Location of Adsorbed Foreign Atoms by Surface Channeling

Abstract

Most location studies are done by Low Energy Electron Diffraction (LEED) which primarily yields the symmetry and structure of the adsorbed layers. Moreover, LEED requires a regular arrangement of the adsorbed atoms. Here we present a direct technique for the location of adsorbed foreign atoms on a monocrystalline surface /1/ by applying “surface channeling” /2/ where fast ions at grazing angles are incident along low index directions of the surface. Then, the scattering of the ions arises from successive correlated collisions with many atoms of the low index surface rows. This allows us to describe surface channeling with continuum row potentials analogous to axial channeling in bulk /3/. Fig.1 shows the projection of the pertinent interaction potential of a (101) Ni-surface with deuterons obtained by superposition of three neighbouring [101] continuum row potentials. At grazing incidence along the [101] direction the ions experience strong steering by this repulsive potential. The important aspect for the atom location experiments is that thereby the ion flux density is reduced at the rows and enhanced between the rows. This effect can be used to locate the position of foreign atoms at the surface, since their interaction with incident ions (e.g. nuclear reactions, characteristic X-ray production) occurs with cross sections which decrease with increasing impact parameter. Therefore, we obtain a reduction in the reaction yield, if the foreign atoms are shadowed by the atomic rows and an increase, if their position is between the rows.