Bulk Contributions Modulate the Sum-Frequency Generation Spectra of Water on Model Sea-Spray Aerosols

Vibrational sum-frequency generation (vSFG) spectroscopy is used to determine the molecular structure of water at a model sea-spray aerosol surface. Both measured and calculated spectra display specific features as a result of third-order contributions to the vSFG response, and these are associated with finite interfacial electric potentials. We demonstrate that theoretical modeling enables separation of the third-order contributions, thus allowing for a systematic analysis of the strictly surface-sensitive, second-order component of the vSFG response. This study provides fundamental insights into the interfacial molecular organization and hydrogen-bonding structure of water, which mediate heterogeneous processes on sea-spray aerosols. Our results emphasize the key role that computer simulations can play in interpreting vSFG spectra and revealing microscopic details at complex aqueous interfaces, which can be difficult to extract from experiments because of the mixing of second-order, surface-sensitive, and third-order bulk-dependent contributions to the vSFG response.