S. A. FitzGerald, L. J. Santodonato, J. R. D. Copley, T. Yildirim, D. A. Neumann, and J. J. Rush; NIST, Gaithersburg, MD, USA. First author: NIST Bldg 235 Room A110. (301) 975-4863.


The C60 lattice has large interstitial sites which can accommodate various atomic and molecular species. Analyzing the structure and dynamics of small molecules placed in these sites provides information about the trapped molecules and the intermolecular potential of C60. We report a neutron diffraction study which demonstrates that hydrogen molecules adsorbed into the C60 lattice sit exclusively at the octahedral sites. Modest loading pressures on the order of 100 bar are sufficient to fill roughly 40% of the available sites. Inelastic scattering results taken on a neutron time-of-flight spectrometer clearly show the ortho to para conversion of the interstitial hydrogen, which occurs via a transition from the J=1 to J= 0 rotational levels. The transition occurs at 14.2 meV slightly shifted in energy from that of the idealized free hydrogen at 14.7 meV. As expected measurements performed with interstitial D2 show the same transition reduced by a factor of two in energy but the transition now appears as a doublet with peaks centered at 7.4 and 6.7 meV. We speculate that the presence of the doublet peak in D2 occurs through the accidental degeneracy of rotational mode with the zone-center phonon mode of pure C60.