Quantum dynamics of interstitial H-2 in solid C-60

We present a neutron-scattering study of the quantum dynamics of molecular hydrogen trapped inside solid C-60 The loading isotherm is shown to deviate significantly from a standard Langmuir response and follows instead an exp onential form, increasing from 40% filling at 130 atm to 90% at 700 atm. Di ffraction data confirm that the adsorbed molecules are randomly oriented an d sit exclusively at the octahedral site. Inelastic neutron scattering clea rly shows the ortho to para conversion of the interstitial hydrogen, which occurs via a transition from the J = 1 to J = 0 rotational levels. The leve l scheme shows relatively minor deviations (on the order of a few percent) from the free rotor model with the splitting in the excited level being the same, 0.7 meV, for both H-2 and D-2. In contrast the shift in the overall level, which is shown to depend critically upon zero-point motion is almost three times greater for H-2 than D-2. We also identify the translational m odes of the trapped molecules which occur at a much higher energy than woul d be classically predicted and have an isotopic shift on the order of root 2.2. Quantum-mechanical model calculations within the self-consistent harmo nic approximation indicate that zero-point motion of H-2 molecules in the g round state play the central role in understanding the experimental results , and in particular the high energy of the translational modes and the magn itude of their isotopic shift. [S0163-1829(99)03133-1].