Ultrafast vibrational dynamics of interfacial water
We report investigations of the vibrational dynamics of water molecules at the waterÂ–air and at the waterÂ–lipid interface. Following vibrational excitation with an intense femtosecond infrared pulse resonant with the OÂ–H stretch vibration of water, we follow the subsequent relaxation processes using the surface-specific spectroscopic technique of sum frequency generation. This allows us to selectively follow the vibrational relaxation of the approximately one monolayer of water molecules at the interface. Although the surface vibrational spectra of water at the interface with air and lipids are very similar, we find dramatic variations in both the rates and mechanisms of vibrational relaxation. For water at the waterÂ–air interface, very rapid exchange of vibrational energy occurs with water molecules in the bulk, and this intermolecular energy transfer process dominates the response. For membrane-bound water at the lipid interface, intermolecular energy transfer is suppressed, and intramolecular relaxation dominates. The difference in relaxation mechanism can be understood from differences in the local environments experienced by the interfacial water molecules in the two different systems.