Inelastic neutron scattering from hydrogen clathrate hydrates

• Published in: Journal of physics. Condensed matter 2008-03, Vol.20 (10), p.104242-104242 (5)
• Main Authors: Ulivi, L, Celli, M, Giannasi, A, Ramirez-Cuesta, A J, Zoppi, M
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Lattice dynamics; Phonon states and bands, normal modes, and phonon dispersion; Phonons and vibrations in crystal lattices; Physics
• ISSN: 0953-8984; 1361-648X
• DOI: 10.1088/0953-8984/20/10/104242

We present the results of a high-resolution inelastic neutron scattering experiment on three samples of ternary tetrahydrofuran clathrate hydrates, containing either H2 at different para/ortho concentrations, or HD. The spectrum due to the H2 molecule excitation shows spectral bands that are assigned unambiguously to rotational transitions, centre-of-mass translational transitions of either para- or ortho-H2, and to combinations of these. We demonstrate that the H2 molecule rotates almost freely, while performing a translational motion (rattling) in the cage, resulting in a paradigmatic example of the motion of a quantum particle in a non-harmonic three-dimensional potential well. Both the H2 rotational transition and the fundamental of the rattling transition split into triplets. This is a consequence of a significant anisotropy of the potential with respect to the orientation of the molecule in the cage, in the first case, or with respect to the centre-of-mass position inside the cage, in the second case. The comparison of our experimental values for the transition frequencies to a recent quantum mechanical calculation is discussed. Spectra obtained at different temperatures confirm an appreciable anharmonicity of the potential energy for the H2 molecule in the cage.