Local effects during ion scattering on adsorbate-covered surfaces

This study addresses the importance of adsorbate-induced local effects during collisions between projectiles and adsorbate-covered surfaces. Various systems are investigated: H- on Na/Ag(111); H- on Na/Cu(111); and Li+ on K/Al(100), by using wave packet propagation methods. From the projectile's perspective, the adsorbate's presence on the surface can be felt from up to a few tens of atomic units away when the projectile is close to the surface, and up to 100 a.u or more when the projectile is on top of adsorbate. This is a well-defined bell-shape region around the adsorbate. However, the scattered ion/neutral fractions are not significantly affected unless the projectile impacts the surface in the close 10-15 a.u. vicinity of the adsorbate. Scattering from adsorbate sites compared to scattering from substrate sites produces very different ion/neutral fractions, that vary not only with the surface coverage, but especially with the distance from the projectile's point of impact on the surface to the adsorbate. Our calculated Li neutral fractions obtained after scattering Li+ from K/Al(100) are in satisfactory agreement with the experimental results.