Absolute measurements of the stretching mode density of states in polycrystalline ice Ih

An incoherent inelastic neutron scattering experiment on polycrystalline ice Ih at T=20 K is reported. The experiment was performed on the high-resolution chopper spectrometer at the Intense Pulsed Neutron Source (IPNS) of the Argonne National Laboratory. Scattering functions in the angular range from 4° to 20° were converted, after analytical corrections, into density of states of the system in the stretching frequency region. Analysis of these results leads to the following conclusions: (i) the number of modes in the stretching region of ice Ih is 8.1/cell which is that expected in the case of free molecules; (ii) the general shape of the density of states is in qualitative agreement with Raman scattering results, however the high energy side of the neutron data suggests that a non-Raman active band exists at about 3500 cm−1; (iii) the overall bandwidth observed in this experiment is in good agreement with what we estimated by a coherent inelastic neutron scattering study on heavy ice Ih.