Core Level Photoemission Evidence of Frustrated Surface Molecules: A Germ of Disorder at the (111) Surface of $C_{60}$ 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 $\mathrm{C}{}_{60}$ 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 $\mathrm{C}{}_{60}$ molecules of the low temperature $(2\times 2)$ surface unit cell, melt about 100 K before the bulk phase transition.