Hydrogen bond network modes in liquid water
David P. Shelton
Abstract
Collective modes and dynamics of dense disordered materials such as water are not well understood, but nonlinear optics provides a sensitive probe to study polar modes in these materials. Here we report the hyper-Raman (HRS) light scattering spectrum measured for liquid ${\mathrm{D}}_{2}\mathrm{O}$ decomposed into contributions from transverse and longitudinal dipolar modes and octupolar modes, using the polarization dependence of HRS. Transverse HRS is observed for orientation and stretching modes, while longitudinal HRS is observed for translation, libration, and bending modes. The HRS observations indicate molecular correlation at distances $>200$ nm for all modes of orientation, libration, and vibration. The rocking/wagging, twisting, and translation modes for ${\mathrm{D}}_{2}\mathrm{O}$ molecules in the hydrogen bonded network are distinguished. The LO-TO splitting is $28\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ for the libration mode and $16\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ for the translation mode, and the relaxation time for libration modes is about 80 fs. The long-range correlation of the orientation and stretching modes is explained as the result of the dipole-dipole orientation correlation in a dipolar fluid. The long-range correlation of the longitudinal polarized libration and bending modes needs further study.