Energy transfer between electrons and ions in collision cascades in solids

This paper attempts an overview of the mechanisms, rates, and characteristics of the several processes by which electronic and ionic energy exchange can occur. This energy exchange raises questions about widely accepted assumptions, and the circumstances in which those assumptions must be questioned. The situations where the problems are most easily seen concern low energies, and especially topics such as whether there is a threshold associated with a band gap or situations where structural or chemical features can be affected by the energy transfer. Yet there are other cases in which ion/electron energy transfer could be important, such as in the highly excited centre of a cascade. In such situations, the observable consequences may be harder to relate to the energy exchange. The issue is therefore one of the final state achieved (and hence affected by annealling of defects and various thermal processes) as opposed to the higher-energy phenomena themselves. I discuss the role of the electronic system in metals as an energy sink and a means of energy transport, drawing analogies with the better-understood insulators. There are strong hints of electronic effects in collision cascades, but their status is still not clear.