Four‐photon coherent spectroscopy of orientational motion of H2O molecules in liquid water

The unexpectedly narrow structure of Rayleigh wing spectra from 5 to 50 cm−1 was detected using four‐photon coherent laser spectroscopy. The observed features were compared with the IR water vapor absorption (from the HITRAN database) and with the spontaneous Raman spectrum in the vapor phase. The fairly good resemblance between our recorded four‐photon spectrum and the pure rotational spectra of H2O molecules in the gas phase allows the suggestion that a discernible proportion of water molecules may exhibit virtually free rotation in the liquid phase.