Energy relaxation dynamics of hydrogen-bonded OH vibration conjugated with free OH bond at an air/water interface

Vibrational energy relaxation dynamics of the excited hydrogen-bonded (H-bonded) OH conjugated with free OH (OD) at an air/water (for both pure water and isotopically diluted water) interface are elucidated via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulations. The calculated results are compared with those of the excited H-bonded OH in bulk liquid water reported previously. In the case of pure water, the relaxation timescale (vibrational lifetime) of the excited H-bonded OH at the interface is T-1 = 0.13 ps, which is slightly larger than that in the bulk (T-1 = 0.11 ps). Conversely, in the case of isotopically diluted water, the relaxation timescale of T-1 = 0.74 ps in the bulk decreases to T-1 = 0.26 ps at the interface, suggesting that the relaxation dynamics of the H-bonded OH are strongly dependent on the surrounding H-bond environments particularly for the isotopically diluted conditions. The relaxation paths and their rates are estimated by introducing certain constraints on the vibrational modes except for the target path in the NE-AIMD simulation to decompose the total energy relaxation rate into contributions to possible relaxation pathways. It is found that the main relaxation pathway in the case of pure water is due to intermolecular OHMIDLINE HORIZONTAL ELLIPSISOH vibrational coupling, which is similar to the relaxation in the bulk. In the case of isotopically diluted water, the main pathway is due to intramolecular stretch and bend couplings, which show more efficient relaxation than in the bulk because of strong H-bonding interactions specific to the air/water interface.

Automated summary

The article investigates the vibrational energy relaxation dynamics of excited hydrogen-bonded OH at an air/water interface through NE-AIMD simulations, comparing it to the behavior in bulk liquid water. The study shows that the relaxation dynamics of H-bonded OH are highly dependent on the surrounding H-bond environments, especially in isotopically diluted water conditions. In isotopically diluted water, the main relaxation pathway is intramolecular stretch and bend couplings, which are more efficient at the air/water interface due to strong H-bonding interactions.