Friction parameter of an ion near a metal surface

The force on an ion located near a condensed-matter surface determines its Brownian motion along the surface. We have studied the dissipative component of this force using a model in which the ion interacts with a metal while moving parallel with its surface. We recover the known asymptotic form of the force for large separations when only electron-hole excitations are accounted for but find a different form when damping of collective states is included.