Direct Evidence for a Surface and Bulk Specific Response in the Sum-Frequency Generation Spectrum of the Water Bend Vibration

Abstract Spectroscopic studies of aqueous surfaces can provide a fundamental understanding of interfacial processes. These studies have largely focused on the OH stretch vibrations of water, which is unfortunate as the bending mode is an attractive feature to probe as it is relatively free of inter- and intramolecular couplings. However, the origin of the response of the bending mode is highly debated and has been assigned to either surface-specific or to bulk effects. Here, we study the bending mode of pure water and charged aqueous surfaces using heterodyne-detected vibrational sum frequency generation spectroscopy (HD-VSFG) to quantify the two effects. Our results show a low - (1626 cm-1) and a high - (1656 cm-1) frequency component which can be unambiguously assigned to an interfacial dipole and a bulk quadrupolar response, respectively. We thus demonstrate that probing the bending mode provides structural and quantitative information on both the surface and the bulk.