Quantized translational motion of ³He and ⁴He atoms in He@C₆₀ endofullerene.

We studied the translational motion of He atom trapped inside the molecular cage of C₆₀ molecule using infrared (IR) spectroscopy. The T dependence of the IR absorption spectra of ³He@C₆₀ and ⁴He@C₆₀ powder samples was measured between 5 and 300K. At the lowest T a single absorption line was observed at 81 cm^-1 (97 cm^-1 for ³He), but as the T increased four new lines appeared above the first transition. We fitted the IR spectra with an anharmonic spherical oscillator model and assumed a rigid C₆₀ cage. The line frequencies and intensities were reproduced with the harmonic (V₂R²) and anharmonic (V₄R⁴) potential and the induced dipole moment (A₁R) terms, where R is the displacement of the He atom from the C₆₀ cage center. The fit gave equal, within the error bars, potential and dipole parameters for ³He@C₆₀ and ⁴He@C₆₀. The translational energies will be compared to the C₆₀ encapsulated translational energies of H₂, HD, D₂, HF, and H₂O. Our results are the test bed of theories describing the interaction of inert gases with carbon surfaces.