Unraveling the Anisotropy of Intermolecular Interactions of Water at the Liquid–Vapor Interface with Terahertz Sum Frequency Generation Spectroscopy Using a Polarizable Force Field

Anisotropic intermolecular interactions of water at the liquid-vapor interface are investigated through calculations of the terahertz (THz) sum frequency generation (SFG) spectroscopy using a polarizable model of water. The spectral calculations are performed at three different temperatures of 280, 300, and 320 K. Our findings reveal that intermolecular interactions of interfacial water exhibit liquid-like spectral signatures rather than ice-like. The observed broadening of the librational mode of interfacial water is attributed to the heterogeneity of the hydrogen-bond network at the interface, as is evident from the self- and cross-correlation contributions to the SFG spectrum. The total SFG spectrum is dissected into contributions from permanent and induced dipole moments, which exhibit oppositely oriented peaks, showing opposite orientations of the permanent and induced dipoles along the hydrogen bonds at the air-water interface. A detailed analysis of the slab-wise dissected spectra shows that the interactions among the first three interfacial water layers from the vapor side make essentially all of the contributions to the total THz-SFG spectrum of an air-water interface.