Interaction potential and infrared absorption of endohedral H-2 in C-60

Abstract

We have measured the temperature dependence of the infrared spectra of a hydrogen molecule trapped inside a C60 cage, H2@C60, 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 H2 and gives rise to an infrared absorption process where one translational quantum is created or annihilated, ΔN = ±1. Some fundamental transitions, ΔN = 0, are observed as well. The rotation of endohedral H2 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 H2. The vibrational frequency and the rotational constant of endohedral H2 are smaller than those of H2 in the gas phase. The assignment of lines to ortho- and para-H2 is confirmed by measuring spectra of a para enriched sample of H2@C60 and is consistent with the earlier interpretation of the low temperature infrared spectra [Mamone et al., J. Chem. Phys. 130, 081103 (2009)].