Probing ice-nucleation processes on the molecular level using second harmonic generation spectroscopy

[摘要] We present and characterize a novel setup to apply second harmonicgeneration (SHG) spectroscopy in total internal reflection geometry (TIR) toheterogeneous freezing research. It allows to monitor the evolution of waterstructuring at solid surfaces at low temperatures prior to heterogeneous icenucleation. Apart from the possibility of investigating temperaturedependence, a major novelty in our setup is the ability of measuringsheet-like samples in TIR geometry in a direct way. As a main experimentalresult, we find that our method can discriminate between good and poor icenucleating surfaces. While at the sapphire basal plane, which is known to bea poor ice nucleator, no structural rearrangement of the water molecules isfound prior to freezing, the basal plane surface of mica, an analogue to iceactive mineral dust surfaces, exhibits a strong change in the nonlinearoptical properties at temperatures well above the freezing transition. Thisis interpreted as a pre-activation, i.e. an increase in the local orderingof the interfacial water which is expected to facilitate the crystallizationof ice at the surface. The results are in line with recent predictions bymolecular dynamics simulations on a similar system.

[学科分类] 几何与拓扑