Vibrational relaxations in liquid water observed by mid-infrared ultrafast spectroscopy

Nonlinear vibrational spectroscopy by use of mid-infrared femtosecond pulses enables the observation of ultrafast vibrational/structural dynamics of molecular systems. This article presents the recent experimental studies on vibrational energy relaxations and energy transfer in liquid water. In pure liquid water, vibrational energy relaxation of OH-stretching and OH-bending excitation occurs at about 200fs and 170fs, respectively. When vibrational energy from individually excited water molecules is transferred to intermolecular modes, a sub-100fs nuclear rearrangement occurs, leaving the local hydrogen bonds weakened but unbroken. Subsequent energy delocalization over many molecules proceeds with the breaking of hydrogen bonds on an 1-ps time scales. The observed vibrational/structral response should be relevant for chemical reactions and structural changes in aqueous environments.