Oxidation of lithium monolayers on silver(111): A study by AES, work function and secondary emission changes

Abstract

The uptake of oxygen by a silver(111) substrate pre-dosed with monolayer and submonolayer quantities of lithium was followed by Auger spectroscopy, work function (φ) variations and changes in secondary electron emission. Oxidation stabilizes the layers by preventing dissolution of lithium into the bulk. The maximum quantity of adsorbed oxygen is proportional to the original lithium coverage, θ Li, up to the first lithium monolayer. The work function variations on oxygen adsorption exhibit remarkable changes as the coverage of pre-adsorbed lithium is increased. Initial decreases (positive-outward dipoles) are observed which indicate incorporation of oxygen in an underlayer. The magnitude of this decrease is proportional to θ Li 4. It is concluded that the underlayer oxygen atoms are “covered” by clusters of four lithium atoms. Long exposures lead to the formation of oxygen overlayers giving increases in φ (negative-outward dipoles). A new model and detailed calculations based on the dynamics of transitions from “on-top” to “underneath” sites, agree well with the observations. Secondary emission maxima are observed corresponding to the minima in φ. The growth mode of lithium is changed when deposition is made in a partial pressure of oxygen. Comparison is made with data for oxygen interactions with alkali metals on different substrates.