Role of ionization in imaging and spectroscopy utilizing fast electrons that have excited phonons

Atomic resolution scanning transmission electron microscopy, based on counting fast electrons that have been scattered to large angles after exciting a phonon, so-called high-angle annular dark-field (HAADF) imaging, is widely used in materials science. Recently atomic resolution phonon spectroscopy has been demonstrated. In both cases experiments are usually modeled taking into account only elastic scattering and the inelastic scattering due to phonon excitation. However, other inelastic processes, such as plasmon excitation and single electron excitation, also play a role. In this paper we will focus on the role of ionization and its influence on imaging and spectroscopy based on phonon excitation. Inelastic scattering due to ionization is mainly forward peaked, which has implications for phonon spectroscopy with a detector in the forward direction. Nevertheless, a substantial fraction of electrons scattered by phonon excitation to larger angles have also lost significant amounts of energy due to also being involved in an ionization event. We discuss the implications of this for HAADF imaging and phonon spectroscopy utilizing electrons scattered to these larger angles.