Low wavenumber Raman scattering in viscous liquids

Low-wavenumber depolarized Raman spectra (−400 to 400 cm−1) for dioxane, water and glycerol are presented. Stokes and anti-Stokes spectra (−400 to 400 cm−1), are used to define a reduced spectrum, in which the effects of temperature and also flat backgrounds and symmetric central components are removed, without knowledge of these parameters. Comparison with the usual reductions shows that (i) all inelastically scattered photons must be considered as ‘true’ Raman signals, due to processes involving Boson-like excitations, (ii) if central scattering exists, then its intensity also depends on the population factor, (iii) the low-wavenumber Stokes signal is more intense than expected for a factor of n + 1 only, owing to non-linear effects, and (iv) the systematic use of reduced spectra is not essential, and can be hazardous in some cases.