Electron collisions with complex targets: Diffraction effects in stimulated desorption

We describe a new electron-scattering technique involving incident beam diffraction in electron-stimulated desorption (ESD) to probe ground-state surface bonding configurations of adsorbates and excited state configurations which lead to cation ESD. We present a qualitative theoretical description of diffraction in electron-stimulated desorption (DESD) and then demonstrate this process by studying the ESD of Cl⁺ from Cl terminated Si(111) (1 × 1) surfaces. The fine structure in the Cl⁺ ESD yields as a function of azimuth is due to interference of the incident electron-beam with the elastically scattered fraction. This interference forms a surface standing wave in the initial state of the desorption process. Comparison of the experimental data to the qualitative theory indicates that desorption is primarily initiated by an excitation localized on the Si atom bonded to Cl. The ∼17 eV threshold energy is consistent with an ESD mechanism involving ionization of the Si 3s level followed by Auger cascading from the σ-bonding surface state and shake-up of an electron in the non-bonding surface π-level. The 2-hole, 1-electron final state produced can lead to the ESD of Cl⁺ due to the hole-hole Coulomb repulsion. Direct ionization of the Si-Cl σ-bonding state can also contribute to Cl⁺ ESD. In general, DESD provides a useful probe of the geometry and dynamics governing low-energy electron interactions with ordered complex targets.