Unexpected Negative-Ion Conversion in Grazing Scattering of Negative Ions on HOPG

An experimental study of 8.5-22.5 keV-energy C-, O-, and F- ion scattering from an HOPG surface has been performed at a grazing scattering angle. For a given incident angle or impact velocity, the F- fraction is the highest and the C- fraction is the smallest among these three projectiles, which is explained with the order of the electron affinity level of the projectile. Moreover, the unexpected observations are that the F- ion fraction increases monotonically with impact velocity and that the negative-ion fraction increases with the decrease in exit angle. It can be explained by the influence of the effective high work function of HOPG and the production of positive ions at short distances from the surface.