Normal photoelectron diffraction studies of selenium and sulfur overlayers on Ni(011) and Ni(111)

Normal photoelectron diffraction was used to determine the structure of $c(2\mathrm{}\times 2)$ S and $c(2\mathrm{}\times 2)$ Se on Ni(011) and of four different overlayers of Se on Ni(111). For $c(2\mathrm{}\times 2)$ Se on Ni(011), Se bonds in the hollow site, whereas for $c(2\mathrm{}\times 2)$ S on Ni(011), it is difficult to choose between the hollow and the top sites. For both a low coverage, disordered Se overlayer and an annealed $p(2\mathrm{}\times 2)$ Se overlayer on Ni(111), Se was found to be situated in those threefold hollow sites which have a vacancy below them in the second layer. In the case of an unannealed $p(2\mathrm{}\times 2)$ Se overlayer on Ni(111), the Se is adsorbed in approximately equal amounts in the top and hollow sites.