Optical spectra of orientationally disordered crystals. VI. The Raman spectrum of the translational lattice vibrations of ice Ih

Abstract

The Raman spectrum of the translational vibrations of polycrystalline ice Ih has been investigated in the range 350–20 cm−1. All the vibrations are Raman active, and there is much fine structure, presumably due to particular points in the Brillouin zone. Tentative assignments are suggested for some of the features. The theory of the Raman scattering by the translational vibrations of orientationally disordered crystals has been discussed and it is shown that the origin of the Raman intensity lies mainly in the disorder in the location of the hydrogen atoms surrounding an O–H⋅⋅⋅O bond. The intensity of scattering below ∼200 cm−1 is much weaker than that above ∼240 cm−1, which suggests that while the vibrations below ∼200 cm−1 are disorder allowed, those above ∼240 cm−1 might be connected with the order-allowed A1g, E1g, and E2g vibrations of the hypothetical crystal with real hexagonal symmetry. The strong Raman scattering near 310 cm−1 is not caused by a splitting of the transverse and longitudinal optic vibrations, such as occurs in ionic crystals, but by significant electrostatic interactions, particularly of the transition moments of neighboring hydrogen bonds. They cause the hydrogen-bond stretching constant to depend on the relative orientation of the two water molecules, and the coupling constant for the stretching of neighboring hydrogen bonds to differ for the symmetric, O–H⋅⋅⋅O⋅⋅⋅H–O and O⋅⋅⋅H–O–H⋅⋅⋅O, and asymmetric, O–H⋅⋅⋅O–H⋅⋅⋅O, configurations. The differing coupling constants cause significant differences in the forces restricting the motion of molecules parallel and perpendicular to their polar axes, and probably causes the peaks in the spectrum at ∼310 and 228 cm−1.