Oxygen adsorption on Ta(110)-supported Pd films studied with photoemission

Abstract

Oxygen chemisorption on Ta(110)-supported palladium films, five monolayers or less in thickness, is studied using photoemission and work function measurements. Palladium monolayers have valence band photoemission spectra which differ significantly when compared to subsequent layers or published results for Pd(111) (i.e. photoemission intensity 0–2 eV below E F is smaller). The photoemission work function change (−0.7 eV) for the Ta(110)-supported Pd monolayer when compared to Pd(111) suggests there is charge transfer from Pd to the Ta substrate. Oxygen absorption increases the work function and the work function increase is largest for the Pd monolayer but becomes smaller with increasing Pd coverage. Oxygen adsorption on Ta (110) produces a strong valence band photoemission feature which peaks near 5.8 eV binding energy. Oxygen interaction with the Pd monolayer produces two valence band features near 4 and 7.8 eV binding energy. A feature at 5.8 eV is seen after the adsorption of 2 or more langmuirs of oxygen. The modification of the Ta4f corelevel and valence band spectra during oxidation of the Pd-Ta bilayer suggests that oxygen enters the Ta substrate and suggests that the absorption of 2 or more langmuirs of oxygen causes the Pd monolayer to cluster permitting direct oxidation of the substrate.