An XPS and UPS study of the interaction of oxygen with sodium-dosed Ag(110)

Abstract

The adsorption of Na and the coadsorption of Na and O 2 on Ag(110) have been studied by XPS and UPS. Adsorption of Na results in a rapid decrease in the work function. Δφ reaching a limiting value of −2.0 eV at θ Na = 0.5 and thereafter remaining constant. In the coverage range 0 < θ Na <1 adsorption of O 2 onto the Na dosed surface always results in an increase in the work function to an almost constant value of Δφ ~ −1 eV. At the same time the XPS data show that the Na/O stoichiometry of the oxygen saturated surface remains essentially constant and independent of the initial Na dose. Calibration experiments using sodium formate as a standard compound indicate that this surface phase has the stoichiometry Na 2O. For θ Na > 1 there is a sharp change in behaviour; the work function of the oxygen saturated surface begins to decrease rapidly, and eventually falls below the value for the Na covered surface itself at θ Na ~ 1.5. The XP spectrum now shows the appearance of a new oxygen peak which increases in intensity as θ Na increases beyond unity. The UP spectra indicate that the binding energy of the surface orbital derived from Na (3s) is increased by ~6.7 eV as compared with the free atom value, and the emission at ~3 eV below E F, which is associated with surface oxygen is not greatly affected by the presence of Na. These results are discussed against the background of information already available from LEED, Auger, and thermal desorption studies, and we attempt to give a consistent interpretation of the properties of the system at coverages both below and above one monolayer.