Core Level Photoemission Evidence of Frustrated Surface Molecules: A Germ of Disorder at the (111) Surface of C60 before the Order-Disorder Surface Phase Transition

Using high resolution core level photoemission, we investigated the disordering transition of the fullerene molecules at the (111) surface of C-60 films. The experimental evidence of a two-step mechanism for the rotational disordering of surface fullerene molecules is provided. The data are consistent with a recent model in which the rotational degrees of freedom of one molecule, out of the four inequivalent C-60 molecules of the low temperature (2 x 2) surface unit cell, melt about 100 K before the bulk phase transition.