Quantum rotation and translation of hydrogen molecules encapsulated inside C 60 : temperature dependence of inelastic neutron scattering spectra

The quantum dynamics of a hydrogen molecule encapsulated inside the cage of a C 60 fullerene molecule is investigated using inelastic neutron scattering (INS). The emphasis is on the temperature dependence of the INS spectra which were recorded using time-of-flight spectrometers. The hydrogen endofullerene system is highly quantum mechanical, exhibiting both translational and rotational quantization. The profound influence of the Pauli exclusion principle is revealed through nuclear spin isomerism. INS is shown to be exceptionally able to drive transitions between ortho -hydrogen and para -hydrogen which are spin-forbidden to photon spectroscopies. Spectra in the temperature range 1.6≤ T ≤280 K are presented, and examples are given which demonstrate how the temperature dependence of the INS peak amplitudes can provide an effective tool for assigning the transitions. It is also shown in a preliminary investigation how the temperature dependence may conceivably be used to probe crystal field effects and inter-fullerene interactions.