NEGATIVE-ION FORMATION IN PROTON-SCATTERING FROM BA AG(111)/

A study on negative ion formation in the scattering of a proton beam f rom two monolayers of Ba on Ag(111) surface is presented. The dependen ce of the negative ion yield on the polar scattering angle and azimuth al orientation of the crystal has been determined for beam energies fr om 500 to 1000 eV. In addition the yield of fast neutrals has been det ermined. Based upon a shadowing and blocking analysis of the angular d istributions of the H- ions, it can be concluded that the Ba overlayer shows the same crystalline structure as the Ag(111) substrate. The re corded negative ion fractions can be explained by a simple model descr ibing resonant charge transfer calculated by the non-perturbative CAM method and taking into account the parallel velocity effect.