STRUCTURAL DETERMINATION OF THE CU(111)(ROOT-3X-ROOT-3)R30-DEGREES-CL/BR SURFACE USING THE NORMAL INCIDENCE X-RAY STANDING-WAVE METHOD

A full structural determination has been made of the Cu(111)(root 3 x root 3)-R30 degrees-Cl/Br surface, which comprises a mixed Cl/Br overlayer with a nominal coverage of one third of a monolayer and a 1:1 stoichiometry, using the normal incidence X-ray standing wave (NIXSW) method. Two Bragg reflections were used, the (111) which uses reflecting planes parallel to, and the (($) over bar 111) which are tilted at an angle of 70.5 degrees to, the (111) surface. Using the (111) data, adsorbate-substrate outermost layer spacings were found to be 1.81 +/- 0.05 Angstrom for chlorine and 2.00 +/- 0.05 Angstrom for bromine assuming zero substrate relaxation. These results, coupled with the (($) over bar 111) data, are consistent with a model in which both halogens, each of which has a coverage of 1/6 monolayer, occupy three-fold coordinated hollow sites on the surface. The adsorbates occupy primarily fee hollows (directly above third-layer Cu atoms) but some occupation of hcp hollows (above second-layer Cu atoms) is also observed. For chlorine the fcc:hcp population ratio is 75%:25% (+/- 10%), while for bromine the rate is 80%:20% (10%). Possible origins of the minor species hcp hollow site occupation are discussed.