Interaction potential and infrared absorption of endohedral H_2 in C_(60)

摘要

We have measured the temperature dependence of the infrared spectra of a hydrogen molecule trapped inside a C60 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, N 1. Some fundamental transitions, 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 V2 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@C60 and is consistent with the earlier interpretation of the low temperature infrared spectra [Mamone, J. Chem. Phys. 130, 081103 (2009)].