Methane in vapor deposited argon: An inelastic neutron scattering study

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Abstract
The inelastic neutron scattering spectrum of a sample of 1% CH4 in Ar was recorded in the energy range below 3 meV. Two strong quantum transitions at 0.90 and 0.59 meV and a broad distribution in the energy range up to 1.2 meV are found in the unannealed sample. Weaker transitions appear at 1.3, 1.7, and 2.2 meV, the latter with a shoulder at 2.3 meV. In the annealed sample (annealing temperature Ta=80 K, annealing time 1 h) the broad distribution and the line at 0.59 meV have disappeared. A new line at 0.45 meV and a shoulder at 0.77 meV merge from the broad distribution. The sharp transitions originate from CH4 molecules in well defined neighborhoods. They are attributed to molecules in an undisturbed fcc site (0.90, 1.3, and 2.2 meV), a fcc site neighboring a stacking fault (0.77 meV), an hcp site in hcp crystallites (0.59 meV), and hcp site within a single stacking fault (0.45 meV). Sites at grain boundaries, lattice distortions, etc., cause the broad background. The annealed sample still contains one stacking fault every 20 lattice planes.