Quadratic induced polarization by an external heavy charge in an electron gas

We have studied the many-body nonlinear interactions between a heavy ion and a solid, modeled by jellium. Once the retarded prescription for the quadratic response function of the medium is obtained, a derivation of the second-order nonlinear induced polarization charge, proportional to (Z1) 2 (Z1 being the ion charge!, and the potential that it originates are presented. These magnitudes are evaluated numerically using the full random-phase approximation to describe the shielded interaction. A comparison with the linear contribution allows us to discuss the validity of the perturbative approach. @S0163-1829~97!07447-X#