Inelastic neutron scattering of a quantum translator-rotator encapsulated in a closed fullerene cage: Isotope effects and translation-rotation coupling inH2@C60andHD@C60

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.