jz6b02678_si_liveslides.zip (5.4 MB)
On the Assignment of the Vibrational Spectrum of the Water Bend at the Air/Water Interface
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posted on 2017-02-08, 20:35 authored by Chayan Dutta, Alexander V. BenderskiiWe 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–1.