GLANCING-ANGLE X-RAY-SCATTERING STUDIES OF THE PREMELTING OF ICE SURFACES

The onset of surface melting at {00.1} (''basal''), {10.0} and {11.0} (''nonbasal'') surfaces of hexagonal ice is studied experimentally by glancing-angle X-ray scattering. The preparation of mirror-like single crystal ice surfaces and a special in situ X-ray chamber is described in detail. The X-ray scattering experiments using synchrotron radiati on give clear evidence that all the investigated high-symmetry surface s exhibit surface melting with onset temperatures T-s congruent to -13 .5 degrees C for the basal and T-s congruent to -12.5 degrees C for th e nonbasal surfaces. The temperature dependence of the thickness of th e quasiliquid is presented and discussed in view of various theoretica l predictions. In addition we found thermal faceting at nonbasal surfa ces as well as a diffuse (''rough'') solid-quasiliquid interface. The thermal properties of the observed surface disorder implies that the c orrelation length within the socalled ''surface melt'' is much larger than the correlation length in the bulk liquid indicating the presence of significant ''substrate'' interactions.