Layer-dependent reactivity in the FeÕMo„110... epitaxial ultrathin film system

Fe films in the 0.4<u<4.7 ML ~monolayer! coverage range were deposited on a Mo~110! substrate at temperatures between 300 K and 700 K, under ultrahigh vacuum conditions. The adsorption of CO, CO2 , H2O, and O2 onto these films from the residual gas within the Ultrahigh vacuum system has been studied using scanning tunneling microscopy, low-energy electron diffraction, and Auger electron spectroscopy analysis. The first, second, and third Fe layers display distinct structures, correlated to the different adsorption characteristics of each layer. Separate mechanisms are identified as the cause of this layer-dependent reactivity. The difference between the first and second Fe layers is attributed to the difference in charge polarization effected between these layers and the substrate. However, our results prove that the different adsorption characteristics of the pseudomorphically strained first layer and the two-dimensionally strain-relieved third layer are directly linked to the film strain.