  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.
