Neutron Scattering Studies of C61H2

ABSTRACTWe report neutron diffraction measurements of orientational ordering, and inelastic neutron scattering measurements of the inter- and intra-molecular excitations, for the 6:5 annulene isomer (C3 symmetry) of C61H2. We confirm previous x-ray measurements[1] which have shown that above 290K this isomer has an fcc structure, with the molecules orientationally disordered such that the methylene groups preferentially occupy “octahedral” sites. Below 290K, the diffraction pattern can be indexed on a simple cubic lattice, in direct analogy with the orientational ordering transition of C60. The observed low energy dynamics reflect this behavior. A peak corresponding to molecular librations is observed below the transition, at ∼2.5meV, while above the transition the peak collapses into a quasielastic line characteristic of rotational diffusion. The intramolecular spectra show several pronounced peaks. The assignments of these modes are discussed.