A high-flux beam of mass-filtered ${\mathrm{F}}^{+}$ at low energy (100–1300 eV) was scattered off Al and Si surfaces to study core-level excitations of ${\mathrm{F}}^0$ and ${\mathrm{F}}^{+}$. Elastic scattering behavior for ${\mathrm{F}}^{+}$ was observed at energies $<300$ (500) eV off Al (Si) for a 90° lab angle. However, above this energy threshold, orbital mixing in the hard collision step results in electronic excitation of F via molecular orbital promotion along the $4f\sigma$ ($\mathrm{F}\mathrm{\text{−}}2p$), significantly reducing the observed ion exit energy. In addition, despite the electronegativity of F, scattering at energies $>450$ (700) eV off Al (Si) produces ${\mathrm{F}}^{2+}$—behavior which is remarkably similar to ${\mathrm{Ne}}^{+}$ off the same surfaces. Inelasticities measured for single collision events agree well with the energy deficits required to form (doubly excited) ${\mathrm{F}}^{**}$ and ${\mathrm{F}}^{+**}$ states from ${\mathrm{F}}^0$ and ${\mathrm{F}}^{+}$, respectively; these excited species most likely decay to inelastic ${\mathrm{F}}^{+}$ and ${\mathrm{F}}^{2+}$ via autoionization.

• Received 15 August 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.257603