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.
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