Druck-Materie 20b.qxd - JUWEL - Forschungszentrum Jülich

the formula also to the libration modes of around 170 (2) and 370 meV (3), and get the effective
masses of 2.71 m and 3.13 m, respectively. They are also close to the free rotation effective
mass. So, the recoil may be due to rotation.
To get the information about the lattice mode in detail we obtained data with an incident
energy of 100 meV. Scattering intensity against the neutron energy is shown in Fig. 2, where
we compared the intensities between methane hydrate and ice. We can see several peaks in
both intensity spectra and there is almost no difference between them. The levels observed are
assigned to TA mode of 8.8 meV, LA mode of 18 meV, TO mode of 27 meV and TO mode
of 35 meV. Librational modes appeared above about 65 meV. This suggests that around this
energy region the neutron cross section of methane hydrate is expressed by linear combination
of each material, methane and ice.
To investigate the low energy level we performed experiments using an incident energy
of 15 meV. Scattering intensity maps are indicated in Fig. 3 for CH4-D2O and D2O. It is
clearly recognized the energy levels originated from methane hydrate by comparing the spectra.
The level below 5 meV would be attributed to the methane mode. Level assignment is
shown in Fig.4.
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TA
Translations
LA
LO
TO
Figure 2. Lattice modes of H2O in methane hydrate and ice
Figure 3. Scattering intensities from CH4-D2O and D2O
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99
CH4-H2O
(13K)
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Librations
H2O