Theoretical investigation of sum-frequency generation spectroscopy at the air/water interface

Water covers approximately 71% of the Earth's surface, rendering water interfaces critically important. The physicochemical processes occurring at these interfaces play indispensable roles in green energy, atmospheric science, materials science, life sciences, and chemical reactions. The complex hydrogen bonding among water molecules is a focus and challenge in research, impacting interfacial structure and dynamics. Sum-frequency generation (SFG) spectroscopy is a powerful nonlinear method for studying the orientation, structure, interactions, and dynamics of various materials at interfaces, offering sensitivity, selectivity, and nondestructive in situ detection. By integrating classical, many-body, ab initio, and neural network-based molecular dynamics (MD) simulations with various methods for calculating hyperpolarizabilities, researchers have overcome significant computational hurdles to elucidate SFG spectroscopy at the water interface. A central finding is that the accurate spectral interpretation necessitates moving beyond the electric dipole approximation and the Condon limit. While the electric dipole interaction dominates the OH stretching signal, the electric quadrupole mechanism is critical for explaining the absence of a resonance at 3200 cm−1 in the low-frequency stretching region, the spectral signatures of the bending and libration modes, the strong non-resonant background, and the controversial shoulder peak near 3600 cm−1. Furthermore, nuclear quantum effects, such as zero-point energy and tunneling, alongside Fermi resonance between the stretching fundamental and the bending overtone, modulate spectral line shapes and amplitude. Future progress hinges on leveraging machine learning, which promises to bridge the gap between computational cost and accuracy in both MD simulations and hyperpolarizability calculations. Ultimately, refining these theoretical models is essential for advancing applications where interfacial water plays a pivotal role.