  Interface-specific hydrogen- (H-)bonding network of water next to a substrate
(including air) directly controls the energy transfer and chemical reaction
pathway at many charged aqueous interfaces. Yet, experimental characterization
of such bonded water layer structure is still a challenge due to the presence
of the ion diffuse layer. We now develop a sum-frequency (SF) spectroscopic
scheme with varying photon momentums as an all-optic solution for retrieving
the vibrational spectra of the bonded water layer and the diffuse layer, and
hence microscopic structural and charging information about an interface.
Application of the method to a charged surfactant-water interface reveals a
hidden weakly-donor-H-bonded water species, suggesting an asymmetric
hydration-shell structure of fully solvated surfactant headgroups. In another
application to a zwitterionic phosphatidylcholine (PC) lipid monolayer-water
interface, we find a highly polarized bonded water layer structure associating
to the PC headgroup, while the diffuse layer contribution is experimentally
proven to be negligible. Our all-optic method offers not only an in situ
microscopic probe of the electrochemical and biological interfaces, but also a
new opportunity for promoting these researches toward high spatial and temporal
resolutions.
