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The libration spectrum of liquid H2O is resolved into an octupolar twisting libration band at 485 cm−1 and dipolar rocking–wagging libration bands at 707 and 743 cm−1 using polarization analysis of the hyper-Raman scattering (HRS) spectrum. Dipole interactions and orientation correlation over distances less than 2 nm account for the 36 cm−1 splitting of the longitudinal and transverse polarized bands of the dipolar rocking–wagging libration mode, while the intensity difference observed for the bands is the result of libration correlation over distances larger than 200 nm. The coupled rock and wag libration in water is similar to libration modes in ice. The libration relaxation time determined from the width of the spectrum is 36–54 fs. Polarization analysis of the HRS spectrum also shows long range correlation for molecular orientation and hindered translation, bending and stretching vibrations in water. 

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 D2O 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 D2O molecules in the hydrogen bonded network are distinguished. The LO-TO splitting is 28 cm−1 for the libration mode and 16 cm−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. 

The polarization dependence of hyper-Rayleigh second harmonic light scattering (SHS) and hyper-Raman light scattering (HRS) measured for liquid CDCl3 show the effect of long-range correlation of molecular orientation and vibration. HRS from the ν1, ν4, and ν5 vibration modes is polarized transverse to the scattering wavevector, whereas HRS from the ν2, ν3, and ν6 vibration modes and SHS from the ν0 orientation mode all show longitudinal polarization. The transverse polarized HRS is accounted for by long range vibration correlation due to dipole–dipole interaction for molecules at 20–400 nm separation. Longitudinal SHS and HRS are due to the combined effect of long range dipole–dipole orientation correlation and the increment in the molecular first hyperpolarizability induced by short range intermolecular interactions.