Eigenvectors of internal vibrations of C-60: Theory and experiment

We have studied the eigenvectors of all internal modes of C-60 up to 75 meV by inelastic neutron scattering. The experimental data are compared in detail with a state-of-the-art ab initio theory and a phenomenological model of the vibrational spectrum of C-60, respectively. We demonstrate that the momentum dependence of mode intensities obtained from measurements on single crystals as well as on powder samples represents a sensitive fingerprint of the eigenvectors which allows (i) unambiguous assignments of silent modes and (ii) discrimination of eigenvectors of modes belonging to the same symmetry class. The latter property can be used for stringent tests of theoretical models. Eigenvectors predicted by the ab initio theory are found to be in very satisfactory agreement with the experimental data, while those obtained from the phenomenological model turn out to be less reliable. We have also performed measurements to study the dispersion of the H-g(1) mode in the low-temperature phase. We found that the data can be largely, although not fully, understood on the basis of a simple van der Waals ansatz for the intermolecular interactions.