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.