Different Processes for Desorption of Ground- and Excited-State Atoms Under Electron Bombardment of Alkali-Halides

Abstract

Single crystal surfaces of LiF were bombarded with 5 to 500eV electrons at different crystal temperatures (300–800K). The temperature dependence as well as the energy dependence of ground- and excited-state alkali atoms were studied. These measurements strongly suggest different mechanisms for the desorption of ground-and excited-state alkali atoms. An excitation of a valence electron into the conduction band and subsequent decay of the formed excited state into an F-H pair will finally lead to the desorption of a ground-state atom, which is well established in the literature. The creation of a core-hole on an alkali lattice-site close to the surface followed by an inter-atomic Auger decay of that core-hole leads to emission of excited alkali atoms and ions. The measurement of the threshold energies for ground- and excited-state alkali atom desorption is a major piece of evidence. The threshold energies are found at 13 and 60eV for the desorption of ground- and excited-state atoms, respectively. This agrees very well with the band gap of 14.2eV and the core-hole excitation of Li⁺(1s-→2s) at 60.8eV and for Li⁺(1s-→2p) at 61.9eVand is clear evidence for a surface intrinsic process for the generation of excited alkali atoms and will be discussed together with the underlying experimental results.