Suppression of projectile-electron excitations in collisions with a free-electron gas of metals - art. no. 052902

Excitation and ionization of hydrogenic projectiles in collisions with meta l free-electron gas are analyzed in the high but nonrelativistic energy reg ime. Transition matrix elements are calculated in the first Born approximat ion, and the free-electron gas response is described by using the Mermin-Li ndhard dielectric function. Projectile excitation and loss probabilities pe r unit length are found to be smaller than those corresponding to the colli sions with a wave packet of single electrons of equivalent density. This be havior is explained in terms of the collective effect (shielding) of the fr ee-electron gas and depends on the energy transferred to the atomic electro n 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.