Abstract
From a glass of water to glaciers in Antarctica, water–air and ice–air interfaces are abundant on Earth. Molecular-level structure and dynamics at these interfaces are key for understanding many chemical/physical/atmospheric processes including the slipperiness of ice surfaces, the surface tension of water, and evaporation/sublimation of water. Sum-frequency generation (SFG) spectroscopy is a powerful tool to probe the molecular-level structure of these interfaces because SFG can specifically probe the topmost interfacial water molecules separately from the bulk and is sensitive to molecular conformation. Nevertheless, experimental SFG has several limitations. For example, SFG cannot provide information on the depth of the interface and how the orientation of the molecules varies with distance from the surface. By combining the SFG spectroscopy with simulation techniques, one can directly compare the experimental data with the simulated SFG spectra, allowing us to unveil the molecular-level structure of water–air and ice–air interfaces. Here, we present an overview of the different simulation protocols available for SFG spectra calculations. We systematically compare the SFG spectra computed with different approaches, revealing the advantages and disadvantages of the different methods. Furthermore, we account for the findings through combined SFG experiments and simulations and provide future challenges for SFG experiments and simulations at different aqueous interfaces.
Highlights
Liquid water−air and solid-state water−air interfaces play a critical role in many biological, chemical, physical, and atmospheric processes as well as environmental science
We provide an overview for the simulation protocols for the Sum-frequency generation (SFG) spectra by introducing the SFG principles, an efficient algorithm for response function formalism, simulation settings, various force field models, ab initio molecular dynamics (MD) simulation and the comparison of SFG experiments and simulations
The message to the simulation community is that calculating SFG spectra offers a unique avenue for accessing the accuracy of the current simulation’s methodologies, which are often inaccessible with the standard benchmarks
Summary
Liquid water−air and solid-state water (ice)−air interfaces play a critical role in many biological, chemical, physical, and atmospheric processes as well as environmental science. On the basis of the successfully modeling of the SFG spectra at the water−air interface, the microscopic structure of the ice−air interface has been investigated using combined SFG experiments and simulations This surface has been measured by Shen and co-workers[66,67] as well as Shultz and co-workers,[68−71] but the physical origin of the SFG spectra of the O−H stretch mode and their bulk contribution were not clear.[72] Recently, the SFG spectra of water at the ice−air interface have been computed and compared with experimental data. The direct comparison of the SFG spectra between experiments and simulations provided a rich understanding of the underlying physics of the surface premelting of the ice−air interface.[73,74] These will be discussed.
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