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We report an inelastic neutron scattering (INS) study of H2 molecule encapsulated inside the fullerene C60 which confirms the recently predicted selection rule, the first to be established for the INS spectroscopy of aperiodic, discrete molecular compounds. Several transitions from the ground state of para-H2 to certain excited translation-rotation states, forbidden according to the selection rule, are systematically absent from the INS spectra, thus validating the selection rule with a high degree of confidence. Its confirmation sets a precedent, as it runs counter to the widely held view that the INS spectroscopy of molecular compounds is not subject to any selection rules.

We report an inelastic neutron-scattering (INS) investigation of coupled quantum translation and rotation of hydrogen molecules trapped inside the closed isotropic cages of ${\text{C}}_{60}$. The low-lying states that characterize the translation-rotation manifold of the hydrogen molecules are accurately determined in our study of the INS peak energies. A comparison between the spectra of ${\text{H}}_{2}$ and HD isotopomers provides quantitative insight into the coupling between rotational and translational angular momentum with HD exhibiting the strongest effects due to mixing of the rotational eigenstates.

A method of measuring tunnel frequencies of molecular reorientation in crystals at low temperatures is demonstrated. Magnetic field variation brings the tunnel frequency to equality, first with the Larmor frequency of paramagnetic impurities and then with twice the Larmor frequency of protons whose magnetism has been saturated. In the first step paramagnetic ions cool the tunneling reservoir, and in the second the tunneling reservoir cools the proton Zeeman system, leading to enhanced proton relaxation.