Perspective on sum frequency generation spectroscopy of ice surfaces and interfaces

This perspective is focused on surfaces and interfaces of crystalline ice studied with sum frequency generation (SFG) spectroscopy by several research groups within these twenty years. The intrinsic interface selectivity of SFG has already enabled one to determine structural aspects of the ice surfaces and interfaces through vibrational signatures of the OH stretch. Here we describe why SFG is suitable for ice surface science and how SFG has revealed the structure and dynamics of dangling bonds, quasi-liquid layer, hydrogen-bond network, and ordered protons at the ice surfaces, ice/substrate interfaces, and ice films on metal surfaces. In particular, we compare SFG spectra of ice reported by different groups to examine consistency between their experiments. We elaborate current controversies on a few SFG studies of ice before presenting future outlook for ice surface science by SFG. Additionally in the appendix we briefly explain some technical issues that may help us resolve conflicts and make progress in the right way.