Real-Time TD-DFT Simulation of Secondary Electron Yields Under Focused Electron Beam Irradiation

Modern scanning transmission electron microscopy (STEM) techniques can utilize a variety of different modes of detection in the study of electronic and structural properties of thin materials. Signals from many commonly applied detection methods — e.g., electron energy loss spectra, which report on excitations induced in materials through inelastic scattering — are well understood from the theoretical perspective, and can nowadays be accurately calculated from first principles. In this presentation, we highlight new developments from our group towards the ab initio simulation of ionization processes (i.e., secondary electron generation) that occur under focused electron beam irradiation. Specifically, this work aims to expand our suite of time-dependent electronic structure theory methods for simulating beam-induced electronic excitations between bound states¹ to also address transitions of material-bound electrons into unbound final states. Motivated by similar approaches for simulating photoelectron generation,² a (real-time) TD-DFT method is put forth in which an idealized focused electron beam-like perturbation is applied to the system while absorbing boundary conditions are imposed to remove unbound secondary electrons. The electron density lost due to the perturbation is then related to the secondary electron yield. Results are presented for some prototypical conducting materials, which have been studied via STEM through secondary electron e-beam induced current measurements.