Electron density fluctuations induced by ion clusters in condensed matter

Valence electron density fluctuations induced around a trailing ion by a leading ion of a swift dicluster moving through condensed matter depend on the orientation of the dicluster axis with respect to the beam axis. These density fluctuations may appear experimentally in the angle dependence of the knock-on collision electrons emitted under bombardment of molecular ions. We present linear-response theory calculations of the density fluctuations at the position of the trailing ion, as a function of the orientation. Second-order perturbation theory calculations of the dependence on the angle of the scattering probabilities of target valence electrons by a two-center Coulomb field moving with different velocities are presented. The applicability of linear theories as the charges and velocities of diclusters are changed is discussed. (orig.)