model, which is based on a classical representation of the nonlinear response of bound (or constrained) charge, is found to be consistent with trends observed in experimental measurements of second-harmonic response as a function of surface charge density. The results of this analysis suggest that SH data are relatively rich in terms of features that can be correlated with local surface structure and that the data should be interpreted by means of a more general model representation than has been previously proposed.
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