Field ionization of O2 on oxygen-covered Pt(111) and W(111) as studied using field ion appearance energy spectroscopy

The local (probe hole) mass-to-charge-resolved retarding potential analysis of O2+ ions originated from a few surface sites of the apex plane of a [1 1 1]-oriented Pt and W field emitter tips was performed. Applying a thermionic cycle, field-dependent values of the binding energies of O2 molecularly adsorbed on Pt(1 1 1) and W(1 1 1) were derived from the field-ion appearance energies of O2+ emitted at 79 K. Two different possible (noble-gas-like and field desorption) mechanisms of the field ionization of molecular oxygen supplied as imaging gas in the field-ion microscope (FIM) are considered on the basis of experimental data.