Alkali co-adsorption and surface coverage effects on electron-stimulated desorption (ESD): CO and NO on Ni(111)

Abstract

We present results on the effect of potassium co-adsorption on the ESD from CO and NO on Ni(111) at 90 K. K co-adsorption strongly affects the yields of positive ions and alters their electron beam energy dependence. At a K-coverage of θ K = 0.25 and saturation coverages of CO or NO, the strong O + ESD threshold associated with O 1s ionization is eliminated. We attribute this to increased metal → CO(2π ★) electron back-donation upon K addition which screens the O 1s hole, and reduces the excitation probability of shake-off/shake-up processes responsible for O + ESD. For fixed K-coverage, the ESD yields depend on adsorbate coverage. For θ K = 0.25 and CO or NO exposures <1 L the adsorbate state is ESD inactive at both valence- and core-excitation energies.