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Abstract

Two distinctly different forms of sub-surface oxygen have been identified and the mechanism leading to their formation clarified. The first of these species is Oa. It is bulk-dissolved oxygen which diffuses interstitially through low-resistance paths such as grain boundaries and higher-indexed crystalline planes. TDS analysis reveals that high temperature oxygen pretreatment results in the formation of crystalline surfaces exhibiting different oxygen diffusion barriers. At elevated temperatures, pure thermal reordering dominates over oxygen-induced restructuring of the silver bulk and surface. The near-surface region is, therefore, comprised primarily of low-indexed crystalline planes. At temperatures in excess of 923K, bulk-dissolved, atomic oxygen (Ob) segregates into these low-indexed planes. This likely occurs via an interstitialcy diffusion mechanism where oxygen substitutes for silver atoms in the lattice. This intercalated oxygen is referred to as Og. A comparison of TDS and XPS results shows that annealing of the silver in vacuum results in surface restructuring to low-indexed planes which subsequently results in a marked increase the amount of Og observed by TDS.