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

2. EXPERIMENTAL
Methane hydrate was prepared by contacting methane gas with crashed ice in a high
pressure container under a condition of pressure of5 MPa and temperature of 268K. The
methane contents in the hydrate were at least 80% and contents of many of samples were
around 90%. We prepared CH4-H2O and CH4-D2O, and also prepared H2O and D2O ice for
comparison.
Neutron scattering experiments were performed on the MARI spectrometer at ISIS, and
on the LAM-80 at KENS. The energy resolution of LAM-80 is about 0.02 meV. The sample
temperature was mainly about 12K. Higher temperatures were also examined on MARI to
study the effect of the temperature on the rotational level, although the results are not described
here..
3. EXPERIMENTAL RESULTS AND DISCUSSIONS
We got wide energy and momentum range scattering data by using MARI. Scattering
intensity map obtained by incident energy of 500 meV are shown in Fig. 1 for CH4-D2O and
CH4-H2O. The left figure is for CH4-D2O and the right figure for CH4-H2O. From the left
figure we recognize recoil like behavior at high Q region not only at low but also at high
energy regions. The lowest curve would correspond to rotation, middle one consist of two
libration levels of 163 and 190 meV and the highest 362 and 374 meV. This situation is
different from the pattern of the right one, which shows the contribution from ice indicating
the energy levels independent of Q. They belong to lattice or vibration modes of ice.
(2)
(3)
)
(1
Figure 1. Scattering intensities from CH4-D2O and CH4-H2O
Where the recoil like behavior comes from? We assumed that the rotation mode would be
2 2
h Q
E = E0
+ EQ
, EQ
= .
2M
represented by energy conservation formula of the recoil,
Here, E0 is the energy level and EQ is the recoil energy. We applied this formula to the
lowest line indicated by (1) and obtained the effective mass of 2.64 m, which is very close to
the effective mass of the free rotation, 2.67 m, where m is the hydrogen mass. Recoil like
behavior of the libration modes assumed to be coupling with the rotational mode. We applied
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