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

A classical example of employing the methods of electronic spectroscop y in the study of chemisorption kinetics is the work on surface oxidat ion of iron and nickel. In the curve which plots the change of stickin g coefficient versus surface coverage with oxygen, a characteristic mi nimum is seen for a coverage less than that on the surface most closel y packed with oxygen atoms. As yet, the increase in sticking coefficie nt has been related to the formation of oxide nuclei. In this work, a model is suggested that elucidates the observed relation between surfa ce coverage and oxygen exposure. This model refers to the actual mosai c structure of monocrystalline metal samples. It is assumed that due t o their mobile diffusion, the oxygen atoms can occupy positions not on ly on the surface, but also on the borders of mosaic blocks. The model was examined by studying the kinetics of Fe(111) surface oxidation an d 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 fo r sulfur adsorption at the borders of mosaic blocks.