Vibrational Sum-Frequency Spectrum of the Water Bend at the Air/Water Interface

We present the spectrum of the water bend vibrational mode (nu(2)) at the air/ water interface measured using vibrational sum-frequency generation (SFG). The blue-shift of the nu(2) frequency from the gas phase value reports on the hydrogen bonding in the interfacial region. The nu(2) line shape of surface water is inhomogeneously broadened and structured. The dominant feature is the least blue-shifted and relatively narrow Lorenztian, tentatively assigned to water molecules straddling the interface, those with free OH bonds. This feature appears at different frequencies in the SFG spectra recorded using different polarization combinations (SSP and PPP for SFG-visible-IR), pointing to possible orientational inhomogeneity. Weaker features are observed at higher frequencies, tentatively assigned to fully H-bonded water molecules. Small but measurable changes of the line shape with temperature are observed in the 0 to +20 degrees C range.