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

Abstract

Articles you may be interested inEffect of pressure on the Raman spectrum of translational lattice vibrations in ice J. Chem. Phys. 69, 5557 (1978); 10.1063/1.436551 Optical spectra of orientationally disordered crystal. V. Raman spectrum of ice Ih in the range 4000-350 cm−1The Raman spectrum of the translational vibrations of polycrystalline ice Ih has been investigated in the range 350-20 cm -I. 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 ~2OO cm-I is much weaker than that above ~240 cm-I , which suggests that while the vibrations below ~2OO cm-I are disorder allowed, those above ~240 cm-I might be connected with the order-allowed A Ig , E Ig , and E2 g vibrations of the hypothetical crystal with real hexagonal symmetry. The strong Raman scattering near 310 cm -I 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 ~31O and 228 cm-I .Many of the strong features observed have been re-

Raman scattering tensors for translational vibrations of ice

“…Wong and Whalley [5] considered the calculated dispersion curves of the low-frequency transverse optic TO and transverse acoustic TA branches in ice Ih known to be in good agreement with the dispersion curves obtained by neutron inelastic scattering [2]. They tentatively assign the spectrum feature between the change of slope at 80cm -1 and the minimum at 87cm -1 to the TA1, TA2 and TO2 branches near the Brillouin zone (BZ) boundary at K point.…”

“…Two maxima at 54 and 60 cm -1 are assigned to the normal modes near the point M of the TA1 and TA2 branches, respectively, and the minimum at 58 cm -~ to the gap between these two branches. A shoulder at 47 cm -~ can be due to the TA1, TAe, TO1 and TO2 branches near the BZ boundary at the point A [5].…”

“…Therefore in centrosymmetric crystals a transition is not simultaneousy active in the infrared absorption and in Raman scattering [25][26][27]. The hexagonal ice Ih has a centrosymmetric space group D~h but the water molecules in ice are orientationally disordered and thus the ice vibrations active in infrared absorption can also be active in Raman scattering [5,[29][30][31][32][33].…”

“…Whalley et al [5,[29][30][31] have calculated the frequencies of the Raman active /'1+ (Alg) and/'6+ (Elg) zone centre modes. On the basis of their calculation they assign the Raman lines at 225 cm -1 and 230 cm -I to those modes.…”

“…These observations serve to distinguish between the two types of oscillations. Faure [4] argues that a (possible) coupling between the translational and the libration and internal modes is in fact weak since the ratio of measured frequencies of H20 and D20 is near to 1.036, confined between 1.015 and 1.055 [5], thus not much different from 1.054 for the purely harmonic translational modes. Therefore it is assumed that the translational vibrations can be characterized by wave vectors and polarization vectors.…”