A New Imaginary Term in the Second-Order Nonlinear Susceptibility from Charged Interfaces

Nonresonant second harmonic generation (SHG) phase and amplitude measurements obtained from the silica-water interface at varying pH values and an ionic strength of 0.5 M point to the existence of a nonlinear susceptibility term, which we call chi((3))(X) that is associated with a 90 degrees 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))(X) approximate to 1-5 chi((3))(wat)(er). 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 versus MgSO4, pointing to the possibility of using heterodyne-detected SHG to investigate ion specificity in interfacial processes.

Automated summary

Research indicates the presence of a nonlinearity susceptibility term chi((3))(X) associated with a 90 degrees phase shift at the silica-water interface. Including this term in the model allows for reasonable estimates of interfacial potentials and second-order nonlinear susceptibilities. The new model demonstrates utility in distinguishing differences in second-order nonlinear susceptibility when different electrolytes are used, suggesting potential for investigating ion specificity in interfacial processes using heterodyne-detected SHG.