Real-time tracking of the intramolecular vibrational dynamics of liquid water

TL;DR

Scientists observed ultrafast vibrations in liquid water using advanced laser techniques, revealing how energy moves through water's hydrogen-bond network.

Contribution

The study provides the first real-time observation of intramolecular vibrational dynamics in liquid water using sub-femtosecond laser pulses.

Findings

The OH stretch mode in water vibrates coherently with a 10 fs oscillation period and 25 fs damping time.

The OH stretch mode rapidly dephases before relaxing through coupling to bending vibrations.

Intermolecular couplings are crucial for high-frequency vibrational dynamics in liquid water.

Abstract

Water’s polarity and hydrogen-bond network give rise to its unique chemical and biochemical behaviour. Its vibrational motions, occurring on a few-femtosecond timescale, govern ultrafast energy transfer within the hydrogen-bond network. However, direct real-time observation of these motions has remained elusive due to the extreme temporal resolution required. 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). Using few-fs ultraviolet (UV) probe pulses transmitted through a 5 µm-thick liquid jet, we monitor the coherent vibrational wave packet dominated by the OH stretch mode, exhibiting a 10 fs oscillation period and a 25 fs damping time. These results reveal the rapid dephasing of the OH stretch mode preceding its relaxation through coupling to the bending vibrations, highlighting the importance of intermolecular couplings of liquid water in the high frequency vibrational dynamics.

Water vibrational motion, which occurs on the few-femtosecond timescale and underpins energy transfer within the hydrogen bonding network, has remained challenging to observe in real time due to constraints in time resolution. Here, the authors investigate the ground state vibrational dynamics of liquid water using a sub-5 fs near-infrared pump pulse and few-fs ultraviolet probe pulses, observing rapid dephasing of the OH stretch mode that precedes its relaxation via coupling to the bend modes.

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