Ion-induced emission of excited atoms from (100) surfaces of transition metal single crystals

The angle-resolved kinetic energy distributions of atoms ejected from Rh{100} and Ni{100} surfaces bombarded with 5.0 keV Ar⁺ ions have been measured for the ground state and two low-lying metastable excited states. The results indicate that the excitation probability varies as exp(- A/av_⊥) at high velocities for atoms emitted in so called 'open' electronic configuration (outermost s shell is not fully occupied) and is velocity independent if the configuration is closed. In the former case, the parameter A/a depends on the azimuthal and polar angles of ejection. The observed variations are attributed to the interaction between the substrate electrons and the ejecting atom. At low velocities the excitation probability becomes nearly constant which is attributed to a recently proposed mechanism of excited atom production due to inter-atomic collisions above the surface. The measurements indicate that the relative contribution of this mechanism increases with the excitation energy. The observed dependence of the excitation probability on electronic structure of the emitted atom is discussed in terms of the orbital de-excitation model.