The role of adsorbates in the Na+-induced kinetic emission of electrons from Ru(0 0 0 1)

Electron emission was measured from clean and adsorbate-covered Ru(0 0 0 1) surfaces under bombardment by 120-1620 eV Na+ ions. A kinetic process is solely responsible for the emission of electrons in these systems, as the low ionization energy of Na excludes any potential emission. A mechanism based on the interaction of the projectile energy levels with the surface bands is proposed. As the projectile nears the surface, this interaction increases rapidly and the excitation of electrons above the Fermi level due to the Heisenberg uncertainty principle gives rise to electron emission. This process is strongly dependent on the velocity of the incident ions and on the surface work function. The model is shown to correctly account for changes in the electron yield from the clean surface as a function of incident ion energy. When Na, Cl and I are adsorbed onto the surface, variations in the electron yield can be largely explained by changes in the surface work function. For O, however, the yield is larger than expected, which is tentatively ascribed to a broadening of the electron energy distributions due to electron-electron interactions involving the oxygen 2p shells