Energy relaxation pathways and their isotope effects of water bending mode in liquid phase: A nonequilibrium ab initio molecular dynamics simulation study

Abstract

Vibrational energy relaxation pathways and the rates of excited bending mode of water molecules (H2O) in bulk liquid water (H2O), and the rates of the excited bending mode of HOD molecules in D2O (isotopically diluted water) were examined via non-equilibrium ab initio molecular dynamics (NE-AIMD) simulation. The NE-AIMD simulation showed a significantly fast relaxation of the bend vibration, at the timescale of 127 fs in pure water and 144 fs in isotopically diluted water. The main relaxation pathway for pure and isotopically diluted water was found to be driven by intramolecular bend–libration coupling (84% in pure water and 96% in isotopically diluted water). In the case of pure water, intermolecular coupling between bend–bend vibrations was small but non-negligible (11%), whereas such intermolecular coupling was negligible in isotopically diluted water owing to a large frequency mismatch between the excited bend vibration and the surrounding bend vibrations.