CHARGE-TRANSFER DURING PYRIDINE ION-SCATTERING OFF CLEAN AND OXYGEN MODIFIED NI(111)

We examined the neutralization of a pyridine ion, C5H5N+, when it was scattered at 20 and 40 eV off a clean metal surface, Ni(lll), an oxyge n adsorbed metal surface, p(2 x 2)O/Ni(111), and a well characterized oxide thin film, NiO(111)/Ni(111). There was a large enhancement in th e scattered ion signal upon going from the clean Ni(111) to the oxygen modified surfaces at both collision energies. At 20 eV, both modified surfaces showed a 30 x enhancement of the scattered ion yields with r espect to the clean surface. At 40 eV, the oxygen covered surface show ed a 75 x enhancement while the oxide showed a 150 x enhancement. Thes e enhancements apparently resulted from oxygen-induced changes in the surface density of states which affected the resonant charge transfer of an electron from the surface to the scattered ion. These enhancemen ts cannot be explained solely by changes in the surface work function or the surface dielectric layer thickness. Our attempts to quantitativ ely fit our data to resonant charge transfer models were thwarted by a variety of experimental and fundamental issues, including the fact th at the scattered ion fragments from a given parent ion and energy disp lay different normal velocities. We also observed that scattered ion s ignals off clean Ni(111) increased with incident ion exposure, an effe ct which we attribute to the chemical modification of the surface: lit tle change was seen for the oxygen modified surfaces.