Light scattering evidence of a central component in ice Ih

Abstract

High resolution Raman spectra of ice I h single crystal in the frequency range 0–30 cm −1 are reported, at various temperatures. For the first time evidence is given for the existence of a Lorentzian central component in all scattering configurations. A polarization analysis of the spectra clearly shows that the central mode cannot be ascribed to the high frequency tails of the Brillouin doublet, nor to the disorder induced phonon background even accounting for renormalization of the phonon propagator. The epectral features indicate the existence of a direct coupling between the light and some relaxation mechanism of the crystal. A model is suggested to account for the excess scattering and for the activation of the acoustic phonon density of states.