Surface plasmon excitations in the wake of hollow atom relaxation at surfaces

We report on distinct peaks arising on the low-energy side of KLL Auger spectra which are observed during the interaction of slow highly charged ions at Al(111) and Si(100) surfaces under grazing observation. The energy offset of these structures, with respect to the dominant spectral feature, corresponds to an energy loss of electrons escaping the bulk which can uniquely be connected to surface plasmon excitations. Describing the bulk interaction of outgoing electrons within the framework of a scattering model, we estimate the electron bulk penetration length following a subsurface K-Auger decay. For high impact energies, we also provide experimental evidence for the ionization of target core levels by these outgoing electrons.