Rotor in a Cage: Infrared Spectroscopy of an Endohedral   Hydrogen-Fullerene Complex
  We report the observation of quantized translational and rotational motion of molecular hydrogen inside the cages of C60. Narrow infrared absorption lines at the temperature of 6K correspond to vibrational excitations in combination with translational and rotational excitations and show well resolved splittings due to the coupling between translational and rotational modes of the endohedral H2 molecule. A theoretical model shows that H2 inside C60 is a three-dimensional quantum rotor moving in a nearly spherical potential. The theory provides both the frequencies and the intensities of the observed infrared transitions. Good agreement with the experimental results is obtained by fitting a small number of empirical parameters to describe the confining potential, as well as the ortho to para ratio. 
S. Mamone, Min Ge, D. H\"uvonen, U. Nagel, A. Danquigny, F. Cuda, M.   C. Grossel, Y. Murata, K. Komatsu, M. H. Levitt, T. R...