Evidence of simultaneous double-electron promotion in F+ collisions with surfaces.

A high-flux beam of mass-filtered F+ at low energy (100-1300 eV) was scattered off Al and Si surfaces to study core-level excitations of F0 and F+. Elastic scattering behavior for F+ was observed at energies <300 (500) eV off Al (Si) for a 90 degrees 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 (F-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 F2+-behavior which is remarkably similar to Ne+ off the same surfaces. Inelasticities measured for single collision events agree well with the energy deficits required to form (doubly excited) F** and F+** states from F0 and F+, respectively; these excited species most likely decay to inelastic F+ and F2+ via autoionization.