Origin of the nonresonant signal in sum-frequency vibrational spectroscopy at the water interface

We theoretically investigate the origin of the nonresonant signal in sum-frequency vibrational spectroscopy (SFVS) in interfacial water. The ground and excited states of water molecules are calculated using density functional theory (DFT) and time-dependent DFT (TDDFT). Transition moments, including electric and magnetic dipole as well as electric quadrupole terms, are computed. The SFVS from the electric dipole contribution is evaluated considering molecular orientations from neural network-based molecular dynamics simulations. The results demonstrate that the nonresonant SSP SFVS is dominated by the electric quadrupole contribution, with negligible contributions from electric and magnetic dipole terms. Theoretical predictions align well with experimental SFVS measurements, underscoring the importance of quadrupole effects in interfacial spectroscopy.