Long-range orientation correlation in liquids

Strong short-range intermolecular interactions result in position and orientation correlations between nearest neighbour molecules in isotropic liquids, and it is generally assumed that such correlations extend at most a few molecular diameters. This assumption is contradicted by results from second harmonic light scattering experiments presented here, which reveal long-range orientation correlations in several isotropic liquids including water. These experiments measure the polarization dependence of the scattered light, and the observations are interpreted in terms of transverse and longitudinal polar collective modes, as well as simple explicit models. The results revise our understanding of the structure of molecular liquids and provide a test of computational simulations.