A neutron-scattering study was made of the quantum dynamics of molecular H which was trapped in this material. The loading isotherm was shown to deviate significantly from a standard Langmuir response, and instead followed an exponential law; increasing from 40% filling at 130atm to 90% at 700atm. Diffraction data confirmed that the adsorbed molecules were randomly oriented and sat only on octahedral sites. Inelastic neutron scattering data clearly revealed ortho-to-para conversion of the interstitial H, which occurred via a transition from the J = 1 to J = 0 rotational levels. The level scheme exhibited relatively minor deviations (of the order of a few percent) from the free rotor model; with the splitting in the excited level being the same (0.0007eV) for both H2 and D2. The shift in the overall level, which was shown to depend critically upon zero-point motion, was almost 3 times greater for H2 than for D2. Translational modes of the trapped molecules were also identified. These occurred at a much higher energy than would be predicted classically, and exhibited an isotopic shift of the order of v2.2. Quantum-mechanical model calculations, within the self-consistent harmonic approximation, indicated that zero-point motion of H2 molecules in the ground state played a central role in understanding the experimental results and, in particular, the high energy of the translational modes and the magnitude of their isotopic shift.

Quantum Dynamics of Interstitial H2 in Solid C60. S.A.FitzGerald, T.Yildirim, L.J.Santodonato, D.A.Neumann, J.R.D.Copley, J.J.Rush, F.Trouw: Physical Review B, 1999, 60[9], 6439-51