Long-Lived Interfacial Vibrations of Water

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Abstract
We have observed long-lived OH-stretch (νOH) excitations (v = 1) in water during ultrafast laser ablation by a mid-infrared pulse tuned to the νOH absorption maximum. The spectrum of excitations is measured using incoherent anti-Stokes Raman spectroscopy. Relative to the equilibrium water spectrum, these excitations evidence a narrowed (100 cm-1 fwhm) and blue-shifted (3600 cm-1 peak) transition. The excited-state lifetime is T1 > 200 ps, compared to 0.2 ps in bulk water. In the early stages of the ablation process, the water mean density decreases rapidly, which breaks up the hydrogen bonding. The long-lived species is attributed to νOH excitations on water molecules associated with interfaces, having broken hydrogen bonds which cannot be rapidly reformed as in the liquid state.