Formation of excess dangling OH bonds during crystallization of amorphous solid water

Dangling OH bonds on ice surfaces are thought to play a central role in surface reactions relevant to planetary, interstellar, and prebiotic environments, yet their direct characterization is hindered by the intrinsic fragility of surface hydrogen-bonding networks, particularly under phase transition conditions. Here, using time-resolved in situ infrared reflection-absorption spectroscopy, we track the evolution of dangling OH bonds during the isothermal crystallization of amorphous solid water into ice I. We observe a transient excess of dangling OH bonds that gradually diminishes as the surface hydrogen-bonding networks reorganize, reflecting competition between nucleation-driven crystallization and surface stabilization. These findings provide direct spectroscopic evidence for a metastable surface state formed during amorphous solid water crystallization and uncover a surface-restructuring pathway that may influence the reactivity of icy surfaces.