Anomalous Slowing Down of the Vibrational Relaxation of Liquid Water upon Nanoscale Confinement

We study the vibrational dynamics of nanodroplets of liquid water with femtosecond two-color midinfrared pump-probe spectroscopy. For the smallest nanodroplet, containing 10–15 water molecules, the lifetime $T_1$ of the $\mathrm{O}\mathrm{\text{−}}\mathrm{H}$ stretch vibrations is equal to $0.85\pm 0.1\mathrm{ps}$, which is more than 3 times as long as in bulk liquid water. We find that the truncation of the hydrogen-bond network of water leads to a dramatic change of the relaxation mechanism.