First Principles Determination of Electronic Excitations Induced by Charged Particles

A time dependent self consistent field based method for determining the rates of electronic excitations induced in materials by the presence of external point charges is presented. The method utilizes the full scalar potential of the external point charge in the interaction Hamiltonian instead of relying on multipolar expansions thereof. A general method is presented for determining the conditions under which dipole selection rules are adequate to describe the electronic response of the material to perturbation by external point charges. The position dependence of point charge induced transition rates between the ground and lowest few excited electronic states was resolved for a small polybenzoid. Notably, electronic excitations that are optically forbidden can be strongly allowed for particular positions of the perturbing point charge. Application of the methods detailed here can lead to an improved understanding of the electronic response of materials under irradiation by beams of charged particles.