Interfacial melting of thin ice films: An infrared study

Interfacial melting of ice has been examined by infrared spectroscopy for the first time. Thin ice films, from 10 to 20 nm thick, were prepared on a face of a germanium prism and studied over a range of temperatures just below the triple point. Interrogation was by attenuated total reflection (ATR) spectroscopy. Interfacial melting regions were distinguished from the underlying ice by comparisons of their spectra with the well established infrared signatures of bulk water and ice. Near the triple point, e.g., −0.15 °C, the spectroscopic profile of the surface melting region is indistinguishable from that of liquid water. This is compelling evidence that the commonly labeled quasi-liquid layer is indeed like liquid water. The extent of infrared extinction from ice films was used to determine the thickness of the quasi-liquid layers. At −0.03 °C the thickness is 15 nm corresponding to 40 monolayers of liquid but at −10 °C less than a monolayer remains. We compare our measurements of surface melting with those of others, sometimes finding discrepancies in thickness by two orders of magnitude. The promise of infrared spectroscopy to the further study of interfacial melting of ice is discussed.