SURFACE MELTING OF ICE I(H) SINGLE-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 I(h) single crystals. The temperature d ependence of the evanescent Bragg scattering upon heating reveals a qu asiliquid surface layer well below the melting point on each investiga ted ice surface. At [10.0] and [11.0] surfaces, thermal faceting is ob served, which is briefly discussed. The ''oxygen-forbidden'' (00.4) Br agg profiles which give direct access to hydrogen long-range order hav e been investigated in the bulk and close to the surface. Although in the bulk the (00.4) width is resolution limited, we discovered prior t o surface melting a strong rotational disorder of the hydrogen bonds w ithin a mesoscopic surface sheet.