Rocking-motion-induced charging of C60 on h-BN/Ni(111)

One monolayer Of C-60 on one monolayer of hexagonal boron nitride on nickel is investigated by photoemission. Between 150 and 250 K the work function decreases and the binding energy of the highest-occupied molecular orbital increases by \A 100 meV. In parallel, the occupancy of the-in the cold state almost empty-lowest unoccupied molecular orbital (LUMO) changes by 0.4\P 0.1 electrons. This charge redistribution is triggered by the onset of molecular rocking motion, i.e., by orientation dependent tunneling between the LUMO Of C-60 and the substrate. The magnitude of the charge transfer is large and cannot be explained within a single-particle picture. It is proposed to involve electron-phonon coupling where C-60(-) polaron formation leads to electron self-trapping.