Production and Transport of Projectile Excited States in Solids
Résumé
Two different types of descriptions can be used to calculate the slowing down of a projectile through materials. For both approaches, improvements towards fully ab initio calculations are attempted by several groups while, in the past, the prediction of stopping power of heavy ions had to rely on the use of "free parameters" like the effective nuclear charge of the projectile. The first description assumes that ion-solid interaction is the result of a series of binary collisions with the target electrons. In this collisional picture, the evolution of charge state distributions of the projectile at the exit of solid targets, involving core state populations, can be successfully predicted in many cases (like for the ETACHA code). In the other one (the dielectric theory), the target electrons are considered to respond collectively to the passage of the projectile. The polarization of the medium can be described as a wake of electronic density fluctuation trailing the ion. The gradient of the wake potential defines an electric field responsible for the stopping power. This electric field also acts directly on the excited levels of the projectile and will induce, among other effects, binding energy shifts as well as Stark mixing of substates. Until recently, only "quasi-free" target electrons were considered in this approach. Up to now, it exists in fact very few experimental results that can be used to decide which of these pictures is the most appropriate and to define eventually their validity limits. In particular, tests on the predictions of the wake model concerning the spatial extension of the electronic density fluctuations are needed. Atomic physics experiments can be used to shed some light on ion-matter interaction. Looking at one partner of the collision, namely the excited state populations of the projectile, the response of the material may be probed.