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 chi((3)), which mainly originates from induced water orientation in the electric double layer. We quantitatively evaluate the chi((3)) susceptibility by molecular dynamics simulation in aqueous electrolyte solutions with varying concentrations and temperatures. We found that the value of chi((3)) decreases with increasing concentration or temperature and that the perturbation on chi((3)) is quite well-correlated with that on the dielectric constant epsilon 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 chi((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.