Geometrical and electronic structure of multiple surface phases: Iodine on Ni{100}

The existence of several surface phases formed by the adsorption of iodine on Ni{100} has been established using qualitative LEED, thermal desorption and Auger electron spectroscopy, and models have been proposed for these structures on the basis of data from these techniques. In particular, two different phases can be found at room temperature, a c(2 × 2) chemisorbed phase and an incommensurate structure identified as a slightly distorted single sandwich layer of NiI₂. Heating of either of these phases leads to some desorption and the formation of a variable-sized centred rectangular mesh chemisorbed phase characterized by varying iodine- iodine repulsive energies. More recent data obtained using surface EXAFS and angle-resolved core and valence level photoemission confirm the structural assignment for the surface iodide and show its electronic structure to be essentially that expected of bulk NiI₂. Studies of core level binding energy changes as seen in photoemission in the range of chemisorbed structures are discussed, and are interpreted as indicating that the Ni-I bond in this system is essentially covalent with the I-I repulsion resulting from roughly equal contributions from through metal and through space interactions.