Ellipsometric study of the transition layer on the surface of an ice crystal

Abstract

The purpose of this work is to examine the properties of the transition layer (quasi-liquid layer, QLL) on the surface of an ice crystal at temperatures just below its bulk melting point (0°C) using ellipsometry. The refractive index n 1 and thickness d of the transition layer were measured on both basal {0001} and prismatic 1 0 1 1 ¯ faces of ice crystals. The ice surface sample used was a slice section of a negative crystal, which is a hole growing to the shape of a sharp hexagonal prism in an ice single crystal. The surface prepared in this way was molecularly smooth and free from contaminations. Transition layers were detected on the {0001} and 1 0 1 1 ¯ faces at the condition of equilibrium vapor pressure. The measured n 1 was 1.330 for both faces, which is quite close to the refractive index of bulk water at 0°C ( n w = 1.3327) rather than that of ice ( n 1 = 1.3079). Consequently, the transition layer should be water-like (namely, QLL), and this result gives direct evidence of surface melting. The critical temperatures ( T w ) at which the QLL was detected on the surface were −2°C for the {0001} face and −2 to −4°C for the 1 0 1 1 ¯ face. The thickness of the layer steeply increases as the temperature approached the melting point. However, the temperature dependences of the layer thickness showed a systematic difference between the {0001} and 1 0 1 1 ¯ faces. That is, T w (0001)> T w 1 0 1 1 ¯ and d (0001)< d 1 0 1 1 ¯ for a given temperature above −4°C. These characteristics were qualitatively explained by theoretical arguments on the basis of the thermodynamics of the surface. Finally, it was indicated that the structure of the interface between the QLL and the ice crystal on the 1 0 1 1 ¯ face changes from smooth to rough at a temperature of −2°C, and that the roughening transition temperature ( T r ) at which the facets disappear from the growing negative crystals or snow crystals is higher than T w .