Surface chemistry of iodine on platinum (111)




Furman, Scott Anthony

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The adsorption of iodine on a platinum(111) single-crystal surface has been investigated using LEED, Auger spectroscopy, and work function measurements. The phase transformations and work function changes have also been measured during desorption. Mass spectroscopy shows that above 300 K the main desorption product is atomic iodine with a small amount of molecular iodine detected as well. The desorption kinetics at these temperatures were studied by different techniques to extract the kinetic parameters and the orders of the desorption reactions. There are two main desorption features, one displaying zero-order desorption kinetics typical of a phase transition and the other displaying first-order kinetics with a coverage-dependent activation energy. The work function changes during adsorption and desorption were shown to be a function of coverage rather than having a site dependence. The adsorption of iodine at temperatures below 200 K was also studied. Multilayers of molecular iodine are formed that desorb with essentially zeroth order kinetics. Two multilayer desorptions were observed with thermal desorption spectroscopy. One of the multilayer desorptions had a significant work function change associated with it. The work function changes were modelled by calculating the hybridization dipole moment using extended-Hückel theory with Bloch wavefunctions. The calculations are sensitive to the atomic position of the adsorbate and require further refinement. Due to the protective nature of the iodine layer and its high polarizability, the iodine layers were used to study the ambient pressure adsorption of fluorinated carbosilane dendrimers. These dendrimers are stable in vacuum but do not form an ordered structure at ambient temperature. Heating the adsorbed dendrimer in vacuum to 1100 K produced a new ordered structure on the platinum surface. This structure was shown not to be an intact dendrimer molecule as two different dendrimers with similar structural moieties produced the same (√19x√19)R23.4° LEED pattern. The ordered structure was studied by Auger spectroscopy to determine the carbon coverage. This structure is proposed to be islands of a coincidental lattice of graphite.



Surface chemistry, Iodine, Platinum