  The interactions between atoms and molecules may be described by a potential
energy function of the nuclear coordinates. Non-bonded interactions are
dominated by repulsive forces at short range and attractive dispersion forces
at long range. Experimental data on the detailed interaction potentials for
non-bonded interatomic and intermolecular forces is scarce. Here we use
terahertz spectroscopy and inelastic neutron scattering to determine the
potential energy function for the non-bonded interaction between single He
atoms and encapsulating C60 fullerene cages, in the helium endofullerenes 3He
and 4He, synthesised by molecular surgery techniques. The experimentally
derived potential is compared to estimates from quantum chemistry calculations,
and from sums of empirical two-body potentials.
