Low-energy hydrogen-ion scattering from metal surfaces: Trajectory analysis and negative-ion formation

A comparative study on negative ion formation in the scattering of a proton beam from both a clean and one monolayer of barium-covered Ag(111) surface is presented. The angular and energy dependence of the backscattered negative hydrogen ions as a function of incoming and azimuthal angles has been determined for a beam energy of 750 eV. The backscattered negative particles emerge from the surface as well as from deeper layers of the crystal. The angular dependence of the outgoing particles shows a very rich structure, which is explained by shadowing and blocking of the incoming and outgoing particles. In addition, the angular dependence of the outgoing neutral particles is determined. The essential features appear the same, but distinct differences can be observed. These are due to changes in the probability for negative ion formation as a function of outgoing angle. The energy distributions of the outgoing particles suggest a large penetration depth along the crystal channels. We have performed classical trajectory calculations that simulate the angular distributions of the backscattered particles very well. These calculations also show considerable penetration of particles into the bulk of the crystal and complicated zigzag trajectories through the bulk before leaving the crystal. The (electronic) stopping inside the Ag solid is at least one or two orders of magnitude smaller ($<0.3\mathrm{}\mathrm{e}\mathrm{V}/\mathrm{\AA }$ at $E=700\mathrm{}\mathrm{eV}$) than the values found in the literature. Comparing the Ag(111) data and the data of Ag(111) covered by one monolayer barium, we conclude that the barium atoms occupy lattice positions of the crystal. The overlayer must contain vacancies to accommodate the large size mismatch between the barium atoms and those of the substrate.