Ultrafast energy relaxation and anisotropy decay of the librational motion in liquid water: A molecular dynamics study

We theoretically investigate intermolecular motions in liquid water in terms of third-order infrared (IR) spectroscopy. We calculate two-dimensional (2D) IR spectra, pump-probe signals, and three-pulse stimulated photon echo signals from the combination of equilibrium and nonequilibrium molecular dynamics simulations. The 2D IR spectra and the three-pulse photon echo peak shift exhibit that the frequency correlation of the librational motion decays with a time scale of 100 fs. The two-color 2D IR spectra and the pump-probe signals reveal that the energy transfer from the librational motion at 700 cm^-1 to the low frequency motion below 300 cm^-1 occurs with a time scale of 60 fs and the subsequent relaxation to the hot ground state takes place on a 500 fs time scale. The time scale of the anisotropy decay of the librational motion is found to be ∼115 fs. The energy dissipation processes are investigated in detail by using the nonequilibrium molecular dynamics simulation, in which an electric field pulse is applied. We show that the fast energy transfer from the librational motion to the low frequency motion is mainly due to the librational-librational energy transfer. We also show that the fast anisotropy decay mainly arises from the rapid intermolecular energy transfer.