Real-Time Tracking of the intramolecular vibrational dynamics of liquid water

Water plays a critical role in chemistry and biochemistry due to its unique properties that arise from its polar nature and the hydrogen-bonding network it forms. Its vibrational motions occur on the few-femtosecond timescale and are pivotal for energy transfer within the hydrogen-bond network. Despite the latter, the real-time observation of these motions has not been achieved so far due to the high time resolution necessary to capture them. Here, we investigate the ground state vibrational dynamics of liquid water initiated by a sub-5 fs near-infrared (NIR) pump pulse via Impulsive Stimulated Raman Scattering (ISRS). By monitoring the transient transmission of few-fs ultraviolet (UV) probe pulses through a 5µm-thick liquid jet, we track in real-time the motion of the coherent vibrational wave packet, which is dominated by the high-frequency OH stretch mode, with a 10-fs oscillation period and a damping time of 25 fs. This reveals the rapid dephasing of the OH stretch mode that precedes its relaxation via coupling to the bend modes, highlighting the importance of intermolecular couplings of liquid water in the high frequency vibrational dynamics.