Mechanisms of free O–H relaxation at the air–water interface revealed by machine learning-accelerated molecular dynamics simulation

Vibrational sum-frequency generation (VSFG) spectroscopy has been widely used to investigate the unique vibrational relaxation dynamics of free O-H groups at the air-water interface. However, there has been ongoing debate regarding the primary relaxation mechanisms-specifically, intramolecular energy transfer (IET) and reorientation (REOR)-and which mechanism plays a dominant role. To explore this issue, we examine the IET and REOR processes of free O-H groups at the air-water interface using machine learning-accelerated molecular dynamics (MD) simulations based on a deep potential (DP) model. From the resulting DP-based MD simulations, we calculate the SFG and vibrational density of states (VDOS) spectra. These spectra consistently show a broad hydrogen-bonded O-H band in the range of 3000-3600 cm-1 and a sharp free O-H peak around 3700 cm-1. Our simulations reveal an intramolecular energy-transfer timescale of ∼800 fs and a reorientational relaxation time of about 900 fs for interfacial free O-H groups, consistent with both experimental and QM/MM simulation results. These findings suggest that both relaxation mechanisms (IET and REOR) contribute comparably to energy dissipation at the air-water interface. Meanwhile, our study reveals that the nuclear quantum effect significantly accelerates both the IET and REOR processes, and the extent of acceleration depends on the specific hydrogen-bonding definition.