Plasmon decay mechanisms in proton-solid collisions

Abstract

A projectile traveling inside a metal can excite bulk plasmons ͑collective oscillations of the electron gas͒ that eventually decay. The two main decay mechanisms are the excitation of a nearly free electron ͑NFe͒, also referred to as a Bloch electron, and the excitation of a pair of interacting free electrons ͑2e͒. In recent publications we developed a model to study these mechanisms for proton impact on aluminum. In this paper, we apply that model to other simple metals. Interesting results are obtained for magnesium, sodium, and potassium. The comparison of the NFe energy and angular spectra sheds light on the role the crystal structure plays. Results for the total probability and excitation power can also be understood in terms of the elements' different characteristics. Some comments are made regarding the relative importance of the two mechanisms for each element and the role that parameters like the electron density, the crystal structure, and the plasmon linewidth might play in this. Also, an approximate scaling rule is found for the plasmon creation probability as well as for the 2e contribution to the plasmon decay probability.