Neutron spectroscopy of C₆₀ fullerite hydrogenated under high pressure; evidence for interstitial molecular hydrogen

Inelastic neutron scattering spectra of a hydrofullerite quenched after synthesis at 620 K under a hydrogen pressure of 0.6 GPa, and of the same sample after annealing at 300 K for 35 h which reduced the hydrogen content by 1.4 H₂ molecules per C₆₀ unit, were measured at 85 K The quenched sample is shown to consist of C₆₀H_x molecules with x ≈ 24 and of interstitial molecular hydrogen. The interstitial molecular hydrogen left the sample during annealing at room temperature, whereas the C₆₀H_x molecules were stable at this temperature The intramolecular and intermolecular vibrations of C₆₀H_x and H₂ in the fullerite are discussed in view of the measured spectra.