POTENTIAL-ENERGY OF ATOMS NEAR A METAL-SURFACE

The dependence of the atom-metal surface interaction potential on the response time of the metal electron gas is discussed. For an atom in its ground state, it is shown that the mirror-image potential of classical electrostatics can be retrieved if the metal response time in the Lifshitz formula is much shorter than the characteristic fluctuation time of the atom, whereas, for the opposite limit, the interaction potential of an atom in a spherically symmetric state is vanishingly small. For an atom in an excited state the interaction potential has additional terms that have the same dependence on the atom-surface separation, but it displays a resonant, dispersive character and may lead to attraction as well as repulsion of the atom.