Charge-Induced χ(3) Susceptibility in Interfacial Nonlinear Optical Spectroscopy Beyond the Bulk Aqueous Contributions: The Case for Silica/Water Interface

The electric field induced (EFI) bulk third-order nonlinear susceptibility (chi((3))) contribution to the second harmonic generation (SHG) signal from charged interfaces was discovered and applied to study the interfacial chemistry of various charged interfaces three decades ago. For both the buried fused silica/water interface and the exposed charged monolayer covered air/water interface, such bulk chi((3)) contribution was all attributed to the chi((3)) term of the polarized water molecules near the charged interfaces. The puzzling experimental observation of the more than one-order of magnitude difference of the EFISHG intensity between the fully charged silica/water interface and the charged molecular covered air/water interface was generally overlooked in the EFISHG literature. Nevertheless, this significant signal difference suggests additional source for the chi((3)) contribution at the fully charged silica/water interface other than the polarized water molecules as in the case of charged monolayer covered air/water interface. In this report, we re-examine the treatment of the chi((3)) mechanism at the charged silica/water interface by including the contributions from the bulk silica using proper boundary condition and image charge distributions for the change screening effects inside bulk silica phase. We show that the chi((3)) contribution from the bulk silica is in similar form as that of the aqueous bulk phase, and it is with more than 1 order of magnitude and with opposite sign. The treatment reported here can be extended to other charged interfaces.

Automated summary

The study identified the bulk third-order nonlinear susceptibility contribution to second harmonic generation signal from charged interfaces and its application in studying interfacial chemistry. Differences in the source of the bulk χ((3)) contribution were observed between fully charged interfaces and charged molecular covered interfaces, indicating distinct mechanisms. The significant signal differences between the two types of charged interfaces were previously overlooked.