ESCA measurement of surface atom oxidation states on chemically modified Mo(100) surfaces

Quantitative surface atom oxidation states are measured in a conventional ESCA spectrometer. Spectra that solely reflect the chemical nature of the surface atoms are generated from a linear combination of spectra measured at two take-off angles. Proper selection of the multiplicative constants results in removal of bulk atom effects from the measured spectra. A simple method which is based on photoelectron attenuation lengths is used to calculate these coefficients. The surface Mo atom oxidation numbers for a series of chemically modified Mo(100) surfaces have been obtained by this procedure. Charge transfer effects resulting from the adsorption of electronegative surface modifiers (B, C, O and CO) are observed. At constant adatom coverage, the Mo oxidation number is linearly dependent on the adatom electronegativity. For a series of partially oxidized Mo(100) surfaces the extent of charge transfer to the metal atoms is not a simple function of the surface oxygen coverage. A model for the initial oxidation of Mo(100) is presented which is in agreement with previous structural and electronic state studies. At oxygen coverages below one monolayer, a chemisorbed oxygen phase exists which produces a constant Mo oxidation number of 1.3. At higher coverages, a rapid increase in the Mo surface atom charge indicates the formation of a surface oxide layer. The electron deficient Mo surface atoms are acidic sites for adsorption of gas phase Lewis bases. The relative strengths of the acidic sites correlate with the metal atom oxidation numbers. This result suggests that the observed adsorption selectivity and reactivity of these surfaces can be discussed in a traditional acid-base context.