STUDY OF ATOMIC RELAXATIONS ON CLEAN AND OXYGEN-COVERED (100)COPPER, (410)COPPER AND (510)COPPER SURFACES BY CHANNELING

Atomic relaxations on Cu(100) and stepped Cu(410) and (510) surfaces w ere measured using a few hundred keV He+ ions in channeling and double alignment geometries. On stepped surfaces, we have also attempted to determine the specific relaxation of edge atoms (both the components p arallel and perpendicular to the (100) terraces). For this purpose, we have analyzed blocking effects on particles backscattered near theta( lab) = 90 degrees, around directions parallel to various major crystal lographic axes of the terraces, the incident beam being aligned along the [100] axis perpendicular to the terraces. The studies were perform ed on clean and on oxygen covered surfaces. On (100) flat surfaces, we find a small contraction of the surface plane on clean Cu (about 2% o f the bulk interplanar spacing), and a marked expansion (about 10%) on oxygen covered Cu (at saturation coverage, whose value was measured b y nuclear reaction analysis). On stepped surfaces, it appears difficul t to discriminate between the specific relaxation of the [001] edge ro w perpendicular to the terraces and the relaxation of the other [001] rows inside the terraces. Concerning the component of the [001] edge r ow relaxation in the terrace plane, our results provide an unambiguous estimate: we find a small contraction (about 0.04 Angstrom) on bare C u and a larger expansion (about 0.18 Angstrom) under oxygen coverage. In all cases, we observed that the presence of oxygen induces a defini te surface disordering which can be described as random small atomic d isplacements of Cu surface atoms around their mean position (on the or der of 0.1 Angstrom). The results obtained on the (100) surface covere d with oxygen can be interpreted in terms of a missing row model which has been proposed by some authors. However, our data can also be fitt ed with no missing row. Similarly, on the oxygen covered (410) stepped surface the data interpretation does not require the assumption of a missing row, but the results are not inconsistent with a description o f the terrace where the fourth or possibly the third [001] row, parall el to step edge, is missing and where the edge [001] row suffers a con siderable outward displacement in the terrace plane (approximate to 0. 3 Angstrom). The limits of high energy ion channeling for the measurem ent of surface atomic displacements, in particular when many sites are involved (for instance in the case of alternate relaxation) are discu ssed and illustrated.