We report an inelastic neutron scattering (INS) study of a ${\mathrm{H}}_2$ molecule encapsulated inside the fullerene ${\mathrm{C}}_{60}$ 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-${\mathrm{H}}_2$ 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.

• Received 15 August 2014

DOI:https://doi.org/10.1103/PhysRevLett.113.123001