Diabatic potential-energy curves for Ar+ and Kr+ interacting with a metal surface are reported. At intermediate distances, the attractive force is considerably smaller than the classical result for an ion attracted to its image charge. This has a substantial impact on the dynamics of electronically stimulated desorption, and on the Franck-Condon broadening of photoemission lines. Quantum effects on ion motion are considered, and the validity of a classical treatment is discussed. © 1989 The American Physical Society.