Probing relaxations of atomic-scale junctions in the Pauli repulsion range

Clean metal as well as C 60 -terminated tips of an atomic force microscope probe the interaction with C 60 molecules adsorbed on Cu(111) and Pb(111). The force measurements unveil a monotonic shift of the point of maximum attraction with the bias voltage. The conventional superposition of long-range van der Waals and electrostatic forces with short-range Pauli repulsion does not reproduce the shift. By phenomenologically including bias-dependent relaxations of the electrode geometry in the analytical expression for the short-range force the experimental data can qualitatively be described.