Image States and Excitons at Insulator Surfaces with Negative Electron Affinity

We discuss electronic excitation processes at two ionic insulator surfaces, LiF(001)-(1×1) and MgO(001)-(1×1), within ab initio many-body perturbation theory. Because of the negative electron affinity of the surfaces, the lowest unoccupied electronic states are image states located in the vacuum outside the surface. Excitations of electrons from the surface layer into these image states are much lower in energy than the bulk excitons. They are responsible for characteristic surface features in the electron energy-loss spectra of LiF(001) and MgO(001).