  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 nonlinear susceptibility term, which we call
chi(3)X, that is associated with a 90 deg phase shift. Including this
contribution in a model for the total effective second-order nonlinear
susceptibility produces reasonable point estimates for interfacial potentials
and second-order nonlinear susceptibilities when chi(3)Xis about 1.5 times
chi(3)water. A model without this term and containing only traditional chi(2)
and chi(3) terms cannot recapitulate the experimental data. The new model also
provides a demonstrated utility for distinguishing apparent differences in the
second-order nonlinear susceptibility when the electrolyte is NaCl vs MgSO4,
pointing to the possibility of using HD-SHG to investigate ion-specificity in
interfacial processes.
