Evaluation of absolute ion fractions from TOF-LEIS spectra

Time-of-flight (TOF) experiments in the low energy ion scattering (LEIS) regime are reported for He projectiles and a copper target for a scattering angle of 129°, aiming at a more profound physical understanding of the neutralization process. In our experiment, the ions are separated from the neutrals by post-acceleration along the flight path between the target and the detector. The spectrum of the neutrals yields information about deeper layers, as in Rutherford backscattering (RBS). Ions originate from scattering by surface atoms, the ion yield being characterized by the ion fraction P+. Our P+ results indicate that at energies >2 keV a second neutralisation mechanism sets in that is similarly effective as the one that is important at low energies. Above the threshold energy for reionization, a non-monotonous behavior of P+ as function of the reciprocal velocity is found. Our data are compared to available data from the literature, yielding mutual agreement within the combined error bars. [Copyright &y& Elsevier]