EFFECT OF ADSORBED IODINE ON THE DISSOLUTION AND DEPOSITION REACTIONSOF AG(100) - STUDIES BY IN-SITU STM

In situ scanning tunneling microscopy has been employed to determine t he structure of iodine adlayers formed on Ag(100) and to evaluate the role of such adlayers in the dissolution and deposition reactions of t he substrate in perchloric acid. In the double-layer potential region, iodine was found to possess a c(2 x 2) structure; this structure was unaffected by anodic dissolution of the Ag substrate. On both bare and I-pretreated Ag(100) surfaces, the anodic dissolution of Ag occurred exclusively at step-edges in a layer-by-layer mechanism. However, wher eas jagged monatomic step lines were found on the I-free surface, rela tively smooth (straight) step lines existed along the direction of iod ine atomic rows ({100}) on the Ag(100)-I surface. Etching rates on Ag( 100)-I were dependent on the direction of the step-edges, which sugges ts that the adlayer near such edges plays a crucial role in the dissol ution reaction. The deposition of Ag on Ag(100)-I was also found to oc cur preferentially at step-edges aligned along a particular direction. A model is proposed to explain the anisotropic dissolution and deposi tion.