Dual role of core electrons in electronic friction

TL;DR

This study uses first-principles simulations to reveal how core electrons influence electronic stopping power and energy dissipation in beryllium during ion irradiation, highlighting a dual role of core electrons.

Contribution

It demonstrates, through real-time density functional theory, that core electrons both enable additional dissipation channels and suppress valence excitations, revealing a new dual mechanism.

Findings

Identification of a Bragg peak structure in electronic stopping power.

Core electrons provide an additional dissipation channel.

Core electrons suppress valence electron excitations via electron capture.

Abstract

Non-equilibrium energy dissipation in multi-shell swift-ion/matter systems remains a fundamental yet incompletely understood problem, with electronic stopping power $\mathcal{S}_\text{e}$ as a relevant observable for electronic friction. Using real-time time-dependent density functional theory, we perform first-principles calculations of $\mathcal{S}_\text{e}$ for beryllium self-irradiation with explicit treatment of all electrons. Our results reveal a Bragg peak exhibiting a distinct structure which lies beyond the reach of standard semi-empirical models. We attribute its appearance to a dual effect of the presence of core electrons, by which their excitation provides an additional dissipation channel while simultaneously suppressing valence electron excitations. An electron capture process by the projectile's core from the host cores is behind such suppression, rather than Pauli blocking. This dual mechanism contrasts with the shake-up effect reported for water, whereby the inclusion of core electrons enhances valence excitation. Our work provides a new perspective on the effect and importance of core electrons in projectile energy dissipation in matter.