On the Assignment of the Vibrational Spectrum of the Water Bend at the Air/Water Interface

We previously reported the spectrum of the water bend vibrational mode (ν₂) at the air/water interface measured using sum-frequency generation (SFG). Here, we present experimental evidence to aid the assignment of the ν₂ spectral features to H-bonded classes of interfacial water, which is in general agreement with two recent independently published theoretical studies. The dispersive line shape shows an apparent frequency shift between SSP and PPP polarization combinations (SFG–​visible–​infrared). This is naturally explained as an interference effect between the negative (1630 cm^–1) and positive (1662 cm^–1) peaks corresponding to “free–OH” and “H-bonded” species, respectively, which have different orientations and thus different amplitudes in SSP and PPP spectra. A surfactant monolayer of sodium dodecyl sulfate (SDS) was used to suppress the free OH species at the surface, and the corresponding SFG spectral changes indicate that these water molecules with one of the hydrogens pointing up into the air phase contribute to the negative peak at 1630 cm<sup>–1