K promoted oxidation of Al and Ta

Core level and valence band photoemission have been used to study the response to oxygen exposure of potassium-precovered, polycrystalline aluminum and tantalum substrates. Deposition of potassium overlayers in the coverage regime from zero to one monolayer, results in an increase of the rate of oxygen uptake for both substrates. The increased oxygen uptake is, however, relatively stronger on aluminum. Correlated measurements of work function and oxygen uptake show that there is a correspondence between these two parameters for low K coverages, i.e., lower work function results in higher oxygen uptake. These results may be understood in terms of a model in which the alkali atoms are increasing the rate of dissociation of oxygen molecules at the surface, as well as lowering the barrier for oxygen penetration into the surface, through a reduction in the system work function.