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

Abstract

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.