Rotational tunneling of ammonia in (NH3)K3C60

The rotational dynamics of NH3 in the ammoniated fulleride (NH3)K3C60 are s tudied by the inelastic neutron scattering technique. Well-defined excitati ons appear in the low-energy spectra at 1.5 K as a broad band centered at s imilar to 350 mu eV with shoulders at similar to 240 and 590 mu eV. The tem perature dependence of their intensities, energies, and widths is consisten t with their assignment to rotational tunneling transitions of the NH3 mole cule. The estimated barrier of the threefold hindrance potential to rotatio n of the H atoms about the K-N axis is similar to 5.7 meV. The existence of minority hindering sites with both larger (similar to 8 meV) and smaller ( similar to 2 meV) torsional barriers shows that orientational disorder effe cts survive in (NH3)K3C60 even at low temperatures. (C) 1999 American Insti tute of Physics. [S0021- 9606(99)01348-3].