Core-level shift spectroscopy on tungsten surfaces. Overlayer and underlayer adsorption

Abstract

Core-level binding energy shifts in the 4f_7/2 level of W previously reported from a range of clean single-crystal surfaces are consistent with a simple linear relationship to an effective coordination number. Here we report data for the adsorption of H₂, O₂ and CO on W{110}, where conventional overlayers are formed, and for N₂ on W{110}, where an underlayer is formed. The observed adsorbate-induced shifts are consistent with a model which accounts for changes in the coordination of surface W atoms due to adsorption, invariably leading to an increase in binding energy, and also changes due to charge transfer between adsorbate and substrate. It is pointed out that these two effects are additive for electron-withdrawing adsorbates, but tend to cancel each other with electron-donating adsorbates. Recent observations for alkali-metal atom adsorption are consistent with this model, and do not imply non-ionic adsorption.