New structural and vibrational opportunities combining Hyper‐Rayleigh/hyper‐Raman and Raman scattering in isotropic materials

Spontaneous hyper‐Raman and Raman scattering together with hyper‐Rayleigh scattering techniques were combined to study and characterize isotropic materials at the molecular range scale. We also demonstrated new transverse optical–longitudinal optical features in hyper‐Raman in the 90° scattering geometry where it is possible to selectively enlighten or cancel longitudinal optical modes. Noteworthy, it is possible to collect both transverse optical and longitudinal optical modes when collecting the scattered light perpendicularly to the plane of incidence. We detail and illustrate in the field of ionic liquids advances in nonlinear vibrational studies that give unique information about the nature and multipolar symmetry of the scatters. The vibrational multipolar approach, which is a generalized procedure to analyze and interpret all these techniques in isotropic materials, is introduced and applied to silica glass. For silica, we proposed a statistical elementary structural unit with nanometric dimension that fits both the octupolar hyper‐Rayleigh response and the multipolar spectral activity. It is derived from the α‐Cristobalite structure, with a local octupolar S 4 symmetry, which consists of a central [SiO 4 ] tetrahedron embedded in a super Si 4/4 tetrahedron with bent Si–O–Si bonds. In that respect, the silica glass is thought to be a random medium with inhomogeneous density at the nanometric short‐range scale. Copyright © 2012 John Wiley & Sons, Ltd.