Molecular Dynamics Simulations of SFG Librational Modes Spectra of Water at the Water-Air Interface

At the water-air interface, the hydrogen-bond network of water molecules is interrupted, and accordingly, the structure and dynamics of the interfacial water molecules are altered considerably compared with the bulk. Such interfacial water molecules have been studied by surface-specific vibrational sum-frequency generation (SFG) spectroscopy probing high-frequency O-H stretch and H-O-H bending modes. In contrast, the low-frequency librational mode has been much less Studied with SFG. Because this mode is sensitive to the hydrogen-bond connectivity, understanding the librational mode of the interfacial water is crucial for unveiling a microscopic view of the interfacial water. Here, we compute the SFG librational mode spectra at the water-air interface by using molecular dynamics simulation. We show that the modeling of the polarizability has a drastic effect on the simulated librational mode spectra, whereas the spectra are less sensitive to the force field models and the modeling of the dipole moment. The simulated librational spectra display a peak centered at similar to 700 cm(-1), which is close to the infrared peak frequency of the liquid water librational mode, of 670 cm(-1). This indicates that the librational mode of the interfacial water at the water-air interface closely resembles that of bulk liquid water.