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 p roton beam from both a clean and one monolayer of barium-covered Ag(11 1) surface is presented. The angular and energy dependence of the back scattered negative hydrogen ions as a function of incoming and azimuth al angles has been determined for a beam energy of 750 eV. The backsca ttered negative particles emerge from the surface as well as from deep er layers of the crystal. The angular dependence of the outgoing parti cles shows a very rich structure, which is explained by shadowing and blocking of the incoming and outgoing particles. In addition, the angu lar dependence of the outgoing neutral particles is determined. The es sential features appear the same, but distinct differences can be obse rved. These are due to changes in the probability for negative ion for mation as a function of outgoing angle. The energy distributions of th r outgoing particles suggest a large penetration depth along the cryst al 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 partic les into the bulk of the crystal and complicated zigzag trajectories t hrough 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 eV/Angstrom at E= 700 eV) than the values found in the literatu re. Comparing the Ag(111) data and the data of Ag(111) covered by one monolayer barium, we conclude that the barium atoms occupy lattice pos itions of the crystal. The overlayer must contain vacancies to accommo date the large size mismatch between the barium atoms and those of the substrate.