Title:Oral: First principles study of the molecular-level structure and dynamics of the air-ice interface using sum frequency generation spectroscopy

The surface of ice is characterized by a disordered pre-melted layer known as the quasi-liquid layer (QLL). The unique molecular structure of the QLL is responsible for ice surfaces acting as catalysts for many atmospheric chemistry reactions in various environments. Understanding the molecular-level structure and dynamics at this interface is crucial for deciphering various chemical, physical, and atmospheric processes. Sum-frequency generation (SFG) spectroscopy is an important tool for probing the molecular-level structure of the air-ice interface as it is a surface specific technique. However, there are several hindrances in the interpretation of SFG spectra. Notably, while SFG is surface sensitive, it is not known what is the depth of the interfacial region that contributes to the spectra, as well as the molecular contributions as a function of their orientation and depth beneath the surface. Here, we compute SFG spectra from molecular dynamics simulations that provide well-converged OH stretching, bending and librational bands of interfacial water at the air-ice interface, utilizing a machine learning potential trained on ab-initio data. Simulating proton-ordered systems we devise a direct connection between local molecular structures and specific features in the SFG spectrum. Such a detailed molecular interpretation of SFG spectra reveals novel insights into the unique hydrogen bonding environment at the surface, microscopic proton arrangement, and surface facet orientation.