Surface melting of iceIhsingle crystals revealed by glancing angle x-ray scattering

We present glancing angle x-ray scattering experiments at [00.1], [10.0], and [11.0] surfaces of ice ${\mathrm{I}}_{\mathit{h}}$ single crystals. The temperature dependence of the evanescent Bragg scattering upon heating reveals a quasiliquid surface layer well below the melting point of each investigated ice surface. At [10.0] and [11.0] surfaces, thermal faceting is observed, which is briefly discussed. The ‘‘oxygen-forbidden’’ (00.4) Bragg profiles which give direct access to hydrogen long-range order have been investigated in the bulk and close to the surface. Although in the bulk the (00.4) width is resolution limited, we discovered prior to surface melting a strong rotational disorder of the hydrogen bonds within a mesoscopic surface sheet.