REINTERPRETATION OF THE ADSORPTION-KINETICS DATA MEASURED BY AES AND XPS

A classical example of employing the methods of electronic spectroscopy in the study of chemisorption kinetics is the work on surface oxidation of iron and nickel. In the curve which plots the change of sticking coefficient versus surface coverage with oxygen, a characteristic minimum is seen for a coverage less than that on the surface most closely packed with oxygen atoms. As yet, the increase in sticking coefficient has been related to the formation of oxide nuclei. In this work, a model is suggested that elucidates the observed relation between surface coverage and oxygen exposure. This model refers to the actual mosaic structure of monocrystalline metal samples. It is assumed that due to their mobile diffusion, the oxygen atoms can occupy positions not only on the surface, but also on the borders of mosaic blocks. The model was examined by studying the kinetics of Fe(111) surface oxidation and of low-temperature sulfur segregation on that surface. About 4 x 10(14) Sites per 1 cm2 of surface area were determined to be available for sulfur adsorption at the borders of mosaic blocks.