Observation of the Bending
Mode of Interfacial Water
at Silica Surfaces by Near-Infrared Vibrational Sum-Frequency Generation
Spectroscopy of the [Stretch + Bend] Combination Bands
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Abstract
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
(ν<sub>comb</sub> = ν<sub>OH</sub> + δ<sub>HOH</sub>) of liquid water at silica surfaces near 5000–5300 cm<sup>–1</sup>, for the first time. The assignments of SFG spectra
are supported by FTIR and Raman spectroscopic measurements of the
bulk water ν<sub>comb</sub> modes. The SFG spectra contain significant
contributions from two combinations, [ν<sub>s</sub> + δ]
≈ 5060 cm<sup>–1</sup> and [ν<sub>as</sub> + δ]
≈ 5300 cm<sup>–1</sup>. 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
