Electron-stimulated alkali metal desorption from the oxygen monolayer-covered tungsten surface

Abstract

Electron-stimulated desorption (ESD) of atoms and ions of Li, Na, and Cs from the oxygen monolayer-covered W(100) surface has been studied by means of a static magnetic mass spectrometer combined with a retarding field analyzer. The ESD cross section of the alkali metal atoms within the monolayer is shown to be independent of the concentration of the deposited metal and is (7±3)×10 −22 cm 2. The ESD cross sections of the Li +, Na +, and Cs + ions at low concentrations are (1.2±0.2)×10 −22, (4±1)×10 −23, and (6±1)×10 −25 cm 2, respectively, and fall off with increasing concentration. The electron energy threshold corresponding to the onset of ESD of the alkali metal atoms and ions is ∼25 eV. The ESD cross section ratio of the Li 6 + and Li 7 + ions is 1.2±0.1 for concentrations N<10 14 atoms/cm 2 and increases up to 1.4 for higher concentrations. The slope of the retardation curves increases as one goes from Li to Cs. The model involving oxygen ion local Coulomb field relaxation permits one to describe the ESD of alkali metals not only qualitatively, but quantitatively as well. It can be used to determine the effective charge of the oxygen, the lifetime of the oxygen excited state on the surface, and the ESD cross sections for the alkali metal ions. The concentration dependence of the ESD cross sections of the alkali metal ions originates from the metallization of adsorbed metal films.