Hot-electron mediated ion diffusion in proton-irradiated magnesium oxide

Highly energetic ions that impact materials have applications from semiconductor industry to medicine, and are fundamentally interesting as they trigger multi-length and time-scale processes. In particular, they excite electrons into non-thermalized energy distributions with subsequent non-equilibrium electron-electron and electron-ion dynamics. In order to achieve a quantitative description of these, we propose a general first-principles framework that bridges time scales from ultrafast electron dynamics directly after impact, to ion diffusion over migration barriers in semiconductors. We apply it to magnesium oxide under proton irradiation and discover a diffusion mechanism that is mediated by hot electrons. Our quantitative simulations show that this mechanism strongly depends on the projectile-ion velocity. This indicates that it may occur only at a specific penetration depth in the target and that it can be triggered by varying the kinetic energy of the particle radiation. Either of these predictions should facilitate direct experimental observation of this effect and significantly advances current understanding of non-equilibrium electron-ion dynamics.