Interfacial Water Structure studied by Vibrational Sum Frequency Generation

Results The VSFG spectrum of the heavy water-air interface is shown in Figure 2. It contains spectral features originating from specific interfacial water molecules: the ‘freeOD’ from water molecules with an O-D bond protruding from the surface (~2740 cm -1 ), as its frequency is similar to the O-D stretch vibration in gas phase, and two peaks in the H-bonded frequency region (centered around 2400 and 2500 cm -1 , respectively). These two peaks contain details about the interfacial water structure; however their assignment has been much debated. By using isotopic dilution experiment (H 2 O:D 2 O 2:1), we provide a completely new assignment of the double-peak structure of the vibrational spectrum of the interfacial water. Our results reveal that the vibrational response originate from symmetric stretch vibrations split by a Fermi resonance (FR) with the overtone of the water bending mode. [2] This assignment is general for interfacial water, and shows that the interfacial water structure is structurally more homogeneous and simpler than previously thought.