Effect of Electric Fields on the Ultrafast Vibrational Relaxation of Water at a Charged Solid-Liquid Interface as Probed by Vibrational Sum Frequency Generation

The effect of the surface electric field on the ultrafast vibrational dynamics of interfacial water is studied using IR pump-sum frequency generation (SFG) probe spectroscopy. At very low salt concentrations, a vibrational lifetime (T-1) of similar to 200 fs, similar to bulk H2O, is observed for the O-H stretch at the H2O/silica interface at pH 6, where the silica surface is negatively charged. However, T-1 increases to similar to 700 fs by increasing the NaCl concentration to 0.01 M. The observation of similar dynamics for a range of salt concentrations, associated with different extensions of the electric field, suggest that the surface electric field is screened faster than predicted by classical electrical double-layer theories and that the Debye length may not be the appropriate measure of the depth sampling of the SFG response. An interfacial excess of cations is hypothesized to explain the faster decay of the static electric field than predicted by the Gouy-Chapman theory.