Coverage dependent shifts of XPS peaks during chemisorption on metals

Shifts of substrate and adsorbate XPS core level peaks and of some Auger peaks have been measured as a function of electron emission angle and of coverage in the adsorption systems oxygen on W (100) and (110), Ru (001), and Ag (110), and CO on Ru (001). The substrate peaks are mostly observed to shift to higher binding energies with increasing coverage, while for adsorbate peaks shifts to higher and lower energies have been observed. Low electron emission angles can be used to maximize the contribution of the first layer in substrate core peaks and valence emission. The shifts show no simple relation to work function changes. Possible contributions to the observed effects are discussed.