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 C-60. Narrow infrared absorption lines at the temperature of 6 K 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 H-2 molecule. A theoretical model shows that H-2 inside C-60 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 relative concentration of ortho- and para-H-2.