Damping by Sequentially Tunneling Electrons

•Damping caused by sequential tunneling through a Cl-vacancy on a NaCl film.•Damping depends on thickness of the NaCl film, tip position and tip-sample bias.•Features of current-induced damping explained by a rate-equation model.•Subfemtoampere tunneling currents are detected by the damping signal.•Maps of the damping signal can be used to inspect the tip shape.