Observation of the Bending Mode of Interfacial Water at Silica Surfaces by Near-Infrared Vibrational Sum-Frequency Generation Spectroscopy of the [Stretch plus Bend] Combination Bands

Vibrational sum-frequency generation (SFG) spectroscopy of interfacial water at mineral/aqueous interfaces is extended to the near-IR range containing the low cross section stretch + bend combination bands (nu(comb) = nu(OH) + delta(HOH)) of liquid water at silica surfaces near 5000-5300 cm(-1), for the first time. The assignments of SFG spectra are supported by FTIR and Raman spectroscopic measurements of the bulk water nu(comb) modes. The SFG spectra contain significant contributions from two combinations, [nu(s) + delta] approximate to 5060 cm(-1) and [nu(as) + delta] approximate to 5300 cm(-1). These measurements provide the first, to our knowledge, reported probe of the bending mode of water at buried interfaces. The data suggest that the interfacial water bending mode is blue-shifted from that of bulk water.