Step-up versus ``Step-down'' scattering asymmetry in the charge transfer of H$^-$ on free-electron vicinal metallic surfaces

We present numerical results based on a wave-packet propagation study of the one-electron charge transfer between hydrogen anions and free-electron vicinal metallic surfaces [1]. An effective potential for the motion of the active electron is derived from Thomas-Fermi-von Weizs\"{a}cker theory, extended to include the image charge effects. The ion-survival probability near the surface is evaluated from a rate equation for projectiles that are incident with a kinetic energy of 50 eV. We find an enhancement of the electron loss near the steps of the surface, caused by the Smoluchowski effect. As a consequence, depending on the orientation of the surface steps, the ion- survival is more likely if the projectiles approach steps from above [2]. \newline \newline [1] B.~Obreshkov and U.~Thumm, Phys. Rev. A {\bf 74}, 012901 (2006). \newline [2] B.~Obreshkov and U.~Thumm, Surf. Sci., in press.