Unveiling subsurface hydrogen-bond structure of hexagonal water ice

The phase-resolved sum-frequency-generation (SFG) spectrum for the basal face of hexagonal ice is reported and is interpreted by molecular dynamics simulations combined with ab initio quantum calculations. Here, we demonstrate that the line shape of the SFG spectra of isotope-diluted OH chromophores is a sensitive indicator of structural rumpling uniquely emerging at the subsurface of hexagonal ice. In the outermost subsurface between the first (B1) and second (B2) bilayer, the hydrogen bond of ${\mathrm{O}}_{\mathrm{B}1}\ensuremath{-}\mathrm{H}\ensuremath{\cdots}{\mathrm{O}}_{\mathrm{B}2}$ is weaker than that of ${\mathrm{O}}_{\mathrm{B}1}\ensuremath{\cdots}\mathrm{H}\ensuremath{-}{\mathrm{O}}_{\mathrm{B}2}$ . This implies that subsurface O-O distance is laterally altered, depending on the direction of O-H bond along the surface normal: H-up or H-down, which is in stark contrast to bulk hydrogen bonds. This new finding uncovers how water molecules undercoordinated at the topmost surface influence on the subsurface structural rumpling associated with orientational frustration inherent in water ice.