Quantum rattling of molecular hydrogen in clathrate hydrate nanocavities

We have performed high-resolution inelastic neutron scattering studies on three samples of hydrogenated tetrahydrofuran-water clathrates, containing either H₂ at different para and ortho concentrations, or HD. By a refined analysis of the data, we are able to assign the spectral bands to rotational and center-of-mass translational transitions of either para- or ortho-H₂. The H₂ molecule rotates almost freely, while performing a translational motion (rattling) in the nanometer-size cage, resulting a paradigmatic example of quantum dynamics in a nonharmonic potential well. Both the H₂ rotational transition and the fundamental of the rattling transition split into triplets, having different separation. The splitting is a consequence of a substantial anisotropy of the environment with respect to the orientation of the molecule in the cage, in the first case, or with respect to the center-of-mass position inside the cage, in the second case. The values of the transition frequencies and band intensities have been quantitatively related to the details of the interaction potential between H₂ and the water molecules, with very good agreement.