One monolayer of ${\mathrm{C}}_{60}$ on one monolayer of hexagonal boron nitride on nickel is investigated by photoemission. Between 150 and $250\mathrm{K}$ the work function decreases and the binding energy of the highest-occupied molecular orbital increases by $\approx 100\mathrm{meV}$. In parallel, the occupancy of the—in the cold state almost empty—lowest unoccupied molecular orbital (LUMO) changes by $0.4\pm 0.1$ electrons. This charge redistribution is triggered by the onset of molecular rocking motion, i.e., by orientation dependent tunneling between the LUMO of ${\mathrm{C}}_{60}$ 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 ${\mathrm{C}}_{60}^{-}$ polaron formation leads to electron self-trapping.

• Received 19 November 2004

DOI:https://doi.org/10.1103/PhysRevB.71.121402