Tensor LEED analyses for three chemisorbed structures formed by iodine on a Pt(111) surface

Three distinct ordered iodine structures on a Pt(lll) surface have been stu died with LEED crystallography in the coverage range 0.33-0.44 monolayers. These surfaces have translational symmetries of the ((root 3 x root 3)R30 d egrees, (root 7 x root 7)R19.1 degrees and (3 x 3) types, and they all invo lve overlayer adsorption on the basically unreconstructed metal structures. The root 3 surface phase is indicated to have essentially all I atoms adso rbed at the regular threefold sites of the fee type (i.e. 3f sites), with n o significant involvement by the corresponding sites of the hcp type (i.e. 3h sites). The root 7 structure has one I on an atop Pt site, and one each at 3f and 3h sites per unit mesh, while the (3 x 3) surface has one I on an atop site and three on bridge sites per unit mesh. The I corrugation is ab out 0.5 Angstrom for the root 7 structure, but is reduced to around 0.1 Ang strom at the (3 x 3) surface. The surface I-Pt bond lengths from these anal yses show a general tendency to follow trends expected with the varying I c oordination numbers. A preliminary discussion is given for uncertainties as sociated with some relaxations indicated in the metallic structures.