Computer prediction of second harmonic generation at interfaces: NaCl aqueous solution in contact with (101) quartz surfaces

We present a computational approach for processing classical molecular dynamics (CMD) computer simulations of liquids at solid/liquid interfaces to determine the second-order susceptibility ${\chi }^{\left(2\right)}$ from the hyperpolarizability $\beta$ of individual water molecules parameterized by quantum calculations. We apply the method to microscopically flat surfaces, but the results can also be applied to scattering from spherical particles (second-harmonic scattering, SHS) in colloidal dispersions. Our ${\chi }^{\left(2\right)}$ values, calculated from molecular trajectories of aqueous NaCl solutions in contact with (101) quartz surfaces, demonstrate the effect of the surface charge density (0 to $-0.12$ C/m²) and salt concentration (0 to 0.8 M) on the second-order nonlinear response. Moreover, we decompose the total signal into contributions from layers at different distances from the interface, allowing us to distinguish the surface-specific contribution from that of the diffuse layer. Analysis of axial profiles of structural (density, dipolar orientation) and electrostatic (charge density, electric field, electric potential) properties allows us to link them with the optical response. The method can also be applied to other solvents and studies of the impact of different types of dissolved ions and molecules on the non-resonant SH signals.