Quantum Electron Dynamics in Helium Ion Injection onto Tungsten Surfaces Based on Time-Dependent Density Functional Theory

The neutralization process of an ion particle on a surface is key issue of the plasma-wall interaction (PWI). We investigated helium (He) ion injection onto a tungsten surface using time-dependent density functional theory (TDDFT) simulation. We developed the TDDFT code QUMASUN for this study, simulating the process of electron transfer from the surface to the He nucleus by solving the time evolution of electron wave function and the classical motion of nuclei simultaneously. Our results show that the probabilities of injected $\text{He}^{2+}$ changing into $\text{He}^{1+}$ and $\text{He}^{0}$ on the surface are approximately 40 percent and 25 percent, respectively. The captured electrons by $\text{He}^{1+}$ and $\text{He}^{0}$ predominantly occupy the 2s and 2p orbitals, corresponding to excited states. In addition, we stated some challenges for applying TDDFT to plasma-wall interactions.