Resonant charge transfer during the backscattering of 3.0 keV Li+ from hydrogen- and iodine-covered Ni(100) is probed with time-of-flight spectroscopy. Hydrogen adsorption on Ni(100) induces only a small increase of the surface work function and the neutralization probabilities for backscattered Li are not significantly affected. Iodine adsorbs with some net negative charge, so that a dipole directed into the surface is expected. Such a dipole would increase the work function thereby decreasing the neutralization probability. iodine adsorption decreases the work function of Ni(100), however, and the neutralization probabilities for Li scattered from the iodine sites are always larger than for scattering from nickel sites. These results suggest that the local charge density associated with adsorbed iodine is not uniform. (C) 2003 American Vacuum Society.