Ice Nucleation at the Water-Sapphire Interface: Transient Sum-Frequency Response without Evidence for Transient Ice Phase.

Heterogeneous ice nucleation at the water-sapphire interface is studied using sum-frequency generation spectroscopy. We follow the response of the O-H stretch mode of interfacial water during ice nucleation as a function of time and temperature. The ice and liquid states each exhibit very distinct, largely temperature-independent responses. However, at the moment of freezing, a transient response with a significantly different intensity is observed, with a lifetime between several seconds and several minutes. The presence of this transient signal has previously been attributed to a transient phase of ice. Here, we demonstrate that the transient signal can be explained without invoking a transient ice phase, as the transient signal can simply be accounted for by a linear combination of time-dependent liquid and ice responses.