A TENSOR LEED STRUCTURAL STUDY OF THE COVERAGE-DEPENDENT BONDING OF IODINE ADSORBED ON RH(111)

Using automated tensor LEED (ATLEED), the structure of I on Rh{111} has been studied in the coverage range from 0.12 to 0.33 monolayers, including the ordered (square-root 3 x square-root 3)R30-degrees phase. Iodine adatoms are bonded in fcc three-fold hollow ''lattice'' sites with an iodine-rhodium interplanar separation of 2.22 +/- 0.06 angstrom and a first to second rhodium interlayer contraction of 3.7%. An iodine-to-rhodium nearest neighbour bond length of 2.71 angstrom indicates an iodine ''effective radius'' of 1.37 angstrom, very close to the covalent radius of 1.35 angstrom assuming a rhodium metallic radius of 1.34 angstrom. Adsorption in hcp hollow sites yielded a minimum Pendry R-factor of 0.40. The possibility of site mixing was examined and an R-factor minimum of 0.27 was obtained for 85 +/- 15% fcc site occupation. The structure at lower coverages (0.25 and 0.12 ML), where only a diffuse p(1 x 1) LEED pattern is observed was examined by diffuse I-V analysis. Diffuse I-V spectra were found to be practically indistinguishable from spectra from the (square-root 3 x square-root 3)R30-degrees structure for identical parallel momentum transfer. An R-factor analysis indicates no coverage-dependent site switching and a coverage independent Rh-I bond length in the coverage range studied (0.12 less-than-or-equal-to theta less-than-or-equal-to 0.33 ML).