Dissecting the Molecular Structure of the Air/Ice Interface from Quantum Simulations of the Sum-Frequency Generation Spectrum

Ice interfaces are pivotal in mediating key chemical and physical processes, such as heterogeneous chemical reactions in the environment, ice nucleation, and cloud micro- physics. At the ice surface, water molecules form a quasi-liquid layer (QLL) with distinct properties from the bulk. Despite numerous experimental and theoretical studies, the molecular-level understanding of this layer has remained elusive. In this work, we use state-of-the-art quantum dynamics simulations with a realistic data-driven many- body potential to dissect the vibrational sum-frequency generation (vSFG) spectrum in terms of contributions arising from individual molecular layers, orientations, and hydrogen-bonding topologies that determine the QLL properties. The agreement be- tween experimental and simulated spectra provides a realistic view of the evolution of the QLL as a function of temperature without the need for empirical adjustments. At lower temperatures, the emergence of specific features in the vSFG spectrum points to surface restructuring, resulting in mixed ice Ih and ice Ic nanodomains at the surface, which may vary depending on how the air/ice interface is prepared. This work not only underscores the critical role of many-body interactions and nuclear quantum effects in understanding ice surfaces but also provides a definitive molecular-level picture of the QLL, which plays a central role in multiphase and heterogeneous processes of relevance to a range of fields, including atmospheric chemistry, cryopreservation, and materials science.