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The polarized Raman spectra of an artificial CO2 clathrate hydrate single crystal have been measured in order to examine the crystal-orientation dependence of the Raman spectra. Since the crystal had crystallographic facets, the orientation of the crystal was determined by using the Miller indices of the facets. When the angle θ between the polarization plane of the incident laser beam and the direction of one of the 〈110〉 axes of the single crystal varied, it was observed that the intensities of the peaks, which were caused by the Fermi resonance of the symmetric stretching mode and the overtone of the bending mode of CO2, and the O–H symmetric stretching vibration mode, varied with θ. Since the tetrakaidecahedron cage in the CO2 clathrate hydrate is distorted along the 〈100〉 axis, the variations of the scattering intensities of the CO2 have been calculated by using a simple model that assumes that the CO2 rotates on the {100} plane in the tetrakaidecahedron cage. The results obtained from the experiments are consistent with the calculations made by using this model. It has been concluded that the anisotropy of the peak intensities of the CO2 show the influence of the cage geometry on the motion of the guest molecule. The anisotropy of the O–H symmetric stretching vibration mode was interpreted with a five-body structure model. As the calculation with the model was consistent with the result obtained from the experiment, it was found that the anisotropy of the peak intensity of the O–H symmetric stretching vibration mode was related to the arrangement of the water molecules. We consider that the result indicates the influence of the motion of the guest molecule on the surrounding hydrogen-bonded network.

A recently developed theory of post-nucleation conversion of an air bubble to air-hydrate crystal in ice is applied to simulate two different types of air-hydrate formation in polar ice sheets. The work is focused on interpretation of the Vostok (Antarctica) ice-core data. The hydrostatic compression of bubbles is the rate-limiting step of the phase transformation which is additionally influenced by selective diffusion of the gas components from neighboring air bubbles. The latter process leads to the gas fractionation resulting in lower (higher) N2/O2 ratios in air hydrates (coexisting bubbles) with respect to atmospheric air. The typical time of the post-nucleation conversion decreases at Vostok from 1300-200 a at the beginning to 50-3 a at the end of the transition zone. The model of the diffusive transport of the air constituents from air bubbles to hydrate crystals is constrained by the data of Raman spectra measurements. The oxygen and nitrogen self-diffusion (permeation) coefficients in ice are determined at 220 K as 4.5 × 10−8 and 9.5 × 10−8 mm2 a−1, respectively while the activation energy is estimated to be about 50 kJ mol−1. The gas-fractionation time-scale at Vostok, τF ∼300 a, appears to be two orders of magnitude less than the typical time of the air-hydrate nucleation, τz ∼30-35 ka, and thus the condition for the extreme gas fractionation, τF ≪ τz is satisfied. Application of the theory to the GRIP and GISP2 ice cores shows that on average, a significant gas fractionation cannot be expected for air hydrates in central Greenland. However, a noticeable (statistically valid) nitrogen enrichment might be observed in the last air bubbles at the end of the transition.

The polarized Raman spectra of a natural air hydrate single crystal from a deep ice core recovered at Dye-3 Greenland have been measured in order to examine the crystal-orientation dependence of the Raman spectra. Since the crystal had facets, the orientation of the crystal was determined by using the Miller indices of facets. When the angle θ between the polarization plane of the incident laser beam and the direction of [111] of the crystal varied, it was observed that the intensities of the stretching modes of the two major guest molecules (nitrogen and oxygen) varied with θ. Since the dodecahedron cage in the air hydrate are distorted along the 〈111〉 axis, the variations of the scattering intensities of N2 and O2 have been calculated by using a simple model that assumes N2 and O2 are on the plane of {111} in the dodecahedron cage. The results obtained from experiments are consistent with the calculations made by using this simple model. We concluded that the anisotropy of the intensities of N2 and O2 was caused by the anisotropic rotation of the guest molecules in the distorted dodecahedron cage.