Suppression of projectile-electron excitations in collisions with a free-electron gas of metals

Excitation and ionization of hydrogenic projectiles in collisions with metal free-electron gas are analyzed in the high but nonrelativistic energy regime. Transition matrix elements are calculated in the first Born approximation, and the free-electron gas response is described by using the Mermin-Lindhard dielectric function. Projectile excitation and loss probabilities per unit length are found to be smaller than those corresponding to the collisions with a wave packet of single electrons of equivalent density. This behavior is explained in terms of the collective effect (shielding) of the free-electron gas and depends on the energy transferred to the atomic electron and on the impact velocity. By comparing results obtained using a binary collisional formalism and those using the dielectric formalism, we estimate the plasmon excitation contribution to the total probabilities.