Second-Harmonic Generation Provides Insight into the Screening Response of the Liquid Water Interface

We use second-harmonic generation (SHG), molecular dynamicssimulation,and theoretical modeling to study the response of the neat liquidwater-air interface to changes in the potential of an externalelectrode positioned near the liquid but out of direct contact. Weobserve a parabolic dependence of second harmonic intensity on theapplied potential. Based on standard theory, we associate this dependencewith the response of the diffuse layer water molecules to changesin the interfacial potential profile. Taking the literature valuefor this response leads to the unexpected conclusion that the electricfields within the diffuse layer are opposite in sign from those originatingfrom the electrodes. This conclusion implies that the traditionalcontinuum-based models of interfacial screening lack the complexitynecessary to properly describe the potential profile of the liquidwater-vapor interface. Effects such as overscreening in thetopmost interfacial layer and extended correlations in the interfacialhydrogen-bonding network may play a role in governing the responseof the water interface to external fields.

Automated summary

We conducted a study on the response of the neat liquid water-air interface to changes in an external electrode's potential using second-harmonic generation (SHG), molecular dynamics simulation, and theoretical modeling. The results show a parabolic relationship between second harmonic intensity and the applied potential. Our findings suggest that traditional continuum-based models of interfacial screening are not sufficient to describe the potential profile of the liquid water-vapor interface, as the electric fields within the diffuse layer were found to be opposite in sign. Effects such as overscreening in the topmost interfacial layer and extended correlations in the interfacial hydrogen-bonding network may be crucial in determining the water interface's response to external fields.