Tuning Charge Transfer in Ion-Surface Collisions at Hyperthermal Energies

Charge exchange in ion-surface collisions may be influenced by surface adsorbates to alter the charge state of the scattered projectiles. We show here that the positive ion yield, observed during ion scattering on metal surfaces at low incident energies, is greatly enhanced by adsorbing electronegative species onto the surface. Specifically, when beams of N^+ and O^+ ions are scattered off on clean Au surfaces at hyperthermal energies, no positive ions are observed exiting. Partial adsorption of F atoms on the Au surface, however, leads to the appearance of positively charged primary ions scattering off Au, a direct result of the increase in the Au work function. The inelastic energy losses for positive ion exits are slightly larger than the corresponding ionization energies of the respective N and O atoms, which suggests that the detected positive ions are formed from surface re-ionization during the hard collision event.