The Origin of the Frequency-Dependence of OH Stretch Vibrational Lifetimes in Liquid Water.

Polarization-resolved infrared pump-stimulated Raman probe spectroscopy was used to measure the lifetimes of OH stretching vibrations in liquid water, giving rise to polarized and depolarized Raman signals. The lifetime of the OH stretch for the polarized Raman was determined to be 200 fs, in excellent agreement with previous measurements. The lifetime of OH stretches contributing to the depolarized Raman signal was determined here for the first time and was found to be at least two times longer (≥400 fs). With the support of mixed quantum-classical line shape calculations, the experimental lifetimes were rationalized in terms of the extent of the coupling between the OH stretching modes as well as their coupling to the overtone of water bending vibrations. OH stretches that participate in an extensive network of coupled vibrations contribute to the polarized Raman spectrum, while mainly uncoupled OH modes give rise to the depolarized Raman signals.