Electrolyte and Temperature Effects on Third-Order Susceptibility in Sum-Frequency Generation Spectroscopy of Aqueous Salt Solutions

Sum-frequency generation (SFG) spectra from charged solid-liquid interfaces include significant contribution from third-order susceptibility χ⁽³⁾, which mainly originates from induced water orientation in the electric double layer. We quantitatively evaluate the χ⁽³⁾ susceptibility by molecular dynamics simulation in aqueous electrolyte solutions with varying concentrations and temperatures. We found that the value of χ⁽³⁾ decreases with increasing concentration or temperature and that the perturbation on χ⁽³⁾ is quite well-correlated with that on the dielectric constant ∈ of the solution. This correlation is understood as both quantities are commonly governed by the response of molecular orientation to the electric field. Accurate evaluation of χ⁽³⁾ in various conditions is important in quantitative estimate of the third-order effect on the SFG spectroscopy, particularly in conditions of high surface charge and ion concentration.