The bonding of chalcogen overlayers on Ni (100)

We present new evidence for the character of two forms of bonding by chalcogen adsorbates on Ni(100) for two different coverages, while the adsorbate site remains the same. Comparisons with data from solid state systems, rather than molecular analogs, are used and may be appropriate sources for models in such cases of strongly chemisorbed adatoms. By comparing recent photoemission data on solid chalcogens (S, Se, Te) with that of chalcogen overlayers on Ni(100), we are able, but for different reasons, to support the previous proposal that with the chalcogen atoms adsorbed in a four-fold site that (a) for the ordered c(2×2) one half-monolayer coverage, the chalcogen adatoms on Ni(100) are bonded to two Ni atoms and that (b) for an ordered one-fourth monolayer coverage, the chalcogen is bonded to four Ni atoms. Two chalcogen adsorbate-induced levels, one largely non-bonding and the other made up of bonding electrons, should be found in the valence region when the chalcogen is two-fold coordinated, as it is in the elemental solid. Several methods for testing these proposals have been suggested. The modification of adatom bonding as a function of coverage implies important adatom-adatom interactions which may be involved in the poisoning of Ni catalysts by chalcogen compounds.