Atomic structures of iodine adlayers on Pt(l 11) and Au(lll), have been described, on our recent results using in-situ STM and ex-situ LEED analysis. The coexistence of two commensurate structures, (3 x 3) and (V7 x V7)R19.1°, has been found on Pt(l 11) in the double-layer potential region for a long period of time, indicating that the transformation between the two structures is surprisingly slow. Incommensurate adlayer structures were found to form on Au(l 11) and Ag(l 11), and the lattice constant was found to vary continuously with the electrode potential using ex-situ LEED, in good agreement with the recent surface X-ray scattering study. The effect of the iodine adlayer on the underpotential deposition of Ag has been briefly described, and a remaining unsolved problem has been pointed out. Finally, it was demonstrated that the iodine-modified electrodes are appropriate substrates for the investigation of the adsorption of organic molecules in solution. Highly ordered molecular arrays were found to form on the iodine-modified electrodes and visualized by in-situ STM, revealing their packing arrangement and even the internal structure of each molecule.

Acknowledgment The author gratefully acknowledges the support from Research Development Corporation of Japan (JRDC) as a project (Itaya-Electrochemiscopy Project) of Exploratory Research for Advanced Technology (ERATO).

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