Depth dependant dielectric constant and second order response at aqueous interfaces
Date
2024
Authors
Yang, Peter
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Abstract
Aqueous interfacial environments have unique properties as a result of their inherent anisotropy; however, analysis of interfacial regions remains challenging. Sum-frequency generation measurements can be used for their study, although determining the linear-optical properties, and the molecular electronic structure required for quantitative analysis remains difficult. From classical molecular dynamics simulations we found the orientation distribution to be invariant with increasing surface number density; we can also therefore expect the ordering of cyanophenol to be a result of interfacial water. We describe the necessity of depth dependence to the sum-frequency response by using interfacial structure from classical molecular dynamics. By exploiting symmetry, we can derive linear relationships between elements of the $\chi^{(2)}$ tensor which shed light on future sum-frequency based orientational measurements. The range of possible values for the interfacial hyperpolarizability ratio has been significantly narrowed with the aid of classical molecular dynamics simulations. We also propose a technique for experimentally measuring the hyperpolarizability ratio that has been derived by exploiting the polarisation null angle method. By using Raman spectra derived from interfacial and bulk-phase \textit{ab initio} simulations we conclude there to be little difference in the hyperpolarizability ratio in the interfacial or bulk environments; by comparing with experimental Raman measurements we also report the existence of asymmetric broadening in the C--N mode.
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Keywords
optics, chemistry