Adsorption of oxygen on electrolytic silver by UPS and work function measurement

The adsorption of oxygen on an electrolytic silver surface has been investigated by ultra-violet photoelectron spectroscopy (UPS), and by the measurements of work function change and kinetics. UPS difference spectra show three positive peaks at about 3.0, 5.3 and 9.0 eV below the Fermi level, which are due to the adsorbate resonances of atomic and molecular oxygen on the silver surface. The peak at 5.3 eV indicates that the orbitals derived from the πg∗ orbital of molecular oxygen are electronically filled in the adsorbed dioxygen complex and the peak at 9.0 eV corresponds to electron feedback from the πu orbital of adsorbed dioxygen species to valence orbitals of silver. The bonding configuration is also supported by the changes of work function. Owing to the transfer of electrons from metal to adsorbate, this adsorption of the atomic oxygen increases the work function (Δφsat = +0.5 eV), and O− (ads) species are formed; with the saturated surface the constant dipole moment per adsorbed oxygen is 0.24 Debye. The kinetic results show the presence of a maximum (Smax = 9.3 × 10−3) in the sticking coefficient curve at room temperature. The relation, S = k exp(550/T), has been obtained by measurement of the kinetics, and the results are discussed in terms of precursor adsorption models.