Anharmonic bend-stretch coupling in neat liquid water

Femtosecond mid-IR spectroscopy is used to study the vibrational relaxation dynamics in neat liquid water. By exciting the bending vibration and probing the stretching mode, it is possible to reliably determine the bending and librational lifetimes of water. The anharmonic coupling between the bending and the stretching degrees of freedom is quantified in terms of a differential absorption cross-section for the fundamental stretching transition carrying one spectating bending quantum. A positive off-diagonal anharmonicity may be caused by the initial excitation of the anharmonic bending mode and is mediated by the hydrogen bonded network.