Noble gas atom as a quantum oscillator in the fullerene molecular cage: THz spectroscopy study.*

Nobel gas endofullerene provides a unique opportunity to study the non-covalent interaction between atoms and fullerene cages. Here, we use THz spectroscopy to study the interaction between noble gas atoms (³He, ⁴He, Ne, Ar, Kr) and the fullerene cage. The temperature dependence of the THz absorption spectra of powdered samples were measured between 5 to 300K.  The translational motion of the atom quantized by the confining potential of C₆₀ defines the THz spectrum.  The spectra of the He isotopes exhibit clearly resolved THz peaks at high temperatures, indicating that the potential energy function V(r) for the encapsulated atom is anharmonic [1,2]. Other studied atoms display one broad peak at elevated temperatures which is also an indication of the anharmonicity of the potential. We fitted the THz absorption spectra with a three-dimensional anharmonic oscillator model including translation-induced dipole moment. The fit results show that the harmonic potential term increases with the size of the noble gas atom. Our results are a valuable input to the quantum calculations of non-bonded intermolecular interactions.