Emission of electrons from clean and modified Ru(0001) surfaces by low-energy Na⁺ bombardment

Kinetic electron emission (KEE) arising from low-energy (120–1620 eV) Na⁺ impact on clean Ru(0001) and Ru(0001) covered with submonolayers of Na, I, Cl, and O is investigated. The observed KEE can be consistently explained by a nonadiabatic electron emission process. As the projectile approaches the surface, electrons in the surface bands are excited by the time variation of the interaction between the projectile and surface. The electron emission mechanism is strongly dependent on the Na⁺ impact velocity v and on the surface work function φ. The interaction is characterized by γ, the inverse value of the interaction length. It is possible to quantitatively reproduce the dependence of the electron yield on v and φ with a single value of γ, even in the presence of adsorbates. For oxygen adsorption and, to a lesser extent, for Cl adsorption, however, the electron energy spectra are broadened. This broadening, tentatively ascribed to electron-electron interactions, has been included in the model and explains the specific features of O/Ru and Cl/Ru quite well. For Cl/Ru and I/Ru, small contributions of a low-energy Auger emission process have also been identified.