Interaction potential and infrared absorption of endohedral H2 in C60

We have measured the temperature dependence of the infrared spectra of a hydrogen molecule trapped inside a C-60 cage, H-2@C-60, in the temperature range from 6 to 300 K and analyzed the excitation spectrum by using a five-dimensional model of a vibrating rotor in a spherical potential. The electric dipole moment is induced by the translational motion of endohedral H-2 and gives rise to an infrared absorption process where one translational quantum is created or annihilated, Delta N = +/- 1. Some fundamental transitions, Delta N = 0, are observed as well. The rotation of endohedral H-2 is unhindered but coupled to the translational motion. The isotropic and translation-rotation coupling part of the potential are anharmonic and different in the ground and excited vibrational states of H-2. The vibrational frequency and the rotational constant of endohedral H-2 are smaller than those of H-2 in the gas phase. The assignment of lines to ortho-and para-H-2 is confirmed by measuring spectra of a para enriched sample of H-2@C-60 and is consistent with the earlier interpretation of the low temperature infrared spectra [Mamone et al., J. Chem. Phys. 130, 081103 (2009)]. (C) 2011 American Institute of Physics. [doi:10.1063/1.3535598]