Low-frequency phonon dynamics of the C₆₀(111) surface

High-quality C60 films, with a thickness of about 100 Å, were epitaxially grown on in vacuo cleaved GeS(001) single crystal surfaces and characterized with helium atom diffraction. Phonon dispersion curves of the C60(111) surface were obtained from high-resolution inelastic helium atom time-of-flight (TOF) spectra. Pronounced inelastic peaks corresponding to the Rayleigh modes and to the frustrated rotations (librations) of the C60 molecules were observed along the 〈110〉 and 〈112〉 directions. The anharmonicity of the librations was found to be slightly larger than in the bulk C60. The contribution of the C60 vibrations (without librons) to the inelastic TOF peak intensities was estimated from lattice dynamic calculations based on the intermolecular potential of Girifalco [J. Phys. Chem. 92, 858 (1992)].